Car tire pressure light TMPS came on in cold weather yesterday
February 11, 2017 8:40 AM Subscribe
Yesterday the tire pressure light came on, but the high was 18 degrees F.
Today the high is 30F.
Do I have to put air in the tires, or is it safe to drive?
Unless the tire is visibly low, you are probably safe to like, go about your daily errands or whatever, but I wouldn't go on a road trip or anything. Just get air in it as soon as is practical.
posted by Rock Steady at 8:45 AM on February 11, 2017
posted by Rock Steady at 8:45 AM on February 11, 2017
Response by poster: I already did this fairly recently when the weather started getting colder. Also, because it is warmer today, I expect the pressure to be higher than the value that caused the TMPS light yesterday.
posted by spacefire at 8:45 AM on February 11, 2017
posted by spacefire at 8:45 AM on February 11, 2017
When you're driving around, your tires get much hotter than the ambient temp, so air temp doesn't play that much of a factor. At the very least you should check all the pressures (regularly) with a good gauge.
posted by hwyengr at 8:50 AM on February 11, 2017 [3 favorites]
posted by hwyengr at 8:50 AM on February 11, 2017 [3 favorites]
Beware, tire pressure can be quite low before it starts to look visibly low, depending on the tire and wheel. I do not recommend using visual inspection to determine if a tire is safe to drive on or not (and even if it's reasonably safe, driving on low pressure can do a lot of damage to your tire in a short period of time).
posted by primethyme at 9:15 AM on February 11, 2017 [5 favorites]
posted by primethyme at 9:15 AM on February 11, 2017 [5 favorites]
Pull into your nearest gas station and check your tire pressure. If you recently put air in your tires and it's low again (not just because of weather) you may have a slow leak.
posted by basalganglia at 9:20 AM on February 11, 2017
posted by basalganglia at 9:20 AM on February 11, 2017
I'm not a physicist but if your air pressure went down went it was cold I don't see how it would go back up when it gets warm again. Air doesn't magically flow back in.
posted by AFABulous at 10:23 AM on February 11, 2017 [1 favorite]
posted by AFABulous at 10:23 AM on February 11, 2017 [1 favorite]
I'm not a physicist but if your air pressure went down went it was cold I don't see how it would go back up when it gets warm again. Air doesn't magically flow back in.
I'm not a physicist either, but I don't think this statement is completely accurate. Temperature is directly related to pressure and volume of gasses (PV=nRT), so that's why if you drive around for awhile, causing the tires to heat up, tire pressure increases. It's not about air leaving and then coming back in (although obviously there is some loss of air in tires over time), but that the gas expands with increased temperature. I'm no expert on physics or cars, so I could be wrong about this, though.
With that being said, I do live in a cold environment, and in your position, I would use a tire pressure gauge to check the pressure of all 4 tires. In particular, be on the look out for one tire having somewhat lower pressure than others. A couple times in the last several years, I've had the tire light come on in my car, and I assumed it was just the cold weather, only to find that I had a slow leak in one tire. (This was not obvious on visible inspection, fwiw).
I'll also admit that I'm pretty lazy, so I will often run errands or drive to work after the tire pressure light comes on, as long as I check the air and see that the air pressure is roughly even on all 4 tires and that it isn't a crazy low value. However, this is driving at low speeds for short distances. I would not hop on the highway or anything like that without out checking for slow leaks and adding air.
posted by litera scripta manet at 10:51 AM on February 11, 2017 [2 favorites]
I'm not a physicist either, but I don't think this statement is completely accurate. Temperature is directly related to pressure and volume of gasses (PV=nRT), so that's why if you drive around for awhile, causing the tires to heat up, tire pressure increases. It's not about air leaving and then coming back in (although obviously there is some loss of air in tires over time), but that the gas expands with increased temperature. I'm no expert on physics or cars, so I could be wrong about this, though.
With that being said, I do live in a cold environment, and in your position, I would use a tire pressure gauge to check the pressure of all 4 tires. In particular, be on the look out for one tire having somewhat lower pressure than others. A couple times in the last several years, I've had the tire light come on in my car, and I assumed it was just the cold weather, only to find that I had a slow leak in one tire. (This was not obvious on visible inspection, fwiw).
I'll also admit that I'm pretty lazy, so I will often run errands or drive to work after the tire pressure light comes on, as long as I check the air and see that the air pressure is roughly even on all 4 tires and that it isn't a crazy low value. However, this is driving at low speeds for short distances. I would not hop on the highway or anything like that without out checking for slow leaks and adding air.
posted by litera scripta manet at 10:51 AM on February 11, 2017 [2 favorites]
AFABulous: Gases have a direct relationship between temperature and pressure. Even if the amount of gas doesn't change. (source)
posted by Wild_Eep at 10:52 AM on February 11, 2017 [3 favorites]
posted by Wild_Eep at 10:52 AM on February 11, 2017 [3 favorites]
I endorse primethyme's plan, above. One of our cars has the TPMS light come on from time to time, seemingly randomly, and go off just as randomly. We live in the bay area and the temperature never gets below about 40, and is rarely above 70 where the car is parked. I've concluded that the "monitoring" aspect of TPMS is just not that good - I keep an eye on the tires, visually, and if they appear deflated, I measure with a physical tire gauge. I would never conclude my tires are unsafe just because of the TPMS light without an additional measurement, although I might be induced to make that measurement based on the TPMS light.
Also I don't mean to pile on AFABulous, but yes the air in a tire does shrink (i.e., pressure goes down) when it gets cold, and expands (i.e., pressure goes up) when it gets hot under the ideal gas law.
posted by Joey Buttafoucault at 11:09 AM on February 11, 2017 [1 favorite]
Also I don't mean to pile on AFABulous, but yes the air in a tire does shrink (i.e., pressure goes down) when it gets cold, and expands (i.e., pressure goes up) when it gets hot under the ideal gas law.
posted by Joey Buttafoucault at 11:09 AM on February 11, 2017 [1 favorite]
Our (older system) TPMS always comes on when it's cold overnight and goes off a couple blocks later. Ours come on as soon as the pressure drops below minimum recommended, so it doesn't so much mean "your tire is flat" but "your tire is possibly .5psi below spec".
Obviously, good pressure gauges are cheap and you can keep one with you, though I've been told that if you're in very cold climates you should keep it inside/in your bag and use it warm, not cold from the glove box. It's just that the early-morning warning has not ever actually meant a crisis for either of our cars, though I will pull over immediately if one comes on after I've been driving and the tires are warm and fluffed.
posted by Lyn Never at 11:23 AM on February 11, 2017
Obviously, good pressure gauges are cheap and you can keep one with you, though I've been told that if you're in very cold climates you should keep it inside/in your bag and use it warm, not cold from the glove box. It's just that the early-morning warning has not ever actually meant a crisis for either of our cars, though I will pull over immediately if one comes on after I've been driving and the tires are warm and fluffed.
posted by Lyn Never at 11:23 AM on February 11, 2017
Did you reset the TPMS after you put air in the tires? If not, you should check the pressure and if you're ok with the pressure, reset/calibrate it.
posted by watrlily at 11:33 AM on February 11, 2017
posted by watrlily at 11:33 AM on February 11, 2017
This is not something you should spend so much time thinking about that you write an AskMe.
Part of driving is checking your fluids and tire pressure with some regularity. A good time to do it is when you're getting gas, you can check your oil and chuck the napkin into the bin provided.
If you have any reason to beleive your tire pressure might be off, just check it. Keep a tire gauge in the glovebox. They're like $2. It only takes a couple of minutes.
posted by teatime at 11:39 AM on February 11, 2017 [5 favorites]
Part of driving is checking your fluids and tire pressure with some regularity. A good time to do it is when you're getting gas, you can check your oil and chuck the napkin into the bin provided.
If you have any reason to beleive your tire pressure might be off, just check it. Keep a tire gauge in the glovebox. They're like $2. It only takes a couple of minutes.
posted by teatime at 11:39 AM on February 11, 2017 [5 favorites]
Best answer: i was delighted to discover that a bike pump will work on car tires.
Personally I always try to regulate the pressure based on the here-and-now. If you put in air when it's cold, and it heats up again, take some out.
posted by rebent at 11:42 AM on February 11, 2017 [1 favorite]
Personally I always try to regulate the pressure based on the here-and-now. If you put in air when it's cold, and it heats up again, take some out.
posted by rebent at 11:42 AM on February 11, 2017 [1 favorite]
"This is not something you should spend so much time thinking about that you write an AskMe."
Seconding this. Why wouldn't you just check it and put some air in if it needs it? It's not like it takes a lot of time or anything. Unless you live like 30 miles from a gas station, in which case you should have one of those air compressor emergency inflator things. Not a bad idea to get one of those anyway.
posted by kevinbelt at 12:20 PM on February 11, 2017
Seconding this. Why wouldn't you just check it and put some air in if it needs it? It's not like it takes a lot of time or anything. Unless you live like 30 miles from a gas station, in which case you should have one of those air compressor emergency inflator things. Not a bad idea to get one of those anyway.
posted by kevinbelt at 12:20 PM on February 11, 2017
I'll add that even though your warning light is no longer on, your tires are almost certainly low still. Maybe not low enough to warn you, but the warm air didn't expand enough that your tires are now fully inflated. It's a good idea to keep your tires full (better gas mileage, etc.), so go fill them up.
posted by kevinbelt at 12:25 PM on February 11, 2017
posted by kevinbelt at 12:25 PM on February 11, 2017
Buy a tire pressure gauge and keep it in your glovebox, they're cheap and more accurate than ones on an air pump. There are a few kinds, I prefer the stick style, consumer reports pressure gauge buying guide
posted by TheAdamist at 1:06 PM on February 11, 2017 [1 favorite]
posted by TheAdamist at 1:06 PM on February 11, 2017 [1 favorite]
Your tires will not hold a full volume of air forever. From time to time, and especially when the outside temperature fluctuates, you will need to be able to measure the pressure and top them up.
posted by Huffy Puffy at 1:19 PM on February 11, 2017
posted by Huffy Puffy at 1:19 PM on February 11, 2017
The tire pressure systems generally come on when the tire is seriously below pressure, but still safe to drive. For example if nominal cold pressure is about 35 psi, the system will alert at about 25.
So it does tend to first give you notice when the tire is extremely cold, but it's a sign that your pressure is not high enough, even if it's going off once it gets to an operating temperature. It just means it got up to 28 psi once the tire got warm. But if your tires are supposed to be 35 psi cold, expected pressure at operating temperature is probably more like 38.
posted by randomkeystrike at 4:29 PM on February 11, 2017
So it does tend to first give you notice when the tire is extremely cold, but it's a sign that your pressure is not high enough, even if it's going off once it gets to an operating temperature. It just means it got up to 28 psi once the tire got warm. But if your tires are supposed to be 35 psi cold, expected pressure at operating temperature is probably more like 38.
posted by randomkeystrike at 4:29 PM on February 11, 2017
Go to Discount Tires, pull in and ask for a free air check. They will fill all your tires to the specs on the tires. Other tire companies do that too, I just like discount. Over in California they are called America's Tire Stores. The big outfits all do free checks.
posted by Oyéah at 6:51 PM on February 11, 2017
posted by Oyéah at 6:51 PM on February 11, 2017
The full-size spare in my 4Runner has a sensor and so far it's usually been the culprit.
posted by achrise at 8:11 PM on February 11, 2017
posted by achrise at 8:11 PM on February 11, 2017
The tire pressure systems generally come on when the tire is seriously below pressure, but still safe to drive. For example if nominal cold pressure is about 35 psi, the system will alert at about 25.
This is extremely misleading. If your car tyre is designed to run at 35psi (cold temp pressure) then it is somewhere between a tangible safety concern and dangerously underinflated at 25psi. It won't damage the tyre itself to drive at that pressure, but the tyre is simply not as capable of producing anything like the grip or support that a properly inflated tyre is required to do. Anyone that tells you otherwise is severely lacking in knowledge of how tyres work.
Will you have an accident around town at 25psi? Most likely not if you're going a few miles. However your braking distances will be increased, the stability of your car is affected and damage to wheels over pot holes is more likely as the tyre pressure directly contributes to the sidewall stiffness (think spring rate) of the tyre. It will be more problematic at highway speeds, to the point of significant stability issues, particularly if only one tyre is underinflated. A lane change avoidance move with tyres at that kind of pressure could conceivably result in a loss of control if done in a hurry as the car will 'bounce' on the soft tyre. It is not something that should be ignored or trivialised.
Something that people often forget about tyres too - as well as many people not realising that tyre pressure fluctuation for temperature is a big deal in safety aspects - is that the quoted tyre pressure is a COLD pressure. So if you are checking your tyres when they are warm (ie above ambient by a measurable amount to the touch) which they will be after a couple of miles usually) then the pressure you set them to needs to be higher. In extreme cases (high volume, high pressure tyres like on my RV, for instance) a cold pressure of 85psi before starting will mean a 95-100psi hot pressure when checked just after coming off the highway. Road car tyres don't hold as much volume of air or such a high pressure (so the effect is less pronounced) but it is a tangible and notable difference.
Check your tyres. Regularly. Check them at home against the manufacturers numbers, then drive 5 miles and check them again. That way if you stop at a gas station to check tyres, you know how much to 'over-fill' them compared to the factory mandated pressure. It will usually be 2-4 psi from hot to cold for a passenger car, more for an SUV or truck.
posted by Brockles at 10:22 PM on February 11, 2017 [1 favorite]
This is extremely misleading. If your car tyre is designed to run at 35psi (cold temp pressure) then it is somewhere between a tangible safety concern and dangerously underinflated at 25psi. It won't damage the tyre itself to drive at that pressure, but the tyre is simply not as capable of producing anything like the grip or support that a properly inflated tyre is required to do. Anyone that tells you otherwise is severely lacking in knowledge of how tyres work.
Will you have an accident around town at 25psi? Most likely not if you're going a few miles. However your braking distances will be increased, the stability of your car is affected and damage to wheels over pot holes is more likely as the tyre pressure directly contributes to the sidewall stiffness (think spring rate) of the tyre. It will be more problematic at highway speeds, to the point of significant stability issues, particularly if only one tyre is underinflated. A lane change avoidance move with tyres at that kind of pressure could conceivably result in a loss of control if done in a hurry as the car will 'bounce' on the soft tyre. It is not something that should be ignored or trivialised.
Something that people often forget about tyres too - as well as many people not realising that tyre pressure fluctuation for temperature is a big deal in safety aspects - is that the quoted tyre pressure is a COLD pressure. So if you are checking your tyres when they are warm (ie above ambient by a measurable amount to the touch) which they will be after a couple of miles usually) then the pressure you set them to needs to be higher. In extreme cases (high volume, high pressure tyres like on my RV, for instance) a cold pressure of 85psi before starting will mean a 95-100psi hot pressure when checked just after coming off the highway. Road car tyres don't hold as much volume of air or such a high pressure (so the effect is less pronounced) but it is a tangible and notable difference.
Check your tyres. Regularly. Check them at home against the manufacturers numbers, then drive 5 miles and check them again. That way if you stop at a gas station to check tyres, you know how much to 'over-fill' them compared to the factory mandated pressure. It will usually be 2-4 psi from hot to cold for a passenger car, more for an SUV or truck.
posted by Brockles at 10:22 PM on February 11, 2017 [1 favorite]
Response by poster: Because of snow on the ground, and the inaccurate pressure gauge on my air compressor, I ended up using the bike pump on all but one tire, where the rim was mounted such that I couldn't fit the pump nozzle into the stem. I'll drive a bit and check the TPMS again tomorrow after the next snow storm (not going anywhere in that car till).
posted by spacefire at 6:14 AM on February 12, 2017
posted by spacefire at 6:14 AM on February 12, 2017
where the rim was mounted such that I couldn't fit the pump nozzle into the stem.
That sounds more like it is a wheel cover then the wheel itself. There should be zero difference in clearance to the valve stem between wheels, but the plastic covers can be mounted in a way that can obscur it. They will just pull off or you can twist them around a bit.
Incidentally, snow on the ground is the only time that lower than specified tyre pressures are an actual advantage (it produces a greater contact patch, with a softer sidewall). So consider that.
posted by Brockles at 7:54 AM on February 12, 2017
That sounds more like it is a wheel cover then the wheel itself. There should be zero difference in clearance to the valve stem between wheels, but the plastic covers can be mounted in a way that can obscur it. They will just pull off or you can twist them around a bit.
Incidentally, snow on the ground is the only time that lower than specified tyre pressures are an actual advantage (it produces a greater contact patch, with a softer sidewall). So consider that.
posted by Brockles at 7:54 AM on February 12, 2017
Response by poster: That's what I meant, isn't that called the rim? I will try to shift it if the indication doesn't go away.
posted by spacefire at 6:51 PM on February 12, 2017
posted by spacefire at 6:51 PM on February 12, 2017
In extreme cases (high volume, high pressure tyres like on my RV, for instance) a cold pressure of 85psi before starting will mean a 95-100psi hot pressure when checked just after coming off the highway. Road car tyres don't hold as much volume of air or such a high pressure (so the effect is less pronounced) but it is a tangible and notable difference.
This statement is exactly backwards from physical reality, it two aspects. First, there is no difference between high volume and low volume tires. From the gas law:
P = nRT/V
The quantity nRT/V is the same and nearly constant for both high volume and low volume tires. So the change in pressure will be the same for high volume and low volume tires. It makes no difference.
Second, the P term in the gas equation is absolute pressure, not tire pressure as measured by your gauge, which is pressure above atmospheric. For any given percentage change in temperature, the percentage change in absolute pressure is the same. For a low pressure tire, say 15 pounds, if the absolute temperature changes by 10%, the gauge pressure changes by about 20%. But for a high pressure tire, say 100 pounds, if the absolute temperature changes by 10%, the gauge pressure changes by about 10%. The percentage change in gauge pressure is much greater for a low pressure tire than a high pressure tire, even though the numeric change is greater. This means that the percentage change from optimal is greater in a low pressure tire than a high pressure tire.
posted by JackFlash at 8:32 PM on February 12, 2017
This statement is exactly backwards from physical reality, it two aspects. First, there is no difference between high volume and low volume tires. From the gas law:
P = nRT/V
The quantity nRT/V is the same and nearly constant for both high volume and low volume tires. So the change in pressure will be the same for high volume and low volume tires. It makes no difference.
Second, the P term in the gas equation is absolute pressure, not tire pressure as measured by your gauge, which is pressure above atmospheric. For any given percentage change in temperature, the percentage change in absolute pressure is the same. For a low pressure tire, say 15 pounds, if the absolute temperature changes by 10%, the gauge pressure changes by about 20%. But for a high pressure tire, say 100 pounds, if the absolute temperature changes by 10%, the gauge pressure changes by about 10%. The percentage change in gauge pressure is much greater for a low pressure tire than a high pressure tire, even though the numeric change is greater. This means that the percentage change from optimal is greater in a low pressure tire than a high pressure tire.
posted by JackFlash at 8:32 PM on February 12, 2017
First, there is no difference between high volume and low volume tires
Well. Of course there is, but it is in the application of those tyres that the difference lies, not in the actual air volume itself. Which was my fault for being imprecise. I was just trying to get across the type of tyre I was talking about (basically - "those big ones you see on commercial vehicles" rather than '"the smaller looking ones on your car"). Commercial vehicle tyres (with a much bigger tyre and much large sidewall) run at much higher load and so see significantly higher normal running temperatures than road car tyres and so get higher pressure rises as a result. I just described the tyres themselves in a lazy manner.
It's not at all 'opposite from physical reality'. It is precisely what happens in the real world, but due to load and hence temperature, not tyre volume. In addition, 'pronounced' was my wording. I never mentioned 'percentage'. A change of 15psi is a more pronounced to the average person change than a 3-6psi change (usual range of difference in a normal usage road tyre).
posted by Brockles at 9:10 AM on February 13, 2017
Well. Of course there is, but it is in the application of those tyres that the difference lies, not in the actual air volume itself. Which was my fault for being imprecise. I was just trying to get across the type of tyre I was talking about (basically - "those big ones you see on commercial vehicles" rather than '"the smaller looking ones on your car"). Commercial vehicle tyres (with a much bigger tyre and much large sidewall) run at much higher load and so see significantly higher normal running temperatures than road car tyres and so get higher pressure rises as a result. I just described the tyres themselves in a lazy manner.
It's not at all 'opposite from physical reality'. It is precisely what happens in the real world, but due to load and hence temperature, not tyre volume. In addition, 'pronounced' was my wording. I never mentioned 'percentage'. A change of 15psi is a more pronounced to the average person change than a 3-6psi change (usual range of difference in a normal usage road tyre).
posted by Brockles at 9:10 AM on February 13, 2017
That's what I meant, isn't that called the rim? I will try to shift it if the indication doesn't go away.
The rim is the steel bit that the tyre itself is actually sitting on. The rim is just the edge of the wheel, and has been brought into common usage. So you either have just a wheel (alloy material, one piece) or you will have a steel (usually painted black) wheel, with a (usually silver) plastic wheel cover/wheel trim on top of it for purely aesthetic purposes.
The tuning industry started calling alloy wheels 'rims' a long time ago. It's not a separate part, it's just the wheel itself.
posted by Brockles at 9:12 AM on February 13, 2017
The rim is the steel bit that the tyre itself is actually sitting on. The rim is just the edge of the wheel, and has been brought into common usage. So you either have just a wheel (alloy material, one piece) or you will have a steel (usually painted black) wheel, with a (usually silver) plastic wheel cover/wheel trim on top of it for purely aesthetic purposes.
The tuning industry started calling alloy wheels 'rims' a long time ago. It's not a separate part, it's just the wheel itself.
posted by Brockles at 9:12 AM on February 13, 2017
This is what you said: "Road car tyres don't hold as much volume of air or such a high pressure (so the effect is less pronounced) but it is a tangible and notable difference."
This is false and dangerously misleading. It is simply not true that high volume tires are more susceptible to temperature changes than low volume tires. By the gas law, they are affected the same.
And exactly the opposite is true for your claim that high pressure tires are more susceptible to temperature changes than low pressure tires. That is dangerous misinformation. For low pressure tires it is even more important to check pressure when the air temperature changes.
If you don't want to do the math yourself you can just look at wikipedia which provides a handy table showing the effects of temperature change on low pressure vs high pressure tires.
For example for a nominal 10 psi tire, the pressure change from high temperature to low temperature is 60%. For the nominal 100 psi tire, the pressure change from high temperature to low temperature is less than 30%, half that of the low pressure tire. This is an extreme case but it demonstrates the principle that low pressure tires are affected more by temperature change than high pressure tires.
What does this mean for tire performance? It means that the low pressure tire will have to flatten by 60%, while the high pressure tire will only flatten by 30%. That's a big difference and both handling and easily visible.
"I never mentioned 'percentage'. A change of 15psi is a more pronounced to the average person change than a 3-6psi change (usual range of difference in a normal usage road tyre)."
This is absolutely false and dangerously misleading. It is percentage change that affects the appearance (flatness) and handling of the tire, not absolute psi numbers. As the physics demonstrates, it is low pressure tires that are affected more by temperature change than high pressure tires. Over the same temperature range, the low pressure tire will go flatter than the high pressure tire.
Safety-wise, this means that people driving on low pressure passenger tires need to be more concerned and diligent about checking their tires when the temperature changes than high pressure commercial tires. This is the exact opposite of the dangerous advice that you gave.
posted by JackFlash at 10:20 AM on February 13, 2017
This is false and dangerously misleading. It is simply not true that high volume tires are more susceptible to temperature changes than low volume tires. By the gas law, they are affected the same.
And exactly the opposite is true for your claim that high pressure tires are more susceptible to temperature changes than low pressure tires. That is dangerous misinformation. For low pressure tires it is even more important to check pressure when the air temperature changes.
If you don't want to do the math yourself you can just look at wikipedia which provides a handy table showing the effects of temperature change on low pressure vs high pressure tires.
For example for a nominal 10 psi tire, the pressure change from high temperature to low temperature is 60%. For the nominal 100 psi tire, the pressure change from high temperature to low temperature is less than 30%, half that of the low pressure tire. This is an extreme case but it demonstrates the principle that low pressure tires are affected more by temperature change than high pressure tires.
What does this mean for tire performance? It means that the low pressure tire will have to flatten by 60%, while the high pressure tire will only flatten by 30%. That's a big difference and both handling and easily visible.
"I never mentioned 'percentage'. A change of 15psi is a more pronounced to the average person change than a 3-6psi change (usual range of difference in a normal usage road tyre)."
This is absolutely false and dangerously misleading. It is percentage change that affects the appearance (flatness) and handling of the tire, not absolute psi numbers. As the physics demonstrates, it is low pressure tires that are affected more by temperature change than high pressure tires. Over the same temperature range, the low pressure tire will go flatter than the high pressure tire.
Safety-wise, this means that people driving on low pressure passenger tires need to be more concerned and diligent about checking their tires when the temperature changes than high pressure commercial tires. This is the exact opposite of the dangerous advice that you gave.
posted by JackFlash at 10:20 AM on February 13, 2017
This is false and dangerously misleading.
No it is not. You seem to be wilfully reading it wrongly.
Safety-wise, this means that people driving on low pressure passenger tires need to be more concerned and diligent about checking their tires when the temperature changes than high pressure commercial tires. This is the exact opposite of the dangerous advice that you gave.
You are absolutely reading my comments wrong. As I stated: "... but it is a tangible and notable difference.". I am not minimising the issue at all. I am saying it is still important. Far more important than people above are making it out to be. I am not giving dangerous advice, you just seem to be not understanding it. I initially thought that was because of my poor method of describing what a commercial tyre looks like, but now it seems wilful.
Safety-wise, this means that people driving on low pressure passenger tires need to be more concerned and diligent about checking their tires when the temperature changes than high pressure commercial tires.
Ignoring the fact that, for a moment, that you seem to think that ANY of this means I am implying people shouldn't check their tyre pressure (See my comment about CHECK YOUR TYRE PRESSURES OFTEN - did you read that bit?), you cannot say that without considering the load and use case. You just seem to be considering retention of tyre shape. There is more than one consideration here - pressure/tyre shape are not the only factor. Commercial vehicles are much more prone to blow out from over heating due to incorrect tyre pressures (a drive along any freeway demonstrates that handily - truck blow-outs are commonplace, passenger tyre blow outs much rarer by comparison) because the tyres are operating much closer to their temperature and designed structural limits than road car tyres are. So that part of your answer is entirely incorrect as the failure of the tyre itself from incorrect pressure is much higher in commercial vehicles (even if they are more resistant to visible deformation for a given percentage pressure change). Commercial vehicles operate much closer to max designed loads whenever fully loaded and at highway speed. This is a LOT of their time. Usually easily 50% of the time. Road car tyres are only near those limits when the car is fully loaded and also being subjected to high cornering speeds and g loading. This is a much, much smaller percentage of the cars life. The majority of cars never even experience the max loads that the car and tyre is designed for.
Again - I have still said it is important to check, and do so regularly. Incorrect pressure in a commercial application are likely to produce tyre failure. Incorrect pressures in a road car are likely to (and easily can) produce adverse handling issues (just as dangerous) long before tyre failure is a factor.
It is simply not true that high volume tires are more susceptible to temperature changes than low volume tires.
Again, that's not what I said. Given that ambient temperature would be the same, commercial truck tyres operate at a higher usual temperature. So the temperature change (delta) is higher. This is not 'opinion' this is measured. I'm not saying they are 'more susceptible' - nowhere have I said that. I have said the range of temperature they are subjected to are higher in normal usage. A greater range of temperature produces a greater variation in pressure. As the gas law dictates, as you have said. Physics is respected.
It is percentage change that affects the appearance (flatness) and handling of the tire, not absolute psi numbers.
To the average person - a needle moving up 3 units is not as pronounced as a needle moving up 15 units. This should be patently obvious. There is nothing at all misleading about that statement and it bears no relevance to percentage, flatness, handling.... anything. Just perception of the user. Dial back your vitriol. The entire point of the comment was 'just because the needle doesn't move as far doesn't mean it is not as important'.
It is percentage change that affects the appearance (flatness) and handling of the tire, not absolute psi numbers
You're ignoring the construction of the tyre, and arguing something about how a percentage change means a tyre is more likely to deform if it is at a lower pressure - which, of course it is. But there is also more to this than just maintaining the tyre's shape. It's not just about maintaining contact patch. So if you are only considering structural shape, then yes I can see why you think a commercial tyre will be able to maintain its shape at a bigger change in pressure than a car tyre. But that is not the entire issue. Spring rate is the bigger aspect in road car/performance tyres, and heat generation from deformation of the tyre (during load cycles over bumps, not static deformation) is key in commercial vehicles. A commercial tyre can grip fine at a pressure loss that would cause issues in a road car, but it will likely explode at highway speeds as it overheats.
In passenger cars there is a significant spring rate effect as a direct consequence from tyre pressure, and an overinflated or underinflated tyre can adversely affect handling significantly without visibly affecting the tyre's shape. It's not just about making sure the structure of the tyre is retained. Which is why I was saying ALL ALONG that even though the needle doesn't move so far it is still important.
Again - my post ended with "Check your tyres. Regularly.". I'm not sure how you are reading that as 'Don't worry if you don't have a commercial vehicle'.
posted by Brockles at 10:59 AM on February 13, 2017
No it is not. You seem to be wilfully reading it wrongly.
Safety-wise, this means that people driving on low pressure passenger tires need to be more concerned and diligent about checking their tires when the temperature changes than high pressure commercial tires. This is the exact opposite of the dangerous advice that you gave.
You are absolutely reading my comments wrong. As I stated: "... but it is a tangible and notable difference.". I am not minimising the issue at all. I am saying it is still important. Far more important than people above are making it out to be. I am not giving dangerous advice, you just seem to be not understanding it. I initially thought that was because of my poor method of describing what a commercial tyre looks like, but now it seems wilful.
Safety-wise, this means that people driving on low pressure passenger tires need to be more concerned and diligent about checking their tires when the temperature changes than high pressure commercial tires.
Ignoring the fact that, for a moment, that you seem to think that ANY of this means I am implying people shouldn't check their tyre pressure (See my comment about CHECK YOUR TYRE PRESSURES OFTEN - did you read that bit?), you cannot say that without considering the load and use case. You just seem to be considering retention of tyre shape. There is more than one consideration here - pressure/tyre shape are not the only factor. Commercial vehicles are much more prone to blow out from over heating due to incorrect tyre pressures (a drive along any freeway demonstrates that handily - truck blow-outs are commonplace, passenger tyre blow outs much rarer by comparison) because the tyres are operating much closer to their temperature and designed structural limits than road car tyres are. So that part of your answer is entirely incorrect as the failure of the tyre itself from incorrect pressure is much higher in commercial vehicles (even if they are more resistant to visible deformation for a given percentage pressure change). Commercial vehicles operate much closer to max designed loads whenever fully loaded and at highway speed. This is a LOT of their time. Usually easily 50% of the time. Road car tyres are only near those limits when the car is fully loaded and also being subjected to high cornering speeds and g loading. This is a much, much smaller percentage of the cars life. The majority of cars never even experience the max loads that the car and tyre is designed for.
Again - I have still said it is important to check, and do so regularly. Incorrect pressure in a commercial application are likely to produce tyre failure. Incorrect pressures in a road car are likely to (and easily can) produce adverse handling issues (just as dangerous) long before tyre failure is a factor.
It is simply not true that high volume tires are more susceptible to temperature changes than low volume tires.
Again, that's not what I said. Given that ambient temperature would be the same, commercial truck tyres operate at a higher usual temperature. So the temperature change (delta) is higher. This is not 'opinion' this is measured. I'm not saying they are 'more susceptible' - nowhere have I said that. I have said the range of temperature they are subjected to are higher in normal usage. A greater range of temperature produces a greater variation in pressure. As the gas law dictates, as you have said. Physics is respected.
It is percentage change that affects the appearance (flatness) and handling of the tire, not absolute psi numbers.
To the average person - a needle moving up 3 units is not as pronounced as a needle moving up 15 units. This should be patently obvious. There is nothing at all misleading about that statement and it bears no relevance to percentage, flatness, handling.... anything. Just perception of the user. Dial back your vitriol. The entire point of the comment was 'just because the needle doesn't move as far doesn't mean it is not as important'.
It is percentage change that affects the appearance (flatness) and handling of the tire, not absolute psi numbers
You're ignoring the construction of the tyre, and arguing something about how a percentage change means a tyre is more likely to deform if it is at a lower pressure - which, of course it is. But there is also more to this than just maintaining the tyre's shape. It's not just about maintaining contact patch. So if you are only considering structural shape, then yes I can see why you think a commercial tyre will be able to maintain its shape at a bigger change in pressure than a car tyre. But that is not the entire issue. Spring rate is the bigger aspect in road car/performance tyres, and heat generation from deformation of the tyre (during load cycles over bumps, not static deformation) is key in commercial vehicles. A commercial tyre can grip fine at a pressure loss that would cause issues in a road car, but it will likely explode at highway speeds as it overheats.
In passenger cars there is a significant spring rate effect as a direct consequence from tyre pressure, and an overinflated or underinflated tyre can adversely affect handling significantly without visibly affecting the tyre's shape. It's not just about making sure the structure of the tyre is retained. Which is why I was saying ALL ALONG that even though the needle doesn't move so far it is still important.
Again - my post ended with "Check your tyres. Regularly.". I'm not sure how you are reading that as 'Don't worry if you don't have a commercial vehicle'.
posted by Brockles at 10:59 AM on February 13, 2017
I assume that we are in agreement that your original statement: "Road car tyres don't hold as much volume of air or such a high pressure (so the effect is less pronounced) but it is a tangible and notable difference" is false and potentially dangerous. It is a complete misunderstanding of how volume and pressure relate to temperature change. Lower volume and lower pressure do not make the effect of temperature less pronounced. In fact it is the opposite, a physical fact, requiring close attention to tire pressure change with temperature for passenger vehicles.
posted by JackFlash at 11:29 AM on February 13, 2017
posted by JackFlash at 11:29 AM on February 13, 2017
is false and potentially dangerous.
No, we are not. It's only false and potentially misleading if you don't understand the statement. Because you seem unable to understand the following clarification:
"Road car tyres don't hold as much volume of air or such a high pressure (so the effect ON A PRESSURE GAUGE when you read the numbers is less pronounced ie the gauge doesn't go up by as many numbers) but it is a tangible and notable difference". In other words "because the pressure rise isn't so big in terms of number of psi as it would be in a bigger tyre that is subjected to bigger temperatures and pressure, it doesn't mean it is less important".
Incidentally, your table you linked to shows this to be true and would demonstrate the effect (on the person, not the tyre) I am talking about:
A tyre filled to 30psi at 20 degrees C, heated from 68 to 104F changes by 3.1 psi. An 80 psi starting pressure would produce an end pressure of 6.5 psi. That, to the average person is a 'bigger change' because it is twice as many psi. Nothing else needs to come into it, at this stage, because we're talking about people being people, not thermodynamics. People don't naturally think in percentages. People would find it easier to dismiss "Its only 3, that sounds low" but see 6 as being "Well that's nearly ten, that sounds big". They'd find it easier to dismiss because 6 sounds like a more pronounced change than 3.
Which, I say yet again, is what I was saying all along, and what my original comment states.
You are reading this with your page an inch from a physics text book. I'm trying to relate it to what the average person would see. A person without any thermodynamic knowledge would see a 15psi rise in tyre pressure and would perceive that as more pronounced a change than seeing a 4 psi pressure rise. They wouldn't factor for volume or starting pressure (or even temperature at usual load), but it would seem more significant because the number is bigger. THAT was the point. Also, the reasons behind that difference are irrelevant to the common person. They don't care. Just because the change is small, doesn't mean it is insignificant, but human nature makes it easier to dismiss. Which is precisely what I have been saying all along.
It is a complete misunderstanding of how volume and pressure relate to temperature change.
No, it is not. It is a complete misunderstanding, by you, of what I was saying and remains so. Nothing I have said is incorrect, and I know quote a bit about Boyles law and its practical applications, particularly and especially in respect to tyre performance. You are reading it in a singular way and not understanding at all what I am talking about. I deal with pressure and temperature changes in tyres and the effect it has on vehicle stability and handling on a daily basis. It is my bread and butter. Well, maybe just my bread, because springs and damping are probably the other half of the meal.
posted by Brockles at 12:21 PM on February 13, 2017
No, we are not. It's only false and potentially misleading if you don't understand the statement. Because you seem unable to understand the following clarification:
"Road car tyres don't hold as much volume of air or such a high pressure (so the effect ON A PRESSURE GAUGE when you read the numbers is less pronounced ie the gauge doesn't go up by as many numbers) but it is a tangible and notable difference". In other words "because the pressure rise isn't so big in terms of number of psi as it would be in a bigger tyre that is subjected to bigger temperatures and pressure, it doesn't mean it is less important".
Incidentally, your table you linked to shows this to be true and would demonstrate the effect (on the person, not the tyre) I am talking about:
A tyre filled to 30psi at 20 degrees C, heated from 68 to 104F changes by 3.1 psi. An 80 psi starting pressure would produce an end pressure of 6.5 psi. That, to the average person is a 'bigger change' because it is twice as many psi. Nothing else needs to come into it, at this stage, because we're talking about people being people, not thermodynamics. People don't naturally think in percentages. People would find it easier to dismiss "Its only 3, that sounds low" but see 6 as being "Well that's nearly ten, that sounds big". They'd find it easier to dismiss because 6 sounds like a more pronounced change than 3.
Which, I say yet again, is what I was saying all along, and what my original comment states.
You are reading this with your page an inch from a physics text book. I'm trying to relate it to what the average person would see. A person without any thermodynamic knowledge would see a 15psi rise in tyre pressure and would perceive that as more pronounced a change than seeing a 4 psi pressure rise. They wouldn't factor for volume or starting pressure (or even temperature at usual load), but it would seem more significant because the number is bigger. THAT was the point. Also, the reasons behind that difference are irrelevant to the common person. They don't care. Just because the change is small, doesn't mean it is insignificant, but human nature makes it easier to dismiss. Which is precisely what I have been saying all along.
It is a complete misunderstanding of how volume and pressure relate to temperature change.
No, it is not. It is a complete misunderstanding, by you, of what I was saying and remains so. Nothing I have said is incorrect, and I know quote a bit about Boyles law and its practical applications, particularly and especially in respect to tyre performance. You are reading it in a singular way and not understanding at all what I am talking about. I deal with pressure and temperature changes in tyres and the effect it has on vehicle stability and handling on a daily basis. It is my bread and butter. Well, maybe just my bread, because springs and damping are probably the other half of the meal.
posted by Brockles at 12:21 PM on February 13, 2017
All I can say is, regardless of your appeal to authority, you don't seem to know what you are talking about. The absolute psi change is irrelevant. What is important to the driver is the percentage change in gauge pressure, not the absolute number.
This is a common misunderstanding about Boyle's Law. It applies to absolute pressure, not gauge pressure. This is the same mistake many commentators made with respect to the football deflategate controversy. It is this difference between absolute pressure and gauge pressure which leads to the seemingly contradictory fact that temperature has more effect on a low pressure tire than a high pressure tire. For any given temperature decrease, the lower pressure tire will go "flatter" than the high pressure tire. For a given temperature change, the effect on handling and performance will be greater for the low pressure tire than the high pressure tire. You seem to be unaware of this fact, as demonstrated in your original statement about volume and pressure.
posted by JackFlash at 1:21 PM on February 13, 2017
This is a common misunderstanding about Boyle's Law. It applies to absolute pressure, not gauge pressure. This is the same mistake many commentators made with respect to the football deflategate controversy. It is this difference between absolute pressure and gauge pressure which leads to the seemingly contradictory fact that temperature has more effect on a low pressure tire than a high pressure tire. For any given temperature decrease, the lower pressure tire will go "flatter" than the high pressure tire. For a given temperature change, the effect on handling and performance will be greater for the low pressure tire than the high pressure tire. You seem to be unaware of this fact, as demonstrated in your original statement about volume and pressure.
posted by JackFlash at 1:21 PM on February 13, 2017
You seem to be unaware of this fact, as demonstrated in your original statement about volume and pressure.
Jesus Christ. Not at all. It's boggling to me that you aren't even reading any of the explanations to correct your misunderstanding of what I was saying. I wasn't trying to give a scientific explanation of the issue, but just a throwaway napkin example to point out that even small numbers matter.
I was saying that people find smaller differences in numbers when they read a bloody tyre gauge easier to ignore than larger ones. Effect/actual difference/Boyles Law/Absolute pressure/Kelvin and magic mushrooms irrelevant. I'm talking about people's REACTION TO THE NUMBERS whatever the numbers are.
People will ignore small numbers in the difference more easily than large numbers. I was saying just because it was a small number doesn't make it less important.
posted by Brockles at 1:51 PM on February 13, 2017
Jesus Christ. Not at all. It's boggling to me that you aren't even reading any of the explanations to correct your misunderstanding of what I was saying. I wasn't trying to give a scientific explanation of the issue, but just a throwaway napkin example to point out that even small numbers matter.
I was saying that people find smaller differences in numbers when they read a bloody tyre gauge easier to ignore than larger ones. Effect/actual difference/Boyles Law/Absolute pressure/Kelvin and magic mushrooms irrelevant. I'm talking about people's REACTION TO THE NUMBERS whatever the numbers are.
People will ignore small numbers in the difference more easily than large numbers. I was saying just because it was a small number doesn't make it less important.
posted by Brockles at 1:51 PM on February 13, 2017
For the last time: In extreme cases (high volume, high pressure tyres like on my RV, for instance) a cold pressure of 85psi before starting will mean a 95-100psi hot pressure when checked just after coming off the highway. Road car tyres don't hold as much volume of air or such a high pressure (so the effect is less pronounced) but it is a tangible and notable difference.
Read it as this: For SOME tyres and applications that aren't directly related to your example here, you can see 15psi or similar in differences between hot and cold temps [subtext - See, people? Anecdotal and potentially understandable real world example that shows tyre pressure can change by a lot more than you maybe expected! I bet you find that surprising]. But with road tyres (because they are different for a variety of reasons that don't need to be explained here for fear of muddying the waters) the raise in psi (what you measure when they are cold and what you measure when they are hot) may show a smaller difference (so not so big a change in number when you look at a tyre gauge) but they still need checking despite the fact you won't see as much of a change in the number on the pressure gauge.
Does THAT make you realise what I was saying? And how it is entirely factually accurate and doesn't question the basics of Gas laws and thermodynamics?
posted by Brockles at 1:58 PM on February 13, 2017
Read it as this: For SOME tyres and applications that aren't directly related to your example here, you can see 15psi or similar in differences between hot and cold temps [subtext - See, people? Anecdotal and potentially understandable real world example that shows tyre pressure can change by a lot more than you maybe expected! I bet you find that surprising]. But with road tyres (because they are different for a variety of reasons that don't need to be explained here for fear of muddying the waters) the raise in psi (what you measure when they are cold and what you measure when they are hot) may show a smaller difference (so not so big a change in number when you look at a tyre gauge) but they still need checking despite the fact you won't see as much of a change in the number on the pressure gauge.
Does THAT make you realise what I was saying? And how it is entirely factually accurate and doesn't question the basics of Gas laws and thermodynamics?
posted by Brockles at 1:58 PM on February 13, 2017
Mod note: This is getting really lengthy and one-on-one; if you two want to keep talking about it, please go to a private channel at this point.
posted by cortex (staff) at 2:01 PM on February 13, 2017
posted by cortex (staff) at 2:01 PM on February 13, 2017
This thread is closed to new comments.
posted by primethyme at 8:42 AM on February 11, 2017 [5 favorites]