under pressure

July 2, 2009 12:53 PM

im looking for a new fangled device designed to measure tyre pressure for cars. you drive onto a ramp and somehow it reads the pressure.

apparantly this is much more efficient than measuring the pressure of each tyre manually. how much do these devices cost and where could i buy them? any useful websites to the topic?

posted by freddymetz to technology (19 comments total)

I found some references to systems that communicate wirelessly with electronic sensors installed on each tire. This U.S. Department of Transportation web page has a brief overview, and says,

Advanced systems that include the driver-interface and sensors attached either to the stem valves through the wheel into the tire well or within the tire well cost approximately $1,200 to $2,100, depending upon the type of system, number of tires, and installation requirements. These systems are available directly through the product manufacturers or original equipment truck manufacturers. Systems that require the use of handheld readers cost approximately $6,000 for each reader, and gate readers range from $10,000 to $12,000.

posted by mbrubeck at 1:12 PM on July 2

Only thing I found were recent patent applications. Nothing on the market yet. One problem I could foresee is that you have to drive onto a ramp to measure the tire pressure. The one thing you don't want to do before measuring tire pressure is drive. That will warm up the tires and skew the pressure measurement.
posted by torquemaniac at 1:38 PM on July 2

Is it possible you're thinking of remote tire pressure monitoring system like this? A sensor is placed inside each tire and wirelessly transmit the pressure to a display inside the car which shows the pressure of all 4 tires.
posted by jaimev at 1:39 PM on July 2

Some cars come standard with the wireless pressure sensors (eg GM's TPMS)
No ramp necessary - the car will email you its tire pressure readings if you like.

From talking to other owners, they don't assume that the sensors are super accurate, the purpose of the sensors is primarily to tell the car computer if there is any kind of problem, and only secondarily as an instrument for measurement. A traditional direct reading with calibrated equipment is considered more accurate, and more desirable if you want to check pressure.
posted by -harlequin- at 1:49 PM on July 2

you drive onto a ramp and somehow it reads the pressure.

This would be completely impossible without some kind of sensor inside the tyre reading the pressure. The system has to read a pressure transducer inside the wheel (common on some modern cars and available after market).

Doing it by weight of car and tyre deflection would be completely inaccurate without detailed knowledge of tyre wall stiffnesses (which changes through the wear and natural degradation over time/UV exposure and so difficult to quantify), exact weight of the vehicle and generally all kinds of fudge factors.

I think the other answers about sensors inside the wheels are the thing you are looking for.
posted by Brockles at 2:00 PM on July 2 [1 favorite has favorites]

I agree with Brockles. Doesn't seem possible without a sensor on the valve.
Costco sells the wireless sensors, starting at 500 bucks.
posted by weapons-grade pandemonium at 3:37 PM on July 2

This would be completely impossible without some kind of sensor inside the tyre reading the pressure.

No, it wouldn't.

If you measured the weight being supported by a particular tire, and you also measured the area of the contact patch, you can back out the pressure inside the tire.

I remember doing something like this in my 10th grade physics class; we rolled a car so its tire was on a sheet of graph paper, traced it, and then counted up the squares inside the line to get the contact area. With that and the curb weight you can get a rough estimate of the air pressure.

If I was going to design something to do this, you'd need a sort of scale that would measure the force being supported by the entire tire, and then a grid of little pressure sensors so you could measure the contact area. If you took some baseline measurements and tried to account for tire stiffness and thickness, you could probably get a reasonably good measurement without any great leaps in technology or engineering.

I don't know why you'd want such a thing — measuring the pressure directly using a gauge is a lot easier and uses a device you can buy at AutoZone for $5 — but you could if you were really hell-bent on doing so. Maybe somebody was. The TPMS readers that I've seen so far are small, PDA-like gadgets, not anything that you need to drive a car specifically onto in order for it to work, so the thing the OP is talking about doesn't sound like them.
posted by Kadin2048 at 10:22 PM on July 2

i heard that the device i described was already in use at various airports (at least in germany) but my google fu has failed..
posted by freddymetz at 2:41 AM on July 3

No, it wouldn't.

Yes, yes it would - if you wanted anything like the accuracy relevant to tyre pressures beyond 'inflated' and 'flat'. It'd be fine in a physics class of perfect and exact definitions of components, but not in the real world of uncontrollable variables. It is theoretically possible, but only in the land of theory.

If you measured the weight being supported by a particular tire, and you also measured the area of the contact patch, you can back out the pressure inside the tire.

You would need to know the precise, exact, weight of the car. You would need to know the precise, exact, weight distribution of the car across the 4 tyres (don't even think of suggesting it'd be equal). You would need the precise sidewall stiffness of the tyre in question - which amusingly is directly dependent on tyre pressure (which screws it all up straight away), age of tyre, depth of thread, style of driving/load life, percentage of UV exposure (just as a start).

So, without some 10th grade convenient assumptions this IS impossible with any sort of accuracy. Particularly with the blinding clanger that sidewall stiffness of the tyre is dependent on pressure. The pressure varies one of the major factors needed to be known in that method of determining pressure - you can't assume a sidewall stiffness without knowing the pressure you are trying to measure.

Don't mistake 10th grade physics for realistic applications.
posted by Brockles at 5:01 AM on July 3

The theory of relativity doesn't apply to much and so tenth grade physics can be used to explain many applications. It can be used to demonstrate that the pressure of the tire on the ground is nearly the same as the pressure in the tire. The difference is due to the weight of the section of the tire that is in contact with the ground. This weight could be ignored as small relative to the weight loading the tire or taken in to account as an estimate or accounted for through calibration. As suggested by Kadin2048 (before getting sidetracked by tire stiffness), the measurement can be made with an array of pressure sensors. The ground can't respond to the stiffness of the object on it. The pressure is force over an area and nothing else. The measurement could also be made with a scale and a contact plate of known area. Pressure arrays are made by Tekscan.
http://www.tekscan.com/industrial/tirescan-system.html
Their systems are designed for research and so are far more than are needed for the suggested application. But they do demonstrate that the pressure transducer need not be in the tire.
posted by llc at 9:34 PM on July 3

But they do demonstrate that the pressure transducer need not be in the tire.

Well, only to a point and with the provisos and prior knowledge that I already mentioned - namely known tyre dynamics, precise weight etc. The research seems to be more aimed at (understandably) the deformations of a tyre for a given load and pressure and tread pattern, rather than any attempt to determine pressure from it. There isn't much use in the tyre industry for establishing and testing tyre performance for badly inflated tyres. You'd have to have a datum run of the tyre at a known pressure to be able to establish the pressure from that once it was changed.

Of course, nothing else would have to have changed for this to work, so a tyre of indeterminate life and history would produce an inaccuracy in the comparison, as would varying weight and tread depth. It'd have to be the same vehicle, same amount of tread depth, no significant time in damaging UV rays (so, sunlight) and only a change in the pressure inside the tyre. While it seems logical that something that has pressure as a variable for some other function may be switched around to be used to establish pressure, it doesn't by any means ensure it is a realistic or accurate method for doing so.
posted by Brockles at 8:57 AM on July 4

Yes, to the point that pressure can be measured. The pressure of an object on the ground does not depend on provisos or prior knowledge. The ground can't respond to the history of the object on it.
posted by llc at 10:32 AM on July 4

I can't get my head around whether you are agreeing or disagreeing, there. But I think you are in line with the basic premise that the pressure on the ground doesn't accurately reflect the pressure in the tyre without an awful lot of parameters being known or at the very least constant (the requirement for which pushes it outside the realms of realistic applications).
posted by Brockles at 10:46 AM on July 4

"All right. Everyone out of the car. Luggage, too. And food. Miranda, you take the baby. And the baby seat. Frank, take the bikes off the back and unhook the bike carrier. Jeremy, you get those ramps out of the trunk, and line them up with the front wheels."

"Whatcha doin' Dad?"

"Measuring the tire pressure. Feels a bit low on the right front."
posted by weapons-grade pandemonium at 7:54 PM on July 4 [2 favorites has favorites]

I had no idea I was being unclear. Brockles, I disagree with everything that you have written. I am completely un-aligned with all of your premises. I think you have no idea what pressure is. Out of all of things that can be made up, I don't know why you chose physics. People are doing what you claim to be impossible.
posted by llc at 2:53 PM on July 5

Oh, that clears it up, then. That just makes you wrong. They are not at all doing what I claim to be impossible - you either didn't read their website or my response properly. They are testing tyre contact patch behaviour for a given load and pressure or range thereof. They could investigate the behaviour of a tyre for a variation of pressure OR load, but only for a known tyre. That system requires constants to produce meaningful results - all of which I've covered. Most testing like that only works effectively and repeatably if you only change one parameter at a time - any more and you must use assumptions and/or lose accuracy.

This is not about making up physics - the pressure on the ground does not, as you say, require any previous history, provisos or prior knowledge. Not at all - I haven't disagreed with that. But to extrapolate that into it being an accurate method of determining a corresponding pressure in only a part of that system is flawed. The force on the ground will be the same if the tyres are all flat, for example. The spread of pressure may be different, but the force on the ground will be the same as it is still holding the car up. The pressure spread (or contact patch) will not just vary with pressure - tyre construction, suspension geometry, weight of vehicle (and temperature of the tyre, even) enormously affect this. Without controlling (or at the very least measuring) those parameters, a load/pressure based system will not be accurate.

Yes, contact patch will vary with pressure, but also a worn tyre and/or one that has degraded through UV exposure will have a very different footprint size and shape to one that is brand new for the same pressure in the tyre. The contact patch will vary if you double the sidewall stiffness of the tyre (but keep the pressure in the tyre the same). It will vary if you change the camber angle of the tyre's incidence with the road. This is why you are wrong. It's not just about basic physics, as basic physics DOES make assumptions (frictionless pivots in momentum calculations, for one excellent example) that produce inaccuracies in real world examples. That system is incapable of having a car drive across it and report the pressure in the tyre without a significant amount of additional information.

Physics produces models. Switching between a model and the real world often produces inherent inaccuracies. This is why the system (which, as I explained, doesn't actually do that which you implied it does) would not be accurate for establishing pressure unless it was purely done with a constant vehicle weight and a constant tyre wear, suspension geometry and age of tyre.

In the tenth grade physics example, you can map pressure against contact patch relatively accurately as long as the behaviour of the tyre doesn't change during that time - it doesn't work in the real world as the stiffness and behaviour of a tyre changes considerably throughout its life. So for a control tyre, like the site you link to, it works. For a tyre through a useful life, it doesn't. For a variety of vehicles included in this wear, it will be inaccurate to the point of useless.

The tekscan system would require accurate sidewall and tyre carcass figures to be able to accurately extrapolate pressure. As I have repeatedly mentioned, this information is only accurate when the tyre is new, unless you want to extensively test your tyre again every time you want to use this system to check the pressures - it is not accurate as the variables change. It looks like it could relatively easily, however, suggest incorrect/correct pressures (within a fairly wide band - possibly 5 psi or so) for a given tyre through the contact patch analysis, but without tyre data cannot determine how much of the load is taken by the tyre and how much is taken by the sidewall. It's not realistic for testing outside a control environment (although very capable within that environment).
posted by Brockles at 3:30 PM on July 5

It looks like it could relatively easily, however, suggest incorrect/correct pressures (within a fairly wide band - possibly 5 psi or so) for a given tyre through the contact patch analysis

The OP's suggestion that this system is used at an airport makes me wonder if that is where the application is usable. As I said, it can (through pressure footprint analysis) establish if any tyre that passes over the sensor is correctly pressured (within a fairly wide band) and would be very good at picking up a badly inflated tyre (under or over). So as a means of checking tyre pressures, it'd work fine.

Giving a number of psi for any vehicle (which 'measuring' rather than 'checking' implies) would be impossible without the additional information for the individual vehicle (tyre age, type, condition, vehicle weight etc).

Perhaps that is where the confusion is - a tyre pressure checking device is not the same as one that measures it to give an accurate pressure reading like a tyre pressure gauge would. In the same way, an in-wheel device only gives you a number - it would need interpreting and doesn't at all tell you whether that pressure is right for your tyre or load conditions. It all depends on which point in the system you want to accurately measure, I guess.
posted by Brockles at 4:05 PM on July 5

No, that isn't where the confusion is.
posted by llc at 8:27 PM on July 5

I meant with the OP's terminology. Also, with what you are saying the system does compared to the limitations of such a system for determining an accurate tyre pressure in any car and tyre combination rather than a control example. It's patently obvious that the machine considers sidewall stiffness and pressure as part of the analysis of the contact patch in the literature you link to - have you even read it? So without a known sidewall stiffness, how does it not lose accuracy?

Regardless, it won't work for driving a random car over to give an accurate tyre pressure reading. If it even gets within 5 psi I'd be surprised. It just can't work without being fed parameters that make it unrealistic for every day style use as implied in the initial question (as I interpreted it and clarified above). I'm not 'making up physics' I am using 20 years of advanced vehicle testing knowledge to come to that conclusion.
posted by Brockles at 8:47 PM on July 5