Should I go easy on potholes or speed right over them?

March 6, 2007 3:36 PM

[Dynamics Filter] Is it less damaging to the wheels to hit a pothole at higher or lower speeds?

Recently, driving my car on a Brazilian road that looked like swiss cheese, I couldn't avoid a couple potholes. It was night, I had never been on that road before and if I swivelled to miss one pothole I might just hit the next. So the next morning I found two slight dents on the front passenger wheel and a flat tire. I suppose it emptied during the night through the gaps caused by the dents.

That got me wondering. If I had been going slower but still hit the same potholes, would there be less damage? More?

I realize there are many details to consider. Tire pressure. Size and depth of pothole. Maybe even road/air/tire temperature.

But provided (a) the potholes are sometimes unavoidable, or not safely avoidable, (b) they are not wheel-swallowing craters, and (c) tire pressure is optimally set, should I go faster, or slow down in order to reduce tire/wheel damage?
I was going about 100-110 km/h (65-70 mph), the car was full and my tires were filled to the pressure indicated on the manual for heavy weight plus road driving.

Some say you can speed right over potholes, some say you should brake before you hit them, some say don't break:

If you approach a chuckhole at speed, don't brake heavily as you near it. Heavy braking compresses the front suspension of the car and will have a tendency to force the tire and wheel down fully into the pothole, potentially causing greater damage than your car might experience if it "skimmed" over it. (here)

Driving the roads at a higher speed (say 50-70 mph) results in a MUCH smoother ride as the tires/wheels literally don't have time to enter the holes - the springs cannot un-compress fast enough (its "hell" on shocks and tires though).

A number of our drivers insist that the higher speed helps them "float" or "fly"over the holes and that slowing down will just make the cars fall into them easier causing more damage to the vehicles. (here)

The impact of potholes on tires increases dramatically with speed and can cause hidden, internal damage that could lead to tire failure weeks, or even months, later. It’s best to avoid potholes entirely, but if that’s not possible, don’t brake during the pothole impact. Instead, apply brakes before hitting a pothole and release them just prior to impact. Braking during the impact sets up the tire and wheel assembly for a “solid hit” against the edge of the hole. Less severe damage occurs when a tire is rolling than when it is skidding over a hole during braking. (here)


So?

posted by AnyGuelmann to science & nature (19 comments total)

Mythbusters: Rough Road Driving
posted by matildaben at 3:43 PM on March 6, 2007

You'll be better off at low-speeds for all but small or shallow holes. (and those ones don't cause damage at any speed, just bumps.)
posted by PEAK OIL at 3:50 PM on March 6, 2007

I'd take Mythbusters over PEAK OIL. Smoother at speed.
posted by mendel at 4:11 PM on March 6, 2007

The Mythbusters show has two problems for telling AnyG how to drive his car. First, they didn't make any effort to see what was harder or easier on the tires/wheels (which was the focus of the question here) and second, they were mostly looking at washboarded roads. (Mythbusters shows in general are pretty unimpressive, but that's another issue entirely.)

For washboards, it is clear that you minimize the pounding on the vehicle by finding the speed (often around 45mph) that puts you in "tune" with the road. The downside, of course, is that doing so means that your tires are in contact with the road only about 2/3 of the time, so you have a lot less braking/steering capacity if there is an emergency. Going faster or slower will radically increase the pounding you and the car receive --- what the right speed is will depend on the frequency and amplitude of the washboards, your car's wheelbase, etc.

But for intermittent potholes (or big speedbumps/topes/judderbars), slower speeds mean less damage to the car. At higher speeds, you run the risk of denting the wheel, pinching the tire (or the tube inside the tire, if your vehicle has those), or damaging suspension components, because the forces are higher.

So slow down for potholes (also because you don't know how deep they are --- a six centimeter pothole is nothing, but a meter-deep pothole is no fun at all), and for washboards find the intermediate speed that keeps the pounding in check.
posted by Forktine at 4:32 PM on March 6, 2007

The big potholes which will pinch your tire and bend your rim need to be handled slowly. If it is night, never drive beyond the combination of the reach of your headlights and your stopping distance. Put your brights on to be able to see a further distance if need be. If there is no way to avoid one when you are driving like a bat out of hell then avoid hitting it head on. Hitting the edge of the hole will do less damage to your tire and suspension than a full frontal assault.

Not all potholes are made equal so there is no one method of surviving all of them. Keep your eyes on the road and drive to the limits of your vision. If you can't see far enough ahead of you to stop for a pothole, what the hell do you think will happen when you find a fully washed out road and a 20-ft deep gully dead 40 feet ahead?

Always remember Newton's laws. The higher the velocity of the mass, the greater the force.
posted by JJ86 at 4:49 PM on March 6, 2007

A road riddled with potholes isn't the same as a washboard road. I've driven over both. Driving to the West Coast Trail in British Columbia I had to go over a very long washboard road. Speeding up made a smoother ride. In fact slowing down caused my truck to start slewing sideways. I also drove the Purdhoe Bay haul road. That was a few hundred miles of the most horrible fucking potholes ever. All driving fast did was destroy my tires faster.
posted by substrate at 4:52 PM on March 6, 2007

I'd take Mythbusters over PEAK OIL. Smoother at speed.

Mythbusters did not test pot holes. They tested smoothness over washboard roads.
posted by PEAK OIL at 5:13 PM on March 6, 2007 [1 favorite]

Is it less damaging to the wheels to hit a pothole at higher or lower speeds?

If it a sizable pothole then more damage will be done at higher speeds.

The real question should be Is is safer to drive on a road with potholes at high or low speed?

And the answer to that is that at lower speeds you have a much better chance of noticing a killer pothole and avoiding it.

I once got a flat from a largish pothole in an asphalt road. The reason I didn't see it was there was a double tired truck in front of me that it didn't phase.
posted by MonkeySaltedNuts at 5:28 PM on March 6, 2007

This depends on so many variables it isn't even funny. Certainly the limiting case of 1 mph would result in little to no damage at all. You'd be gently rolling into the hole and then rolling back out of it. No big deal.

Alternately, consider you were going about 56 mph (25 m/s) over a pothole that is perhaps a foot or two (let's say 0.5 m) wide. That means that one wheel cruises over the pothole in 0.02 seconds. Plugging into your basic physics formula, your tire would drop a grand two millimeters into the hole during this time.

Clearly, this is a situation where the middle ground, a moderate speed, results in a bigger impact than either going very fast or going very slow. I am certainly not an automotive engineer, so I am not sure what kind of variables to look for to know what will and will not damage a car.

Now, for a more complex scenario (rather than my simplistic tire rolling down the road), I'd have to consider things like the suspension, because that suspension will push the tire down into the hole as the car moves over it, resulting in a larger displacement than two millimeters. You'd probably need a spring constant and a few other numbers to find out how fast that tire wants to descend.
posted by adipocere at 5:55 PM on March 6, 2007

That means that one wheel cruises over the pothole in 0.02 seconds. Plugging into your basic physics formula, your tire would drop a grand two millimeters into the hole during this time.


Your tires do not drop because of gravity. They drop because there are large springs pushing them down. As such, your calculation is wrong. i'm not going to try to guess how wrong it is.
posted by PEAK OIL at 7:26 PM on March 6, 2007

There is clearly less damage to the wheels/tyres at lower speeds - the only way there would be any real chance of the wheel "flying" over the top of a pothole would be if the pothole was very small in diameter or if your speed was ridiculously high. Also, it is far easier to avoid the holes at lower speed, because you have more time to react and swerving is less likely to lead to lack of control at lower speeds.

That's not to say that driving faster would not be more comfortable for you, because it depends on the harmonics of the suspension and what they are set up for. it may well be that driving faster allows the suspension to reduce how much the bump is transmitted into the car (because the up/down motion takes place over a shorter period of time?), but the impact on the wheel/tyre is still going to be higher because the impact speed is higher.

Unlike corrugated roads, there is no possibility of "tuning" the speed of your car to the road, because potholes are too random. Corrugated roads (if you are going to reference Australian roads, at least use the right language ;-) all have a speed that works best - too slow and you will shake the shit out of the car and everytihng in it, too fast and you will lose traction completely and hit the first solid object in a straight line from where you are now.
posted by dg at 7:29 PM on March 6, 2007

I do not see where I said the word "safe." I do see the phrase "I am certainly not an automotive engineer," though. I obviously did not intend for the rest of what I wrote to be ignored, since that would be pointless of me to write. I do see that I wrote that there are a lot of variables to consider and suggested that back of the envelope calculations are probably not helpful.

When it comes to safety, I might question the wisdom of dangerously driving slowly on a Brazillian road, at night, given that the rest of the drivers might not be. And, you know, dangerously randomly braking for potholes.
posted by adipocere at 8:41 PM on March 6, 2007

Some people could interpret "your tire would drop a grand two millimeters into the hole during this time. " as an implication that it is generally safe.

I simply wanted to point out that this is incorrect. It is unsafe to speed over pot-holes for any number of reasons.

Effective wheel rates are essentially unimportant here. The original poster dented a rim. Unless you are running on exceptionally low-profile tires, with fragile (racing) rims, this is hard to do, and requires a very serious pot-hole. Going faster over such a hole would almost certainly have caused more damage, and exposed the driver to more danger.

As such, I see no point in intellectual discussions about the few cases where it would be okay. They do nothing to answer the question, and can easily be interpreted by careless, or non-scientific readers as advising incorrect actions.

It's nothing personal. it's just that your post is not only useless, but also dangerous.
posted by PEAK OIL at 9:24 PM on March 6, 2007

Not that it's all that germane to the question asked, but then again, it may be.

If you damaged your rim and tire as described, you almost certainly damaged your front suspension, too. Even a MacPherson strut suspension can have its control arms bent by those kind of forces. Slow down, way down, man, when faced with this kind of road. 25 km/hr max. 1/2 that, if the potholes are wider than your tire diameter, or deeper than your tire radius.
posted by paulsc at 10:30 PM on March 6, 2007

If you damaged your rim and tire as described, you almost certainly damaged your front suspension, too.

This is an excellent point. It hadn't occurred to me that somebody would fail to check this/have it checked, but most people aren't car people.

Common front suspesions (MacPherson, double-wishbone) are very good at handling vertical loads, but have very limited ability to withstand the sort of horizontal load that would accompany a wheel crashing into the far side of a pot hole.
posted by PEAK OIL at 6:10 AM on March 7, 2007

Again, forget the math, forget the physics. I've driven on both terribly bad roads and washboard roads. The way you treat them is different. If you try to speed over a road with lots of deep and sharp potholes you're just going to wreck your tires, possibly your rims, possibly your suspension.

On the Purdhoe Bay haul road even with reasonable caution we had to patch holes quite often and make use of one of our spares. The one time I overdrove the road I ended up shredding my tire. If I'd have damaged anything else we would have been hosed until an infrequent truck passed.

Mythbuster fanboyism is great, I love the show, but you're giving this guy dangerous advice.
posted by substrate at 6:26 AM on March 7, 2007

Well, this guy is actually a girl ;-)
I will absolutely have the suspension checked, thanks for that!

The thing about low speeds is that it's just not possible to go slow when you're on a busy highway shared by large menacing trucks. The least you can go is 80km/h, is not even safe to go any slower. So if really low speeds are out of the question, I guess the doubt lies between high speeds and higher speeds.

Of course, as I said, huge asphalt craters are beside the point as they are probably fatal to the wheels at 80, 120 or 140km/h. And in that case this thread would probably be about broken axles rather than bent rims.

Instinctively I feel that at 80km/h the bang is stronger, or at least more head-on to the wheel and more likely to tear the tire/bend the rim/both. At +120km/h it seems to me the impact is lighter even if the pothole is relatively large - over 15cm deep and maybe wider/longer than the wheel. I guess the potholes I hit were about that large. I was going 90-100km/h. That's the doubt in my girly-driving mind.

(FYI the car is a 2007 Volkswagen Fox.)
posted by AnyGuelmann at 7:08 AM on March 7, 2007

I forgot to mention something important.
The amount of blown tires and pieces of rubber you see along the road in Brazil is no joke.

Knowing very well that Brazilian road drivers are not exactly a cautious breed and the absolute majority drives above 120km/h, I wonder if my instincs (previous comment) are all wrong about this subject. And that wouldn't exactly be surprising :-)
posted by AnyGuelmann at 7:14 AM on March 7, 2007

Mea culpa. Even though I'm not a dude, I plead guilty to Male Answer Syndrome. Mythbusters has nothing to do with this question and I apologize for bringing it up.
posted by matildaben at 10:01 AM on March 12, 2007