Would it make sense to expect a car to hydro-plane LESS when the tires are under-inflated because there is marginally more tire (think of how a flat tire squashes) making contact with the wet road?
Or would it be reasonable to expect MORE hydro-planing as more of the tire (and the car’s weight) is distributed over the wet road and less water is channeled away?
Or is there some other physics or general explanation?
Your understanding of what happens when a tire is under-inflated is incorrect.
When a tire is under-inflated, the center area of the tread does not make contact with the road, and only the edges of the tread make contact with the road. Visualize a concave tread area and you will get the idea.
Conversely, when a tire is over-inflated, only the center of the tread makes contact with the road, and the tread edges do not. Visualize a convex tread area in this case.
Neither situation is good, from the standpoint of hydroplaning, or from the standpoint of handling, or from the standpoint of even tread wear. You want the full width of the tire’s tread to be in contact with the road, and that means using the correct inflation pressures for your particular vehicle.
In other words, you should be inflating your tires to the pressure recommended by the vehicle manufacturer on the placard affixed to the door jamb. NEVER use the inflation pressure listed on the tire’s sidewall as your guide to normal inflation pressure.
On dry pavement, no tread is needed on the tires. However, treadless tires would slide all over the place on wet pavement because of the thin layer of water between the road and the tire. The tread is designed to throw off the water and prevent hydroplaning. Indianapolis race cars run “racing slicks”–tires with no tread. The race is called after so many laps if it starts to rain or postponed if it is raining before the race starts. This is done because the racing tires would have no traction on the wet track (and the race cars aren’t equipped with windshield wipers).
Any tire that is either over OR under inflated will have more of a tendency to hydroplane AND to slide on slick surfaces than a tire that is properly inflated.
Tha also applied to a tire that has a significant amount of the tread worn off. Contrary to popular belief, a tire worn down to where the wear bars are obvious is nowhere near as good as a tire with over 50% of its tread left…and a new tire is even better. Wear bars only designate a legal limit. Not a safe limit for all but the absolute best climates.
If you are talking about a few psi under or over, then I doubt the effect is measurable. Now if you are talking about a car/tire combo that calls for 32 psi and you are running either 8 psi or 46 psi ( 24 psi under or over), then the tire is not going to handle as well in the rain, but then it won’t handle as well on a dry road either.
For hydro-planing, be more worried about the tread condition
-and-
wider tires will hydroplane when narrow ones will not …at the same speed on the same weight car.
Think of surfing on the beach on a boogie board -vs- a 2x4.
The wider the more surfable.
I agree with @kengreen and @keith and others completely. Hydro planing is more about the conditions vs the tread design, speed and tread wear with tire pressure being a marginal contributor. Obviously, having the correct air pressure is somewhat important, but newer radials are more forgiving as far as road contact is concerned. If you are driving so fast as to prevent the water from being displaced correctly, no tread design will help, even over water you could easily handle at lower speeds. Simply put, lower speeds with slightly incorrect tire pressure is much more beneficial then going too fast with perfectly prepared tires.
Super wide tires with added contact area may help cornering in the dry, like bald tires are better on dry pavement as well. They quickly become problematic as both speed and water depth increase as a car tire begins to plane. This is much the same way some planing boats do on water with their flat aft surfaces and wide contact areas as their speed increases. When that happens, at least a boat has a rudder while a car doesn’t.
So, my vote is to slow down. It’s more effective and easier then changing the air pressure !
Tread style can certainly make a difference in hydroplaning, if it is endangering your life new tires are a cheap alternative. Hydroplaning is as real as the ratings for hydroplaning on tires.
Here’s one way to think about it: a tire that’s twice as wide has twice as much water to displace as it drives along. So a low tire is wider, and more prone to hydroplane. Also a low tire has less pressure per square inch, which makes it harder to squish out the water. A lose-lose.
Other factors being equal, SPEED plays an important role here as well…Hydroplaning can almost always be avoided by slowing down…
Theoretically even my neighbor’s bald tractor tires won’t plane, regardless of the water depth…slooooow.
One more point: a car out of alignment or a car with worn out parts or irregularly worn tires is more likely to hydroplane, slip, slide, and do other unwanted behaviors. A tire bouncing along has no way to keep a grip on a wet road, Or a dry road. Or ANY road. A tire trying to glide sideways due to being toed in or out wants to slide. Give it a wet surface and it just might. A car with the wheel leaning in or out (bad camber) cannot get a proper grip on the road. A wheel…well, you get the idea.
Keep your car in good shape.
So, in conclusion.
- keep good tires on the car with plenty of tread.
- use tires rated for the climate you drive in.
- check your inflation regullarly and adjust as needed.
- keep the car in good shape…wheels out of alignment do not interface with the surface properly. Slipping and sliding can easily result.
- when in doubt…SLOW DOWN