hydroplaning

[George Harris]

I think it's the 'contact patch' thingie where the theory falls apart. 
With an aggressive tread, less of the rubber makes contact at the same 
pressures. Take the snow tire as an example. The contact patch is 
relatively small compared to a touring tire at the same pressure. The 
rubber in the tread is supporting the snow tire on the knife edge of the 
tread allowing it to cut through the puddle and squeeze the water out 
through the valleys between the tread.

If the contact patch were the same for all tread designs at all tire 
pressures then I would agree with the theory, but I don't believe it is. 
With an aggressive tread, less rubber actually meets the road at the 
same tire pressures. The pressure per sq inch is greater on that rubber 
due to the support given by the rubber in the tread. The tires in 
question, a belted touring tire, and a belted snow tire or rain tire, 
deflect the same amount presenting the same 'contact patch' but on the 
rain tire with aggressive tread, the actual rubber meeting the road is 
smaller.

If this is not true, why does every manufacturer have special designs 
for wet road handling such as the Michelin HydroEdge?

Huw Powell wrote:
> 
> I think this is the way that rule of thumb formula works.  At those 
> speeds, as long as there is a layer of water, the tire can no longer 
> penetrate it fast enough to make any contact - tread pattern does not 
> matter because the "contact patch" is doing the hydroplaning.
> 
> While I'm sure there might be some small variations (it sounded like a 
> rule of thumb, after all, not "hard" physics), it does make sense.
> 
> Think of what happens when you hit a puddle, say, at least 1/2" deep - 
> no matter what the tire is, if you're going fast enough it simply rides 
> on top of the water.
> 
George Harris wrote:
 >
 >> IMHO this theory that hydroplaning depends only on the pressure in the
 >> tires, and is independent of tread design and contact patch, just
 >> doesn't make sense.
 >>
 >> Think logically about what happens. If you have a vary aggressive
 >> tread with deep grooves, water in all its forms, wet, snow, mud, can
 >> squeeze out through the grooves and the tread will make contact with
 >> the pavement.
 >>
 >> Taking it to the extreme, a very narrow tire will not hydroplane as
 >> easily as a drag racing slick.
 >>
 >> If the above is wrong, my thinking on this subject has been wrong for
 >> the last 45 years so I hope someone can show me some experiments that
 >> prove it's wrong :-)
 >