Re: Hydroplaning

[Don Hoefer <hoefer@eng.pko.dec.com>]

Huw,

> Not air pressure not air pressure not air pressure, it's weight on the
> tire that matters
> 
> air pressure just affects hydroplaning by changing the footprint...

Yes and no.  A heavier vehicle with the same tire pressure will tend to
hydroplane at the same speed.  Since you are pushing against a fluid,
force per unit area is key.  Unless you are driving on the rims, force
per unit area is provided by the tire's inflation pressure.  When you
change the footprint of a given tire on a given vehicle, you change the
force per unit area.  A narrow tire will hydroplane at a higher speed
than a wide tire at the same inflation pressure because there is a smaller
area of water that it has to push against.  More of the water will be
able to flow to the side (where you still need a force to keep it there),
whereas a wide tire has to push more water along with it as it will not
be able to flow to the side as readily requiring more force to do more work.

The confusion here is that there are two factors at work.  Tire inflation
pressure and contact patch shape.  Tire pressure affects both for a given
load.  A true test would be to increase tire pressure and increase the load
to yield the same contact patch.  Now you have a case where hydroplane speed
is solely dependent on tire inflation pressure.

The force required to push a fluid increases as the cube of speed (speed^3).
That means that the hydroplane speed formula given is only approximate within
a limited range of tire pressures.

Don Hoefer
'82 Coupe

Massachusetts, USA

P.S.  Any fluid mechanics masters on the list?