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>> Seriously, I'm curious why inflation pressure is the primary factor.
>Tire pressure determines the downward force that is exerted on the water.
>You need this force to accelerate and move the water out of the way.
>To drive in the rain without hydroplaning you need a small tire contact
>patch (high pressure, high force per unit area) and low speed.
Now this is starting to make some sense. Perhaps the original post
( sqrt(pressure) * 9 ) originated with a vehicle whose contact patch was
proportional to inflation pressure such that this formula was found to
hold (ie, derived empirically).
For a different tire and vehicle weight, the contact patch woule also vary
with inflation pressure, but by a different constant of proportionality
(assuming it *is* constant). That means the formula given could not be
expected to hold for a different tire+vehicle, although a relation of
identical form exists, and would have to be derived emprircally for every
In other words, for *your* car it might be: sqrt(press) * 12 or something.
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