Cornering In The Rain
(Traction and Hydroplaning)
By: James R. Davis
Though it may be obvious to most of you it's not that obvious to some.
quote:
The question is fairly simple: do you recommend significantly reducing your cornering speed on wet roads? I understand that there are many variables to be considered, so I'll narrow it down to a specific set of conditions. On a road with which you are fairly familiar, in a moderate rain but with no standing water (i.e. no visible risk of hydroplaning), do you take corners at the same speed you would if the road was dry? I guess what I'm getting at is, to what degree does water decrease available traction/friction?
There are many variables to take into consideration so there are no set answers but there are some 'rules of thumb' that we can talk about:
On dry pavement
- Your tire traction can handle up to about 1.1 g of acceleration
- When your tire demands exceed that amount of traction you skid which reduces their ability to handle acceleration by about 25%
On wet (not standing water or with depth less than about 1/4 inch)
- Your tire traction can usually handle up to about .8 g of acceleration
- When your tire demands exceed that amount of traction you skid which reduces their ability to handle acceleration by about HALF
If standing water or depth greater than about 1/4 inch
- Traction available at speeds less than about 50 MPH remains able to handle about .8 g acceleration (though I have low confidence in that number)
- When your tire demands exceed that amount of traction you HYDROPLANE which reduces their ability to handle acceleration to essentially ZERO
Hydroplaning is more probable
- The faster you go
- The wider your tires
- The lower the air pressure in your tires
- The deeper the water is
This is true regardless of whether you are accelerating (in a straight line or turning) or not.
It is most likely that your front tire will hydroplane before the rear one does because you ride a single-track vehicle. That is, the front tire squeezes most of the water off the roadway by the time the rear tire gets there. But those of you who have put an extra wide tire (perhaps even automobile type) on the rear wheel will find that you have changed that dynamic. The odds of the rear tire hydroplaning first in that scenario goes up dramatically.
One last thought ... water drainage on a freeway (any wide roadway, actually) drains to the right (in the U.S.) and that means that the depth of that water is greatest in the SLOWEST LANES. That should suggest that the odds of hydroplaning is about equal in every lane (slower movement but greater depth makes the slow lanes as dangerous as the faster lanes.)
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(James R. Davis is a recognized expert witness in the fields of Motorcycle Safety/Dynamics.)
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