MadSci Network: Physics

Re: Do tires skid easier at high speed or low speed?

Date: Thu Nov 9 20:24:32 2000
Posted By: Arnold Anderson, Staff, Tribology/Friction systems, retired (Ford Scientific Laboratory)
Area of science: Physics
ID: 973622221.Ph

While a car moves along a road, the contact surface of the tire 
temporarily has a static contact with the road surface.  This is the same, 
whether the car is moving fast or slow.  However, there are differences 
with speed, duration of contact and rate of loading.  For this reason, a 
tire-road surface, when wet, is much more likely to produce a skid as the 
vehicle speed increases.  It does not really matter what loads the tire 
contact.  It could be from the brakes, the steering, or even a wind gust.  

When the tire-road contact is dry, many variables influence skidding.  You 
asked specifically about a tap on the brake pedal.  For most brake 
linings, the friction will be lower at high speeds--for the same brake 
pedal tap.  Thus, the tires will have a bit less loading at higher speeds.

I looked at data from proving ground tests on cars to get some specific 
data.  These tests were performed to stop the car as quickly as possible, 
without skidding.  Brake applications were made at 30, 60, 80, and 100 mph 
(about 50, 100, 130, and 160 kph).  A small difference with vehicle speed 
did appear.  However, no human would be able to reliably detect this small 

You should be aware that there are exceptional conditions, where things 
would be different.  For example, some racecars have wings that increase 
the down force on the tires as speed increases.  Some passenger cars and 
light trucks may have the opposite effect--there can be some aerodynamic 
lift.  In addition, the braking action also can vary with speed, depending 
on the type of brake (disc, leading-trailing drum, duo-servo drum), and 
some brake specifics (brake lining type, rubbing surface runout).

To answer your question simply--when road conditions and brakes are good, 
there is no significant speed effect.  

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