Braking at high speed vs low speed.

[Komodo86]

I am interested in the forces at play determining wheel lock up at different speeds.

Lets say a car is capable of locking the front wheels up at 130mph.

Does it lock up the wheels significantly easier at 30mph? If so, why?

If we assume it can generate the same acceleration level from both speeds, so the same load transfer, what would cause the tyre to give up grip sooner at low speed than at high speed?

 

[imcjoek]

If we assume it can generate the same acceleration level from both speeds, so the same load transfer, what would cause the tyre to give up grip sooner

This statement is contradictory. If they generate the same acceleration level, how do they also "give up grip sooner"?

At high speed, aerodynamic drag helps you stop.
At high speed, aerodynamic lift doesn't.
Unless you have downforce. In which case it does.

 

[dynatune]

Easy to ask, difficult to answer, here a few thoughts

1) How about aerodynamics of the car ???????? .....As mentioned Drag helps but Lift makes things worse .... could be worse could be better at 30 mph ..
2) How about tire contact patch deformation of the tire due to centrifugal forces ? ... probably better on 30 mph ....
3) Does the car have Electronic Brake Distribution ? ... if so this can influence significantly the stopping distance giving room for more stability at high speed braking ...probably better at 30 mph
4) Not even getting into ABS/ESP ....
5) What kind of brakes/pads are on the car ... at 30 mph they might not get to optimal temperatures whereas at 130 mph they might do .. or vice versa ....
6) .....

Cheers,
dynatune,

http://www.dynatune-xl.com

 

[GregLocock]

But at least in the speed range 115 to 10 kph the deceleration rate using an ABS car is just about constant, to within experimental accuracy.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[Komodo86]

Easy to ask, difficult to answer, here a few thoughts

1) How about aerodynamics of the car ???????? .....As mentioned Drag helps but Lift makes things worse .... could be worse could be better at 30 mph ..
2) How about tire contact patch deformation of the tire due to centrifugal forces ? ... probably better on 30 mph ....
3) Does the car have Electronic Brake Distribution ? ... if so this can influence significantly the stopping distance giving room for more stability at high speed braking ...probably better at 30 mph
4) Not even getting into ABS/ESP ....
5) What kind of brakes/pads are on the car ... at 30 mph they might not get to optimal temperatures whereas at 130 mph they might do .. or vice versa ....
6) .....

Well, quite. There is a lot of variables! I am not convinced that it easier to lock up the wheels at lower speeds. But some people are.

To my mind, all else being equal, there is the same normal force on the tyres in either case, so the same brake force can be applied. What is not constant (and what the driver is acutely aware of) is the amount of pressure they are putting on the pedal, and the ratio of pedal pressure to brake force is not completely linear from a high speed stop owing to the effects of temperature on the friction co-efficient of the pads.