Rally Car Brake System Design

Hello,

A few questions while going through the Brake System Design Course:

1. What is a typical (assumed) peak deceleration value for a rally car running 99.5% of stages on a gravel stage? Here in the States we have very little tarmac stages.

2. Can I assume the coefficient of friction will be the same as the peak deceleration as in the worked example in the course?

3. Are most rally cars set up for equal brake bias as they would want the back end to rotate easier?

Thanks a lot, love the course!

p.s. a glossary of terms and definitions, might be helpful for some, my $.02

~Bergen

My 5 cents from gravel sprints and hill-climbs - mileage may vary.

1/ it's going to depend a great deal on the tyres used their size, the vehicle weight, and - of course - what is meant by 'gravel", etc. Smooth, hard, swept 'gravel' can be very close to tarmac with good tyres, but if it's wet or has a lot of loose dirt or 'gravel', it can be a lot worse.

2/ I haven't seen that, but see above - there's a close relationship between CoF and peak deceleration.

3/ Depends - front wheel drive, rear wheel drive, AWD? Heavy or light? Weight distribution? A lightened Japanese FWD with a 70% front bias is going to be rather different from a Porsche 911 with a rearward bias - and, yes, they are rallied on gravel in Europe [for decades, just another part of their global motorsport success, like the Paris-Dakar] and sound GLORIOUS! Have a look on YT for them.

Yes, a rear-ward bias is sometimes used, but that's proportional to the weight bias, too, with it being probably most used with FWD, to reduce understeer. Unless one is skilled, and practiced with it, it's an easy way to proceed directly to the accident - do if you try it, be careful.

Depending on what vehicle you're using, a braking bias of around 70% may be a good starting point, or maybe 60% with an adjustable pedal box and/or pressure limiting, or proportional, valve on the rear lines. A driver adjustable box is a good idea, anyway, as conditions can change rapidly between stages, or even during one.

Hi Daryl,

Some good info from Gord, but I'll add my thoughts as well.

1). gravel is quite different, as depending on the depth, the gravel will build up in front of the tires under braking and form a wedge, which can be helpful. Again it's going to really depend on the vehicle and what tires you're running, and data will be the best way of knowing.

I would assume though that the deceleration values would still be less than that of a car on asphalt, simply due to less grip.

2). Yes still make this assumption, it'll keep things simple. Basically this value is used to calculate the load transfer and also braking torque capacity, so it'll still be a good assumption. Just be sure to not overestimate this value in the first place.

3). I would say definitely don't assume equal brake bias. It'll most likely be far too rearward unless you are in something very very rear-weight bias. However, due to less grip, you will transfer less weight, so a more rearward bias will be suitable. In terms of targeting a more rearward bias for rotation though, that really depends on the driver and how snappy the car is, I wouldn't design the braking package around this, rather I'd use adjustment of the bias bar or proportioning valve to achieve this if necessary.

I'll just add that many rally drivers make heavy use of left foot braking, so that should be accounted for in thermal capacity and balance, in addition to using the brakes to actually slow down.

Mike has a good point there - skilled drivers may use the brakes and power at the same time to help balance the vehicle, with the power effectively reducing brake torque at that axle. Also, especially if anti-lag isn't an option, it's not unusual to apply some degree of throttle to reduce lag on exit, or to keep boost levels up with full throttle but use the brakes to moderate the torque to the ground to reduce tyre slippage.