Brake Design - Road Race 2

[wayno1391]

I am in need of basic brake layout design for road race car. Master cylinder size for front vrs. rear? Bias bar vrs. proportioner? looking for short throw/hard pedal and ability to adjust bias from front (dry track) to rear (wet track). I have master cylinders 5/8, 3/4 and 13/16 in hand. Also a hydraulic porportioner and a bias bar set up. Calipers are equal on all four corners. Smaller tires on front. Origional design information has been lost. Looking for where to start.

 

[swall]

Cobra kit car? Do a search on "brake bias bars" and you will find a couple of useful threads.

 

[wayno1391]

No, formula car has been full bodied to sports racer. Thanks.

 

[mishar]

Start with putting the largest one for the front and the smallest for the rear, bias bar at 60/40 and than experiment. It depends highly on aerodynamics. Down force can make rears way more useful at high speeds, but what will happen when you slow down?

I didn’t really understand why you would want more rear braking in wet conditions.

 

[BrianPetersen]

It's probably more correct to say you want "less front" braking in the wet. Less deceleration G-force means less forward weight transfer in braking. Whether that's meaningful, compared to countless other simultaneous effects, is another matter. Personally, I want the fronts to lock first no matter what, otherwise one goes spinning off the track.

There are more factors than the original poster has mentioned; center of gravity height and position being most significant. It's easiest to figure out graphically. To get an idea of the front/rear brake balance, draw a scale diagram showing the tire contact points with the center of gravity at the correct location with regards to its height and the fore/aft location in the wheelbase. Draw a line from the CG down to intersect the ground at an angle whose slope corresponds to the maximum expected deceleration G-force (e.g. if you expect 0.75 "g" deceleration then the slope has to be 0.75 inch/mm forward for each 1 inch/mm down - hard to explain in words but if you do the drawing it's easy to figure out). Where that intersects the ground, the inverse of the proportions of the lengths from that point to each tire contact patch gives you how much force is on each contact patch and that's the brake balance. Err a bit on the side of front, and there is your first-guess.

This does not account for aerodynamic effects, which can be significant on winged race cars at very high speeds. If the aerodynamics are right then there should be downforce on both the front and rear ...

 

[NormPeterson]

If braking is in essentially a straight line and the rear tracks overlay the front tracks, rear tire grip may be aided by the evacuation of water provided by the front tires. Think of this as a split-mu situation turned 90°.


Norm

 

[wayno1391]

The info received here, coupled with that from Wilwood tech dept, results in a 3/4" front M/C, 13/16" rear M/C, bias bar at 50/50 and hydro proportioner in rear line set at "0". I'll start with this and fine tune at the track. Thanks for all the input.