Are vented rear discs 'worth it' on a FWD track car?

[David Redszus]

I did a lot of track time in a warmed over Lotus Elan with 50/50 static distribution and braking balanced approximately 65/35, as per stock. To be fair it could have used more braking overall, given substantially increased tire grip, but never found an issue with fore/aft balance either on track or street.

Assume a Lotus Elan with the following:
Overall weight (assumed to be fully loaded) = 1517 lbs
Wheel base = 84 in
CGH = 16 in
Weight distribution = 50/50%
Tire/road Mu = 1.28 max

Under braking at tire maximum grip;
the front tires will need a force of of 1443 lbs
the rear tires will need a force of 497 lbs, (left and right combined).
Total required brake force is 1940 lbs.

The required brake bias: front 74.4%, rear 26.6%.
A bias of 65/35 will cause a rear tire lockup while attempting to reach
maximum vehicle braking force.

How did you measure the brake bias? What is the tire grip limit Mu?

[chimpvalet]

My approach to the calculations was just to compare caliper piston and rotor diameters as the car had a single circuit system with no pressure limiters or proportioning valve. Front calipers were 1.89" piston, rears were 1.34 as I recall. Rotors were 9.6" front and 10" aft. As stated the car certainly could have used more brake than it had but it was sold on before exploring further.

[David Redszus]

My approach to the calculations was just to compare caliper piston and rotor diameters as the car had a single circuit system with no pressure limiters or proportioning valve. Front calipers were 1.89" piston, rears were 1.34 as I recall. Rotors were 9.6" front and 10" aft. As stated the car certainly could have used more brake than it had but it was sold on before exploring further.

Do you remember the MC diameter, number of caliper pistons, front and rear, and tire size?

[chimpvalet]

The M/C was .625, 2 pistons per caliper all around, tire OD around 22.6". I'd fitted Panasport 14x6 alloys with Hankook 185/60-14's which were native to some Dodge Neon spec race series. Beautiful little car with faultless handling as I'd set it up. Taking guidance from David's analysis it seems it could have used larger bore front calipers.
Cheers

[David Redszus]

The M/C was .625, 2 pistons per caliper all around, tire OD around 22.6". I'd fitted Panasport 14x6 alloys with Hankook 185/60-14's which were native to some Dodge Neon spec race series. Beautiful little car with faultless handling as I'd set it up. Taking guidance from David's analysis it seems it could have used larger bore front calipers.
Cheers

Your MC, piston, disc and tire diameters will result in a wheel force distribution of 65.6% front and 34.4% rear.
To reach maximum vehicle tire grip you will lock up the rears well before the fronts reach their grip limit.

There are two easy solutions to achieve the correct brake balance and increase stopping force.
Install a brake force proportioning valve in the rear brake line. It should be selected so that the rear
brake line pressure is reduced to 70% of the front line pressure. This will produce a brake bias ratio of
75/25, and allow increased force to be applied to the fronts without locking up the rears.

Once a reasonable brake balance is accomplished, the bias can be tweaked to induce a slight
braking corner oversteer to assist corner entry.

Another approach is to use rear pads with lower Mu values, or a combination of both techniques.
Done properly, braking will be improved, as will tire wear.

On real race cars, we use data logging to measure brake line pressures, pad temperatures, and corner wheel
slippage and compare them with linear deceleration Gs.