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

[BLSTIC]

I'm (hypothetically) looking at getting in some track time with a lightweight FWD car (830kg, 100hp). It's got 234mm vented front discs and 180mm drums on the rear. There are multiple bolt on upgrades for the front (up to 262mm race-quality discs and pads because Integra parts are used), but only a 234x9mm solid disc option for the rear in pedestrian quality.

Are ventilated discs a worthwhile upgrade over solid discs on the rear of a lightweight FWD? The path from drums to solid discs is a simple enough upgrade, maybe half a day off the ground. But to go to ventilated discs would mean developing everything myself (probably use the existing front discs on the rear, but FIIK what caliper to use).

Do solid rear discs ever really overheat on FWD track cars? Would I be likely to experience this in, say 8 minutes of track time? Or would I only have to worry for endurance events (i.e. never because it's for track thrashing once in a while not real racing)?

I'm not particularly worried about pads for the rear as pads can usually be easily modified to fit.

[youngfg]

I have endurance raced a 300ZX for the last ten years with solid rear rotors with no problems. Races were from 7 hours, up to 24 hours long.

The car made about 200HP to the wheels and weighed about 2500lbs, the stock solid rear rotors and brakes were never a problem. The front brakes were another story.

And being FWD you car would be even easier on rear brakes.

[BLSTIC]

Well that's promising. Also I read the part numbers wrong. Rear disc conversion on mine is 10mm thick, same as yours.

Math time!

Assumptions:
Weight distribution changes are proportional to braking force changes
50mm of swept area both discs, only swept area counts towards heat capacity

300zx (200whp)
2500lb = 1132kg (I grew up metric)
Weight dist = 50/50
Disc Diameter = 290mm = pi*(145^2)-pi*(120^2) = 20813mm^2 heat capacity
1132*0.5/20813 = 0.0272 heat load

Sirion (80whp est)
830kg
65/35
234mm disc = 16415 capacity
830*0.35/16415 = 0.0177 heat load

272/177= 65% of the heat loading

It looks like my rear discs despite their miniscule size are dealing with proportionally much less heat (because of the weight and weight distribution). And that's only if we are braking from the same speeds. With your 5.66kg/hp and my 10.38 (55% of the power-weight ratio) that seems unlikely even if our corner speeds were identical.

So basically even if I screwed up royally with my assumptions and calculations and was out by 50% I'd still be within the realms of using the available solid disc conversion on my car for short events as long as I had a suitable pad.

Thanks for your input.

[PackardV8]

Many years ago, I had a conversation with an OME engineer who was pissed he had to add rear disc brakes just to satisfy marketing. He said, "We've tested it and on a stock FWD under racing conditions, the rear brakes are just along for the ride. The weight transfer is such, it's difficult to keep them from locking up.

[FC-Pilot]

Many years ago, I had a conversation with an OME engineer who was pissed he had to add rear disc brakes just to satisfy marketing. He said, "We've tested it and on a stock FWD under racing conditions, the rear brakes are just along for the ride. The weight transfer is such, it's difficult to keep them from locking up.

Yep! In many oem type stuff that is often the case. We saw with the chargers about 7 years ago that they needed more braking in the rear, but that was a rarity. Now performance stuff that has lowered and much stiffer suspensions that the rears can take more than the factory type setup, as long as the vehicles setup is well sorted out.

To the OP, I think you will be just fine as long as you pick a suitable pad compound. Also ducting can make a big difference to keep you on the safe side. If I were in your shoes I would run the solids.

Paul

[Kevin Johnson]

The early 1970s FWD Renault 17 R1313 used solid disks in the rear versus the lower performance R1312 using drums. Very similar cars save for output.

I used to have examples of both -- looking at the factory parts manuals right now.

[David Redszus]

Optimization of vehicle brake systems is a very complex task. We have written several
programs to assist the engineer in the process of component selection.

There is no improvement in stopping power between discs and drum brakes; its mostly
a matter of hydraulics and force balance.

A proper brake design process would include:
vehicle corner weights
wheel base
CGH
front and rear tire radius
tire mu values

for disc brakes we need:
master cylinder diameter
caliper piston diameter and number
mean disc diameter
pad friction mu

for drum brakes we need:
wheel cylinder dia and number
drum diameter
shoe friction mu
and prop valve characteristics, if used.

The first step is to calculate weight transfer and corner friction forces; longitudinal force balance.
Next we calculate the hydraulic forces at front and rear based on master cylinder, caliper pistons,
and wheel cylinder sizes.

Then we determine the force transfer to the ground for proper force balance.

Then we do it all over again to adjust the brake bias ratio that will permit braking to the
tire grip limit and yet not lock up any wheel. This becomes tricky when braking into a corner.

Then we worry about fluid displacement, pedal travel, pedal force, and heat issues.

My guess is that 95%+ race cars do not have optimized brake systems and that bolt on
after market products make matters worse rather than better.

Here is a very simple test. Take the car on the road to about 25 mph and slam on the brakes.
Can you lock up all four corners evenly? If so, you're nearly done. If not, you have some work to do.

You can determine whether the fronts or rears are locking up prematurely by measuring the skid marks.

Once upon a time I modified the brakes on a BMW for the old Firehawk series. I thought I had done
a great job and told the engineers in Munich about my accomplishment. They made an ass out of me by
pointing out how much I had not considered and how much stopping power I had totally missed.

[shoe]

We endurance raced a 1600lb 110hp civic for years. They are momentum cars.
All your braking will be quick and short. Your straightaway speeds are low and your corner speed is high.
The rears will need a lower temperature pad compound as they will never really get too hot. So forget about cooling.

You will be amazed at how long the rear brakes will last. Fronts too.

[FC-Pilot]

My guess is that 95%+ race cars do not have optimized brake systems and that bolt on
after market products make matters worse rather than better.

I could shake your hand right now! Oh how true. As a brake testing specialist I agree that most aftermarket kits screw more up than fix. “But it has to be better because this is there whiz bang front kit and these guys hooptie rear kit.” The brake system is just like a race engine, great individual components alone don’t mean it will work, but a well designed combination of components will provide superior performance.

Paul

[Kevin Johnson]

I read a little further to discern why Renault -- which was very active in racing -- would discontinue using the solid rear disk setup in the R17 (itself a variant of the R12).

It looks like they decided not to pursue racing it any further:

Renault abandoned plans to contest the World Rally Championship which it won in 1973. Instead, the factory developed a high-performance version of the 17 coupé at the Alpine Competition Factory which used many of the A110 bits to compete in "selected" European events. The Gordini-developed engine had two twin-choke Webers, a hot cam, an 11.5 compression ratio, big valves and tuned extractor exhaust system. The body was very light, featured fibreglass doors, boot and bonnet panels, plus plastic windows and a stripped interior. The factory said the car's weight was lowered by more than 25 percent.

Its most famous outing was the "Press-on-Regardless" WRC in the United States, in Michigan, 1974. The Rally was the US section of the World Rally Championship. The car that won the rally was a Renault 17 Gordini driven by Jean-Luc Thérier and Christian Deiferrer, with a similar car coming third.

I believe that in rallying, the ability of the disk brake system to shed contamination (water, dirt, sand) would be invaluable.

I know from personal testing ("cough") that the top speed listed was at redline in top gear (I did not push it). It certainly would have gone faster. I had versions with the ~120 hp output and also one with the RenaultSport 160+ hp engine mentioned above (100 hp per litre NA from a bolt-in kit). I tested both the wedge head engines and cross flow. The wedge head transplant came from a Europa. Both engines propelled the car easily to the top speed of approximately 112-115 mph. The Europa S2 had a listed top speed of around 120 mph.

The 1972 P.R. 961 parts manual that I have was given to me by my old friend, Jean-Claude, who ran Franco-American Motors (later P.F. Engineering) in Glendale, CA. It contains many of his notes.

Jean-Claude_1994.jpg

[lefty o]

all depending on how serious you are. as the others mentioned, the rears arent doing too much of the braking. mostly imo they are there to keep the car braking in a straight line. whether drag racing, or road course, id be more concerned with the mass of the brakes, and the rears of a fwd car typically arent very heavy in comparison to the front. you have to weigh cost vs. benefit to you. high buck 2 piece rear rotor for a typical fwd car might lose 1.5-2lbs for a rotor, or something mid priced like a wavespec rotor you might lose .5-1lb for said rotor. again cost vs. benefit. a set of good pads alone will be your biggest benefit for stopping per $ spent.

[chimpvalet]

Not the slightest case to be made for the vented rears. If your car's static weight distribution is anywhere near 65/35 the fronts will be doing nearly all the work. 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.
What solid rear discs should provide, if done well, is more consistency as they either heat up or get wet but the points listed in prior posts above must also be considered.

[PackardV8]

Probably the best reason to go to rear discs on a FWD car is it has become very difficult to find good rear brake shoe lining material and anyone to reline the shoes.

Since asbestos is too hazardous to use, there's been little thought or engineering given to drum brake shoe linings. I work with Studebakers and whether they're four drums or two discs and two drums, those without experience complain about the braking and want to go to an aftermarket four disc setup. The problem is always either they bought modern hard linings, can't look beyond the corner tire store for brake work or can't service the Hydrovac.

jack vines.

[Kevin Johnson]

Probably the best reason to go to rear discs on a FWD car is it has become very difficult to find good rear brake shoe lining material and anyone to reline the shoes.

Since asbestos is too hazardous to use, there's been little thought or engineering given to drum brake shoe linings. I work with Studebakers and whether they're four drums or two discs and two drums, those without experience complain about the braking and want to go to an aftermarket four disc setup. The problem is always either they bought modern hard linings, can't look beyond the corner tire store for brake work or can't service the Hydrovac.

jack vines.

After I experienced total brake fade halfway through a three hour misadventure on a hilly logging road I had the linings redone with a Kevlar based material (four wheel drums on a 66 Plymouth Valiant). I had to put it in second gear and use the engine to slow the car on hills. Thank goodness it was over a holiday and no trucks were using the road.

[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?