Rear brake upgrade ( with pics in last page )
 

Hi everyone, as many other c4 owners I did the c5 brake upgrade for the front brakes including stanless steel brake lines and slotted and drilled front and rear rotors and bias spring, my brakes improved a lot since then and that was 2 years ago, last week I had to hit the brakes hard to stop the car after coming out of a curve doing 50 miles per hour and getting surprised with unusual long line of cars at the 1/2 mile away traffic light, by the way my abs is not working and the dealer couldn't figure out how to connect his tech1 with its computer and told me to live without it!! as a result from that hard breaking with no abs I was able with a help of an ingle to stop the car without killing anyone but I got a flat spot on my 6 months 6k miles tires and that was causing a lot of vabration and because of that and I had to replace the tires again, the damage was on the front tires only, which tells me that the rear brakes didn't do much in that stop (didn't lock) it got me thinking of using the rear c5 calipers that I bought as a set with the front ones and they been setting in the garage since then, I did some research and found somebody who his name is dale who will sell his adapters for a reasonable price, using c6 13" rotors, but I will loose my e brake, he makes adapters for early c4 too with the shoe style e brakes inside the rotors and those early c4 get to keep their e brakes even when installing the c5 rear brakes calipers, sorry for my long story BUT my question is : can I install the early c4 e brakes in my 91? So I can have all c5 calipers and all 13" rotors and e brake the same time ????

Anyone knows if installing the early c4 e brake system on a 91 is possible or not?

Exactly
So putting even bigger brakes on the rear will do nothing

As far as getting the dealer service depatment to service the ABS, I'm surprised they made that statement. The Tech-1 needs a Brake Module that plugs into the unit for ABS diagnosis. It should have been in the kit.... Many other aftermarket scanners can test ABS. Snap-On and Matco have scanners that will work.

Going with a rear C5 brake conversion may not help braking performance a lot on the street. In your incident, were you driving too fast for conditions? Were you aware there was a traffic light and that it would be possible to have a line of cars waiting?

What pads are on the car? Brakes on street driven cars rarely see operating temps that would allow performance pads to really work. With ABS not working (and knowing it), it becomes a mater of knowing where brake lockup can occur and then applying braking effort to just short of that point. You will find that many serious track day people will disable ABS during events. Those serious drivers that don't can apply braking to the point where they car stop the car quicker than if they relied on ABS. Even then, it's still a matter of knowing what's ahead and being aware especially on the street.

So is teh rear conversion going to be worth the money? Probably not if you are only going to drive the car on the street.

Is it still not understood that 80% of ALL/ANY braking effort is on the FRONT wheels ONLY? The rears almost leave the ground during a panic stop situation.

Thats physics...its law in this universe and cannot be changed.

Its called "weight transfer" when the loads are thrown forward in the same direction as the motion of the car when the brakes are applied. That in effect suddenly makes the rear end light as a feather...and useless to braking. The ONLY thing rear brakes do in a panic stop is help keep the rear end behind the front end by that tiny bit of drag that is has..
You can get by with brakes off of a 10 speed bike on the rear of most cars. There is NO braking force there that can be transfered to the ground....its all up front.

People swap rears out for looks, for track purposes maybe where they want more drag on the rear under certain race conditions, or for a uniform brake system. But for effectiveness...its pointless.

Weight Transfer. Bigger rear brakes on a street car are useless. Dead weight.Pretty but useless. A buddy recently spent a PILE of money on getting some wilwood 6 piston calipers on all FOUR wheels. The shop had to special order those rear calipers because they were not even listed for street cars...useless and a waste of money. An extra $2000 becauswe this dude could not comprehend weaight transfer and why the rear brakes do not need to be as huge as the fronts...little wilwoods work just as well as big wilwoods on the rear...

Your ABS however is why you almost hit someone and IF you were able to lock up the fronts at 50 mph I'd say that you might have more of a traction issue than a braking problem...
Get the ABS fixed. Brakes do next to nothing to stop a car if the wheel is not turning.

Hi again rodj, but when only the front brakes are locking in a hard stop like that one, doesn't that mean I do need more braking for the rear? My thinking is if my brakes were balanced the way they supose to, I would be able to stop the car faster, if my rear brakes were good enough i should be locking them too with my foot stepping that hard on the brakes!!

That is right. However, more "testing" is required before coming to the conclusion that you need better rear brakes. It's true that max braking will occur when all 4 tires are at the threshold of lock up.

Your one-time incident isn't enough data to go parts-changing yet. Go some where vacant, safe, and try to learn more about your brakes. Then if you still feel you need more rear brake bias,
1. Did you install the DRM brake bias spring?
2. Buy pads for the rear that have more bite/higher coefficient of friction. WAY easier and cheaper than changing rotors/calipers, etc.

After all that, get your ABS fixed so you can maintain 4 wheel threshold braking in variable traction situations.

This was the first thing I thought of. I remember a thread many years ago where the member didn't install the bias spring & his braking was very front heavy.

Oops. My bad. Sorry for not reading thoroughly.

:thumbs:, thanks Rodj and everyone else here giving all these tips, i have also dealer pads on all 4, brake peddle doesnt travel a lot in order to stop the car, i thought i had the best brakes in the vette from all the cars i had, tires are less than 6 months old with 8k miles on them, i did the brake upgrade myself at my garage and i even tried it first for couple days without the bias spring to see the diffrence and i was feeling that i had brakes in the front only! and after i installed it it made big diffrence but the peddale became stiffer a bit

But if the car's brake were designed to have about 30% braking in the rear and 70% in the front then by installing the c5 brakes which has 25% bigger pads and 1" bigger rotors, then that changs the persentage again and even by installing the DRM bias spring it still not enough brake in the rear, and thats why the rear tires didnt lock in hard braking as of the front, what leesvet said about the wight transfer to the front is true but that would be another reason to make the rear tires lock faster and easier!!!

Actually a .5" radius change is not going to have a huge impact. And the pad size no impact at all. What's missing is the piston area in this equation.

Now let me pls put my orignal question again in this form: can i install the 85 to 87 emergency brakes on my 91? i will upgrade the rear brakes since i have the c5 brake sitting at home and it will coast me about $200 only to get the job done, and if i see after that the rear tires are locking before the front does then i will remove the bias spring and that should take care of it, but i dont want to loose my e brake and if i'll be able to install the shoe style e brakes then that would be great

and if i see after that the rear tires are locking before the front



If your rears lock before the fronts you have a variety of choices here. Debate for which to choose may rage tho.

You can:

Decrease the rear pad Cf
Increase the front pad Cf
Install (maybe) smaller piston rear calipers
Install (again maybe) larger piston front calipers
Increase the diameter of the front rotor via an optional kit
Install an adjustable prop valve allowing you to balance to your needs

The fit of a 1" larger rotor is about a 4.5% change only. That can be overcome with some crappy pad compounds in the rear if it were too much. Normally exploiting an extra 5% in the rear is not uncommon at all.


Reading back again...you came from too much front and are now asking about too much rear. Until you have exploited too much rear you'll never really know how much you can use. The original problem was all front lock up. Other comments on surface conditions etc aside (but correct) you'd be right in trying to exploit more rear braking. Most street cars have proportioning set up to cut rear line pressure so not to lock the rears..! This is done by various tools and under hard application of the pedal that's when it's at the max reduction. Ideally as pointed out you'd like all four to be at the limit. Pretty hard to do on a street car. But all that being said adding more rear brake and testing the results can be both fun and challenging.

Trailbrake into a fast apex and you will know if their working when your back end starts comming around. :D

Exactly!
Then back it off a turn.

yes but with lots of provisos
Early hub components are shorter than late so you would need custom spacers made to retain late outer yoke or if you used the early outer yoke your wheels would be 3/4" in from stock position

I see, well I don't like spacers and I don't want my wheels to be more in from now because that would effect the handling plus the look, I think I'll have to say good bye to my e brakes which I almost never use unless I'm parking down or up the hill

I don't understand how the bigger pad doesn't make a diffrence or how the half " extra rotor doesn't make diffrence either!!! Try to put ur car on the jack stand and wear a glove and spin the front tire and try to stop it by using ur fist first and then try to stop it by using the whole palm applying the same pressure, my understanding is when using the same pressure the bigger the pad means more friction therefor easier stopping , and also the bigger the rotor the less pressure needed to have the same stopping power , c5 rear brakes uses 12" or 12.5" rotor ( not sure ) but if I do the upgrade I will use the c6 z51 rotors which are 13" and that's why I'm expecting to have to remove the bias spring if not more like what u said about the valves etc