Too much front brake- need different pads front and rear?
 

I'm having a problem with early front wheel lockup, and I'm hoping someone can suggest brake pads to fix the issue.

The car is a 2300 lb RS look with 930 brakes, an SC type master cylinder, no proportioning valve, and stock porsche brake pads all around. The car has a 993 motor in back, which makes it pretty rear heavy. I'm locking both fronts up quite early in street driving and at the autocross, and I haven't found any other problems with the brakes. I was thinking I'd try and fix the issue by putting higher friction brake pads in the rear.

To that end, I'm looking for pads that work well cold and at low temperatures that would give me more rear brake than front. Any suggestions? Based on the following links, I was thinking maybe the Pagid RS4-2-1 (black) front and RS 4-2 (blue) rear might work, but if you look at the data it's a bit contradictory.

http://www.needforspeed.co.uk/htmlpages/techBRP03.htm
http://www.pagidusa.com/pagidracing/techinfo.html#frictiongraph
http://forums.audiworld.com/s4/msgs/1915589.phtml

Any ideas? I have used RS-19 (yellow) and RS-14 (black) front and rear in my last car, but they really need to be up to temp to work correctly- not ideal for autocross.

if i was in the same situation as you i would add a adjustable proportioning valve . i doubt if you will get the results needed out of brake pads plus the added wear on brake rotors with agressive pads. race pads work well in track day /de events but on the street you cant get them hot enough to bite. i would think that a proportioning valve would actually be cheaper than pads with just a little more work involved.

First install a 23mm m/c

a p/v does absolutely no good when the fronts are locking first

check you alignment, don't go overboard on neg. camber in the front

Hopefully you have at least a 225 tire in the front?

an lsd helps a lot, 40/60 or 40/70 if you track a lot

a final tweek is to use diffewrent pad compound f/r, for example Pagid RS19 yellow front and RS 14 black in back.

Alignment and corner weights are good. 225s front, 245s rear, toyo RA1s. It has a limited slip...

So I'm down to pads. As I said, I've used RS19s front with RS14s back on my last car, but you just can't get the heat in them to work well for street/ AX. I used to drag the brake getting to the starting line... I think I need to find split friction level pads that work at lower temps; some must exist...?

Listen to Bill. The M/C neds to be upgraded to a 23mm from the 930 era. The 930 caliper pistons are too large for a 19mm M/C to effectively handle.

http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=4


I had the same problem which I solved by reducing my negative camber in front as close to 0 degrees as possible.

What was really scary with all that camber was how easy it was to lock them up when the road got wet.:eek:

Cheers,

Joe

I'm checking on the master cylinder- it's possible the PO replaced the master cylinder as well. It certainly doesn't feel like a soft pedal.

0 front camber comes at a heavy price in cornering and tire ware...

I'm sure that there are many split coefficient pad sets, I am mostly just familiar w/ Pagid

for better cold maybe try
RS4-2-1 black front and RS19 yellow in back. The RS4-2-1 are not going to be very good for a lapping day, but both work well cold w/ the RS19 will always have relatively better friction by the same amount in the back. So you get predictable performance at all temps and better friction in the back.

I wouldn't go to zero camber just don't go as crazy as some of the track guys like to.

My car is completely stock and, I don't know about your cars but, my front always much stronger than the rear. Many agressive braking locked up the front. Front tires slide, smoke everywhere but the rear acted as nothing.

You have a p/v that is apparantly doing it's job. You want the front to lock a tad before the rear.

I've got 930 brakes on my 2460 lb SC and I feel I have early front lock up as well, has not happened on the track yet but in a "almost panic" stop the fronts lock up seemingly too fast.

I am running a 23mm 930 MC and Padgid orange all around.

PS.they squeal like a MF on the street. I'm swapping out to a regular street pad for non track use.

Same boat/issue. 930 brakes on an SC, with the turbo MC and the fronts lock up, it seems like, way early. Can you use an adjustable pv on the front circuit to even it up some?

He has the 23mm MC.

For AX only, I'd try the oranges in front and blacks in the back.
Your rear friction levels would be nice & high - even when cool compared to the oranges that came with your car when you got it.

Let this be your guide:
http://www.rennstore.com/content/00/...ent/249775.vmg

Thanks for the input. One thing has me confused: The graph in Craig's post above is the same as the 2nd link I'd found, but I notice that the table below doesn't agree at all, giving orange and RS14 black very similar friction levels at higher temperatures (this is from the first link)...

http://forums.pelicanparts.com/uploa...1161914653.jpg

Is one out of date or wrong?
Also, no issue mixing carbon and ceramic based pads?

I may try Orange front pads with stock rear pads. All this data seems to suggest that will make things worse, at which point I'd try the blacks in the back (and go back to waking the dead with brake noise on the street).

No, the graph just shows what happens across operational temperature range.

If you you look at the RS 4-2-1 vs the RS 19 you can see that they are almost pareallel. Thats good, consistant response not matter what the temp.

I wouldn't use RS4-4 w/ RS 19 or RS 4-2-1 because there wil be a diffewrent respoinse at low and hight temps.

another possibility for A/X would be RS4-2 front and RS 4-4 rear, same thing, parallel response curves but less of a differential between them.

Never ever put a p/v on the front, the results are frightening.

Did some research:

For AX - a rear PV would be your best bet.
Stay with RS44s F&R

Craig- But I'm locking the fronts already, so a rear pv wouldn't help much...

Bill- Look at the RS4-4 and RS 14 data at 300C on the table- almost equal friction. If you check the graph at 300C they are no where close... They can't both be right.

The graph that Craig posted is from The North American Pagid distributor, they probably got it from Pagid. The table looks to have been put together from the same source, but apparantly has inaccuracies.

I would believe the graphs

I was having front lock up issues with a different type of car that is light weight nad has lots of power. I was not having this issue on the track only in Autocross.

The biggest part of my problem was me. I was pushing the car past its limits. I was asking too much from it. Once I smoothed out and picked better braking points along with better turn in points my lockup issue is non-existant. All of this smoothness enabled me to consistantly score Fast Time of the Day at almost every event with an AP class Cobra being the one to spoil that for me.

I am not saying you do not have issues with the car but in some cases when everything has been checked and double checked, maybe its time to check the nut behind the steering wheel. ;)

Sounds like you'll have to do track events..... rather than autocross!;)

I stopped Autocrossing 2 years ago, but, what I have learned Autocrossing has proven to be so helpful on the road course.

I was getting bored with Autocrossing so I moved on the DE's for now and possibly NASA racing in about a year or so when I feel I have learned enough not to be a rolling chicane in a race.

Couple questions.

My 911 has 930 brakes.
The 23 MC gave me brake feel and eliminated the 21 MC 2+ inches of brake pedal travel.
The factory front brake bias made the car nose dive during threshold braking.
I had Pagid oranges all around.

I kept the oranges in front and went to blacks in the back.
That did move the bias to the back, car no longer nose dived.
I am considering a dual master cylinder set up and then returning to Pagid oranges all around.

Fyi my car has 275 and 335/17 710s.
Camber f and r 2.8/2.5
The way the car warms up I think the wide rear tires and 20 quarts of oil take longer to heat up than the brakes.

The graphs are interesting.
With my current set up should I be concerned that during my warm up lap(s) that either the front or rear brakes will heat up first creating a temporary brake imbalance?

Are my cold car handling concerns more critical due to the black rear (higher friction) brakes warming quickly and the wide 335 tires warming slower?
In a similar situation street driving what tendency's would my brakes/car have if a sudden stop were required after the brakes had been cooled for a half hour on the freeway. Lockup in the rear first?

2 more cents, rear brake bias is preferred in rain racing to avoid any front lock up, though too much rear brake bias can make a car push at turn in.

That's why you want parallelism in the friction graphs as much as possible, if you use the same pads f/r that is guaranteed to a point(the fronts will always be a little ahead of the rears). If you use different pads f/r then at least try to get the curves as close to parallel as possible for the sake of consistancy.

To much rear under any condition will make the back want to turn more than the front, I don't believe that is called push, but I am not 100% up on NASCAR talk.

In street driving the dynamics are so slow compared to on a track that it won't usually make a difference, I suppose you can come up w/ all sorts of possible scenarios where there would be an issue but the likely hood of all of them is low.

I have used Both RS4-4 and RS 19 on street in crisp conditions(unexpected snow), no problem

Bill Verburg,
What is a p/v Bill? Power Valve?

p/v - proportioning valve aka pressure limiting valve. It limits line pressure seen at the rear wheels.

ahh, thanks Bill

Everything I have seen on this thread relates to premature front lock-up. I have the opposite problem on my '80 - 911SC in that the rears lock early in D/E situations. I am running all stock brakes and M/C with 205/225 tires. The only suspension modification is adjustable Koni's in the front. Anybody have any ideas? I am about to replace the M/C in an effort to rectify.

The m/c has nothing to do w/ bias. Normally functioning SC brakes will lock the front just slightly before the rears.

Check to be sure that the fronts are functioing @100%. It's easy for the pads to become glazed or oil soaked, or the rubber lines to swell internally. All of these things reduce the effectiveness of the afflicted units.

If you still have issues after that the do the opposite orf the above use more agressive pads in front than in back.

Thanks. I'll check the pads for oil. I have replaced the original lines w/SS so they should be good . . . .but I'll check them anyway. So, if the M/C doesn't bias F/R, and there is no biasing valve, how do they balance the system?

Usually piston size, but Bill knows the specifics better than I -

It is always a good idea to get the brakes working hard on cars that have been parked a lot or driven casually. Pads can glaze, and rotors can surface harden, leading to the need to turn or replace them. The rear brakes on my SC were a touch grabby this spring, but an hour of fun sorted that out quite nicely.

Medium braking, working up to hard braking can do wonders deccelerating from 60 to 25 or so. A couple or three in a row, then drive for about 1/2 mile and repeat for few cycles. Drive a few miles before parking, and don't forget to check your mirror frequently. YMMV

As Craig mentioned, caliper pistons, specifically the ratio of the area of the front caliper pistons to the area of the rear caliper pistons and rotor diameters

Seems like a rather crude solution, but I guess they know what they are doing! I will look at the rotors for glazing, they are original although not a lot of miles (70k) but they have seen multiple DE's and a lot of elapsed time. If I replace the rotors, any thoughts on stock style v. cross-drilled?

No, it's basic engineering.

the pads glaze from over heating not the rotors.

also look at the pads for signs of contamination.