Front right brake/wheel locking up before front left on 87 Carrera

[Maxhouse97]

My 87 has a set of 996 brakes on it. Happens in a straight line at threshold braking. Tugs the wheel which makes it very annoying.

Troubleshooting completed:

Bled brakes
Changed fluid
Changed pads
Professional corner balance
Watched Pistons extend while someone pressed the brake pedal (this is where I noticed a piston on the side that doesn't lock up extending slower than the others)
New master cylinder
Rebuilt all 4 calipers with new pressure seals and dust boots, checked pistons

I am at a loss. Would love to check clamping force at each caliper - is there a system to so this? How would a spec 911 guy troubleshoot this (most modern race shops only deal with ABS cars).

[gliding_serpent]

You look to be doing this right, and i am no pro, but i see no brake line mention. soft lines? If one is getting fatigue they can deform, and this delay braking force on the bad side. Go stainless or at least new.

[Flieger]

What were the corner balance numbers? The usual method is to get LF+RR = RF+LR but with the offset driver weight this means the front weights will not be equal at the same time as the diagonals being equal.

If you are talking on the limit threshold braking in a straight line that might be enough.

[KTL]

Which brand of seals did you use to rebuild the calipers? Reason I ask is because some guys have found the aftermarket seals don't fit the pistons too well and make for difficult movement of the pistons.

[Bill Verburg]

Originally Posted by Maxhouse97

My 87 has a set of 996 brakes on it. Happens in a straight line at threshold braking. Tugs the wheel which makes it very annoying.

Troubleshooting completed:

Bled brakes
Changed fluid
Changed pads
Professional corner balance
Watched Pistons extend while someone pressed the brake pedal (this is where I noticed a piston on the side that doesn't lock up extending slower than the others)
New master cylinder
Rebuilt all 4 calipers with new pressure seals and dust boots, checked pistons

I am at a loss. Would love to check clamping force at each caliper - is there a system to so this? How would a spec 911 guy troubleshoot this (most modern race shops only deal with ABS cars).

Sounds like one or both calipers need to be rebuilt, I'd do both at the same time. while in there new brakes lines would be good.

[Maxhouse97]

Yes brake calipers were all just rebuilt, using Racing Brake seals ($$ ... I actually tried Centric in the rear and they didn't fit right).

New stainless steel brake lines were installed 3-4 years ago. I ding suspect that they are bad, but I guess I could have gotten a bad one.

When I installed my 996 brakes, my shop essentially "cleaned out" the stock 87 Carrera brake bias switch that limited brake pressure to the rears (because the 996 setup did not need this). I thought perhaps they somehow messed that up, but that should only effect fore and aft, not left and right.

However upon further thought I think the proportioning valve may be the problem, as again my mechanic said he would essentially remove the internals that limited brake pressure to the rear and leave the body intact. If there was a second circuit or if he didn't evenly clear them that could be the problem.

I was also told by someone on RL that a tool exists to measure brake force. That's encouraging. More to follow.

[Flieger]

The proportioning valve won't affect the pressure going to the front, it only limits the pressure to the rear.

What are your corner balance numbers? If you try it with equal front weights you may like it better than equal diagonals.

[Bill Verburg]

Originally Posted by Maxhouse97

Yes brake calipers were all just rebuilt, using Racing Brake seals ($$ ... I actually tried Centric in the rear and they didn't fit right).

New stainless steel brake lines were installed 3-4 years ago. I ding suspect that they are bad, but I guess I could have gotten a bad one.

When I installed my 996 brakes, my shop essentially "cleaned out" the stock 87 Carrera brake bias switch that limited brake pressure to the rears (because the 996 setup did not need this). I thought perhaps they somehow messed that up, but that should only effect fore and aft, not left and right.

However upon further thought I think the proportioning valve may be the problem, as again my mechanic said he would essentially remove the internals that limited brake pressure to the rear and leave the body intact. If there was a second circuit or if he didn't evenly clear them that could be the problem.

I was also told by someone on RL that a tool exists to measure brake force. That's encouraging. More to follow.

he p/v only affects the rear circuit

just because they were rebuilt doesn't mean they are right
proof is that the time response of one side is different from the other side, it's impossible to tell which is the culprit
ss brake lines don't fail internally like rubber ones so that can probably eliminated as an issue

a root cause is the excess front bias that this set up gives
if you have 996 calipers and rotors in front and back the bias is 1.811 where you want close to 1.5
if you have 996t calipers and rotors in front and back the bias is 2.072

any irregularity at a corner will be exacerbated by this basic fault in bias.

if you have other than 996 specific rotors bias is changed a bit hard to analyse w/o facts, but most likely the bias won't get anywhere near where you want it to be.

You can try using higher Mu pads in back but this is a band aid not a cure

[Matt Romanowski]

Here is the tool Innovative Products of America 7884 Brake Pad Apply Pressure Gauge Kit | Network Tool Warehouse I think Longacre makes one too.

[Jeff Alton]

Originally Posted by Flieger

The proportioning valve won't affect the pressure going to the front, it only limits the pressure to the rear.

What are your corner balance numbers? If you try it with equal front weights you may like it better than equal diagonals.

This....

[Maxhouse97]

So I had the car corner balanced before, and eliminated that as a cause. But now that I look back at the numbers, could this be it?

With 185 lb driver weight:

LF 625
RF 554
LR 922
RR 849

That's 70 lbs less on the front right than the front left. Could that be the cause? I think the tech said they couldn't do much more to balance it. What are my options? Or is this amount of left to right variation normal? I've had a person in the right seat and still have the problem, which is another reason why I ruled this out...

[Maxhouse97]

Thanks for the tool Matt - that is exactly what I was looking for! Question is whether I can find a shop that owns one, or if I have to buy it myself.

[3rd_gear_Ted]

The car is so front biased that the slightest difference in the brake components magnifies the problem into what you are experiencing.
Your correct 911 brake bias is back there in the garage within the brake parts you changed out with all due respect.

[rkeele]

Bill Verburg is the expert on this, same problem I had with my old setup 944T fronts and SC rears, always on the right no matter how many times it was cornered, seems you have to change rears to get proper bias, your Carrera rears are just not big enough although they work OK with 944T fronts the 996 fronts are probably to big for the Carrera rears, changing all 4 fixes that, also changed my Master cyl. to 930 you get better petal pressure, needed for bigger calipers. Bob

[Flieger]

Originally Posted by Maxhouse97

So I had the car corner balanced before, and eliminated that as a cause. But now that I look back at the numbers, could this be it?

With 185 lb driver weight:

LF 625
RF 554
LR 922
RR 849

That's 70 lbs less on the front right than the front left. Could that be the cause? I think the tech said they couldn't do much more to balance it. What are my options? Or is this amount of left to right variation normal? I've had a person in the right seat and still have the problem, which is another reason why I ruled this out...

You can get equal fronts if you want, but at the expense of unequal diagonals. It might take a few rounds of iterating the ride height and alignment but it's possible. Your mechanic may not have wanted to reindex the rear bars and that may be why he said he couldn't do any better.

Here are my numbers with me in the seat FWIW.



And this is without me in there (so it might be a similar distribution to the case of carrying a passenger)

[Maxhouse97]

Sorry guys to clarify, my car has stock 996 (non-turbo) calipers front and rear with a 930 master cylinder. So I have the same brakes as a 996 out of the factory, which I understand (996) does not have a proportioning valve. This is why my mechanic took out the guts of the proportioning valve that my 87 came with. Although I guess it has ABS, so perhaps it doesn't have to worry about this as much.

As far as the stock set-up (996) have too much forward bias, I haven't heard of others that have updated their G-series cars to 996 brakes having this issue. I thought it was a common upgrade (I bought the Instant-G kit). I have read about trying higher friction pads in the back only - I guess I could try that.

I rebuilt the calipers myself with high quality seals. So although I could have messed it up, I don't think I did. The pistons looked good.

Someone from RL helped me with a cross-weight calculator below ... seems like my corner balance might not be that problematic after all.

My plan is to try the brake force measurement and go from there. At least then I can rule out the brake system. Then if the bias is truly an issue I can try the pads. After that I guess it's a new brake set-up? I picked it (996 system) specifically because it came from the factory as a set - it would be frustrating if I end up having to change it out again.



----------------

You need to put those numbers into a cross weight calculator to see the real picture.

https://robrobinette.com/corner_weight_calc.htm


You're at 50% cross.
40%-60% front to rear
52%-48% left to right

[Maxhouse97]

What's the downside of unequal diagonals?

[Flieger]

Originally Posted by Maxhouse97

What's the downside of unequal diagonals?

The commonly touted response is that equal diagonals give the most consistent handling between left and right turns, although I haven't tried to prove it to myself.

If you look at in a pure lateral acceleration situation to make things simple, then in one direction you basically have more load on the front outside tire than in the other direction. The mass distribution stays the same though, so that means that in one direction it should be more towards oversteer and in the other direction more towards understeer. With a left hand drive car this would mean left hand turns it would push and right hand turns it would be loose.

If you look at my numbers and divide the LF/LR and RF/RR the left side is .597 and the right side is .600 so a difference of +0.4%. Your ratios are left .678 and right .653 so a difference of -3.7%.

Those numbers are probably not really the balance shift you see though because the inside tires still contribute to the lateral acceleration and in the limit of 100% lateral load transfer you have simply the total weight bias and the corner balance doesn't matter. So I think the effect of unequal diagonals should be minor and more down to things like the difference in inclination and toe angles between the inside and outside tires, since you are dividing the weight between the inside and outside differently between the front and rear.

And in the end you should be able to find a happy medium where you aren't too far away from equal diagonals or equal fronts.

If you are right on the limit of braking and generally brake in a straight line then you will probably want to try the equal fronts. You may need to go somewhere in between if you start getting a rear locking up though, because to make the fronts equal you are making the rears less equal.

It's always a compromise with a sports car because of the inherent weight imbalance of the offset driver so if that doesn't work you could try buying a Formula Ford or something.

[Bill Verburg]

Originally Posted by Maxhouse97

Sorry guys to clarify, my car has stock 996 (non-turbo) calipers front and rear with a 930 master cylinder. So I have the same brakes as a 996 out of the factory, which I understand (996) does not have a proportioning valve. This is why my mechanic took out the guts of the proportioning valve that my 87 came with. Although I guess it has ABS, so perhaps it doesn't have to worry about this as much.

As far as the stock set-up (996) have too much forward bias, I haven't heard of others that have updated their G-series cars to 996 brakes having this issue. I thought it was a common upgrade (I bought the Instant-G kit). I have read about trying higher friction pads in the back only - I guess I could try that.

I rebuilt the calipers myself with high quality seals. So although I could have messed it up, I don't think I did. The pistons looked good.

Someone from RL helped me with a cross-weight calculator below ... seems like my corner balance might not be that problematic after all.

My plan is to try the brake force measurement and go from there. At least then I can rule out the brake system. Then if the bias is truly an issue I can try the pads. After that I guess it's a new brake set-up? I picked it (996 system) specifically because it came from the factory as a set - it would be frustrating if I end up having to change it out again.



----------------

You need to put those numbers into a cross weight calculator to see the real picture.

https://robrobinette.com/corner_weight_calc.htm


You're at 50% cross.
40%-60% front to rear
52%-48% left to right

You can choose to believe whomever you want, but a few points to ponder
you car is not a 996,
996 has a longer w/b, i9t's dynamics aare different
996 has ABS
996 does use a p/v as do all modern rear and mid engine Porsches, the factory designs in excess front bias in all their street cars as an anti law suit feature, they want the fronts to lock first as they consider it safer and more defensible in court.

even if the final clamping forces are exactly the same on both front calipers that isn't the issue, it's the transient instantaneous response combined w/ excess front bias that is causing the issue. Any corner balance weight issues can only make it worse. A smaller m/c would also make it worse.

[Maxhouse97]

So I guess you're saying I need a new brake system? Oy vey!

Maybe it's time to swap to that 930 setup I've always wanted so I can fit 15" Fuchs...