Front Brakes Locking in Pit Lane

We ran into an interesting problem at Mid-Ohio this weekend and I'm still not sure of the cause. I had a brand new 5/8" Girling master cylinder with a reservoir extension (also new) on my F1600 RF97 VD with ICP 20 calipers on the front and LD 19s on the rear. I would run a practice session and come into pit lane after the session to have tire temps/pressures recorded and when I would go to leave pit lane the front brakes were locked. We had to release some fluid out of the system to get them to release. The pads had a little more than 1/8th of material on them and had been shimmed so that the piston doesn't protrude very far out of the caliper as I had been directed by others with ICP calipers. Based on the data, it doesn't appear that there was brake drag on the track. We eventually solved the problem by replacing the master cylinder with one without a reservoir extension and by replacing the front pads but I would really like to know what the true cause of the problem was. Has anyone else experienced this?

Ray

I have had that when I held my foot on the brake pedal after coming in hot. The fluid expands in the calipers due to the brake heat, and with the opening to the reservoir shut off (as it should be with the master piston depressed), the fluid has nowhere to go and locks the brakes. When you release the pedal, since the pressure has expanded the master piston seal, it is stuck and won't retract.

Maybe the integral-reservoir masters have a different design on the passage to the reservoir, or the seals are not so prone to expand and stick as noted above.

If it happened and you did NOT have your foot on the brake, then the seal to the passage was not opening properly when you released the brake pedal, and the hot fluid could not expand.

Thanks Dave. I just looked at my data and it does look like I was riding the brake too much coming into pit lane. And I might have had my foot on the brake when the car was stopped. The engine was shut off so I can't see that in the data -- but now I know what not to do!!

Ray

The cause may have been the incorrect pushrod in the mastercylinder, or the retaining washer for the mastercylinder pushrod in backwards. It is critical to keep a girling pushrod with a girling master cylinder, lockhead with lockhead etc. They all seem to have different end profiles.

If the pushrod is incorrect, it sometimes won't let the piston stroke come back far enough to release the pressure, eventually locking on the brakes. This may not have been the case on yours as it only happened in the pit lane, I would have expected it to happen at random times, not the same place each time.

David Clubine

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Originally Posted by David Clubine

The cause may have been the incorrect pushrod in the mastercylinder, or the retaining washer for the mastercylinder pushrod in backwards. It is critical to keep a girling pushrod with a girling master cylinder, lockhead with lockhead etc. They all seem to have different end profiles.

If the pushrod is incorrect, it sometimes won't let the piston stroke come back far enough to release the pressure, eventually locking on the brakes. This may not have been the case on yours as it only happened in the pit lane, I would have expected it to happen at random times, not the same place each time.

David Clubine

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David,

I'm glad you bosted this because I did change the pushrod for the new master cylinder as well. I had bought one from pegasus that was for Girling but was extra long so that it could be cut to size. However, I did notice that the end of the pushrod was different then the one I replaced. The brakes didn't seem to be locking on the track but it sure makes me wonder.

Ray

David,

I just looked at the pushrods that pegasus sells and it looks like they sent me a Lockheed pushrod instead of a Girling so I bet that was the problem! It is wierd that it only happened in pit lane but the pushrod definitely ain't right.

Thanks,

Ray

I'd never run pads that thin (just over 1/8").
They were junk way before they got that thin.
The fact that you had to run spacers behind the pads is a red flag of sorts to me.
Bin those pads and get a fresh, proper thickness set, w/o the need for extra backing plate.

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Originally Posted by glenn cooper

I'd never run pads that thin (just over 1/8").
They were junk way before they got that thin.
The fact that you had to run spacers behind the pads is a red flag of sorts to me.
Bin those pads and get a fresh, proper thickness set, w/o the need for extra backing plate.

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Thanks for the advice Glen. I guess I was going by the street car approach where the pads are ok as long as there is brake material. Why isn't this the case with the pads we use on our race cars and how often do you end up changing your pads?

Ray

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Originally Posted by racinray

Thanks for the advice Glen. I guess I was going by the street car approach where the pads are ok as long as there is brake material. Why isn't this the case with the pads we use on our race cars and how often do you end up changing your pads?

Ray

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I have often used old bare pad plates as spacers for thin pads, especially for test sessions, where ultimate braking is not a requirement. If they are warped, then it is not good, but otherwise I've had no problems doing that. IMO, it's better than throwing away half-used pads, or conversely, running the caliper pistons out too far.

The thin/worn out pads will hold heat longer, as there's no "insulation" so-to-speak.
The material that is left has been heat cycled to death when they are that thin, so there's compound/hardness changes happening.
The biggest thing is the pistons are not designed to retract from such an "outward" operating position.
I realise you had the spacer shims, but it's sounds super Mickey Mouse to me.

had this exact problem, similar car, similar brake set up. Was riding the brakes to keep a tow line taught on the way into the pits and he fronts locked just as we made a 90 degree off the pit lane into the paddock. damn near flipped me over before I could get the truck driver's attention!.

Bottom line - heat buildup with brake application more than several seconds without a release is a problem!

So what is the minimum thickness of friction material before replacement is needed?

is hold it up against a new one. Somewhere around a little bit more than half gone and I consider it "used".

All the issues that Coop said is spot on with my past experience. I'm Cheap and I push my pads to a maximum of 2/3's wear compared to a new set. Why skimp on trying to use the pad to its full life when the performance of the pad falls off as it gets to thin. The thinner the pad material the less heat it can absorb and thus transfers into the caliper. Racing on marginal pads doesn't do much for consistent driving and confidence. We spend a lot of money trying to reduce lap times and the cost to replace pads is cheap in comparison.

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Originally Posted by glenn cooper

The thin/worn out pads will hold heat longer, as there's no "insulation" so-to-speak.
The material that is left has been heat cycled to death when they are that thin, so there's compound/hardness changes happening.
The biggest thing is the pistons are not designed to retract from such an "outward" operating position.
I realise you had the spacer shims, but it's sounds super Mickey Mouse to me.

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I agree with what you said, but, to clarify, the added thickness and holes in the old plates act similar in insulating ability to thicker pad material and keep the pistons from extending too far. I also said that I do not use them in competition, just for practice or testing, where brakes don't usually get taxed to their maximum.

So, while what you said is on the safe side, IMO, you can definitely get more life (value) from the pads without negative effects if you use spacers.

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Originally Posted by DaveW

I agree with what you said, but, to clarify, the added thickness and holes in the old plates act similar in insulating ability to thicker pad material .

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Dave, what holes are you referring to....the mounting holes or have you made a modification to the old plates?
I like the idea and want to be sure to get it right.
Thanks.

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Originally Posted by Nardi

Dave, what holes are you referring to....the mounting holes or have you made a modification to the old plates?
I like the idea and want to be sure to get it right.
Thanks.

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On molded pads, there are usually holes that the pads compound molds into for better retention to the plate. Those are the holes I am referring to. I clean all the old pad material off the plates and out of these holes before using them, and these holes are then empty. Make sure the plates are flat within 0.020" or so before using them.

I don't modify the plates.

Obviously, as others have said above, don't go crazy using the pad down to the last few thousandths. 0.10" pad material left is a good time to replace them. So, if you do that (on LD20 or D044 pads), you have used ~3/4 of the available material, and that is, IMO, a good compromise.

Depending on how hot your brakes get, if they are run too thin the brake material can start to crumble and disintegrate. I have seen that on thin pads. Best thing is to call the maker of the brake pads you are using and ask them what the minimum usable thickness is and replace them when they get that low. Plus brake pads rarely wear flat. The thinner they get the worse they become. This out of flatness causes extra stress on the pistons in the bore of the caliper and can lead to excess wear of the piston/cylinder walls causing the pistons to start to hang up and not retract.

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Originally Posted by lancer360

...Plus brake pads rarely wear flat. The thinner they get the worse they become. This out of flatness causes extra stress on the pistons in the bore of the caliper and can lead to excess wear of the piston/cylinder walls causing the pistons to start to hang up and not retract.

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Very good point, and very important.

I should have, but did not think to mention this, because this is somewhat of a separate issue. When I reuse, or inspect and continue to use, ANY pads, I ALWAYS make sure they are not tapered more than 1/32" in any direction (LD20-sized pads). If they are, you can sometimes rotate them in the calipers to even out the wear. If that doesn't work, then, if possible, surface grind or machine them back to flat. If they are tapered over 1/8" (again, LD20-sized pads), I would not reuse them because of the points you mentioned.

Dad always said "son there are two things you never pinch pennies on brakes and condoms" as usually he was correct.

A long time ago, I came directly out of light formula and S/R cars into the big sedans -- a Trans-Mustang.

I couldn't believe what a cake-walk this was going to be. At Blackhawk Farms, folks in the big sedans were braking before the #4 braking marker! I could go nearly to marker 2 -- and still get a good late apex early-on-throttle launch off the corner.

Until, I learned what was really going on. I'd come into the pits for a gulp of fuel as our practice session continued. All good. Rolled down to track entrance at the end of the pit-lane... pushed on the brake pedal when the flag-man watching up-stream traffic indicated for me to stop... and the pedal went right to the floor. Pumped like hell -- could get no pedal. Killed the ignition, left the car in First gear, clutch engage, and chugged-chugged to a kind of a stop, but not without first bumping the tower at the end of the pit-lane (and scaring the absolute hell out of everyone in the second floor of the thing).

The brake fluid had boiled, creating a massive vapor-lock -- probably at all four corners of the car.

I learned that braking just before the 2nd braking-maker at Turn One (and doing the same thing at every other corner on the course) was not a good idea in the big-bore, very heavy, stock-brake-equipped A/Sedans! It was a matter of time before your brake fluid would boil... and etc. -- and, of course, this could easily happen at speed as well -- no amount of air-flow over the binders would cool them.

Started braking before marker #4 at Turn One just like everybody else. But for a few laps, man, I thought I was gonna be the category's King!

Lesson learned.

Chris

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Originally Posted by glenn cooper

The thin/worn out pads will hold heat longer, as there's no "insulation" so-to-speak.
The material that is left has been heat cycled to death when they are that thin, so there's compound/hardness changes happening.
The biggest thing is the pistons are not designed to retract from such an "outward" operating position.
I realise you had the spacer shims, but it's sounds super Mickey Mouse to me.

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Originally Posted by DaveW

I agree with what you said, but, to clarify, the added thickness and holes in the old plates act similar in insulating ability to thicker pad material and keep the pistons from extending too far. I also said that I do not use them in competition, just for practice or testing, where brakes don't usually get taxed to their maximum.

So, while what you said is on the safe side, IMO, you can definitely get more life (value) from the pads without negative effects if you use spacers.

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The old plates are steel and steel transfers the heat to the caliper pistons much more readily than the brake pad material.
You are using double the steel and half the pad material, that results in a lot more heat transfer to the caliper piston. Sounds like penny wise and pound foolishness to me. Brake pads are such a miniscule portion of the cost of racing that is doesn't make sense to go beyond their designed useful life span.

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Originally Posted by Brian C in Az

The old plates are steel and steel transfers the heat to the caliper pistons much more readily than the brake pad material.
You are using double the steel and half the pad material, that results in a lot more heat transfer to the caliper piston. Sounds like penny wise and pound foolishness to me. Brake pads are such a miniscule portion of the cost of racing that is doesn't make sense to go beyond their designed useful life span.

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My calipers were made for 3/8 discs originally but now I am running thinner discs so I use these spacers to keep the piston extension to a minimum.

I believe the resistance to heat transfer through the pad material is independant of the backing material, so using an old pad plate as a spacer will result in an increase the thermal resistance because of the longer heat flow path to the piston.

Jonathan

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Originally Posted by Brian C in Az

The old plates are steel and steel transfers the heat to the caliper pistons much more readily than the brake pad material.
You are using double the steel and half the pad material, that results in a lot more heat transfer to the caliper piston. Sounds like penny wise and pound foolishness to me. Brake pads are such a miniscule portion of the cost of racing that is doesn't make sense to go beyond their designed useful life span.

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That would certainly be true if the backing plates were full surface with no holes. However, since they have empty holes where the pad material had been molded into them, the net heat transfer is likely a wash between using old backing plates as spacers or the equivalent extra pad depth.

On the other hand, however you view it, this discussion is all supposition, and the proof is "does it work well for you?"

Since it has historically, and does now, work well for me, I'll leave it at that. ;)