Wear on Aluminum Calipers

Walt Fricke · 09-30-2010, 09:11 PM

I've been running 4 piston aluminum calipers on my track car since the mid-late '90s. These are a defunct brand called Bremtek, which was a company owned by a clever Denver guy named George Rhodes. He started out with what looked like a copy of the Wilwood of that era, but improved upon it in various ways, including different sized pistons in both fronts and backs to lessen taper wear. He even made a full sized front caliper with 3" spacing, for those with those front struts. Alas, the company is long out of business.

These have been virtually bombproof. I've never had to replace the seals, and I can run ATE without bleeding or changing for years.

However, last year I started getting some irregular braking - swerving under heavy braking into some corners on the track. When I examined the calipers closely I saw this wear:

This is the area where the bottom (with respect to rotor rotation) of the pad backing plate bears on the caliper. The photo also shows, most clearly on the right side, the effects of my change from stock 2.7/SC rotors to the thicker 3.2 rotors. The grooving is in two general places, with a ridge between them.

The inside wear is shorter fore and aft because this is a rear caliper, and the inside pad has to have its "ears" cut off in order to fit without bumping into the mounts, which are at 3" spacing instead of the 3.5" spacing up front. In fact, perhaps the apparently deeper grooving there is due to higher loading per surface area for the inner pad.

Anyway, since the pistons were not sticking, the calipers were square to the rotors (one was 0.008" off over the length of the pad, which I shimmed - did you know that Diet Pepsi cans have a 0.004" wall thickness? Just right for emergency setting of valve lash.), and nothing else seemed amiss, this pretty much had to be the source of my problems.

I temporized by smoothing this part of all four calipers some with a file. And smoothing the rather rough sheared edges of the pad backing plates. Got me through a race weekend without incident.

But that was no long term solution. I noted that Wilwood now has at least some calipers with a fancy, spring loaded stainless insert on this surface. To reduce wear and avoid rattling, they say. Aha.

I took the calipers to my neighborhood machinist and we talked. He promptly hauled out a set of Alcon 4 piston aluminum calipers, and said "Like this?." The Alcons had steel inserts in this location. So got to work with his mill, and fabricated and installed some inserts for me.

Worked great at the one DE since I had it done.

If I ever switch back to thinner rotors (weigh less, and I wasn't having heat issues, just changing for changes sake I guess) I think I will have to countersink and use beveled flat headed screws. And I am watching the wear on the upper surfaces, though I don't think it is significant.

A lot of you racers/DE guys are running 930 calipers, or 944 Turbo calipers, or other aluminum calipers. All are going to have issues like this, seems to me, as the loose pad slams down on that aluminum surface over 15 or 20 years of track use. Anodizing hardens the surface, does it not? But it has its limits. And none of those calipers have a steel wear surface built in, do they?

Something to check for, anyway. Nothing like having your car jump over a lane, or even half a lane, under heavy braking for a slow corner at the end of a fast straight to get your attention.

Anyone noticed this with their calipers?

Walt

Steve@Rennsport · 09-30-2010, 10:21 PM

Hi Walt,

Take a look at part #17 and part #18. These stainless steel plates protect the bridges from pad wear.

Each & every 930 caliper has had those since 1978. No wear issues with these things; the only issues these have arise from poor maintenance.

Walt Fricke · 09-30-2010, 10:34 PM

And the screws to hold them, too - #19!

Thanks - I had never looked at a 930 parts diagram (or owned those fine calipers, either). Those Porsche race engineers thought of everything.

The design makes adding those easy. The machinist who did the work on my Bremteks underbid the work, because using a long 1/8" cutter required a lot of passes. And the fastening screws had to go outside.

The old S calipers, I think, don't have this (I owned a set once, but a while ago)? What about 944T, Big Reds and their ilk, Boxster monoblock?

Steve@Rennsport · 09-30-2010, 11:31 PM

Quote:

Originally Posted by Walt Fricke

What about 944T, Big Reds and their ilk, Boxster monoblock?

Every one of the 944/964/993 calipers have those: I'd need to check the monoblocks to see if they are present.

Bill Verburg · Bill Verburg's Garage

On the 964/993 calipers those ss plates actually locate the pad in the caliper, the primary issue is that mung tends to build up behind the plate, this makes it harder to fit pads(some folks have been known to grind down the edge of the pad instead of fixing the root issue) it is also a place for corrosion to start once there is a buildup of road salt and moisture.

on the later monoblocks they came up w/ a different method of locating the pads so the ss end plates were eliminated, along w/ the issues. but of course there are always new ones such as it's harder to change pads

KTL · 10-01-2010, 06:46 AM

Quote:

Originally Posted by Walt Fricke

The old S calipers, I think, don't have this

You're right. Nor do the other typical Ate iron calipers. I know the S calipers are "aluminum" alloy. Just pointing out that the other Ate castings of iron don't have them either. I can see why the iron units wouldn't have the plates, given that iron is a fairly hard surface.

Regardless of plate presence or not, people should make a point of cleaning that area on occasion. Simply sliding a flatblade screwdriver down into the caliper pad box is enough to remove pad deposits and crud buildup. Track cars especially get all sorts of crapola in there- pebbles, tire rubber, sand,.... you name it.

Walt Fricke · 10-01-2010, 11:23 AM

Looks like this is yet another instance of the aftermarket being behind the factory in important details. So only cheapskates like me, using some models of aftermarket stuff, may have this problem to fix.

Not an issue on the stock A/M iron calipers on my SC.

Bill Verburg · Bill Verburg's Garage

Quote:

Originally Posted by Walt Fricke

Looks like this is yet another instance of the aftermarket being behind the factory in important details. So only cheapskates like me, using some models of aftermarket stuff, may have this problem to fix.

Not an issue on the stock A/M iron calipers on my SC.

S/A/M calipers use a method of holding the pads in place that is similar to the late monoblocks so do not need the ss pad holders.

in S/A/M calipers the pads are hung from and positioned by these pins and secured by the top springs

the 964/993 Brembos use the ss end springs and a largish top spring to hold the pad in place, the pad is positioned by the 2 ss end plates and held in by the large top spring

the monoblocks use a modified version of the older pin setup, a single hanger pin, w/ an upper spring plate and lower positioning pins cast into the caliper body

Bill Verburg · Bill Verburg's Garage

I had meant to include Bremtek which is most similar to the monoblock w/ single hanger pin and w/ positioning blocks cast into the calipers. I don't recall whether or not Bremtek also used an upper spring or not

Trackrash · Trackrash's Garage

On my S calipers the pads do hit and rub on their edges. The holes for the retaining rods are elongated and do not take the force of braking.

I always put a THIN coat of hi temp grease where they rub.

In my opinion the wear issue is as much from abrasion of the pads moving as from pounding.

Walt Fricke · 10-01-2010, 10:22 PM

Bill

The calipers in my pictures are Bremteks. They use a single long, thick cotter pin through "loops" on the tops of the pads to keep the pads in place. No positioning spring. Don't need it. Mine give an incredibly high pedal, so the rotor kicks them back just a hair. They use an O cross section O ring for their seal, so not much retracting force there. The cotter pin doesn't have to keep the pads from getting too deep toward the axle, because these pads have "ears" on their ends for that purpose.

But I don't believe that any caliper design of the type we care about uses the removeable pins to transfer the force of the braking to the caliper. Seems to me that all designs rotate (or slide or push or whatever) the pad downward so the leading edge of the pad backing plate hits a fixed part of the caliper, and the force is transferred there (or most of it is). Where well designed, this interface is a steel part. It could be "springy," but after compressing a very small amount butts up to something solid.

Is that what those four funny cylinders of the Boxster caliper are for? Keep the pads straight, but also transfer the force when the brakes are applied?

Pins mostly keep the pads from falling out, or from falling in, as it were, in some designs.

The S/A/M pins absolutely are not strong enough to transfer these forces.

Bill Verburg · Bill Verburg's Garage

Quote:

Originally Posted by Walt Fricke

Bill

The calipers in my pictures are Bremteks. They use a single long, thick cotter pin through "loops" on the tops of the pads to keep the pads in place. No positioning spring. Don't need it. Mine give an incredibly high pedal, so the rotor kicks them back just a hair. They use an O cross section O ring for their seal, so not much retracting force there. The cotter pin doesn't have to keep the pads from getting too deep toward the axle, because these pads have "ears" on their ends for that purpose.

But I don't believe that any caliper design of the type we care about uses the removeable pins to transfer the force of the braking to the caliper. Seems to me that all designs rotate (or slide or push or whatever) the pad downward so the leading edge of the pad backing plate hits a fixed part of the caliper, and the force is transferred there (or most of it is). Where well designed, this interface is a steel part. It could be "springy," but after compressing a very small amount butts up to something solid.

Is that what those four funny cylinders of the Boxster caliper are for? Keep the pads straight, but also transfer the force when the brakes are applied?

Pins mostly keep the pads from falling out, or from falling in, as it were, in some designs.

The S/A/M pins absolutely are not strong enough to transfer these forces.

The pins serve 2 purposes they hang the pad when in static situations and guide the pad axially when in use, they do contribute somewhat to longitudinal placement, especially in lightly loaded situations, the vast bulk comes from the fact they they are clamped to the rotor very tightly. The 964/993 Brembos do not use pin type pad hangers because they have the plate type instead

the high pedal comes from the ration of slave to master piston sizes

Walt Fricke · 10-02-2010, 08:43 AM

Well, you would think high pedal is a result of the MC/caliper relationship. However, I switched from two 0.75" MCs with my Bremteks to two 0.70" MCs. My legs are not getting any stronger, and I never lock wheels in a straight line at high speeds.

Pedal was just as high as before. I'm considering trying 0.625" MCs.

Little front hub bearing wobble and minimal rotor runout seem to be key factors in having a high pedal, at least with the Bremtek O/O ring seal. And solid mounting of the MC. I look on the cylinder ratios more in terms of how hard you have to push on the pedal to get the braking you want than how far you have to push before it feels firm.

We are talking at cross purposes on some of this, I think.

What I got interested in was the force path, if you will, from rotor to strut, which means to caliper. Since it goes rotor >pad>eventually caliper, where does it enter the caliper. My conclusion is that it has to do so directly, with the pad backing plate pushing, as it were, on some fixed part of the caliper. It is the rotor, via the pad material, which is doing the pushing, to be sure. But where does that push go? I'm not talking about the piston clamping forces here, though of course they are what causes the pad to want to move with the rotor.

Do you see this differently?

Bill Verburg · Bill Verburg's Garage

Quote:

Originally Posted by Walt Fricke

Well, you would think high pedal is a result of the MC/caliper relationship. However, I switched from two 0.75" MCs with my Bremteks to two 0.70" MCs. My legs are not getting any stronger, and I never lock wheels in a straight line at high speeds.

Pedal was just as high as before. I'm considering trying 0.625" MCs.

Little front hub bearing wobble and minimal rotor runout seem to be key factors in having a high pedal, at least with the Bremtek O/O ring seal. And solid mounting of the MC. I look on the cylinder ratios more in terms of how hard you have to push on the pedal to get the braking you want than how far you have to push before it feels firm.

We are talking at cross purposes on some of this, I think.

What I got interested in was the force path, if you will, from rotor to strut, which means to caliper. Since it goes rotor >pad>eventually caliper, where does it enter the caliper. My conclusion is that it has to do so directly, with the pad backing plate pushing, as it were, on some fixed part of the caliper. It is the rotor, via the pad material, which is doing the pushing, to be sure. But where does that push go? I'm not talking about the piston clamping forces here, though of course they are what causes the pad to want to move with the rotor.

Do you see this differently?

isn't that what I said here

Quote:

the high pedal comes from the ration of slave to master piston sizes

though somehow an extra n got added to 'ratio'

the difference at the pedal will be small, I guarantee you that there is more travel the smaller the master piston is. Certainly the pad is dragged along by the rotor on initial application, w/ a single pin hanger it also rotates, 2 things stop both motions, the physical contact at the end plates and positioning pins and friction, in a 2 pin set up there will be far less rotation, and in the plate type holder almost none, when the pads are squeezed against the rotors it is going to be very difficult to move them w/ rotor rotation, it's like trying to pull a book out from under an elephant standing on it

certainly pad knock back can be an issue that will affect pedal travel

Walt Fricke · 10-02-2010, 02:40 PM

The pad is certainly dragged along by the rotor when you apply the brakes. My thinking concerns what stops it from being dragged farther than the tiny bit it does get dragged.

My take is that it is "the physical contact at the end plates" which is the force path, and that the pin hangers have nothing to do with this part of things. The Bremtek's use pretty basic long cotters. I bought a bunch of replacements early on. Haven't used a one of them. These cotters are easy to bend, so could not hold much force.

Ditto with the pins in my SC: all made in 1977, I think. I wonder from time to time if they ought not to be replaced, but they work just fine.

I do wonder how much the pistons are involved. They apply a lot of clamping force. The backing plates end up with impressions in them from the pistons. However, the piston bores, whether in iron or aluminum calipers, don't show much, if any, wear (in my experience). If the pads were trying to force the ends of the pistons downward (in the direction of rotation - I guess where calipers were mounted behind the wheel it would be the upward direction), and there were any significant pad movement involved, I'd expect wear in the bores.

Wilwood suggests adding a spacer behind the pad when the pad gets thin, but this is not to reduce the lever arm effect on the piston. It is to keep the inner end of the piston back behind the seal.

So I conclude that the force path (if that is a reasonable way to look at things) is not through the pistons (aside from the fact that they create the friction which creates the force), but through what I gather should be called the end plates (or pins, in the case of the Boxster?).

Walt