36 replies to this topic

[av8tor]

My 91 PTE car keeps bending front brake pads. Stock OEM brakes.  Always the inside pad.  The backing plate gets bent and the pad cracks.  Using PFC #97 and #11.   At first I thought I had worn spot on the caliper that was grabbing the pad, but last weekend I bent the inside pad on both fronts.  Anyone?  Bueller?

Kevin

'91 - 121 PTE NASA

'99 - 121 SM SCCA

 

 

[callumhay]

I cracked 2 sets of PFC pads on the left rear of my 90. The uper slide pin was loose and I assumed was causing the caliper to bind. Tightened the pin as tight as I could get with an Allen wrench using threadlocker and so far OK. To me the issue on the front could be coming from the pins binding or locking in the slides . I guess anything that affects the alignnent of the caliper, pins, bracket and even the hub/bearings could be a possible cause what you are seeing. Per Dave Wheeler, I use silicone lube on the slides and try to lube them every weekend or second weekend. Hopefully someone here has seen it on the front and can shed some light on it and at it's an easy solution.

[Bench Racer]

Never had bent backing plates, did have non free calipers. I found the Permatex green pin lube will attack the rubber bushings/calipers not free. Went back to using Sil-Glyde, issue solved.

[chris haldeman]

I've seen bent inner backing plates a few times over the years. Only cause i could figure was improper install. It is easy for the pad too drop out of place when assembling especially when the piston is pushed all the way back. Then when first pumped up the pad catches, bends and pops into place. I have cought pads that have dropped down during install countless times and check everytime before pumping them up. Reference point for my conclusion is during normal use the piston doesn't move enough in and out too bent the backing plate the amount I have seen

[SaulSpeedwell]

Best Answer

A few thoughts.  The SM has a single piston floating caliper.  The pins float in plain metal bushings with a bit of support and guiding from a rubber bushing/boot.  When you mash the pedal, the single piston pushes the inner pad into the rotor.  That action tries to "pull" the caliper and the outside pad into the rotor with equal but opposite force.

 

The problem is this - unlike when you are playing with the brakes on the jackstands or pulling into Starbucks at 7 mph on your 185/70-14 Fuzions with stock brake pads, when you mash the pedal at the 2 board on the track, the brake is resisting torque of 100-200 ft-lbs (halfassed napkin math).  So the pads are being jammed into one side of the caliper bracket, and the floating part of the caliper is being "cocked" within the slop allowed by the pins and bushings.  In the case of your 1.6, the calipers are rearward of the bearing, and the pads are thrust "upward" when you mash the pedal.  That is why tapered wear is normal.  It is also why "real racecars" use multi-piston fixed calipers.

 

Recommendation:  Take everything off except the bare caliper and bracket. Sideload ("cock") the caliper with your hand and see if it slides in and out.  No?  That could be why you are bending inner pads. Your sliding caliper is actually a "clutch" that grabs when sideloaded.  I went through dozens of caliper brackets to find ones that didn't do this.  Wirebrush your caliper brackets where the pad backing plates mate, install the smooth stainless slider clips (use a dab of high temp RTV if you don't want them flipping out during pad changes), file/polish your brake pad backing plates until it all slides nice and smoothly, even when "tipped" a few degrees in any direction.  The stainless "clips" will eventually dimple and need changed (or rotated to the "reverse" side that doesn't matter except when braking in reverse).  And feel free to "rotate" your pads as if they were tires.  If you do it frequently, your pads will be squarer and last longer than if you never rotate your pads.

 

Many will tell you that they run no pad hardware and life is great and they've never had a problem.  Just consider that when you are braking at the 2 board to make a TV-worthy pass for 15th place at the Smitty McSmitterson Memorial Double Regional, that pad is being jammed into one side of the caliper bracket at 100-200 ft-lbs.  That pad's ability to move freely and squarely (and thus your ability to "modulate" the brakes) is entirely a function of the friction in the floating caliper - the piston, the pins, the pad/clip/bracket.

 

Some pads fit tighter than others.  Some pads have stronger backing plates than others.  If the front pad is not up to the task, the high pressure portion (directly under the piston) turns to cheese while the ends stay firm ... and you can bend the pad even if everything else is as good as it can be. 

 

I have also seen what Haldeman is advising - often when someone pushes the piston all the way back but is re-using partially worn pads.

 

Typically one end bends more than the other - and it is the end in compression under braking.  When both ends bend near-evenly, it is the "center-turned-to-cheese" failure mode, and means you are overdriving the capability of the compound.  Add cooling, brake less, or move to a higher temp compound.

[SaulSpeedwell]

Sidebar:  Pad kickback can't happen when the pads and rotors are new.  More precisely, when the caliper piston is fully retracted into the caliper, and there is only, say, 0.5 mm of possible clearance between the pad and rotor, then the (nominal) maximum kickback is nothing more than the (nominal) clearance of 0.5 mm.  Pad wear and rotor swelling makes "nominal" into YMMV.  The piston also has the least chance of cocking when it is fully retracted into the caliper.

[Bench Racer]

My 91 PTE car keeps bending front brake pads. Stock OEM brakes.  Always the inside pad. 

 

My process when changing/flipping/anything with the brake pads is to check pads are in home position, inside pads with flash light/mirror and or head stuffed behind caliper.

[Tom Hampton]

My process when changing/flipping/anything with the brake pads is to check pads are in home position, inside pads with flash light/mirror and or head stuffed behind caliper.

Fingers work too, if you know what to feel for.

[Bench Racer]

Fingers work too, if you know what to feel for.

Still talking about brake hardware.

[Tom Hampton]

Lmao. Well, yes I was. But, one can always appreciate a good double entendre.

[davew]

Pad cracking/bending is almost always caused by the pad not being properly seated in the bracket. The bend will be about 2/3 from one end, With the bend point at the witness mark from the caliper piston. Almost always on the inboard pad. Here's what happens:

 

You install new pads properly in the mount. The rotor flops and knocks the pad out of position while you are installing the caliper. You can see the outer pad, so you reposition and start over. You don't see the inboard pad and the caliper goes back on. You naturally pump up the brakes, which bends the pad. You get a funky feel to the brakes. It feels fine in the pits, but is soft for the first part of travel, then gets hard like normal, but longer than normal. Car stops almost normally. the piston is getting pushed back into the caliper by the bent pad acting like a spring. The pedal is long and soft until you bend the pad enough to take up the gap, then it returns to normal. Eventually the flexing of the pad will crack the friction material and the friction material may fall off.

 

How to prevent: After installing the rotor on the hub, put 1 lugnut on, finger tight. This prevents the rotor from flopping around. Use the anti rattle clips, they help hold the pad in place. These do not work with Cobalt brand pads as they have wide mounting ears and do not fit with the anti rattle clips in place. Be aware of what can happen and double check your self.

 

If you have tapered pad wear, you have something binding or worn. Minimal taper is always expected, maybe 0.015-0.020". Unless you are using a caliper, you won't see or notice this. If you have severe taper, do not reposition the pads. the taper usually occurs in mirror image between the inner and outer pad. Flipping severelly tapered pads will create a "V" shape contact surface that gives you a long pedal as the tapered pad acts like a spring and pushes the piston back in.

 

Anytime you have a rear caliper pin come loose, replace the caliper bracket and the pin. When the pin comes loose it wobbles out the threads in the bracket. No amount of torque or locktite is enough to hold it permanently. AT least not to my satisfaction for brake parts. When any part of the brake system fails, it is usually a bad time. Any rebuilt part (calipers, Starters, water pumps, etc.) have been sand blasted at the factory, then they get painted. Any sand/debris in the threads is now sealed in place by the paint. EVERY rebuilt part with an exposed thread, should be cleaned with a proper tap or die to remove this debris prior to install. Next time you buy a rebuilt part, lay a clean piece of paper on the bench and run a tap through the mounting holes and look at how much debris falls onto the paper. You will be amazed.

 

As with most things, attention to detail will make life much better. Chase the threads and pay attention to the position of the inboard pad.

 

Road America is probably the worst track for breaking brake parts. We are there about 10 times a year. I have seen tooooo many times what happens when brakes fail. It is never pretty. Heed my warnings, please.

 

dave

[Jason J Ball]

"How to prevent: After installing the rotor on the hub, put 1 lugnut on, finger tight. This prevents the rotor from flopping around. Use the anti rattle clips, they help hold the pad in place."

 

Always do this ^^^ when swapping rotors. I also, always slide the caliper in and out making absolutely sure the pads are properly aligned. With the piston fully compressed, you still have room to do this with new pads.

[surlynkid]

This is exclusively due to mis-installation. The pad moves and is not parallel to the rotor face when the pedal is pushed, and the end of the pad gets bound up in its little channel. It is usually when you compress the piston way further than needed and have slop between the pads and rotor. I always hold them finger tight while my wife pushes the pedal to ensure proper seating.

[av8tor]

This is exclusively due to mis-installation. The pad moves and is not parallel to the rotor face when the pedal is pushed, and the end of the pad gets bound up in its little channel. It is usually when you compress the piston way further than needed and have slop between the pads and rotor. I always hold them finger tight while my wife pushes the pedal to ensure proper seating.

If I opened the door and asked my wife to crawl into that thing, she would fall over laughing.  Luckily I can convince my son to push the brake pedal.  I like the concept!!

[surlynkid]

My bonus is that it is her race car. She jumps in and does her part. She is 5'4" with a 14" seat. I can't get in it.

[Alberto]

My bonus is that it is her race car. She jumps in and does her part. She is 5'4" with a 14" seat. I can't get in it.

 

Does she have a sister?

[Todd Green]

This is exclusively due to mis-installation.

 

That is not my experience.  Back when I was having brake pedal issues I was running PFCs (among other pads).  Common wisdom differed between running with and without the clips.  So I called PFC and asked if they were designed to be run with our without the clips.  They said without, so that's how I ran them.  (Too long ago to remember if I also ran them with clips and/or springs).  Anyway I bent two sets of pads.  Several other sets with no issues.  This was not due to installation issues.  I ran several race weekends before they bent and I've also had zero issues with Carbotechs or Hawks bending.  I think there is something about their design and not running clips that causes issues with the ears binding.  OP are you running the clips? Edit: I should add that I'm still running PFCs on the rear and never had an issue with them.

[Ron Alan]

^^^^This is why Saulspeedwell post is so good...there is more than one reason pads may bend. And I think the degree to which a pad is bent may also indicate how it bent...just a thought?

[surlynkid]

Not using the clips just makes the ends more prone to hang up on one end. I use high temp brake slide pin grease in there to prevent it.

[Steve Scheifler]

Years back we bent multiple sets in both cars, fresh calipers and paid close attention to all mentioned possible causes and still they bent. Never bent any before that or since with any other brand. As I recall those had slightly thinner backing plates and worse yet holes in the plate exactly under the edge of the piston. Under ideal conditions and full thickness they may have been fine, but there will always be a little wobble so they need to be strong enough to deal with that. Obviously if you are bending pads that many other people are running without issues, the problem is likely your parts or installation.

As for Cobalts, the rears are available two ways, standard and "optimized". They got tired of people running the standard version without the little spacers then complaining about problems, so the optimized pads have smaller slots. To me they seem sub-optimal because the slots in the pad are of uniform width but the bracket is not (inside seat is narrower as I recall) The spacers provide a uniform seat for the slot and less slop overall, so I run the standard version.