Avid BB5 outside pads contacting rotor crooked

Stevob · 09-21-2009

The problem is more pronounced on the front than the rear. I've adjusted both sets as per instructions a couple of times, but last Friday, as I was overhaulling them to find the cause of the previous night's front brake failure, I removed the pads to find that only the outside pads are worn on an angle on two axis. Like so...

What is perplexing is that the inner pads on both sets have worn straight. I was able to save some pennies by re-using these for the rear only. When I was cleaning the calipers, I removed the inner adjustment dial and noticed that the outer pad pistons have some movement in them. This seems to me to be the cause of the problem. Closer inspection of the brake's operation indicates that the outer pads don't always come in straight with regards to the caliper, not the rotor. I removed the plastic shield to see if I could disassemble the whole unit, but didn't like the idea of trying to remove the weird star shaped nut with a set of pliers.

My questions are therefore...

Is the outer pad piston supposed to be loose?

If so, is there a readily available tool for disassembly?

Has anyone else come across this before? (I did do a search, but most problems seemed to have been caused by incorrect caliper adjustment)

On a side note, has anyone come across something that has made correcting warped rotors easy? I was thinking of applying something to the rotor, then spinning the wheel to look for the cleaned spots from pad contact.

Stevob · 09-21-2009

anyone?

GlassTrain · 09-21-2009

Not sure if this has anything to do with it, but I believe the actuating piston has a pin that fits into the center of the pad. Maybe the pin doesn't let the pad touch the piston itself (let's it 'float on the pin"), or maybe it does let it touch and when the rotor flexes the pad doesn't lean with it. I hope this makes sense.

I have noticed on my front disk that the scoring from the actuating pad doesn't extend all the way down like the rear. I'll have to look at this and see if mine is eventually going to do the same thing.

Stevob · 09-21-2009

Quote:

Originally Posted by GlassTrain

Not sure if this has anything to do with it, but I believe the actuating piston has a pin that fits into the center of the pad. Maybe the pin doesn't let the pad touch the piston itself (let's it 'float on the pin"), or maybe it does let it touch and when the rotor flexes the pad doesn't lean with it. I hope this makes sense.

I have noticed on my front disk that the scoring from the actuating pad doesn't extend all the way down like the rear. I'll have to look at this and see if mine is eventually going to do the same thing.

Thanks for the reply. The pad has a corresponding hole to match the pin on the piston. They appear to line up correctly on mine when I insert the pad. However, that doesn't explain the loose pistons.

eta...the new pads appear to have the same problem. I'll have to dismantle the caliper to inspect it.

GlassTrain · 09-21-2009

Now that I've looked at them, there is some play in the actuating piston at rest. From what I can see, the whole assembly is a big screw, ie; the caliper acts like a big bolt clamping against the rotor when the cable is pulled. When the clamping bolt that holds the cable is tightened, it kind of skews the whole caliper against the threads, so maybe it was over torqued and damaged either them or the caliper 'threads' on one side (making the caliper lean when pressed against the rotor).

Also, the star nut under the red plastic cap moves the actuating caliper in and out from the rotor, so I'm not sure if that would release the piston on that side.

Stevob · 09-22-2009

Here we go Ron...

After removing the red plastic cap, this is what we see. The star bolt on mine was only finger tight, but that wasn't the problem...

Here we have the return spring which locates on the caliper and in turn on the acutator (where there is a pre-load adjustment hex screw also)...

Said spring removed...

Here we see our problem...

The piston assembly can then be pushed straight out of the caliper via the inner pad hole. Piston assembly removed for your viewing pleasure.There is very little play between the black part of the piston and it's locating hole in the caliper when assembled. The two parts of the piston assembly aren't freely moving, it seems to click out of a square position and then hold that position until clicked back in..

The two parts snap together and apart with the assistance of a spring clip...

Now showing the back of the piston assembly, which incorporates the angled bearing races that push the piston inwards...

I hope this gives some idea of the problem I have...

Now for a possible solution. If I was to find a washer with the right thickness, I could place it between the two parts of the piston assembly and eliminate the play therein. Hmmmm...

GlassTrain · 09-22-2009

Wow! Boy did I have the innards all wrong with the 'large bolt' concept!.

So the piston sticking at an angle seems to be the problem?

When I look at that bottom picture, it looks like the rotor isn't centered, but that's actually because the brakes are locked down, pushing the rotor over against the stationary pad, right?

If so, that is quite a gap between the top and bottom of the actuating side's pad. It's almost as if the silver piston isn't following the bent over contour of the rotor. Or it's sticking with the bottom portion kicked out not letting the top of the pad make contact.

Also... Don't know if it's just the picture, but the bottom CPS bolt looks bent or something.

JoeyDurango · 09-22-2009

In my experience with BB5's (never had them personally, but worked on plenty), the pad's uneven contact seems to be a design flaw inherent in just about every BB5 I've ever seen. Brand new they are hard to set up because even when the caliper is perfectly aligned, the outer pad's lower edge will rub, although the upper edge is far enough away from the rotor that when applied, the brake has a mushy feel. To get them to feel good at the lever, you almost have to be okay with some brake rub.

Looks like you've done a good bit of work on disassembly and diagnosis. Good work, but just know that it isn't only your brake. Every BB5 I've seen has this problem to some degree or another. Not a problem with the BB7's.

GlassTrain · 09-22-2009

The pad he shows at the top seems uneven side/side, not top to bottom. Is that common?

Stevob · 09-22-2009

The pad is uneven side to side and top to bottom. In the last pic, there was no pressure on the brake lever. I'd just refitted the wheel and this pic was take before I'd re-aligned the caliper, but it gives some idea of the mis-alignment present with regards to the caliper. I should have taken the pic with the wheel removed.

I have every confidence that I can solve this problem with the correct sized washers inbetween the 2 parts of the piston assembly. I will let you guys know how it goes when I can locate some later this week.

Stevob · 09-22-2009

Quick pic without the rotor. Damn it's hard gettin the focus right when up close on a pos p&s.

This pic is taken from the front of the bike. It's a little difficult to tell, but the pad is angled inwards towards the rear as well.

Stevob · 09-23-2009

Ok, here we are again.

I took the brakes apart again this arvo, with the intention of taking the offending parts and finding some suitable washers at the hardware store. A quick look at the parts told me this wasn't going to be easy to do, as the proposed mating surfaces weren't exactly flat, like so...

Can see here but...

Easier to see the issue here. A normal washer wouldn't be able to sit flat against this surface.

So I had a look around my messy work bench, and found a couple of these. They're the little washers that sit either side of the plastic dolly wheels on some derailleurs. Flat, but with a raised lip. Perfect! I had to drill them out so they would fit first.

Fitted...

And the result is good...

So I wait and see if they will handle the pressures/temperatures involved in braking, but I'm quietly confident that it'll do the job.

Note: The little star shaped bolt that I removed to get acces to this was too tight on the rear brakes to undo with pliers, but I was able to prise the silver part of the piston off with a small screw driver when I compressed the brake lever and exposed the back of this part through the little gap in the caliper where the brake pad "handles" sit. After fixing the rear brakes I had to insert the pads and compress once more to "click" the silver part back in properly.

Stevob · 09-23-2009

I don't know about anyone else, but I find it a pain in the proverbial when my rotors are warped and I try to find the point at which to apply the pressure.

So I got a permanent marker and "painted" the rotor surface on both sides like so...

I waited for this to dry then spun the wheel and adjusted the brakes until they started to rub. They removed the marker where they rubbed, making it easy to apply the pressure in exactly the right spot. Hard to tell but it's here.

The result, straight rotors, straight pads and hopefully much, much, much better braking (and with less effort).

GlassTrain · 09-24-2009

Those pads are dead straight now. Awesome!

I don't have the wear problem like you do yet, Stevo, but I do have a problem where the outside pads sit crooked at rest and have to be way back from the rotor to clear it. Haven't trued the rotor so when the bent portion goes past a crooked pad it makes this weird creaky door sound at slow speed, or a quick howl at high speed. When it comes form the rear it sounds like someone off in the distance behind me is yelling.

Maybe shimming it will solve this (as well as truing the rotor).

Stevob · 09-24-2009

The sad thing is I'll have to wait until Sunday to try them out.

If you can find the right type of shim Ron, you'd save some time in the process by doing it as I did the rear set, by not removing the whole piston assembly. It can be a pain in the butt trying to get it all back together.

Good luck with it all.

mikesnowdon · 10-15-2009

Hey Stevo!

Great work. Was the result as you expected = better braking?

BeaverTail · 2 Weeks Ago

^^^^how did it work out?

Lule · 2 Weeks Ago

Where's your pad springs? I feel that your washer is going to eliminate the 'wobble' of the piston that's necessary to follow the flex of the rotor during braking.

Frankie Figs · 4 Days Ago

I just got a 160mm BB5 front for $15 and a 160mm BB5 rear for free and I want to make them work. Soo, did it work?