Dark ring on rotor

[Globalti]

Looking again at the picture of the rotor I notice that the braking area only seems to be 2/3 "swept" by the pads, which is worrying. The centre and the "spokes" of the rotor look like a satin finished stainless steel to me and the outer two thirds of the braking area is getting polished, as it should. So why only the outer two thirds? Did you put the new pads back in fully? I know that with BB7s its possible to have the pads only partially pushed home, which leaves them at a slight angle and creating a horrible noise as they vibrate around. They have to go home firmly until there's a click.

If the brakes judder and make a horrible squawk as the bike rolls to a stop they are definitely contaminated. Put the pads in the dishwasher to decontaminate them and clean the rotor with glass cleaner, IPA, meths or ethanol.

 

[Willam]

Looking again at the picture of the rotor I notice that the braking area only seems to be 2/3 "swept" by the pads, which is worrying. The centre and the "spokes" of the rotor look like a satin finished stainless steel to me and the outer two thirds of the braking area is getting polished, as it should. So why only the outer two thirds? Did you put the new pads back in fully? I know that with BB7s its possible to have the pads only partially pushed home, which leaves them at a slight angle and creating a horrible noise as they vibrate around. They have to go home firmly until there's a click.

If the brakes judder and make a horrible squawk as the bike rolls to a stop they are definitely contaminated. Put the pads in the dishwasher to decontaminate them and clean the rotor with glass cleaner, IPA, meths or ethanol.

Thanks, I'll dounble check the fit tonight when fitting.

I was never comfortable with the fit, thinking as the m615s dint seem to have a default pad to buy, the ones I was buying were not quite the right fit, there didn't seem to be any space left for the rotor, not thinking it could be me just not fitting them correctly.

Here's hoping!

 

[Jody]

Did you push both pistons back as far as they would go when you changed the pads and discs. Also you did you measure the discs that were on previous to rule out having put 165mm onto a 160 mount etc.

@Globalti I don't think the pads can go in the caliper wrong on those brakes. If the pin went through they were in correctly.

 

[Willam]

Did you push both pistons back as far as they would go when you changed the pads and discs. Also you did you measure the discs that were on previous to rule out having put 165mm onto a 160 mount etc.

@Globalti I don't think the pads can go in the caliper wrong on those brakes. If the pin went through they were in correctly.

Crap,there are 165mm rotors! I was thinking there were not 180mm so must be 160mm...not sure how to check now as the old ones are long gone, but I'll have a good check on the fit.

Yeah I was thinking if the pin went in they must be fitted correctly but I'll give them an extra push to see if they sit lower down, as the old ones seemed to sit at an angle.

Yes the pistons were pushed back at the time of fitting as I bleed the brakes at the same time.

Thanks

 

[Jody]

I don't mean to panic you and am only thinking out loud. What bike do you have and what year? We're the Shimano brakes standard fit or have you swapped them.

 

[Willam]

I don't mean to panic you and am only thinking out loud. What bike do you have and what year? We're the Shimano brakes standard fit or have you swapped them.

It's a GT avalanche 1.0 2007 and yes I changed the brakes a couple of years ago.

 

[Tom B]

Crap,there are 165mm rotors! I was thinking there were not 180mm so must be 160mm...not sure how to check now as the old ones are long gone, but I'll have a good check on the fit.

Yeah I was thinking if the pin went in they must be fitted correctly but I'll give them an extra push to see if they sit lower down, as the old ones seemed to sit at an angle.

Yes the pistons were pushed back at the time of fitting as I bleed the brakes at the same time.

I didnt know that either. When I changed mine from Tektro HDC-300 to Shimano M615 i just got on with it. I notice that not as much of the disk is swept as I would expect, but given that they work and the pads dont get lipped as i would expect if part of the pad wasn't catching I figured all was well. I just assumed that the extra width on the disk was for pads with a larger "footprint".

With regards to pushing the pistons back, are you double sure, I always find they go back that little bit further than I expect.

 

[Willam]

I didnt know that either. When I changed mine from Tektro HDC-300 to Shimano M615 i just got on with it. I notice that not as much of the disk is swept as I would expect, but given that they work and the pads dont get lipped as i would expect if part of the pad wasn't catching I figured all was well. I just assumed that the extra width on the disk was for pads with a larger "footprint".

With regards to pushing the pistons back, are you double sure, I always find they go back that little bit further than I expect.

Yeah I know what you mean... think my next purchase is going to be a piston-push-a-backer tool.

Anyway I put the bike back together and the back seems fine, the front has not got the stopping power I would have hoped for, but I'll give it a couple of days for them to bed in before I start thinking what to try.

 

[Jody]

Yeah I know what you mean... think my next purchase is going to be a piston-push-a-backer tool.

Anyway I put the bike back together and the back seems fine, the front has not got the stopping power I would have hoped for, but I'll give it a couple of days for them to bed in before I start thinking what to try.

A plastic tyre lever is fine for pushing the pistons back or a screwdriver used carefully between the pads when in position. No need for a special tool.

As for what to try. Just make sure the pads are contacting the disc with no overhang then just ride it if they are preforming OK.

 

[keithmac]

If you can't push the pistons back (after a good clean) with your fingers they are sticking in the calipers..

 

[I like Skol]

Be careful pushing back some of the Shimano calipers (and maybe others?). They might have ceramic pistons and if forced back crooked or roughly these can chip or crack.

 

[Jody]

Be careful pushing back some of the Shimano calipers (and maybe others?). They might have ceramic pistons and if forced back crooked or roughly these can chip or crack.

Don't think 615's have them

 

[Willam]

Thinking about last time I changed the pads the pistons were hard to push back in...so that could be part of the problem here, should they push back in very easily? I guess so as the little clip that comes with the pads is expected to do it.

 

[Globalti]

No that little clip is just there to prevent the pads from rattling around. It doesn't have anything like the power to push the pistons back.

Most people think that a brake piston slides back and forth inside the seals when you apply the brake but they probably haven't ever tried to remove a piston and realised how tightly it fits in the seals. Getting a piston out is a swine of a job and is probably best done by splitting the caliper and applying steady pneumatic pressure on the pistons with a track pump. They will slide out slowly then come out with a loud pop and fly across the workshop so it's best to cover them with a cloth. There is also a spray of brake fluid, so great is the air pressure needed.

In fact what happens is that the pistons are held solidly by the seals and it's the distortion of the seals that allows them to move and contact the rotor. They do creep very slowly through the seals but only over a period of months as the pads wear.

 

[keithmac]

Must be completely different to motorcycle calipers then as when I've refurbed one of those you can slide the pistons in with one finger.