Pinching disc brakes

Cqc

Hi all, my avid elixir 5 brakes are rubbing on both pads. How does one stop this happening? There is no bite point adjustment. I have tried sanding the pads, repositioning the callipers, sanding the pads, and cleaning the rotor, but no change! How can I stop this? Thanks

nicklouse

Go use them.

If you have bled them you could well have over filled them.

The Rookie

Most people overfill Avids when bleeding as Nick points out, you MUST use a spacer to hold the pistons fully out.

The other common AVID issue is the calliper not being aligned correctly due to twisting on the CPS washers, you need to nip each bolt up a little at a time, alternating to stop this.

poah

The Rookie wrote:

you MUST use a spacer to hold the pistons fully out.

oh god I've never done this and its been fine

The Rookie

Until you put new pads in it usually is.....

Cqc

I haven't bled the front one which is the one that's pinching, so i don't think it's that... Also the caliper aligning won't do anything as they're rubbing on BOTH sides. when I lever the pistons apart, they move back into the caliper, but then when I pull the lever, they spring back...

The Rookie

Well they should do that......sounds like either it's not aligned now or the pads were worn off straight with it not aligned. Some light rubbing is totally normal, is it just rubbing or actually slowing the wheel?

Cqc

Actually slowing the wheel, if I soon it as hard as I can it only does about one revolution

The Rookie

Could perhaps be the pistons seizing out, that I think is the only other possibility!

Antm81

Elixirs seem to suffer from fluid expansion, my 7s seem to lock on after not being used for a while, bleeding them cures it.

Cqc

Antm81 wrote:

Elixirs seem to suffer from fluid expansion, my 7s seem to lock on after not being used for a while, bleeding them cures it.

Cheers

Antm81

Just be careful not to overfill them after, mine get that bad after leaving them a while that the whole wheel is locked, I'm going to replace them with a shimano set at some point instead

cooldad

Bleeding removes air from the system. Doesn't sound like the problem to me.

Antm81

If done properly it'll only leave the right amount of fluid too, bearing in mind that will be fresh fluid and unlikely to have absorbed moisture unlike the old fluid which is the likely cause of the expansion in the first place.

I've had this happen a few times, cured it once by bleeding after not being able to ride for a couple of months, the second time it happened was after a few weeks of not riding due to work, this time I just loosened the bleed port on the caliper and pushed the pistons back, removing some fluid. This only happens with the bike in the shed, doesn't happen If I keep it in the house instead.

cooldad

So more likely sticky pistons sorted whilst fiddling about 'bleeding'. A bit of fluid on the seals, or a bit of use normally cures it.

The fluid will not absorb moisture in the short term because the system is sealed - if air/water is getting in, fluid must be leaking out somewhere.

Bleeding is for removing air and not random unrelated issues, many of which are caused by unnecessary attempts at bleeding.

Antm81

I was just pointing out what sorted mine with similar symptoms and my pistons weren't stuck but I'll bow down to your superior judgement

taffygriffiths

If avids use dot 4/5 glycol bases fluid, then the brake fluid is hygroscopic.

I.e it absorbs moisture in the lines. Given the small volume in bike brakes and sealed systems, could this lead to essentially having more 'fluid' in there?

Shimano use mineral oil which wouldn't have the same affect?

cooldad

But where's it coming from? Car systems are open, bikes are sealed. So it would take a long, long time for enough water (vapour) to get in there, and for anything that gets in, something must get out, so total volume shouldn't change. Or that's what my logic says anyway.

Be careful with numbers. DOT 4 and 5.1 are glycol based and suitable for bike brakes, and you can mix them together with no problem (except Shimano and anything else that uses mineral oil.) DOT 5 is silicone based and doesn't mix with DOT4 or 5.1.

The Rookie

taffygriffiths wrote:

If avids use dot 4/5 glycol bases fluid, then the brake fluid is hygroscopic.

I.e it absorbs moisture in the lines. Given the small volume in bike brakes and sealed systems, could this lead to essentially having more 'fluid' in there?

Shimano use mineral oil which wouldn't have the same affect?

Mineral oil is hygroscopic as well, but less so.

You need to explain how moisture is getting in, but bare in mind that if it did the volume of brake fluid plus water will initially DECREASE (you can do this with a glass of water, dissolve some sugar in it and the volume of water reduces as the dissolving substance acts to pull the brake fluid molecules closer together), but as CD has explained, there is nowhere for the water to get in anyway!