Warped discs? need a sense check

[Speed addicted]

I think I've got warped discs my 85 BMW K100 cafe/rat/thing, but I need a sense check before spending loads on new discs.

Symptoms,
The lever quite often needs a couple of pumps to work properly, feels like air in the lines. I've bled them normally, I've bled them with a pressure bleeder, I've even used the pistons to push fluid back up. I'm pretty sure there isn't air in there.
Once they're pumped up they work normally, firm lever, no give. I'd expect air to be consistently soft.

On the last couple of runs I did notice pulsing at the lever, but this is a new addition to the issue.

So far it's had new braided lines, new seals in the calipers, new master cylinder seals, multiple bleedings.

Previously I was thinking that failing seals in the master were allowing fluid to bleed back, but I've replaced them and the piston. The brakes are good once pumped but it's extremely exciting coming into corners at speed not knowing what you'll get!

What I think is happening now is that the warp in the discs is pushing the pads back in slightly so I need to take up that space before they make contact again.

This is the old bd when it had the low bars on

Edited by Speed addicted on Tuesday 23 April 11:19

[Jazoli]

Easiest way to check for warped discs is to raise the front wheel and use a dti and measure the runout on the discs, a cheap ebay one will do, https://www.ebay.co.uk/itm/Dial-Indicator-Test-DTI... like that.

I don't think the discs are causing the symptoms you are suffering personally, for the amount of runout they would need to push the pads back it you would certainly notice major pulsing at the lever at all times, but who knows.

Edited by Jazoli on Tuesday 23 April 11:26

[dern]

Jazoli said:

Easiest way to check for warped discs is to raise the front wheel and use a dti and measure the runout on the discs, a cheap ebay one will do, https://www.ebay.co.uk/itm/Dial-Indicator-Test-DTI... like that.

I don't think the discs are causing the symptoms you are suffering personally, for the amount of runout they would need to push the pads back it you would certainly notice major pulsing at the lever at all times.

You'd probably also notice it pushing the bike around. Measure the runout as above though.

When you've had the pads out have you noticed that they're wearing more on one side than the other or that they not covering as much of the disc face as you'd expect? I've had similar symptoms on kit cars where the caliper alignment isn't quite right or the components haven't been fitted correctly or a slider pin is worn. Are the calipers twin piston or is it one piston and slider? If it's the latter then check the slider is moving freely and isn't worn.

[anonymous-user]

Doesn't sound like warped disc to me.

When I got one on my blade it was like I had no brakes. And the pulsing was really high.

[dc2rr07]

Jazoli said:

Easiest way to check for warped discs is to raise the front wheel and use a dti and measure the runout on the discs, a cheap ebay one will do, https://www.ebay.co.uk/itm/Dial-Indicator-Test-DTI... like that.

I don't think the discs are causing the symptoms you are suffering personally, for the amount of runout they would need to push the pads back it you would certainly notice major pulsing at the lever at all times, but who knows.

Edited by Jazoli on Tuesday 23 April 11:26

This plus check the bobbins are free before checking runout
https://m.youtube.com/watch?v=1qoPqN2GBdw

[Jazoli]

dc2rr07 said:

This plus check the bobbins are free before checking runout
https://m.youtube.com/watch?v=1qoPqN2GBdw

I don't think they are floating discs

[Androgynous]

Jazoli said:

Easiest way to check for warped discs is to raise the front wheel and use a dti and measure the runout on the discs, a cheap ebay one will do, https://www.ebay.co.uk/itm/Dial-Indicator-Test-DTI... like that.

I don't think the discs are causing the symptoms you are suffering personally, for the amount of runout they would need to push the pads back it you would certainly notice major pulsing at the lever at all times, but who knows.

Edited by Jazoli on Tuesday 23 April 11:26

You could also get an idea using a feeler gauge between the pad and the disc as you rotate the wheel.

Obviously not as good as a DTI but better than nowt.

[Speed addicted]

Jazoli said:

dc2rr07 said:

This plus check the bobbins are free before checking runout
https://m.youtube.com/watch?v=1qoPqN2GBdw

I don't think they are floating discs

No, weight reduction didn’t appear to have been on the brief when they built the K100. The discs are riveted to solid carriers.

[Speed addicted]

Ok, I’ll have a look at the disc runout tonight. I don’t have a dial gauge handy so I’ll start with clamping a rule to the forks and go from there.

I’ll also have a look at the pads to see if there’s uneven wear, or if the callipers are spaced oddly.

[dc2rr07]

Jazoli said:

dc2rr07 said:

This plus check the bobbins are free before checking runout
https://m.youtube.com/watch?v=1qoPqN2GBdw

I don't think they are floating discs

You could be correct

[Speed addicted]

Ok, we have weirdness.

I think I’ve worked out what’s happened.
When I rebuilt the callipers I fitted HH pads in an attempt to modernise the braking experience slightly and make swapping between bikes less of a shock at the first corner.

These appear to be thicker than standard, so the outside pad in both callipers is rubbing slightly, and the piston is all the way in. This leaves the inside pistons to take up the slack as the discs are off centre.

I imagine this was less of an issue with thinner pads as they had somewhere to go.
The callipers/disc appear to be about 1-2mm off centre.

Centralising it would require removal of material from somewhere, it’s the wrong direction to shim.
The wheel is powder coated though, I’ll check under the discs to make sure I removed the coating where they meet.

[Tango13]

I'd hazzard a guess that the r/h piston as you look at the picture is sticking and not pushing the pad onto the disc enough.

[Androgynous]

Speed addicted said:

Ok, we have weirdness.

I think I’ve worked out what’s happened.
When I rebuilt the callipers I fitted HH pads in an attempt to modernise the braking experience slightly and make swapping between bikes less of a shock at the first corner.

These appear to be thicker than standard, so the outside pad in both callipers is rubbing slightly, and the piston is all the way in. This leaves the inside pistons to take up the slack as the discs are off centre.

I imagine this was less of an issue with thinner pads as they had somewhere to go.
The callipers/disc appear to be about 1-2mm off centre.

Centralising it would require removal of material from somewhere, it’s the wrong direction to shim.
The wheel is powder coated though, I’ll check under the discs to make sure I removed the coating where they meet.

That does look odd, it's as if you're missing a spacer/shim to centralise the caliper. RH pad can't move to it's natural position due to interference from the caliper body.

[Speed addicted]

Took it all apart last night, there was some powdercoat under the disc mounting but not enough to change anything really.
all pistons move freely when the discs aren't in the way. New seals etc all round less than a year ago helped.

There does appear to be a slight warp in one of the discs, but it's really slight so may have been there all along.

Due to the way it's mounted there's no adjustment, I'd need to file the calipers, fork mounts or wheel to make the necessary clearance.

Think it's time to question the owners club beardies and see wtf is going on before I go postal with a file!

[Androgynous]

Could you shim the disc outwards?