So I rebuilt the front caliper on M1 because it was very lightly dragging and nothing I did maintenance wise would stop it (including removing the front brake lever to be sure the relief port could do it's job) and it's a little better but the inner pad is still not as slack as it should be.
I've got the original 99 M2 front wheel and caliper mated to X1 forks and an EBC disc fitted.
When I look closely I find that the disc is not centered in the caliper, being about half a disc thickness to the right i.e away from the centerline of the wheel. Now - if it where the other way I could shim it but in this direction I can only remove metal to rectify the problem
What is happening is that the inner pad is "kissing" the disc always and when I apply the brake it is kicking the disc over to meet the other pad which is delayed from contact due to it's greater distance.
The caliper is retracting just as it should when I check it off the disc and , thinking back, I was in the habit of fitting new pads frequently before I altered the bike from standard because I found it made an extraordinary difference going from half worn to new.
Now, I suppose new pads would help again but now I understand the mechanics of what is going on I'd rather sort it properly. Any clever people out there with ideas?

Are the external spacers the correct sizes? Have the bearings been installed from the non-disc side, i.e. Non-disc side bearing to shoulder, spacer, disc side bearing to internal spacer?

Edit - corrected the order of bearings...

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08 Specialized Langster

The axle design gives the overall design away. One side is free in the fork, the other side bolts up against the fork. Everything from the left hand bearing shoulder on the left hand side is critical to the positioning of the disc. So that means distance of inner face of slider to centreline of fork, spacer width, bearing width and distance between bearing and shoulder (should be zero).

Then coming over the top to meet the disc, the caliper mount face to caliper centreline, the caliper mount face on the slider to fork leg centreline, the fork centreline to centreline distance...

A common problem on the mechanical speedo bikes is the loss of the small spacer (see useful info depository in How To) which pulls the wheel to the left (thanks ). You have an electronic speedo so not an issue with you.

Are your fork tubes/sliders bent?

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08 Specialized Langster

Thanks for your speedy and succinct replies Nick - I have discounted the disc side bearing as I thought that if it wasn't inserted to full depth it would have the opposite effect ( although that's only my instinct!) I haven't touched the front end since Maz built it up for me at which time he rebuilt the forks and I would be very surprised if he hadn't spotted something was bent or otherwise. What he did say was that the brake had a "odd" feel about it but the pads were buggered to be fair!
As regards the spacer dimensions, I think I have a spare set somewhere but am pretty sure they are identical from past memory because at the time I compared them whilst fitting the X1 front end.

Which way is the disc offset in the caliper? To the left or right as you sit on the bike?

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08 Specialized Langster

To the right side as you sit on it. When you release pressure on the brake the disc goes back to the left and uses up virtually all of the retraction of the left pistons so the pad is "kissing" the disc whilst the right side pad is well clear.

What happens if you loosen the clamp on the right hand slider and bounce the forks then tighten the clamps?

What happens if you pull the bearings and install the right hand bearing first then the spacer and the left, i.e. get the disc side bearing against the shoulder?

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08 Specialized Langster

Thanks - will try slacking, bouncing and new pads for now. Still think that the bearing is up to the land because if it wasn't then it would move the wheel the way I want if you know what I mean?

None of the above should matter. Although the ideal is to have the disc centered in the caliper, it won't affect operation as long as it is not actually touching the caliper and there is sufficient room for the (new) pad (try a part worn pad...)
Once the pistons have moved up to the pads on the first application they should stay there (less a fraction clearance)... If there is then constant pressure on a pad it will be because the piston or the pad is partially seized. If the pad is excessively clear of the disc it will be because the piston or the pad is partially seized.
For the disc to distort to the pad rather than for the pad to move to the disc, my money is on the caliper itself, the seals are wrong size or contaminated with the wrong fluid, or there is an internal blockage in the caliper passage to the outer pistons or your outer pad is not free to move.

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'98 M2 Razorback, it's a keeper.

Well rik, you have said a lot of things I have thought about calipers recently and that is why - although it seemed ok I.E. retracting nicely - I rebuilt it using new pistons and seals. I made absolutely sure the seals were fitted correctly with the ridge towards the inside etc, well lubed and off the bike the pistons do retract equally and evenly. I pushed the pistons back and started afresh etc. but I haven't tried new pads yet as my last resort is that the old pads (which have plenty of meat on them) have taken on a "set" due to uneven wear?
There is one thing that crossed my mind and it is that, in a perfect world, you would want the pistons to be equally extended because a more deeply inserted piston will have more drag on it than one which is further extended and may move at a different rate. Perhaps that effect is negligible in the real world?

Ridge? IIRC they are just a square section?
Were the seal grooves clean and clear?
OK so they retract nicely but what's more important is how nicely they pump out...remove a pad and block two of the three pistons with slivers of wood to check each individual piston, squeeze the lever a few times but don't let the piston emerge too far! ...it should progress evenly with a tiny shrink back after each pump... then push back and repeat for the other pistons...)
Can you confirm what fluid you're using? And what assembly lube you used?

The only drag is (or should be...) the seal, its surface area in contact with the piston does not alter; the force and the drag are both linear regardless of how far extended a piston is.
Is the inner pad free to jiggle and clear of the disc when you first fit it, ie before the first squeeze of the lever? If not, experiment with a part worn pad, does it rub after it's been moved up to the disc?
Is the outer pad free to jiggle? Unless it is seized, I just don't see why the inner pad deflects the disc over to the non moving outer pad unless the outer pistons are tight or not getting any pressure behind them.

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'98 M2 Razorback, it's a keeper.

And just one more point , the pistons should only retract back enough to take the pressure off the pads never far enough that they need pumping back out to meet them, if the disc is springing back away from one set of pads then that again indicates one side of the caliper must have tight pistons.

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2001 X1 2009 1125CR 2006 Uly X3 2010 Uly a fully Maz'd S1 and a 1991 RS1200 Westwind.

Were the seal grooves cleaned out thoroughly (a Dremel with a disc type wire brush works wonders here) before caliper rebuild?

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08 Specialized Langster

Thanks for the input gents - I am far from an expert on brakes but I can confirm that I cleaned the grooves to spotless level, used the assembly silicon grease on both seals and pistons, checked that the seals had the correct arrangement of the high end of the wedge towards the inside (I checked with my finger), that the pistons do all move out and back a little as you pump them out.
I have used a Buell rebuild kit which included all seals, pistons and assembly lube and until you fit
it and pump the pistons out to the disc everything feels spot on. Now, I will say that the pistons don't move as one when you pump them out from pushed right back but I've never had a caliper that does!
This is why I'm asking for input because the only thing that strikes me as wrong is the fact that the caliper is not straddling the disc centrally which seems bad engineering. If I hadn't inserted the disc side bearing correctly (ie down to the land) I believe that the problem would be in the opposite direction to that which I have?
Apart from anything else I guess that I'd like to know if they are "all like that sir" as regards centralisation or is mine unique?

I'll check some bikes when I get home.

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08 Specialized Langster