Disc brake alignment issues

[ishaw]

I've been trying to set up a front disc brake (hope x2 on a Reba post mount) and it's been a right pain. I think I've discovered a couple of issues but wanted to check I wasn't going mad and if these coukd be the cause.

Issue 1: front wheel alignment.
I've recently had the front wheel tried by my lbs but I think it's not dished right as when it sits in the fork drop outs, it's not centred. To compensate for this I have to angle the wheel in the drop outs which results in the rotor also being at a slight angle too I.e. not straight within the calliper. Am i right?

Issue 2: post mount wear on the fork.
The forks are new to me, but second hand. It looks as though the previous owner has over tightened the brakes they had fitted at some point, as there is evidence of an I sent that corresponds to the shape of a disc calliper mount. This results in the calliper not seating squarely and is probably compounded by issue 1.

So, what are my options? I've tried a thin washer between the calliper and fork mount, but this gets distorted when I tighten it all up. How much room do I have to use a thicker washer without moving the calliper too far away from the rotor? I don't think I can add a V brake/avid style washer and cone without moving the brake too far out. I guess I could try and 'face' the fork mount but how would I do this at home without making a mess of it?

Any other options open to me? I need to rebuild the master cylinder as it's got a slight leak, but keen to use the forks I have fitted if I can.

I'll be visiting my lbs again to get the wheel sorted so that should take care of issue 1 at least.

 

[integerspin]

Forks Donald ducked, I will give you a fiver for them;-)
Check and re-dish the wheel then file/machine the mount.

 

[mechanical_vandal]

I have to angle the wheel in the drop outs

This is a bad bad way to sort your problem. It's very unlikely the wheel will stay in the position you set it. Your weight and bumps from the terrain you're riding will be working to seat the axle fully into the dropouts, add in the rotation/pulling from the disc brake and you're asking for trouble.

Issue 2: post mount wear on the fork.

This is something I'd have a go at sorting myself but there are tools to do it properly, a good bikeshop should have them. To do it yourself I'd be trying to use a long file that can hit both posts at the same time, you need to try and ensure you're applying pressure evenly and keeping the file square in all directions. I'd probably only diy it if the damage is light. If there's a lot of material to be removed I'd have a shop with the relevant facing tool do it.

 

[ishaw]

Cheers for the input.

I have to angle the wheel in the drop outs

This is a bad bad way to sort your problem. It's very unlikely the wheel will stay in the position you set it. Your weight and bumps from the terrain you're riding will be working to seat the axle fully into the dropouts, add in the rotation/pulling from the disc brake and you're asking for trouble.

I agree, which is why I'll be revisiting the shop to get that issue sorted.

Issue 2: post mount wear on the fork.

This is something I'd have a go at sorting myself but there are tools to do it properly, a good bikeshop should have them. To do it yourself I'd be trying to use a long file that can hit both posts at the same time, you need to try and ensure you're applying pressure evenly and keeping the file square in all directions. I'd probably only diy it if the damage is light. If there's a lot of material to be removed I'd have a shop with the relevant facing tool do it.

I will probably give this a go. It's not a huge amount, just enough to off-set the calliper and make alignment that bit more tricky, though that said, using thicker washers is certainly helping, I'll see if after the wheel is sorted whether that does the trick or if further work is needed.

 

[cherrybomb]

Re:

Ian,

How is the disc alignment if you seat the axle fully into the dropout? given you have disk brakes, other than the aesthetic annoyance of the rim/tyre being slightly off centre, the incorrect dish shouldn't actually cause too much of a problem in the short term.

As for the damage to the mount. Quite a lot of callipers come with a set of convex & concave washers which allow for some vertical, as well as the usual horizontal adjustment for alignment. I might have some kicking around, but you should be able to get a set off blag for a couple of quid I'd have thought.

 

[ishaw]

Haven't tried aligning the caliper with the wheel central in the drop out. I'll be honest and admit that I must have fitted the wheel to sit straight and only realised after spending a lot of time unsuccessfully setting up the brake so that the disc didn't rub.

I could give the concave/convex washers a go, my worry is that I'll be pulling the caliper away from the rotor and the pad/rotor contact will be reduced. I was going to use some from some v brake pads, but they are quite meaty and will add to the above won't they?

 

[mattr]

It might be that the fork lowers (or crown) is twisted. Hence your recently trued wheel seeming to be out of dish. Or you might just have a poor lbs.

Try flipping the wheel to see if it's off centre the other way. Or try it in another fork.
Try it without a tyre too.

And adjusting it to sit evenly between the fork legs will double the error at the ground if my maths is right......

I've done filing of post mounts before, the lbs wanted 400-500 sek to do it and I didn't have the tools. My mate just wanted the bike on the road immediately. So I filed. Took about 4 strokes with the file. And AFAIK is still being ridden about now ~5 years later.
Turned out that the shop didn't have the tools either. But that's another story........

 

[mattr]

If the caliper/mount isn't designed to use a concave/convex pair. Don't.

You'll either have to keep filing the friction surface to remove the newly formed lip. Or one day you'll suddenly have no brakes as the two lips will meet.

 

[ishaw]

It might be that the fork lowers (or crown) is twisted. Hence your recently trued wheel seeming to be out of dish. Or you might just have a poor lbs.

Try flipping the wheel to see if it's off centre the other way. Or try it in another fork.
Try it without a tyre too.

And adjusting it to sit evenly between the fork legs will double the error at the ground if my maths is right......

I've done filing of post mounts before, the lbs wanted 400-500 sek to do it and I didn't have the tools. My mate just wanted the bike on the road immediately. So I filed. Took about 4 strokes with the file. And AFAIK is still being ridden about now ~5 years later.
Turned out that the shop didn't have the tools either. But that's another story........

Cheers for that. The forks are fine, it's actually the second wheel they have trued that has come back like this, so I think they might be a bit poor at building wheels. The problem with an LBS is that it is local....

 

[hamster]

Firstly check whether it's the wheel or the fork alignment. Flip the wheel and check if it comes up offset on the other side.
If so, then take the wheel back to be dished correctly by the shop.
If not, then you have a fork twist problem, which is setting everything else out of alignment. YOu will probably be able to shim and fiddle to get it all to 'sort of' work but in reality you are probably better off biting the bullet and replacing the fork.