Checking brakes - hydraulic disks

[Doki]

About a year ago I had a fairly fast crash on my mountain bike. I bent the
rotor, replaced it with a genuine magura one, but still can't get it to run
quite right. There's still always a slight rub. I've noticed today that the
QR is also a bit bent. Now, what are the odds that something else is not
quite right? Could the actual hub itself be out of true, or the caliper? How
could I check?

 

[Paul Boyd]

On 26/05/2007 13:07, Doki said,
> About a year ago I had a fairly fast crash on my mountain bike. I bent
> the rotor, replaced it with a genuine magura one, but still can't get it
> to run quite right. There's still always a slight rub. I've noticed
> today that the QR is also a bit bent. Now, what are the odds that
> something else is not quite right? Could the actual hub itself be out of
> true, or the caliper? How could I check?

Have you checked the axle? The best way to do this is to take it out
and roll it along a piece of glass - any bend will be obvious that way.
Otherwise, just keep tweaking the rotor! I would have thought it
unlikely that the hub body would be damaged.

A final thought - have you checked the the pads are still absolutely
square on to the rotor, both in through their height and front-to-back?
If they're not square, any slight movement of the rotor will be more
likely to catch.

--
Paul Boyd
http://www.paul-boyd.co.uk/

 

[Doki]

"Paul Boyd" <[email protected]> wrote in message
news:[email protected]...
> On 26/05/2007 13:07, Doki said,
>> About a year ago I had a fairly fast crash on my mountain bike. I bent
>> the rotor, replaced it with a genuine magura one, but still can't get it
>> to run quite right. There's still always a slight rub. I've noticed today
>> that the QR is also a bit bent. Now, what are the odds that something
>> else is not quite right? Could the actual hub itself be out of true, or
>> the caliper? How could I check?
>
> Have you checked the axle? The best way to do this is to take it out and
> roll it along a piece of glass - any bend will be obvious that way.
> Otherwise, just keep tweaking the rotor! I would have thought it unlikely
> that the hub body would be damaged.
>
> A final thought - have you checked the the pads are still absolutely
> square on to the rotor, both in through their height and front-to-back? If
> they're not square, any slight movement of the rotor will be more likely
> to catch.

This is the thing - the brakes are magura julies. The manual sanctioned way
to adjust them is to fit the bracket to the IS mount, then loosen the bolts
attaching the caliper to the bracket (the caliper has oval bolt holes), pull
the brakes on, then tighten bolts. This has previously worked perfectly, but
I can't get anywhere now.

How would I go about removing the axle? The disc seems to be running
perfectly true. I'm beginning to suspect it might be a caliper problem, and
getting tempted to take it to the LBS.

 

[Paul Boyd]

On 26/05/2007 14:44, Doki said,

> This is the thing - the brakes are magura julies. The manual sanctioned
> way to adjust them is to fit the bracket to the IS mount, then loosen
> the bolts attaching the caliper to the bracket (the caliper has oval
> bolt holes), pull the brakes on, then tighten bolts. This has previously
> worked perfectly, but I can't get anywhere now.

Without seeing it, that does sound like you might have bent something
about the caliper rather than the hub. If the mountings are bent, the
caliper will not seat straight to the rotor no matter how you adjust it
- you won't have the ability to make the correct adjustment. (I think
I'm right in thinking that Magura brakes don't have the concave washer
fixings like Avids do, and work in a similar way to cantilever or
V-brake blocks).

To be honest, it might be best to get a bike shop to check it. At best
you might find that facing the mounts will be enough to bring it all
into line, at worst you might need a new caliper or even forks :-(

Incidentally, Shimano have the same "loosen it off, squeeze the brakes
and tighten" method. I can never get that to work! I just align the
caliper by eye so that the disc is centred at leading and trailing
edges, then bop the pads in and squeeze. Works every time (although I
need to set the back pads with my weight on the bike to avoid rub.)

--
Paul Boyd
http://www.paul-boyd.co.uk/