frame-mount cargo rack makes rotor wobble?
 

I just added a frame-mounted Ibera PakRak (disc-brake version) to my MTB. I noticed right after that the rotor rubs on the brake pads (didn't before), so I looked up how to adjust the calipers & messed with it a bit and now instead of a consistent rubbing noise, it's intermittent, and I noticed the rotor appears to wobble between the brake pads. If I remove the cargo rack from the frame (at least the mounting points on either side of the frame by the rear wheel), it stops wobbling - leading me to believe the extra inward pressure from the rack is pushing something every so slightly out of alignment. In more messing with the calipers I haven't be able to resolve the issue.

I've gotten it to the point where, while the rotor still appears to wobble between the pads, it doesn't rub against them. But braking with that setup will cause uneven wear over the long run, won't it?

The rotor isn't warped - it wasn't wobbly before, and nothing hit or even came in contact with it since installing the rack.
Tektro IO disc brakes
Ibera PakRak
all on a 2007 Raleigh Mojave 4.0

Is this a common problem with frame-mounted cargo racks on disc brake bikes?
Is there a solution for it?
(Still fairly new to having & maintaining a bike, so pardon if I overlooked something obvious).

Post some photos of the setup.

Why is your rack putting inwards pressure on the frame?
Don't you bend and angle your rack legs so they match the frame width and bolt on squarely before installation?

I'll typically spread the legs by hand to match frame width, then use a vice to bend the tips back to parallel

permanently bent rack should line up with mounting eyelets in a relaxed state, no springing or forcing it into place to make it fit. It exerts no inwards/outwards pressure on the frame.


Now, if putting heavy cargo onto said rack then misaligns your disk calipers; that's another problem.

If the rotor is touching intermittently when spinning the wheel off the ground,
Then the rotor IS warped. Can't see any way around that.
If the rotor is touching intermittently while riding, with loaded rack.
Then Id suspect swaying load is twisting the rear dropouts a little.
Merely mounting the rack MIGHT change basic setup but really shouldnt have any dynamic element.

[MENTION=65684]cobba[/MENTION] - I'll add photos once the sun's up. I've only got the one light in the living room where the bike is.
[MENTION=135358]xenologer[/MENTION] - how would I go about bending it to fit? I'm in student housing so I don't have a garage or workshop. Seems like the most promising solution, if I could somehow get a setup and step-by-step on how to do it.
If it's not something I can do in my living room with only a household toolkit, should I head to my LBS and ask? (Might be worth it to have them realign the caliper after I've been messing with it).
[MENTION=114951]dabac[/MENTION] - Like I said, it wasn't wobbling at all before mounting, so it can't be the rotor. I haven't gone for a ride with any load on the rack, just the rack itself.

If it's touching intermittently, then you have to have something causing that periodicity.

A tight rack might warp things, but they'd remain warped. A set degree of deformation.
A rack that tightens and untightens simply isn't reasonable.
So you have to look for something that can vary.
A warped disc would do that.
A loaded rack swaying while riding would do that.
A frame flexing under pedalling would do that.
But I'd rather suspect that would have happened before you mounted the rack too in that case.

maybe the rotor is slightly untrue, and the addition of the rack reduces clearance enough so that frame flex from pedalling suddenly have become noticeable.

Or its something completely different - maybe an axle/bearing issue - that simply happened to coincide with mounting the rack

A cast iron storm drain will provide ample leverage and won't get upset by someone trying to bend a rack stay in it.
As for spreading, grasp and pull.
Or turn the rack upside down on the floor. Get a heavy friend to stand in the rack. Pull at one stay at a time.

Maybe you bumped the rotor when you were installing the rack. Stress on the frame from the rack might alter the rotor alignment but not make the rotor wobble.

'Frame mounted' should mean there is no interaction with the fork at all ..

Yes really need pictures to have any Idea of What you are talking about..

My Brompton uses the head tube to carry the load.. Via a mounting block so bag frame is removable with the bag.





.....

Bing! Bing! Bing! :D

As for the rack being so much more rigid than the frame that the caliper would be displaced... not likely IMO.
Steve

There is absolutely nothing on the frame that could cause the ROTOR to move back and forth unless the caliper was physically loose and sliding back and forth.

Your rotor is warped. Fix it or buy a new one. My guess is you whacked it when installing the rack. (Or dropped the wheel on the disc brake side.)