Front disc brake rub issue when under load
 

Hi there,

I'm building up a monstercross type bike. I have an issue with the front disc rubbing when i'm sat on the bike. When there is no load there is no disc rub. Just wondered if someone here could diagnose and help me resolve this. I am beginning to wonder if the lower fork bearing race might be out of alignment forcing the wheel out of of true or the wheel? The problem happens in both laternal and forwards movement.

Parts
160mm rotor, SRAM road Rival caliper on a 160-180 adapter plate. Hydraulic hose as yet unattached. 2.6" tyre, 29er mtb wheel rim, carbon mtb fork with QR drop outs. The front wheel bearings feel tight and both the wheel and the rotor are true.

Any ideas?

To diagnose this you have to focus or possible movement between the disc brake mounts and the disc itself. Anything not within those "boundries" doesn't factor.

I'd consider fork flex near the ends, hub bearing play, movement of the axle in the dropouts, etc.

For starters I'd pull the rim across toward a fork blade while watching for movement of the disc in the brake.

FWIW my prime suspect would be axle play, which is easy to confirm and fix or rule out.

This ^^^^^

The first thing I think of is fork slop between the uppers and lowers. Andy

Interesting.

I would have thought that the brake bridge at the top would have provided enough rigidity to the lower fork/wheel assembly.

But, you work on far more suspension bikes than I ever have or will, so I defer to your experience.

"monstercross" bikes are rigid fork.
i've seen only a couple "carbon suspension forks" EVER... two Super-high dollar Lefty Cannondales... they were MTBs.

As to the issue the OP is having.. i'd be looking at the Dropouts REAL CLOSE, and also the triple clamp/tube junctions...

there's a strong chance that the Quick Release is bottoming out on the axle before the thing can clamp solidly, too......

OR The AXLE IS BROKEN. ;)

Carbon tubes fitted into aluminum Triple clamp and dropouts, right? Brand? Model?

The first thing I’d do is replace the skewer with a closed cam design if you can’ haven’t already. It can be hard to get the skewer tight enough to work well with a disc otherwise.

Also check the alignment of caliper carefully. If the disc tabs aren’t machined perfectly perpendicular to the disc you may be able to get it to not rub in the stand, but not have enough clearance in real world conditions. Having the mount faced at a shop with appropriate tooling isn’t a bad idea.