3

I ride a road bike with carbon frame and fork an hydraulic flat mount disc brakes. A fews days ago the front disc was slightly rubbing on the braking pad, so I readjusted the position of the braking saddle (loosen mounting scews, pull brake, thighten screws back accordingly to producer manual Nm). This worked fine and the rub is gone and hasn't come back.
However, when I put force on the handlebars - and thus the fork - like pulling handlebars when restart after red light stop or when ridding out of the saddle, there is a slight rub when applying force. It immediatly stops rubbing when ridding "normal".
So should I be concernd about this/reajust the brakes? Or is this just the fork flexing under pressure and nothing to worry about?

  • +1 my new fork exhibits the same behavior, so I'd like to know the answer too, but it's probably a combo of tight pad clearance, taller rotor (140 to 160) and fork flex. – Klaster_1 May 20 '18 at 14:53
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    It's worth checking the bearings of the hub and to tighten them if necessary. The rim can move under load if the bearings need re-adjusting. – Christian Lindig May 20 '18 at 16:44
5

Flex causes this and it's not going to hurt anything. However, it's frequently the case flex is just the contributing factor that pushes the system into rubbing.

Look down at the gap between your rotor and pads, and slowly spin the wheel. If you were to see consistent .2-.4mm-ish gap on both sides, a true rotor (no warbles), and good parallelism between the pad and rotor, there's not really anything you could do to improve things at that point, and you'd know it was just flex causing the rub.

If you see an out of true rotor, off-center pads, or very minimal gap all around, then that means the rotor is getting closer to the pads in certain spots than it should be able to, and while it may not rub unloaded (in the stand etc), adding flex is getting it there. So to solve it you may need to true the rotor, face the frame/fork, reset the pistons, and/or adjust the caliper position.

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    "1.5mm gap on both sides" sounds like a lot, aren't all hydraulic disc calipers limited to 0.2-0.5mm of retractable piston travel? – Klaster_1 May 21 '18 at 3:09
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    @Klaster_1 I spitballed that number and was way off. Shimano's working number is .2-.4mm. Edited. – Nathan Knutson May 21 '18 at 3:29
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I’ve had the same thing happen with my Cervelo c3. After a two mile descent the front brake rubs whenever I parallel hard out of the saddle. Never did it for 18 months before going down the hill but now does it every time. I think something has changed but my local shop where I got the bike say “they all do that”. If it had done that from day one I would have taken it back and demanded a refund. What does anyone else reckon?

  • 1
    Welcome to Stackexchange. Please do go take a moment to read the tour specifically that this is a Q&A format not a chatty web forum. You've posted a "me too" that doesn't answer the question. Could you please consider using edit to include what you've done to fix the problem, and how that worked for you. – Criggie Jun 10 at 8:29

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