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My front brake has a tendency to dig in or dive when I use it on bumpy roads. There seems to be a degree of positive feedback in this effect, causing me to sometimes brake much harder than I intend to if I hit a bump. It can be a bit alarming and I'm concerned I might accidentally lock the front wheel.

The bike is a semi-BSO mountain bike. I've replaced a lot of the components including the brakes, forks and headset.

The brakes are Shimano BL-MT501/BR-MT520 hydraulic 4-piston disk brakes which are Deore groupset. They're not loose (caliper and adaptor bolts torqued to about 8Nm). I bled them recently. The pads (Shimano D03S Resin) and rotor (Shimano SM-RT76) are not overly worn but should be bedded in (both fitted at the same time, around 700km subsequent use)

I replaced the extremely low-end suspension forks with steel non-suspension forks (Salsa Firestarter). The geometry of the forks is extremely close to the forks that came off, accounting for sag - they are intended to be a drop-in replacement for suspension forks.

The rear brake doesn't have the same behavior. It's using the original no-name rotor that came on the bike, which has maybe 1200km additional wear on it (I replaced the brake but not the rotor on the rear, with no apparent ill-effects)

I'm guessing that the issue with the front brake could just be that the brake is simply too powerful for my riding conditions (it's four piston) and maybe has poor modulation?

Questions:

  1. What could be causing the problem?
  2. Is there anything I can do to mitigate it (choice of pad material, style of rotor, adjustments to the brakes, braking technique, etc)?

Thanks

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  • Clarify: does this happen when riding along normally, or immediately when you activate the brake, or after the brake has started biting ?
    – Criggie
    Oct 21, 2023 at 19:57
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    The scenario tends to be limited to when I'm already braking and then I hit a bump. Braking is generally smooth and predictable provided that the road surface is smooth - i.e. I'm already braking, without any issue, and then a bump causes the brake to snatch Oct 21, 2023 at 20:00
  • Dang that's a hard one. Normally brake dive comes from suspension, but your front end doesn't have any.
    – Criggie
    Oct 21, 2023 at 20:05
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    Yeah, I wondered if it could be caused by the forks bending as I go over bumps and that changing the geometry, but surely they can't flex that much? I'm pretty confident the geometry is otherwise unchanged compared to the original forks with a person's weight on them - I mean I've not stuffed it up Oct 21, 2023 at 20:07
  • You don't happen to have the brake lever closer to your wrist than the handlebar, do you ? Unlikely but that would lead to positive braking feedback loop. This used to be a thing when people would rotate their dropbars but change nothing else.
    – Criggie
    Oct 21, 2023 at 20:39

1 Answer 1

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I'm inclined so say there's no actual hardware problem here and you're just not used to riding such conditions (perhaps because you previously didn't have equipment that allowed it)!

When you hit the front on a bump then this causes a push not only upwards but backwards too, no way around it. So it's basically a bit of braking without the brakes actually being involved. In case of a suspension fork on a slack bike, this backwards jolt is absorbed partly by the fork travel, but with a rigid fork this can't happen and it does feel like the brake is suddenly over-grabbing. Yes, this is scary and can send you over the bars.

The solution is to anticipate it. Basically, you should just not brake into bumps - pay extra attention where your wheel is going whenever you're braking, and if you know it's about to hit a bump briefly let go of the front brake. (There will still be the backwards jolt, but the break in braking compensates for it.) Bonus points for also shifting your weight back just before hitting the bump, this makes it easier for the wheel to clear it. On larger bumps you can make the motion stronger and basically lift the wheel over the obstacle completely.

Experienced MTB riders do all of this quite automatically and may not even realise they're doing it.

If these were some shady brakes then I would admit the possibility that they're actually doing shenanigans here. But I really don't see how this could be happening, and Shimano hydraulic brakes are very trustworthy.


And, this effect does also happen on the back, but there it's much less noticeable. The reason being that the backwards impulse alone shifts your weight forwards and therefore allows the rear wheel to get over easily. But on the front that same effect is of course working against you!
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  • In the absence of any mechanical explanation, this seems most likely to be what's going on. The issue is happening less often as I'm being more careful to simply avoid using the front brake going over bumps. I'm trying to pick my way through bumpy sections, but traffic and sheer amount of lumps means it's not always practical Oct 25, 2023 at 14:34

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