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I have a foldie which has always had bad/weak rear brakes, which keep getting worse. To preempt the response that they are expected to be weaker than front brake, this isnt a question about weight transfer, or traction being lower in the back, or the rear wheel locking up sooner. Right now, if i were to brake the rear while moving, I barely feel any braking at all. I have tried to adjust it to be angled correctly and hitting the rim correctly but it seems to make the issue worse.

I recently noticed that both the rim surfaces are a bit curved (the centre is about 1 mm thiner/deeper than the top/bottom edges (if that makes sense). One side is worse than the other, which made me feel it was a manufacturing defect, but now i see that the other side has a similar (but less pronounced) curve, which makes me feel it is just a wear thing.

I also notice that the rear brake pads are very smooth to the touch. They still have the grooves vertically, which indicates that they arent worn out yet? Should i try to sand it down a bit to try and improve the braking?

Is there anything else I can do? Is the rim wearing down normally or is that the cause of the braking issue? (I have been sorta compulsively trying to adjust the back brake nearly as long as i can remember, cant get them working as I would like.)

The bike is ~2 years old, ~1300km riden. Sorry for the rambly post.

Edit: The rim is curved, but the wear indicators are still quite pronounced. Does this mean the rim has bent rather than worn out? The brake pads are quite worn out as well, one of the edges of one of the pads has reached the wear indicator, so I have ordered a new pair. Removing, servicing, and reseating the pads seems to have given me some braking power for now. Remains to be seen if it lasts long. I could find a couple of small metal pieces(like 2-3 sub mm size) on the pad, but not enough to explain the wear. I wasn't able to hear them when I brake either so might not have been a major factor.

  • If cleaning rims and new brake pads have not fixed it, next most common cause of loss of braking power is cable problems. – mattnz Jul 6 '18 at 10:40
  • if there's as much as a 1mm curve in the brake track on the rim i'd suggest that's the most likely issue. Most likely only part of the pad is generating any significant braking force. – Andy P Jul 6 '18 at 11:14
  • @AndyP pretty sure the pad is following the curve, so it should still contact fully.. Especially when i pull like im mad.. – Karthik T Jul 6 '18 at 12:00
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    Do you brake a lot? To wear the tracks down by 1mm in 1300km is impressive or points towards a bad quality material for rims or unsuitable pads. – Carel Jul 6 '18 at 19:47
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    If you have not adjusted or replaced the cables i would start there, they stretch naturally over time and need to be adjusted or pulled tighter – Nate W Jul 6 '18 at 21:14
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I agree with David Richerby: The curve in the rim is bad. This indicates an enormous amount of wear, that is not justified by your mileage, whatsoever. There is a chance that this was caused by some abrasive debries being caught up by the rubber of your pads, which was then regularly pressed against the rim when braking. You should have been able to hear this, but apparently and unfortunately, you didn't notice. This could have had an effect like replacing the pads with sandpaper, so even if it was temporary, it may have had a quite lasting effect. You say that your pads are smooth now, so probably the pads are fine again. Irrespective of the state of your pads, the rim is a security risk now, and should be replaced ASAP.

Concerning your inability to tightly grab the rim with your break pads: You should check the lever you are using. There are different brake types out there, some require the cable to move a large distance, some require the cable to transmit a lot of force. Your lever either delivers one or the other, not both. V-breaks require the long-move. If you are using a wrong lever, the distance that you can move your break pads will simply be too short. So, make sure that you are indeed using a V-break lever, or, if your lever is adjustable, that it's used in V-break mode.

  • OP would have to measure the rim sidewall thickness with a special caliper to find out how much material is remaining. A crude way without tools is to check if the rim wall deforms at maximum pressure: Deflate the tires, put the brake pads as close to the rim as possible (almost rubbing) and then inflate the tires to maximum (or slightly higher) pressure and see if they start rubbing. You can also measure the difference in outer width between low and high pressure with a normal caliper. – Michael Jul 7 '18 at 5:47
  • So I am able to still feel the groove that indicates wear of the rim, which should mean that they are still ok wear wise? I wonder if this is more a case of the rim bending due to me pressing hard rather than wearing off with constant braking. – Karthik T Jul 9 '18 at 7:58
  • If that makes sense, could that be a manufacturing issue? – Karthik T Jul 9 '18 at 7:58
  • @Michael that looks like something I can try, if I were to do the "crude" way, how much expansion is too much? or is it not supposed to expand at all? – Karthik T Jul 9 '18 at 14:54
  • @KarthikT Yes. But that really puzzles me now. The wear indicator should not be more than a millimeter deep, so the wear cannot explain the curvature's depth. I'm at a total loss at explaining the curvature in your particular case, then. I would doubt that you can deform a rim by pressing too hard on the brakes: You would need to counteract all the pressure from your tire, and put as much of force on top to even reverse the force on the rim inward. I find it more likely the rim was bent outwards. Anyway, if the rim was bent in either direction, that would make replacement much more pressing. – cmaster Jul 9 '18 at 19:14
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A 1mm curve on the brake track suggests that the rims are worn and you need new wheels; on the other hand, it's strange that they'd be so worn after only 1300km. I'm not sure how much that will affect your braking performance but the rims weaken as they're worn away by the brakes (of course) and will eventually fail if not replaced.

  • The name for that wear on a rim brake track is "scalloped" or "scolloped" – Criggie Jul 7 '18 at 7:58
  • I am still able to feel the rim wear groove, it is nearly as pronounced as the front wheel, which makes me feel this is not just wear related. Does that make sense? Could it be bent rather than worn out? – Karthik T Jul 9 '18 at 7:59
  • That's interesting. The rims shouldn't have bent so I'm not sure why they'd be scalloped and still have an intact wear groove. I'd recommend taking it to a bike shop to see what they think after examining the wheels. – David Richerby Jul 9 '18 at 9:16
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Sounds like a cable issue. In my experience the orientation of the brake pads and the cleanliness of the rim and pads has much less effect on stopping power than proper, low-friction cables and housing. You might want to replace the housing and cable.

Are you able to move the levers smoothly and with very little friction/force up to the point where the pads engage? Since the brakes and levers seem to be no-name parts: Is the front brake (which seems to use the same parts) working satisfactorily?

Edit: While you are at it you could also replace the brake pads for high quality pads (e.g. the green Swissstop or the salmon Kool Stop pads) which should improve braking power, especially in wet conditions. They will probably also reduce rim wear.

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