Magura rim brake does not work well on a new rim

Question

I have a bike with 20inch wheels and Magura HS11 rim brakes. On the rear wheel I have just replaced the rim, because the old one had split after many thousand kilometres. The new rim is an Alex DM24 406X24. The old one was another wide aluminium rim.

The Maguras were braking fine with the old rim. In contrast now with the new rim the brakes do not work well: After some initial moderate stopping power the brakes do not grip harder when I pull the lever harder.

To explain it a bit more in detail: When I press the lever slightly (say, of a “feels-like’ amount of 10%) the brake reacts as expect by braking of “10%”. But when I press hard, say 50% or 100% (=as hard as I can), the breaking power does not increase at all, it stays at around 10%.

I am pretty sure it is not an issue with the brake system (oil, etc), because when pulling the lever I feel that the pressure against the rim increases. (Furthermore, not oil is leaking out and the brake behaviour is consistent over days.)

I guess it must have to do with the brake pads an the rim surface. The rim looks to me like it has normal alloy sides, so I kept using the old black brake pads.

So, what might cause this strange behaviour? Should I replace the pads with pads of another type? Or is there another fix?

Answer 1

As it turned out, the problem were not the brake pads. The issue was cause by the fact that the new rim is slightly narrower. The brake cylinders still touched the rim, but were the already nearly fully extended out. This lead to the particular situation that the brake started breaking lightly, but full force coupled not build up, because the two cylinders had reached their maximum extension before that.

The fix was easy: I moved the brake cylinders (in the released position) further in and now they are gripping the rim again properly.

Answer 2

It is expected that new braking rim surfaces, brake disks and pads are less efficient in the beginning. You need to bed-in them.

Additionally, because you've changed the rim it is possible that pads need re-alignment as they do not fully engage with the new rim's surface. Watch closely if they contact the braking surface at the right angle from both sides, do that with their full width, and that they avoid touching the tire.

Answer 3

Magura rim brakes were/are known as "rim-crushers" because they were capable of putting out a lot more pressure than the caliper brakes of the day. I had them on a tandem and I could lift the rear wheel with the front brake, when riding it solo.

So pressure is unlikely to be the problem.I'd take the wheel out and have a close look at the brake pads - I bet they've got a layer of something that has rubbed off the rim.

You could clean off the braking track using Isopropyl alcohol and clean tissues. Probably unmount the tube and tyre first, and left the rim stand for a few hours after cleaning and before reassembling.

Last resort before replacing the pads is to see if they will swap left for right. That way the edge that meets the rim will be clean or cleaner than the other end.

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Magura pads come in 4 colours

• Black - Standard pads suitable for all rims offering good performance and good wear in most conditions.
• Grey - Standard pads designed to work better on anodised(hard-coated) or ceramic coated rims offering good wear in most conditions.

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• Kool stop (Red) - Performance pads for all polished/uncoated aluminium rims offering greater stopping power particularly in dry conditions.
• Green Frog - Performance pads for all hard anodized(black) and ceramic rims offering greater stopping power particularly in wet conditions.

So for dull aluminium brake tracks use RED and for ceramics and shiny aluminium use GREEN.