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For trial bicycles there is a technique to improve braking performance by roughening the rim surface using a grinder.

Ground rim

How-to video

Apparently, this technique is not widely seen outside the trials competition. There are other ways to improve the braking efficiency (better pads, cables, housings and technique) worth trying before messing with power tools. Not to forget that decreasing rim's thickness shortens its lifespan.

I recently discovered (purely by accident¹) that application of a certain kind of household cleaning fluid to the rim surface had improved braking power on a few spots of the rim.

The only explanation I have now is that the rim's surface has gotten chemically etched, which roughened it in those spots. I wonder now if this is another viable hack to try for braking performance.

Is such chemical rim treatment a thing to consider? Is using a sandpaper and roughening by hand an alternative?


¹ I was cleaning a neglected chain from a nasty thick layer of dried teflon chain lube. Apparently, I was not very accurate and a section of the rim got covered by the cleaning liquid. The same liquid has demonstrated to be able to remove anodizing from a cassette, meaning it is quite powerful (base? acid? no idea). I should probably stop experimenting with it…

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    I'd be concerned also for the tyre that would have been wetted by the cleaner.
    – Criggie
    Apr 26 at 19:57
  • If you are going to try this, I'd definitely prefer mechanically treating the surface to doing it chemically. If there are any minuscule cracks or pores you may end up affecting the strength of the material beyond just the surface effects you're expecting.
    – DavidW
    Apr 26 at 20:32
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    My v-brake maintenance routine includes using a scoring pad to clean the surface of the brake pads to remove glazing and a soft scoring to clean the rim without damaging it. Both actions assisted with non abrasive kitchen soap (liquid). That is enough for me to do XC riding.
    – Jahaziel
    Apr 26 at 20:37
  • What were the ingredients in the cleaning fluid?
    – Chris H
    Apr 27 at 9:48
  • Annoyingly I've just got rid of some old (mainly damaged) wheels. I could probably have run a static test treating section of a rim with various cleaning fluids, applying a constant braking force, and measuring the force on a pedal to overcome this. OK that's static friction, but it's easier to arrange than stopping a spinning wheel, and the e-bike wheel I got rid of wouldn't fit the available frame for testing
    – Chris H
    Apr 27 at 9:59
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As I said in a comment under another answer, you may well have very well degreased those sections, on a rim that had traces of stubborn grease even when apparently clean (either from road grime or chain lube getting flicked at it).

Another possibility is that by stripping the anodising off the rim you've revealed the far softer aluminium underneath. This will wear away faster and can pick up grit more easily.

Aluminium does naturally oxidise, but this layer is thinner and weaker than what you'd get from anodising, at least initially. The reformed native oxide layer may well end up behaving the same as the original rim surface.

Certainly I'd want uniform braking. A grease spot is noticeable, but not too bad; a grippy spot could cause you to lock the wheel if braking maximimally on the rest of the rotation. Luckily it's the back, and I hope you're not braking to the limit of your traction while cornering.

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When I was into freestyle BMX in the early 2000s, we had a few tricks to improve braking. Much of the tricks were designed, like in the case of trials, toward having brakes that can be locked up solid with the least amount of effort so that you can consistently lock them up with 1 finger. So modulation was not usually a goal.

First of all, contrary to the trials example provided, we did not find that rougher rims were better. The friction characteristics of rubber on rims is complicated, and apparently making the rims rougher can also reduce the surface area in contact with the pads, and may also scrub the pads away faster. But whatever the reason, and while I'm not doubting the trials technique could work, I'm just saying that we didn't find that rougher rims were better. In fact, usually rough rims got better after they had worn smooth, and for people to find that their brakes actually got worse after sanding the rims. And it was widely believed that chrome-plated rims, which were nearly mirror-smooth, were the best of all for braking.

About applying substances, the usual suggestions were Simple Green or Coke. In my experience both worked, but Coke didn't last very long, and both usually caused squealing during non-lockup braking.

Some household cleaners contain lye, which will etch aluminum and remove anodizing. One example is Purple Power. If you are looking for a substance to experiment with, lye seems as good as any (although it can be dangerous), but I have no experience with using it to improve braking. Most all my bikes have disc brakes.

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    This. I think it's pretty well-established that the more complete the surface contact, the more frictional force; of course that requires both surfaces to be clean and dry, which is where a roughened surface may have the advantage of allowing at least partial contact through a layer of contamination.
    – DavidW
    Apr 26 at 20:30
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    You remind me that while the chrome rims were the best stoppers when dry, they were also the worst performers if they happened to get wet. Wet chrome rims bad almost zero stopping power when wet, which is probably why chrome rims never caught on for MTBs. Apr 26 at 22:25
  • @BetterSense I got rid of my last chrome rims recently. They were dreadful in the even the slightest damp. They seemed to pick up enough water on each revolution that even braking early to dry the rims didn't work (unlike with Al, which is also better to start with)
    – Chris H
    Apr 27 at 9:40
  • Lye (caustic soda/sodium hydroxide) will etch aluminium, though the effect will vary with both concentration and contact time. The previous surface (anodising, mechanical refinishing etc.) will also make a difference. It will also react with any grease to form a slippery soapy layer. Once this is rinsed off you'll have an effectively degreased rim.
    – Chris H
    Apr 27 at 9:44
  • The surface topology of a rough surface is interesting - imagine taking a flat surface and adding little ridges widely spaced - very little contact area. Add little grooves instead and you've got most of the original flat surface, but some edges to catch, and maybe channels to remove water/mud. Even the direction of the grooves will make a difference. But in general pads and braking surfaces conforming to each other helps, which is why improving the pads alignment of rim brakes that have been misaligned for a while doesn't always have an immediate effect
    – Chris H
    Apr 27 at 9:47

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