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I got a mid-80's Puch in great condition, and I replaced the original front wheel (steel) with a newer, aluminium one. But after the switch I have noticed that it doesn't brake as well as before. What could be the problem? Do the brake pads need some time to "break in" i.e. to align with the rim surface better?

Just to note, it's the old style caliper brake, the pads can just be moved up-down and I aligned them well with the new rim.

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    Rim brakes need a little time to leave a bit of coating of brake material on the rim surface. They shouldn't move significantly for a "break in". Make sure the rims and pads are clean of oils as well as being properly adjusted (you can do a good wipe down with citrus cleaner or rubbing alcohol) and the brake is mounted securely and centered properly. You can also try a set of high quality pads (like KoolStop salmons or whatever). If none of this works, you may want to try a light rub of fine sand paper (600 grit, lets say)
    – Batman
    Nov 6, 2013 at 0:10
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    I would recommend replacing the pads, if they're original or pretty old. And, as Batman suggests, you may need to clean oils off of a new rim.
    – Daniel R Hicks
    Nov 6, 2013 at 1:48
  • On my commuter bike, I often notice very small aluminum splinters in the pads. After cleaning the rims with brake cleaner and a good brush with a wire brush (for the pads!), they brake very well indeed.
    – arne
    Nov 6, 2013 at 5:49
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    Pads are old indeed (I believe they are original ones), but they are not spent, and they did brake absolutely fine on original steel rims (I could have locked the wheel with no problem). The only explanation I have is that the pad material is not suitable for aluminium as it is for steel. I'll try changing the pads next.
    – Mladen Jablanović
    Nov 6, 2013 at 6:51

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Aluminium rims should give superior braking performance, as mentioned in this answer (Steel vs. Aluminum Wheels) and this blog (steel vs. aluminum wheels).

Check out the shape of the rims and how the brake blocks make contact. If the brake blocks are not in full contact then you will need to adjust them. See also @Batman's suggestion. That could involve further vertical adjustment of the blocks. Make sure the blocks are adjusted to be as close to the rim as possible without touching when the brakes are not applied, so the maximum braking effort can be applied.

Also check whether the calipers are moving as the pressure is applied (avoid riding the bike for this, if you can, because of the obvious danger). Because of the better grip the blocks get on the aluminium rims, it could be the old calipers are not rigid enough for the new job required of them.

If further adjustment and gentle riding does not rapidly improve the situation, invest in new brake blocks. After that, welcome to the expensive world of upgrades: new brakes, new cables, new ...).

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    Can you clarify the following: "Because of the better grip the blocks get on the aluminium rims, it could be the old calipers are not rigid enough for the new job required of them." If the grip is better on aluminium rims than on steel ones, the force needed would be lesser, hence caliper rigidity would be less important, right? BTW, I haven't seen an article which claims superior braking performance of aluminium rims over steel in dry conditions.
    – Mladen Jablanović
    Nov 6, 2013 at 6:45
  • @MladenJablanović I think what andy256 is getting at it that the calipers may flex in the direction of wheel rotation, rather than in the direction of their own motion. I've never seen this though, and the force in that direction would be the same for the same stopping force, so no different.
    – Chris H
    Nov 6, 2013 at 8:57
  • @ChrisH: I've seen (very old, very cheap) calipers flex quite a bit in the direction of wheel rotation, but those were (it seems) cut from sheet metal and rounded, not die-cast or forged.
    – arne
    Nov 6, 2013 at 9:00
  • @arne, OK, I don't think I've ever seen calipers that cheap - certainly never ridden on them.
    – Chris H
    Nov 6, 2013 at 9:02
  • I know that kind of calipers, this is not the case, anyway, as Chris said, force in rotational direction would be the same as with steel rims (which brake fine), so no problem there.
    – Mladen Jablanović
    Nov 6, 2013 at 11:15

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