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When I upgraded the brakes on my bicycle recently, the shop asked if I wanted to also switch from my (chromed) steel rims to aluminum rims, saying the aluminum rims would provide better braking power. I opted not to at the time to save money; with the brake upgrade (old-style sidepull caliper to dual-pivot), I have plenty of stopping power when dry.

In the rain, however, my brakes are almost worthless. Do aluminum rims provide significantly better braking when wet?

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I find that my disc brakes work extraordinarily well including when wet. –  ChrisW Jun 8 '11 at 13:57
    
I also find that Chris's disk brakes work well when wet. Not that fitting disk brakes is likely to be possible for the OP, but it is a valid point. –  Мסž Jun 8 '11 at 22:52
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3 Answers 3

up vote 7 down vote accepted

Since you have gone for steel rims and need to stop in the wet and are unlikely to upgrade to alloy rims straight away, I think you are deserved of a practical answer...

In the wet, with regular brake blocks, the alloy rim will have a better coefficient of friction than the steel rim. This much is known and these brake blocks are the wrong ones for your steel rims. You need the brake blocks designed for the job, the ones with a leather face:

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Typically these are nice and cheap, they also last a very long time. In the wet they work remarkably well on steel rims, although it can take a wheel revolution for the braking power to kick in. To work around this 'turbo lag for braking' you can put the brakes on extra early to clean the rims of water and get into a habit of having your brakes ready on tricky descents, junctions etc.

You can run them just on the rear wheel first, in that way you can see how they work and use your existing rear pads on the front when the existing set run out.

Don't let their hum-drum appearance put you off - if you have steel rims they are a performance product.

If you want the in depth on 'coefficient of friction' as it applies to bike wheels and different materials, here you go:

http://books.google.co.uk/books?id=0JJo6DlF9iMC&lpg=PP1&pg=PA248#v=onepage&q&f=false

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The shop are correct. Steel or chrome plated steel rims will give poor braking power in wet conditions. Even under normal dry conditions braking power is considerably lower when compared to aluminium rims. You should expect a braking power improvement of up to 4 times by switching to aluminium rims.

From wikipedia:

Rim brakes are cheap, light, mechanically simple, easy to maintain, and powerful. However, they perform poorly when the rims are wet. This problem is less serious with rims made of aluminium, than on those with steel or chromed rims.

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Yes, aluminium rims help wet braking performance a lot compared to chrome plated steel rims. You still usually need roughly one rotation of the wheel for the brake block to wipe off most of the water film before you get really good braking, but there's still almost no comparison to steel rims.

One thing to keep in mind though: if you do a lot of riding in wet weather, the brakes can wear out aluminium rims fairly quickly. When the rims are wet, a lot of road grime tends to stick to them, and when the brakes clamp on that, it grinds down the surface of the rim relatively quickly. We're not talking about a ride or two (or anything like that) here, I've seen people who commute all the time wear out a set of rims in a couple of years or so (whereas I've rarely seen a fair-weather rider wear rims out at all -- they usually last until they either get bent in some kind of accident, or the rider gets the itch for something newer and/or lighter).

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