Rim Weight

Posted by: Jan Heine Category: Hubs/rims, Testing and Tech

Rim Weight

At Compass Bicycles, we are not obsessive gram counters, but none of us want to carry extra weight on our bikes. A few grams here and there add up quickly to a couple of pounds, and that can make a difference not only in how the bike performs, but also how it feels when you ride it.
There are some components where you cannot save weight without undue compromises, and rims are a very good example. A well-designed clincher rim weighs about 450-500 g (650B) and 480-530 g (700C). To reduce this weight further, you only can remove material in four ways:

  • Thinner sidewalls. Example: 1.3 mm instead of 1.6 mm. Savings: 30 g.* The rim sidewall abrades as you brake. You need at least 0.7 mm of sidewall thickness to keep the tire from exploding. With the thinner sidewalls, your rim can lose only 0.6 mm until it is worn out, instead of 0.9 mm. That means your rim will last only 2/3 as long.
  • Narrower rim: Example: 20 mm instead of 23 mm. Savings: 24 g. On the down side, the rim no longer supports wide tires well.
  • Thinner rim “floor” (the side facing the hub): Example: 0.9 mm instead of 1.0 mm. Savings: 11.4 g. Now there is less material to counter the stresses imparted by the spokes, and the rim is more likely to crack.
  • Bottom of the well (the side facing the tire): Most rims already have the bare minimum here, so there are no further savings possible.
  • Rim shape: A more triangular shape can save material over a traditional box section, but the brake tracks are much shallower. You will have to adjust your brake pads frequently as they wear, otherwise they will cut into the tire (with sidepull and centerpull brakes) or dive under the rim (with cantilevers).

None of these weight saving options are very appealing. In the end, a good rim has a certain weight, and there is little you can do about it.
If you really must have a lighter rim, just ride it, and it will get lighter every time you brake. Of the two rims shown above, one is 70 grams lighter. Both rims started out from the same extrusion (Mavic MA-2/MA-40). The rim on the left is brand-new. The rim on the right has been used for a few years. The abrasion of the brake pads has removed 0.7 mm from each sidewall. The rim has lost 70 g of weight, but it is close to the limit where it becomes unsafe to use.
Even if you are willing to compromise durability and tire support to save weight, all the potential savings add up to only 65 g per rim. If you use a disc-only rim, you save another 30-40 g, but the separate brake disc weighs more than you save on the rim. (And using “disc-only” rims with rim brakes is a bad idea for obvious reasons: the brake tracks are worn out from the onset.)
Here are a few better ideas to reduce the weight of your wheels:

  • Folding tires: A Kevlar bead saves about 70 g per tire. Such a tire performs the same as one with a wire bead.
  • Superlight Tubes. On a 650B x 42 mm tire, using a superlight tube saves about 45 g. The down side is that you’ll have to inflate your tires every couple of weeks, as the air leaks out of the thinner tube more quickly.

Together, the weight savings from lighter tires and tubes are more than you’ll ever get by using a “pre-worn” rim. Light weight components have their place, but they should not come with undue compromises.
* The circumference of a 650B rim is 183 cm, the sidewall is about 1 cm tall. The aluminum removed by making the sidewall 0.03 cm thinner is 183 x 1 x 0.03 = 5.49 cm3. There are two sidewalls, so you remove 9.98 cm3. Aluminum has a specific density of 2.7 g/cm3, so the savings are 9.98 x 2.7 = 29.6 g.

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