12/22/2008

Dish = bad, off-center rim = good



On a typical multispeed bike, the width of the gear cluster causes rear wheel offset, or “dish”.

The above diagram is from Jobst Brandt's book The Bicycle Wheel, and shows that a much higher spoke tension is required for drive-side spokes to balance the lateral (side-to-side) forces on the rim. [Ignore the red lines for now.] The weight of the bike and rider are therefore supported almost entirely by the drive-side spokes, which effectively halves the number of spokes in the rear wheel. The diagram shows an old six-speed hub; the offset is even greater for current ten-speed hubs. This results in a wheel that is less sturdy and more prone to go out of true than a comparable front wheel.

In order to compensate for dish, some wheel designs move the non-drive hub flange closer to the center; this helps even out spoke tension but does not add lateral strength to the wheel. Other designs use twice as many drive-side spokes as non-drive-side, which requires a rim that can support the tension of three closely spaced spokes.

One clever solution to rear wheel dish is the off-center (or asymmetric) rim. The idea is simple: move the spoke holes to the left side of the rim. This allows for a rear wheel with both wide spacing between the hub flanges and equal spoke tension. In the diagram above, the red lines show what the spoke angles would look like if the rim had off-center spoke holes.

Mavic’s top-of-line, carbon-spoked R-Sys wheelset is built with an asymmetric rear rim. The shop has several of these wheelsets in the store and you ought to check them out, if only to marvel at their light weight (1355g the pair) and multiple high-tech features. A cross-section of the front and rear rim shows the asymmetric shape of the rear:

[One quick note: The voluntary recall of Mavic R-Sys wheels has nothing to do with its asymmetric rear rim. In fact, they're not even recalling the rear wheels. Whether carbon spokes are a good idea...well, that's an entirely different topic. 12jan2009]

In a slightly more traditional vein, Velocity USA offers two excellent off-center alloy rims. The Aerohead OC is a light (418g) semi-aerodynamic rim that is built for speed, while the Synergy OC is a burlier (490g), wider rim, perfect for the tourer, the clydesdale, or mountain biker.

Both rims come in a variety of sizes and spoke hole drillings, and can be purchased individually or in pre-built wheelsets. The guys at the shop can help you decide which options would best fit your needs.

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