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.

Tour of Missouri

So, I just flew home from the Tour of Missouri and boy are my arms tired. Ok, all kidding aside. My arms are tired, but that is because I was on moto duty the whole week. Riding around on the back of a moto carrying a set of wheels for six stages (I was along side the road for the TT) makes the arms a bit tired. I'll fill in with some more details later.

Musings from the jump seat of Mavic 2 (or How I spent my Labor Day weekend)

I got the call a while ago that I would be on the Mavic crew for the US Pro Championships weekend in Greenville. Labor Day weekend couldn't get here fast enough. I had some major news throw a kink in life early in the week, but by the weekend, I was ready to go.

I arrived on Friday, just in time to take care of some of the mundane tasks involved with the running of a neutral support crew. That involved washing the truck (the BIIIIG yellow one - fortunately, the trailer was staying at the hotel for the weekend so we didn't have to wash that), sorting the wheels (yes, all of them) and neutral bikes, and just making sure everything looked good for the sponsors. The first lesson I learned was to make sure you have plenty of clearance when jumping into the truck. I jumped in to grab some gear and BAM, caught a SRAM Force rear derailleur in the top of the head. OUCH, lesson learned and only minor bleeding.

Saturday was the time trial. We set up five pits along the circuit. I spent the day under a yellow tent watching the riders scream through a pretty hairy off-camber turn. I was sure someone was going to stack it in that corner, but everyone made it through safely. These are pros after all. However, you could still definitely tell who had spent some time training on their TT bikes. They were sailing through the corner in their aerobars and maintaining speed. The others...SKETCHY. Sitting up, grabbing the brakes, and changing lines several times through the turn.

We finished our official duties in time to head out for a ride. A group of 6 of us went and did the long loop of the road race. This included the ascent of Paris Mountain. Paris Mountain is a nasty little bump that gains about 1,000 feet over 3.5 miles. The pros would tackle it 4 times, we opted for only once and it HURT!!!!!!

Stay tuned for the road race report...