Monday, May 01, 2006

Some Thoughts on Wheels

I haven’t finished the Wheelbuilding page on the website yet, but I do indeed offer a wheelbuilding service. While waiting, here are some basics to ponder.

Ride Quality – Many folks describe wheels are riding harshly or softly yet bicycle wheels offer very little vertical deflection. What deflection exists is masked by the much greater deflection of tires (even when pumped to very high pressures), fork, stem, seatpost, handlebars, and saddle. Still, these folks perceive something. Where does it come from?

Perception is a tricky thing. Our mind can often be tricked into perceiving something that isn’t there. A good example is a taste test conducted with unflavored gelatin that is colored red. Despite the fact that the gelatin has no flavor, most test subjects report that it has a cherry or strawberry taste. The red color has created such a strong expectation of flavor, the mind perceives that flavor despite what the tongue actually tastes.

A wheel that has a deep section aero rim might create the expectation of a harsh ride. Similarly, a carbon wheel that has a unique sound when ridden may color someone’s perception of ride.

We can see this in actual test data. Francois Grignon tested the vertical deflection of various wheels and found that a wheel that is consistently regarded as harsh riding, like the Specialized tri-spoke, actually has a relatively low radial stiffness. Deep section rim wheels like the Mavic Cosmic and Campagnolo Shamal are slightly stiffer but still far from the stiffness of a conventional wheel built with 36 spokes and a Mavic GL330 rim. See details of the test here.

In Damon Rinard’s test of lateral stiffness of various wheels, you can see a similar phenomenon. In general, wheels with more spokes are stiffer than those that have fewer spokes. While stiffness does not necessarily indicate durability, lateral stiffness is generally considered to be desirable. A wheel that is stiffer laterally will provide more stable handling with a heavy load. While this is not critical for racers, since they are often lightweight and carry no gear, it might be an important consideration for touring cyclists.

Spoke Selection – Rinard’s test data show that lateral wheel stiffness is affected by the number of spokes, spoke thickness, rim weight and height, and hub flange spacing. So, having more spokes will produce a stiffer wheel, but using a heavier rim might allow you to maintain that stiffness with fewer spokes. Wheel durability generally increases with the number of spokes, and a broken spoke will have less effect on a wheel with more spokes than a wheel with less.

Still, the answer is not always to build 48-spoke wheels. The number of spokes must be balanced with the application and type of wheel being built. The same is true for spoke thickness. I always build with swaged (butted) spokes since the cost is only slightly more than straight gauge spokes while wheel durability is increased. Spokes that are swaged have a thinner middle section, and the ability for the middle of the spoke to flex in the middle will move the stress from the elbow and threads, the most common failure points.

I generally use 2.0/1.8 mm or 1.8/1.6 mm spokes. If the difference between the thickness of the spoke ends and spoke middle is too great, as on 2.0/1.5 mm spokes, it will be too difficult to bring the spoke up to proper tension. Achieving high tension on spokes is dependent on keeping the nipple turning on the spoke threads instead of twisting the spoke. With a large differential in spoke thickness, the middle of the spoke will begin to twist at an unsuitably low tension. The nipple will not turn any further on the spoke unless the tension is relieved by pressing inward on the rim. There are commercial fixtures available to do this, but these do not help the home mechanic when the wheel needs retruing.

Can a reliable wheel be built with 2.0/1.5 mm spokes? Yes, if more spokes are used to compensate for the lower tension. For example, using 36 2.0/1.5 mm spokes might result in a wheel that is as durable as a 32-spoke wheel built with 1.8/1.6 mm spokes. The 32-spoke wheel will be just as light so there is no apparent advantage to using the 2.0/1.5 mm spokes.

In fact, there is likely a disadvantage. When spokes are not tensioned high enough or if not enough spokes are used, a spoke can become completely untensioned under load. When a spoke is untensioned, the nipple can turn resulting in an out-of-true wheel. So, while the wheel might be durable, it may require more frequent retruing.

To be continued . . .

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