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Wheel Stiffness

by John Neugent

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John Neugent probably knows more about bicycle wheels than anyone else alive. Maybe more about bikes as well. He's spent his life in the bike business, at every level. He now owns Neugent Cycling, a firm devoted to delivering world-class equipment at the lowest possible price. If you are in the market for a set of wheels, please, check out John's site. He really knows his stuff. —Chairman Bill

There are three types of wheel stiffness. Torsional—the force rotating the wheel, vertical—how the wheel absorbs vertical shock, and lateral—side to side. When we discuss wheel stiffness the vast majority of people are referring to the lateral stiffness.

Lateral stiffness is affected primarily by three factors—rim profile, spoke gauge, and spoke angle. There has been an evolution in the way we build wheels as materials evolved and the importance of aerodynamics is better understood. In the old days, a good set of custom wheels would most likely be built with a Mavic Open Pro rim and 32 or 36 double butted spokes. The rim is a relatively small box section but the number and gauge of the spokes makes up for any shortfall the rim lacks in stiffness.

Mavic Open Pro

Mavic Open Pro cross-section

As spoke and rim material evolved, larger section alloy rims because more popular. They are deeper and wider making them much stiffer. This also allows for the use of fewer and lighter gauge spokes.

AC30 rim

31mm deep AC 30 rim cross section

The popularity of carbon rims now takes that to an even greater level. Most of the pros race on 50 mm deep rims that are 25-28 mm wide. This not only makes for an incredibly stiff rim but also increases the angle at which the spoke exits the hub flange because the rim is much deeper.

C50 Open Tubular

C50 Open Tubular rim

In the old days double butted spokes were primarily 2.0x1.8x2.0. Spoke makers have now developed spokes that are much lighter and thinner. The older spokes were about 6 grams and it’s now possible to get light and thin spokes that weigh slightly more than 4 grams. They have also increased the quality to a point where most quality wheels have 44 spokes (20 in front and 24 in the back). The spoke weight alone had decreased from about (64X6) 384 grams to (44X4.1) 180 grams. Spokes also act like egg beaters in the wind so the elimination of 20 spokes is a significant aerodynamic advantage.

Spokes

Spoke design changes

Spoke gauge however, is only one of the factors in wheels' stiffness. The thicker the spoke the stiffer it is and as the spokes not only come down in count but also in thickness you can reach a point of no return. Many of the top carbon wheels are now coming with 20 spokes and riders are noticing that even with their brakes open they can experience wheel rub. Most attribute this to rim flex, but those big carbon rims hardly flex at all. What is happening is the low spoke count is allowing the hub to move in the center of the wheel. Tightening spokes does no good because spoke tension is not a factor in wheel stiffness. The solution is to add more spokes. That’s why some of the pro riders are using 24 spoke rear carbon wheels even though the store bought version has 20.

John Neugent was was one of the first to establish quality hand building in Taiwan around the turn of the century. He now owns Neugent Cycling, a firm devoted to delivering world-class equipment at the lowest possible price.