Paul Lew talks wheel dynamics
Reynolds’ tech guru explains all you need to know about carbon wheels
Paul Lew, Director of Technology & Innovation at Reynolds Cycling, has written a series of revealing articles on wheel dynamics that are available on the Reynolds website.
Part one deals with wheel stiffness.
“The qualifications for a ‘good’ wheel always incorporate stiffness and compliance, or more specifically, wheels that are laterally stiff and vertically compliant,” says Lew. “In engineering terms this means that the wheel has minimal deflection side-to-side and at the same time moderate to large deflection in the [up-and-] down direction.”
Lew says that the hub is responsible for 40% of a wheels stiffness and compliance.
“A well-designed and manufactured hub will not suffer from the static and dynamic forces of a high-tensioned wheel,” he adds. “A wide flange spacing decreases side-to-side deflection and at the same time increases vertical deflection, both contributing to a well-riding, ‘good’ wheel.”
According to Lew, spoke stiffness and spoke count are also responsible for 40% of wheel stiffness, but spoke crossing patterns and spoke tension don’t have a significant impact on this variable.
“Increasing the spoke count in a front wheel from 16 spokes to 20 spokes will increase the wheel's resistance to deflect laterally by approximately 30%. Increasing the spoke diameter and profile from a 2.0/1.8/1.5mm triple-butted spoke to a straight-gauge 2.0mm spoke will increase the wheel stiffness approximately 35%. The combination is additive, so increasing the spoke count from 16 to 20 spokes and the spoke diameter to a 2.0mm straight-gauge spoke will increase overall wheel stiffness by 65%.”
He says that the belief that a crossing spoke pattern will increase vertical deflection while minimising lateral deflection is incorrect; it will typically increase both.
The final component of the wheel, the rim, determines 20% of wheel stiffness, says Lew. The material used and the section aspect ratio both come into play here, the section aspect ratio being the more important.
A low aspect ratio rim – where the width dimension is greater than the height – will typically have minimal side-to-side deflection and moderate to high vertical deflection, but it won’t match a high aspect ratio rim – where the height is greater than the width – in terms of aerodynamics.
“Carbon fibre has the unique quality of low mass, stiffness tune-ability, high strength, and an unlimited fatigue life not possible to achieve with metal alloys which makes it an ideal material of choice for high performance rims and wheels,” says Lew.
In part two of the series, Lew again deals with spoking, comparing the stiffness of wheels with various spoke crossing patterns.
“The test revealed that a radial spoke pattern results in a less vertically compliant wheel than a crossing spoke pattern, but only by a very negligible amount,” Lew says. “I think most people would be surprised to see how little influence a crossing pattern has on vertical compliance and rider comfort. “
“Rider comfort and vertical deflection is best controlled by tyre pressure, not the spoke lacing pattern,” he concludes.
Lew deals with carbon-fibre variability in his third article, discussing the types of carbon-fibre and resins used in wheel manufacture.
“As a general rule, as the modulus increases, the strength decreases and the fibre is more brittle,” he says. “The grades of modulus that are associated with “best practices” for bicycle wheel production are Standard and Intermediate.”
He’ll be looking at the carbon-fibre moulding process in a future article so if you’re interested keep your eyes on www.reynoldscycling.com.