[CR]Tyre aerodynamics, ..how to get indisputable data

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From: "Norris Lockley" <norris.lockley@talktalk.net>
To: classicrendezvous@bikelist.org
Date: Tue, 25 Sep 2007 00:24:52 +0100
Subject: [CR]Tyre aerodynamics, ..how to get indisputable data

I have to pin my colours on the mast, right up front here, and admit tha t I really don't know much at all about tyre aerodynamics - the subject bei ng pretty irrelevant to my riding as I never manage to go fast enough to fe el any difference, but I do know a bit about the aerodynamics of wheels and quite a lot more about Chris Boardman's escapades at the Barcelona Olympic s in 1992 and his World Hour Record ride at Bordeaux in 1993. Needless to s ay, but these bits of knowledge are interrelated.

Before contributing my ten-pennyworth I thought it wise to check out a f ew points with "The Fastest Man on Two Wheeels", the account by Phil Ligget t of Chris' career up to 1994 and including both of Chris' inspiring rides, and also with a TV documentary about the two UK record breakers of t hat era, Chris, and Graham Obree.

I was on the sidelines of the development of the Lotus, very much in the background, not really involved but observing the deroulement. With the Re cord attempt I was very involved in that I was responsible for getting Chri s on board that beautiful yellow Corima. Liggett's account of the Lotus is embroidered in some ways, and the tests on the bike were not quite as profe ssional as it might be thought, bearing in mind the company that was carryi ng them out. Although wind-tunnel tests were carried out they were no t dynamic, and they did very little to improve the aerodynamics of the Lotu s or of the Mavic 3-spoke wheels used, but they did help to improve e xtensively Chris' own position on the bike.

When Chris got linked up with CORIMA, the team down there in Loriol, led by Christian Pascaud, realised that Chris' position had been pretty well o ptimised and that their job would be to test which of their wheels would be the most aerodynamic and efficient over the one-hour ride. Chris knew by t his time just how low he could get down on the bike, and the peculiar flat bars were one of his stipulations. He provided the drawing, with the vital& nbsp;measurements from which CORIMA developed the shape of the frame a nd bars.

CORIMA is only a small company but one with extensive connections in the aerospace industry and Government contracts. They arranged for the wheels to be tested dynamically and in doing so were able to measure airflow, drag , turbulence and energy consumption. Chris had expressed a wish to use two disc wheels, but the tests showed that the 4-spoke deep-sectioned wheels ou t-performed the discs in all departments for a record ride involving consta nt high speeds over a long duration.

The results, and I have the maths somewhere, showed that a deeper r im gives better penetration of the air and less turbulence, and that the fe wer spokes the better as turbulence decreases as the number of spokes decre ases. The deeper rim also helps to stiffen the wheel as the number of spoke s is reduced.

As for tyres, the tests also showed that below 23mm there was a reductio n in drag, given the same pressures, but that the tendancy towards discomfo rt as the tyre became thinner, was counter-productive. For that reason it i s very seldom that European Pro riders will use tyres or tubs less than 23m m wide. On a number of occasions I rode in the women's Tour de Fr ance in a team car and was involved with putting the tyres, clinchers in th is case, on the wheels. On one particular day I was in the lead car, in a t ime-trial, behind the French woman , Catherine Marsal,who was at the t ime both World Champion and also race leader. Thinking that I could give he r a little bit of extra "edge" over her rivals if I shod her wheels wi th some 19 or 21mm tyres I set about in my best British time-trial fas hion, only to be pulled up short by the chief mechanic who was quite appall ed at what I was trying to do. "Jamais!" (Never!" He claimed that the the h arshness and lack of comfort of the thinner typres would in fact detract fr om Catherine's performance resulting in a slower time.

I feel certain that some company such as Michelin, Continental, Vittoria etc must have a formula, something on the lines of those tangent, cotangen t ones, that would allow a cyclist to permutate the variables to best effec t.

I've just realised that this contri could be judged by Dale to be "off-t opic" ie post 1983, but I reckon that the aerodynamics etc being discu ssed have been of importance over a far longer time-scale.

Norris Lockley, Settle.UK where I have just learned that I can buy a clincher tyre ( I hate that word) that rolls up just like my tubulars

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