Buyer's guide: bicycle geometry

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Buyer’s guide: bicycle geometry

A step-by-step guide to the tubes, angles, and profiles of the chassis of your next bike


Top-tube

The top-tube governs the length between the seat-tube and head tube and so has a significant impact, in tandem with seat angle, saddle position, and stem length, on the ‘reach’ of a bicycle (more of which later).

For Gribaudo, the length of the top-tube is closely related to the purpose of the bike: the way in which it will be used. “For a cyclo-sportive bike we generally have a shorter top-tube and a bit higher headtube just to be in a bit less aggressive position, with better balance between the height of the saddle and the height of the handlebar,” he says.

The top-tube, along with the seat-tube, is often the first item to be decided by the designer

“For a racing bike, usually riders like to have a very low handlebar or hand position. On  a competition road racing frame, we have a longer top tube and shorter head tube, just to have the position in the front lower and further to make sure they can really lay down on the bike to be as aerodynamic as possible.”

Olsen says that reducing the top tube length for so-called sportive bikes, likely to be ridden in a more upright position than the low, aerodynamic profile adopted by professional cyclists, is sensible “within reason”.

“A lot of bikes I think get it wrong, because they raise the headtube and shorten the top tube. You’ve basically got a double hit there. If you raise the headtube, the handlebars come up and back towards you, and that reduces the reach of the bike.”

The shape of the top tube is another important consideration. Its front end is subject to torsional forces from the handlebar, and various profiles are deployed in a bid to defeat flex in this area. Olsen has continued a tradition that dates as far back as the 1920s with FW Evans bikes, and been used more recently in the mountain bike sphere by Cotic, by deploying an ovalised profile for the top tube of steel and aluminium frames. “Having that wider top tube helps stablise it from side to side,” he says. Olsen distinguishes between stability and stiffness, however, and argues that much of the comfort of a frame comes from flex in the front triangle, rather than the rear.

The top-tube on many carbon bikes broadens en route to its junction with the headtube. Gribaudo, who works almost exclusively with carbon fibre, and whose designs including the Lapierre Xelius EFI and Aircode machines raced by the FDJ.fr WorldTour team, concurs with Olsen on the purpose of a broadened or ovalised top tube profile at the junction with the headtube, and also on the need for flex at the middle of tube. “The headtube in combination with the fork will help to absorb a lot of impact coming from the front wheel, so we try to always get a bit of flex in the top tube.”

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