Using Body Measurements to Determine bike set-up

The numbers on this page come from the book 'Road Racing: Technique and Training' by Berhard Hinault and Claude Genzling.I found it very useful when specifying new bikes for unusually tall and unusually short riders who were unable to get a satisfactory position on their existing bikes.

I found that those in the middle ground were normally able to get a close enough position on a medium sized bike using stem and saddle adjustment to specifiy the right sized frame based on an existing one rather than purely by calculation.

Frame Size

An approximation for the correct frame size can be established from the length of the lower leg. Remember that frame size is not a limiting factor in setting saddle height - there are plenty of long seat posts on the market. Frame size limits the distance below the saddle that the handlebars can be set and, for off the peg frames, will probably determine the length of the top tube. For a time trialling bike it is worth going for an undersized frame.

Angle

An approximation of the correct angle can be established using the ratio of the thigh (femur)to the lower leg (Tibia). To get the orthodox equilibrium position the knee joint should be directly above the pedal axle with the cranks horizontal. Minor adjustments can be made by sliding the saddle on the rails. The table below gives an indication of the appropriate angle for different femur(F) to tibia(T) ratios.

Most off the peg frames have seat angles of 73 or 74 degrees which, with a suitable seatpost and saddle, caters for most needs. Again, the tallest and shortest riders face the biggest problems.

Reach

Reach is very much a matter of preference though too much difference from the norm can affect the saddle set back.
I’ve not seen a calculation or table of reach. The way to do it seems to consider Joe Normal with body ratios as follows:

• Trunk/inside leg = 0.76
• Arm/inside leg = 0.87
• Forearm/inside leg = 0.4

These are the relevant dimensions because they relate horizontal length (in the riding position) to saddle height.

This assumes a stem of approximately 1/5 the length of the top tube. Use these relationships to determine an approximation for reach.

This table gives Joe Normal figures. If your ratios are different you'll need to adjust accordingly. The reach figure is from the front of saddle to the centre of bars. Unfortunately this doesn't easily give the precise top tube length. If you know the frame size, saddle height, length of saddle, seat angle, head angle and the height difference between saddle and bars you can calculate it!

To use the information in the table to decide what frame size you need, or whether a frame is likely to fit, you must assume that these are for a 'square' frame, i.e. one which has seat tube and top tube the same length. If you are close to Joe Normal a frame sized according your inside or lower leg should be OK. If you are longer or shorter you might need a larger or smaller frame.

Establishing reach for tri-bars is largely experimental.

Saddle height

The magic starting number is 0.885 x Inside Leg. There are other formulae based on this to compensate for different crank lengths and pedal types (this figure is based on 172.5 cranks and Campagnolo Quill pedals!). This is a starting figure. It should put you within a couple of centimetres of your ideal position. The final adjustments are down to feel and expert eye.