's Forum Archives - General

Archive Home >> General(1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 )

formulas for bike fit(4 posts)

formulas for bike fitpina
Jan 27, 2003 7:06 PM
Trying to get estimate for my wife's road bike size. Know the lemond formula, any other's better in your opinion? The lemond formula doesn't work for me with long torso and short legs.
Try theseScot_Gore
Jan 27, 2003 7:43 PM
Cannondales never fit anyone!!!!!philthy underwear
Jan 27, 2003 9:21 PM
don't need formulas...C-40
Jan 28, 2003 9:43 AM
All you need to establish the vertical size of the frame is an accurate inseam measurement. A real bike with a horizontal top tube makes a great inseam measuring tool. Find one small enough to standover with some amount of clearance and block up the wheels until you get saddle-like crotch pressure, in bare feet. Measure from the floor to the top of the top tube to get an accurate inseam measurement.

The standover height of the frame should be 2-4cm less than your measured inseam. Tall riders might tolerate up to 6cm of standover clearance. Long torso riders may need to stay close to the 2cm mark to get sufficient TT length.

Formulas and body measurements used to predict top tube length, stem length and total reach are pretty worthless. These measurements are inherently inaccurate.

A fit bike or any regular bike with know geometry that's close to the needed size will be a much more accurate indicator of the appropriate frame geometry.

You should strive to get a frame with a STA that allows the saddle to be nearly centered on the seatpost with the knee directly over the pedal spindle. This will permit the largest amount of adjustment for future fine-tuning of the saddle fore-aft position. Sometimes a change in the seatpost style will help achieve this. For reference, each degree of STA will move the saddle about 1.2cm.

The appropriate TT length will allow a comfortable position without the use of an excessively short or long stem.

The geometry variations between different brands are generally pretty small, with the one exception of LOOK frames. The majority of LOOK frames use a 72.5 degree seat tube angle even on small frames, which shortens their effective top tube length. If you need a short top tube, LOOK might be the answer. Most of the rest will have TT lengths that rarely vary more than 1cm.