Frame Fit

There are several frame geometry factors that should be taken into consideration when fitting a person to a bike as well as several components on a bike that affect the fit of the bike. First I am going to talk about the geometric factors then the components that affect bike fit.

Bill Boston has several articles on frame geometry and bike fit which start with an analysis of seat tube length, which he concludes with that the seat tube length is not a measurement of frame size but rather a the resulting dimension of the top tube height, bottom bracket drop and seat angle. His conclusion is a warning that you cannot size a bike by its seat tub length, which is what some bike manufacturers used to do.

Boston goes on to talk about top tube height saying that for road bikes there should be 3/8 of an inch to one inch of crotch, Downs agrees in his book and adds that for mountain bikes there should be one to two inches of crotch clearance. Boston explains the effects of higher and lower top tubes, saying that higher top tubes create a more upright riding position in addition to allowing a longer top tube and stem. The inverse is true too, that lower top tubes force the rider to hunch more and create a shorter tube length and stem length.

Next Boston talks about crank arm length and how it affects the fit of the bicycle. Generally speaking longer legged people should use longer cranks and short-legged people short cranks however there is more to crank length than leg comfort. Longer cranks allow for more leverage while shorter cranks give you less of a mechanical advantage. Also, as Boston points, out longer cranks generally require a shorter top tube or for the handlebars to be raised accordingly.

One last thing about frame fit that I’m going to mention from Boston’s articles is how seat tube angle affects riding. A shallow seat tube angle changes the rider’s sitting position, which can decrease pedaling efficiency.

Downs briefly talks about bike fit in his book covering stems position and seatpost height. Stems vary in length and angle, the shorter the stem the sharper the handling and the opposite is true as well, the longer the stem the softer the handling of the bike is. The angle of the stem affects seating, a stem with a large angle will have the rider seated more erect than a stem with no angle or a negative angle. Stems also can be raised or lowered on the steer tube, which also changes the seating position of the rider. Ideally the stem should be raised or lowered so that with the pedals leveled out and arms locked while griped on the breaks there is a 45 degree angle between the rider’s back and the bike’s top tube.

The seatpost can be raised or lowered to accommodate the height of the rider, also the saddle can be moved forward and backward to change knee angle. The ideal position for the seatpost is so that when seated there is a slight bend in the knee when at the bottom of the pedal stroke with the balls of the feet placed over the center of the pedals. Then the saddle rake should be adjusted forward or backward so that when the pedals are level a theoretical vertical line would be able to pass though the pedal axle to half an inch behind the kneecap.

• Anderson, Karl. "An Introduction to Bicycle Geometry and Handling.." dclxvi.org. N.p., n.d. Web. 17 Mar. 2010. .
• Boston, Bill. "Bicycle Fit." Cutting Edge Bicycle Technology. N.p., n.d. Web. 17 Mar. 2010. .
• Brown, Sheldon. "Revisionist Frame Sizing--by Sheldon Brown." Sheldon Brown-Bicycle Technical Information . N.p., n.d. Web. 17 Mar. 2010. .
• Downs, Todd. The Bicycling Guide to Complete Bicycle Maintenance and Repair: For Road and Mountain Bikes(Expanded and Revised 5th Edition). 5 Rev Exp ed. Emmaus, Pa.: Rodale Books, 2005. Print.
• Nicol, Scot. "Ibis Cycles, Inc :: Materials 101." Ibis Cycles. N.p., n.d. Web. 17 Mar. 2010. .
• Wilson, David Gordon. Bicycling Science, 3rd Edition. 3 ed. London: The Mit Press, 2004. Print.