Monday, November 30, 2009

Bike Fit triangle

Bike fitting is a topic that evokes strong feelings amongst cyclists. Some think it is absolutely essential to have the perfect fit. They tend to think of that perfect fit as the one and only optimal solution. It exists somewhere in Plato's cave and it takes a real expert to discover it. Once discovered untold magical benefits will befall the fitted.

I am quite skeptical when it comes to fitting. I believe that humans are very adaptable and that they can get "used to" and perform well using a wide range of possible fittings. I know from personal experience that I did well on frames as small as 51 cm and as large as 61. I am 6'1'' or 182 cms tall and when I plug my measurements into a fit calculator the recommended frame size is always 58 cms (the old fashioned c-to-c seat tube or equivalent length). Currently I ride a 56 cm frame because I feel better in a slightly more upright position.

There is a way to bring some order to the chaos of fitting. Basically I view fitting as a compromise represented by a triangle of power, comfort, and aerodynamics. The optimal fit, or the range of optimal fits is somewhere in the inside of the triangle. Even though it is customary to think of the triangle as equilateral, the "exact shape" really depends on the person. In some people optimal power and optimal aero are close together, whereas in others optimal power is closer to optimal comfort. In most individuals however, we have a real triangle and serious compromises need to be made.

Furthermore, not the whole inside of the triangle is accessible to all. There are limits to how far an individual can go along the major axes. The range represents how adaptable a person is. Some people may never be able to get close to optimal power while in aero while others may never be aero while comfortable.

Boonen, road bike

Even in the textbook case of an equilateral triangle where the whole internal area is accessible, the optimal fit is not defined. It depends. It depends on the type of riding a person does and there are optimal fits for every riding experience. Many competitive cyclists will have two fits, one for regular racing and one for time trialing. They will use different bikes for these activities too.
Cancellara: road position

Cancellara TT position

If you predominantly race in packs, you probably want to bias your fit towards optimal power. Although aero is important, it is far less important in pack riding where you can draft. Comfort too can be compromised if the usual races are short 1-3 hour events. If this is your typical ride you will do best with a power fit.

If you predominantly ride alone and compete in long time trials or triathlon you will prefer a good aero position. You can give up some power to get there as the overall result will still be favorable. Experience has shown that nearly everyone has to give up power to get good aero. How much you should give up depends on how adaptable you are.

Finally, if you are an endurance cyclist and often ride in excess of 100 miles, you will value comfort more. Even ironman athletes will want to give up some aero and some power for comfort, especially when comfort means keeping the legs fresh for the run.

It also matters whether you prefer to climb or sprint on the flats, and whether you mainly ride on the track or on the road. Once again, you need not find a globally optimal solution and you can easily train two or three optimal but different positions. The only thing to keep in mind, is to do your race training using the position (and bike) you will ride in the race. Otherwise you may be in for a nasty surprise.

Today I rode 30 miles.

No comments: