Interpreting Bike Fit Data

Your question seems rather straightforward: "...how do I interpret that into a frame size?"

It begs a simple answer: You practice. After you fit about 50 bicycles, the first 20 being not that great honestly, you'll begin to get the hang of it.

Bike fit and position isn't difficult, it's mostly common sense and experience. It takes a lot of fits to learn how things really fit and how they may be identical to, somewhat different than or utterly different than what a geometry chart or stack and reach table says. It takes more fits to learn how to help a person with a complaint about their position.

I'm a bike fitter who works in a bike shop. I've done a few thousand triathlon specific bike fits and about 250+ triathlons. One of the things I learned about fit and position- after doing a few hundred when I was convinced I knew what I was doing- was that I did not know. I also learned to importance of observation and listening to the person I was fitting. There is a lot to be gained from those two things.

It would be nice to think that fit and position are as easy as some measurements from a website and some calculations. Those are an excellent start point. They get you thinking about fit and position and they familiarize you with the terminology so when you do visit a good bike fitter in your area you will both speak and understand the same terms describing fit and position.

A couple points though: There are three fitters in our store. If I gave all three the same customer and the same measuring equipment they would likely come up with different dimensions for that customer. However, if they then translated those positions into a fit and then positioned their customer on that fit each of the three fitters work would be within millimeters and degrees of one another.

It isn't science. It's simply practice, and practice is simple. Do it over and over until you start to produce good, repeatable, functional results- understanding the dynamics of fit and positon and understanding your customers. It simply takes practice.

That's a funny little cat thingy doing push-ups you've got there.

Edited by Thomas Demerly 2008-09-22 10:28 PM

I hear ya. Firstly, I think test rides aren't necessarily a good tool to evaluate a bike for potential purchase. Here's why:

http://www.bikesportmichigan.com/store/testdrive.shtml

The majority of sales in our shop, overwhelmingly, are remote sales- sales other than through the door. That means phone sales (mostly) and internet sales. I think it is better if we can have a conversation with a fellow over the phone before we make decisions about what they would be best off buying.

Truth be told, selecting the right frame size and geometry isn't that difficult for a guy who understands both his product and his customer well. It boils down to a rather sophisticated process of elimination- eliminating sizes and geometries one by one until an optimal one remains. That is realtively easy to do over the phone. That's the good news.

The bad news is that positioning is very tough over the phone- I'd go so far as to say impossible. Technologies such as Darfish Motion Capture Software combined with good webcams are improving those capabilities, but by and large, they are a ways off. WE use motion capture in our store for positioning and will have the capability to do it remotely early next year. I think many shops will.

If you get a good feeling from the person you are working with over the phone- a sense of confidence in their recommendations, that they aren't just handing you a sales job and they are asking good, logical fit and position questions and questions about the equipment you;ve used and the events you intend to do then you have likely found a competent fitter. They can make solid recommendations over the phone. Be sure you "own" or understand why they are making recommendations- ask questions, understand why you are buying what you are buying so you have some "ownership" of the fit.

Here's another thing I wrote recently on buying on the web or over the phone:

http://www.bikesportmichigan.com/store/mailorder.shtml

Best of luck in your search My Friend.

Edited by Thomas Demerly 2008-09-22 11:32 PM