An admitted shoe geek waxes philosophical about running, triathlon, and life in general.
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Monday, April 25, 2011

Road bike fit

In my last poll, I asked about what bike aspect was most mysterious. Of the six votes cast, two were under "fit". Now this is a big subject, and depends on the type of fit sought.

I'm not trying to take the place of a professional fitting, and I'm not an expert on Fit Kit or other fitting methods. I'll be talking about generalities and goals of bike fit.

The main difference between a road fit and a tri fit is the emphasis on aerodynamics. In essence, the positions are pretty similar -- the tri fit is just a road fit that's been rotated forward about the bottom bracket. But the positions are built from opposite ends. Road bike fits are made from back to front, and tri fits are made from front to back.

Firstly, a road bike is built to be stable at speed. "At speed" will generally mean anything north of about 10 mph. There are a lot pieces of the bike's geometry that go into that stability, but the end result is that the bike wants to keep going in a straight line, unless you do something to it. Part of the design of the bike is the distribution of the rider's weight between the two wheels. They're designed to be best with about 45% of the rider and bike combined weight on the front wheel. This is why, when triathlon was in its infancy, and we were all using various methods to "get low and forward" on our bikes, they got all kinds of twitchy, getting a mind of their own. Too much weight was going onto the front wheel on a bike that wasn't designed for it.

But aside from the bike's stability, there is also the comfort of the rider. I'll ignore the fact that, for a new rider, a bike saddle is NOT going to be comfortable. There are some things to which we just have to get acclimated. But one of the big things that new riders struggle with is the hands. Numbness, tingling, tired arms. This is NOT something to which one just gets acclimated! In fact, if you try to put up with this for too long, things like carpal tunnel syndrome can result. The goal of a road fit is first to get the balance at the back so there isn't so much weight on the hands. How is this accomplished? By sitting back far enough, and having the core strength to hold your upper body. Something that helps with this is the amount of force generated by the legs on the primary power phase (pushing down on the pedals), as this gives part of the moment (one of those physics/engineering terms that just means a force that tends to rotate something around an axis) that holds the body up. But most new riders won't have the specific strength to produce this force in abundance for very long. It takes time to build up. Don't worry, it'll come.

So back to the idea of having your weight back... Think of squatting down into a sitting position. The farther you lean forward, the more of a tendency there is to fall forward. Same thing on the bike. The saddle should be back far enough that the weight on the hands is minimized. You should almost be able to take the hands off the handlebars and not fall forward. As experience and bike-specific strength develops, the position can come forward if desired. Also, sometimes physical weight distribution of the rider will change over time... Periodic re-evaluations aren't such a bad thing.

Once this point is established, then comes body lean. This is determined by two things -- comfort and flexibility. Aerodynamically, you want to lean forward as far as you can. But with the weight pushed back, you're likely not going to get your back level with the ground. Kudos to you if you've got the gluteal flexibility to make that happen, and hopefully your neck will be able to handle the hyperextension... Anyway, you want to make sure you're not going to the limits of your range of motion, but also it's surprising just how close you can get when you're not dealing with eccentric muscle contractions. The forward body lean can affect that forward/aft weight distribution, so sometimes adjustments need to be made there to accommodate. This is a little bit of an iterative process.

Next comes the location of the handlebars. I'll leave width alone, as that is mostly just a matter of what the rider likes (I've got everything from 36 cm on my track bike to 44 cm on my commuter). But you want the bars where they can easily be reached in all hand positions (tops, hoods, hooks, drops), and not over extend the elbows or rotate the shoulders too far out. The hands should be able to rest in a relaxed manner on the hoods, with a slight bend at the elbows. Going to the bar drops, there is more body lean involved, but not so much that the legs have to splay outward to avoid hitting the stomach (whatever size it happens to be).

The best approach is to get all this done BEFORE you get the bike, not only so you know what size bike you need, but which bikes will fit. Some bikes are made with very slack seat tube angles, which will more easily accommodate a rearwards seat position if needed, and not have so long of a top tube that you can get the handlebars in the correct location for you. We just went through this with my wife, and she started with a professional fitting done at Sammamish Valley Cycle. From there, we could narrow down the field to those bikes which would fit her. That's not to say that whatever bike you have can't be adjusted to fit -- seatposts and stems are some of the least expensive bike components out there (as long as you're not emotionally attached to carbon fiber).

So as you can see, the idea of "reaching the ground, and handlebars obscuring the front hub" method of fitting a bike really doesn't cut it. But it's not a black art, either. The goals are pretty simple -- sit on the bike such that you can be mostly relaxed, and the pedaling movement aids in this comfort.All this so you can ride more and get acclimated to that saddle... :^)

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