An admitted shoe geek waxes philosophical about running, triathlon, and life in general.
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Friday, October 5, 2012

Tri Bike Fit

A while back I wrote a generalized piece on the basics of road bike fit, and how it's built from back to front.

Tri bike fit has a different goal, and because of this, it has a different method of fitting, and is built completely backwards to that of a road bike fit. Yup, it's built from front to back.

While road bike fit is about finding a weight balance on the bike, tri bike fit is about minimizing aerodynamic drag while keeping as much power transfer as possible, while balancing the limitations of flexibility and comfort. I'll get into weight balance in another post, as it feeds into frame geometry and how the bike handles as well.

Articles and diagrams on SlowTwitch will often cite some ideal body angles saying that it is a classic triathlon fit. People will often regurgitate these angles when some one asks for a fit critique, overlaying them onto the picture of the person on the bike. And while this is typically a proper answer (ignoring the person's flexibility and physiology limitations), they don't go into WHY these angles are the classic fit.

There are a series of considerations to take into account when developing your position: 
* Shoulder flexibility 
* Upper back flexibility 
* Neck flexibility 
* Hip angle for maximum power generation

I'll take you through my own "front to back" fit process and where the limitations come into play.

To start, put your hands up like you're a boxer protecting your face. They're probably very close together, maybe touching (good), elbows at about shoulder width, hands about level with your mouth. Or at least they should be. How raise them up until they're about level with your forehead. Does this cause any tension in the lats or posterior deltoids? Any pain? If it does, then you're bumping up against one of your potential limitations. You want your elbows very close to directly under your shoulders (relative to forward/aft) in the aero position so that your skeleton is supporting your upper body weight, not your muscles.

Now start bringing your elbows together. If you can get them to touch each other without having to PULL them together with your pectorals (chest muscles) then congrats -- you're one of the lucky few that can really use an extreme parallel and narrow hand and elbow position on the bike. For the rest of us, that point where it starts taking some effort to get the elbows together is the limit for how narrow we should go with the aero-bar pads.

Next comes the neck. Or, more appropriately, your ability to see up the road. The best aero position in the world does you no good if you can't see what you're about to run into (or over). So rest those elbows on the desk in front of you, with your hands relaxed ahead (be careful of what you press on the keyboard -- maybe you should move it out of the way), and start standing up out of your chair. Still able to read this? Keep going. Don't strain the neck. Keep your chin low and forward...

Okay, at some point you needed to start raising your eyebrows to see. When this happens, you've passed the point of your saddle height relative to the bar pads. Some people will have no trouble with a saddle height that is WELL above the aero bar pads. Some people need to have the pads above the saddle (this is not to say that it can't be optimized, as getting the hands and arms narrow will do near 80% of the job). The neck is why -- the ability to "turtle" (keeping the chin low and forward) and see up the road is the limitation.

People often seek a more aggressive position, wanting to get lower in front. I'd suggest that it's a backwards thought process brought about by trying to fit a person to a bike, rather than fitting a bike to a person -- it's more about getting higher in back relative to the bars by strengthening the neck and increasing flexibility. But when one is looking at a bike already in the stable, I can see how this "lower in front" concept comes about.

So now that the front end is determined, and the relative saddle height, we can look at the hips. This is going to determine the effective seat tube angle at which the rider will set up the bike. Set up all those front-end angles and pads-to-saddle differential on a fit bike...

Start with something fairly standard for a road bike seat tube angle, say 74 degrees (which is on the slightly steep end of standard). Do you need to splay out your knees so that they aren't hitting your chest (or other body parts)? Then move the bottom bracket back.

What? Move the bottom bracket back? YES! The converse of that is to move everything forward. But those points in space have already been determined, so move the bottom bracket backwards.

You will find a point as the bottom bracket moves backwards where the knees track straight, power is at a peak, breathing is easy, and you're not working against your hamstrings. Now add about two degrees. Really. You're going to find that you'll creep forward on the saddle anyway in your aero position, so bring it under you now. If you have a road bike that's dialed in, you'll find that the hip angle will be pretty close to the same on a tri fit.

Now you have all the points in space that you need to find the bike that fits your position. If you're trying to fit this onto a bike which you already own, you may find that it becomes un-rideable. This is a sign that the bike is just not configured to fit your best position. Meaning you can do one of two things: compromise your position, or get a new bike.

"But what about aerodynamics?" you ask. These things will get you there -- or within 95% of "there" -- trust me. Yes, you could spend a lot of money to get yourself into a wind tunnel and save a few more grams of drag, and likely all it will do is make your wallet lighter and your position less comfortable. If you're only doing short races, then it may be money well spent as long as it's "comfortable enough" for the time you'll be on the bike. If you need to spend 5 to 6 hours or more on your bike in an 140.6, compromising that comfort may be a day-ender.

So why are those classic SlowTwitch angles so classic? Because they're a boiling-down of this process over hundreds of riders -- an average of a bunch of positions that went through the trial-and-error process to arrive at a position that takes the above into account.

I'll go into the fit of the bike to your position in another post very soon, as it brings frame geometry and stability into play. I'll also chronicle how I landed on the bike I ride now.

My own position has evolved over some 25-plus years of riding, learning along the way. It's very compact, with an elbow and shoulder angles that is tighter than a the "classic" position, and my hands are higher. I wouldn't use this position for a long-course race, but my racing is at the Olympic distance and lower. I don't put up my position as an example of what any one else should strive toward, as it's based on my own comfort (for the distances I'm racing), flexibility, and limitations.

1 comment:

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