Remote tri bike fit
Written by: Dan Empfield
Date: Tue Nov 11 2008
The bike maker shipped this racer's new frame here, to be joined with the gruppo and the aerobars sent by the sponsors providing those parts. We built the bike in the Slowtwitch workshop, and when it was done I was pretty sure that it would fit its new owner nicely. I was confident his team's mechanics would only have to make minor modifications to cause the bike to fit. How did I know what his fit coordinates were, never having met this rider?
I thought I would share my process with Slowtwitch readers, as it could and should impact the normal course of business for both click and brick and mortal retailers. I built this bike and generated the correct cockpit distance, armrest elevation drop, saddle position versus the bottom bracket every positional element of this bike knowing only two of the rider's fit coordinates: his saddle height, and his overall height.
Of course, when the rider arrived, we went through almost two hours of fitting. During this session I "optimized" the rider at several seat angles, allowing him to self-select armrest elevations and cockpit distances. In essence, he chose his optimized position; I just helped him find it.
Yet I was confident my advance guess as the fit he would choose would be close to the mark. In fact, it was right on the mark, save one variance: I guessed in advance he'd choose during the fit process an armrest elevation that was 17cm below the saddle. During the fit session he settled on 18cm.
The set up I chose was based on a group of educated guesses and calculated risks. I will list them below. But I must also say that bike racers especially tall ones, such as this 6'2" rider I had in front of me are in several ways easier to position, and I'll explain why that is.
There are three basic assumptions I make when setting up a rider's bike assumptions I always make for a fit, trim, experienced aerobar rider. The first such assumption is that he'll choose to sit at 79 degrees of seat angle, such angle measured through the bottom bracket and the center of the saddle's rails. I don't care what the seat angle of the bike is, I just position the saddle fore/aft so that the saddle rail center passes through that slope.
Of course, not everyone wants to sit at 79 degrees. Some want to sit steeper, others shallower. I'm playing the odds. I know that this is the mean among those who are positioned correctly, and if they choose an angle other than 79 degrees it's going to be 80 or 78. That range, 78 to 80 degrees, measured how I just described, applies to 8 or 9 out of every ten riders who ought to be on a tri bike.
Then there are those who race triathlons, but who ought not to be on a tri bike, rather they ought to be on a road bike, set up as a road bike. That's a discussion for another time.
The second guess I made is that this rider will choose a cockpit distance exactly the same as his saddle height. The distance from the nose of the saddle to the ends of the aerobars (not including the bar-end shifters) would be, I wagered, 82cm long. I reckoned this because the rider's saddle height, bottom bracket to saddle-top, was 82cm. This is probably the first time you've heard of this symmetry between cockpit and saddle height. It's not something I generally teach, because it's a rough guess and can be off by several centimeters. It is subject to where one sits on the saddle, as well as by one's morphology: A rider short in the torso will have a shorter cockpit than saddle height; a short-legged rider with a long torso will have a cockpit distance longer than the saddle height.
This is why I asked this pro cyclist both for his saddle height as well as his overall height, so I could gauge whether he was long- or short-waisted.
For armrest drop, I applied a quadratic equation that has served me well over the years: saddle height squared X .005 (minus) saddie height X .1925 (this latter number is a coefficient that varies with seat angle and .1925 is indicated for a seat angle of 78 degrees). This yields 18cm worth of drop. This is another guess. It's based on an ever-growing dataset, and would be adjusted if and when the data indicates. I chose to place the aerobars at 17cm of drop, because my instinct told me to select a drop slightly less aggressive. In fact, my instinct was wrong and my formula right: he chose 18cm.
So there you have it. But, I had a specific problem with this fellow: the UCI rules he must follow. His saddle nose was set about 2cm behind the bottom bracket. But we needed to be 5cm behind because of rule 1.3.013 of the UCI rulebook. So I moved the saddle rearward, to 5cm behind the BB. But I did not shorten the cockpit to match, because I didn't mean for the rider to sit further back, just for his saddle to sit further back. When I position bike racers, I initially do so without any consideration of the rules. Once the optimized position is achieved, then we pull the saddle back, and the rider simply nose rides (probably in a way he's never contemplated nose riding before).
This means that the cockpit distance is now no longer 82cm, but 85cm. This does not mean the rider is "stretched out," because he's still sitting with that happy right angle formed by his upper arm and a line down his torso the angle you'd expect to find with a proper cockpit distance. The rider would no-doubt prefer to have more of the saddle underneath him, but he can't, as long as the UCI's rules prevent it.
In a happy circumstance, this coincided with the second UCI rule I needed to consider, 1.3.023 of the UCI rulebook: bottom bracket to handlebar tip. But first, in anticipation for this, I asked my rider if he routinely asks for a morphological exception. He indicated that he did. It is typical for UCI commissaires to disregard the rules as they are written in the rulebook, and to simply grant or not grant an "ME" based on a rider's height. If he's short, he gets the saddle height ME. If he's tall as my rider was he gets the bar-tip ME, which means the ends of the aerobars can extend 80cm in front of the bottom bracket, instead of the 75cm allowed were the ME not granted.
The rider's cockpit distance, now 85cm, just works, because the saddle nose is 5cm behind the BB, yielding the max 80cm allowed by rule 1.3.023.
In Part II of this article I'll describe how the above methodology impacts, or ought to impact, tri bike manufacturers and the retailers who sell them.
In the second and final part of this series, we look at how remote bike fitting aids the retailer who'll invest the time, and thought, and energy to the display of the tri bikes he inventories. 12.01.08
There are three ways to ride a bike: road race (the way pros do it); triathlon (the way pros do it); and that strange morphing of the two which we call the "tweener." 12.29.08
This article exists as part of a series of eleven on tri bike fit, and describes the proper armrest drop for a triathlete; how to measure it; how to determine it. 9.13.07
This article describes how we find a rider's proper cockpit length. 9.13.07
Slowtwitch means different things to different athletes, but tri bike fit is the thing we're probably best known for. Herein is a description of our process for tri bike fit. 9.13.07
Accommodating UCI regs
Masters TT rider
Reviewed by: Donald Gillmore, Dec 28 2009 6:47PM
"Good" bike fitters are NOT over rated.
Reviewed by: JT Lyons, Jan 5 2009 3:39PM
At the same time, I've seen many fitters that are not "good" charge the same amount, or not charge anything and sell a bike with a "good" free fit.... it's not the same... not even close.
Are bike fitters overated?
Reviewed by: Chad Walton, Nov 19 2008 4:43PM
[Editor's note: Yaqui bikes are built by Ves Mandaric. He was my master builder when I owned Quintana Roo. He builds the Exit Cycling fit bike that I use. He and I make all the up-to-date tools that measure stack and reach. It's no accident that your Yaqui fits you well. But it's still a static fit. It's like sending you a pair of running shoes based on the length and width of your foot, with no knowledge of how you run. It's functional, but not ideal.