Devising a static tri-fit system

June 29, 2009

Static versus dynamic fit systems

What do I mean by a static fit system? It's where you determine your bike position solely by morphology, not by riding style.

Here's another way to look at it. Using a dynamic fit system, you're required to ride a bike in order to learn how you best fit aboard it. Using a static fit system, all that's needed are measurements of a few body lengths and levers.

If you want to know your seat height, using a static method you only need to know your trochanter height. Or, your inseam. Famous is Lemond's formula: seat height = .883 * inseam.

What is the dynamic analog to this? It's to determine your seat height based on your knee angle at the bottom of your pedal stroke. You might sit aboard a fit bike, or you might just sit aboard your own bike as it's fixed into a trainer. Your saddle is moved up and down, and a measuring device determines your knee angle you form at "bottom dead center" when you ride.

The dynamic system has this over a static fit system: it allows you the flexibility to ride with a toe-pointer pedal stroke, or a heel-dropper pedal stroke, or anywhere in between. In other words, if the Lemond method assumes everyone is going to ride heel-dropper style, then if you're a toe pointer and your saddle height is achieved using the Lemond method, your saddle is going to be positioned too low, isn't it?

Let's consider how we choose running shoes. Is choosing a shoe only about how long and wide your foot is? Does it not matter how you run? Whether you're a midfoot striker or you tend to strike more fore or aft? Whether you're heavy or light, and how that downward pressure affects your contact points?

This is the typical drawback with static fit systems: They assume you're going to ride your bike in a certain way.

Static road versus static tri

Unless you feel compelled to mandate "no toe pointing" you've got to take one's pedal dynamics into consideration when determining saddle height. You've got too many variables in the equation. Until you determine ankle angle (or plantar angle) at bottom dead center, you can't possibly determine seat height based only on inseam.

Otherwise, though, a static method for fitting one up for a road bike is not necessarily a bad way to go, if you stipulate that your static method establishes a starting point for your rider, not an ending point. Once your rider starts pedaling a bike, he'll immediately (or quite soon) begin to think about saddle pitch, saddle fore/aft, cockpit distance, and so forth. You can only learn so much about how a person interfaces with his or her bike prior to the actual riding of the bike.

On a road race bike there is one spot on which your weight can be supported skeletally: on the saddle. As you move forward, your body weight shifts to the front of the bike, and rests on the handlebars. This weight is supported muscularly, and you can't do that for very long. For this reason, you're pretty much anchored into one possible, sustainable, position. This is why a static method of bike fit works with some utility (or grants, at least, a decent starting point) for road bike fit.

On a tri bike, you're suspending your weight skeletally in two places: the saddle, and the aero bars. You can rest your weight skeletally in both spots. For this reason, you have more fore/aft options available to you. Where do you want to sit? With the saddle at 73° of seat angle, as is the case on your road bike? How 'bout 76°? Or 78°? Anyone care for 79°? What about 80°, or even 81°? All are possible.

No static measure exists that'll tell you where you'll prefer to sit along this fore-aft gradient. This is one reason why static measures are less reliable when dealing with tri bike fit, even if you're simply using it to establish a "starter fit."

The Static method for tri bike fit

Nevertheless, there is a way to devise a static method for tri bike fit. Why do this? One use is for mail order companies that ship tri bikes to those who don't have good fitters proximate to them. Second, this system may give you a broad sense of whether your fit is in the ballpark of a position typical among good athletes and their bikes. Finally, as we describe the system we'll describe it's shortcomings, in order to champion dynamic systems.

To use this static fit sytstem you'll have to agree to some assumptions. What we're trying to derive are fit coordinates. In road cycling, the most important are the "core four": Saddle height, saddle setback (from bottom bracket), saddle nose to handlebar clamp, and elevation drop from saddle top to road bar top. Know these four coordinates, you'll know how it is you fit aboard your road race bike.

Tri bikes start the same same way. You need to determine saddle height, and saddle setback. We need elevation, but from saddle top to aerobar pad top. Finally, yes, we need cockpit distance, but there are two measures: saddle nose to armrest back, and saddle nose to aerobar extension end, specifically to the shifter pivot bolt.

Saddle height

Our system of tri bike fit upon which we rely uses a dynamic fit protocol. If you want to use a static method to determine the 5 coordinates above, fine. The Lemond method is as good as any: .883 x inseam. Or if think tri bike saddle heights should be slightly higher than road bike saddle heights, how 'bout .885? Or whatever floats your boat.

Saddle setback

This is the stickiest part of trying to squeeze tri bike fit into a static fit system. Where do you want to ride? At 77° of seat angle? Or 80°? I have never found a good way of determining that, absent optimizing a rider's position at various seat angles and, using his own feedback and my own sense by observing his cadence, power and body language. If you employ a static system of measure, you're telling the rider what seat angle he ought to ride, rather than letting him tell you what seat angle feels best for him.

What I can do is play the odds. If you measure seat angle through the bottom bracket and the center of the saddle's rails, the median seat angle preferred by those whom I optimize at a range of angles is 79.5°. This, after some thousands of trials. So, there you go. That's the angle. If you're going to use a static system, everybody gets set up at 79.5°.

You can figure this with a SmartTool (a digital level that measures inclines), or with a chart that backs out the X/Y coordinates. In that latter case, you might say that, at 80cm of saddle height, the rider is at 79.5° of seat angle with the nose of the saddle even with the bottom bracket (I'm just raw guessing, I haven't worked it out). But at 72cm of saddle height, the saddle might have to be 2.5cm in front of the BB to give you that same 79.5° seat angle.

In a future article, I'll create this chart, though be forewarned: Profile Design's TriStryke saddle measures 21.5cm from rail-center to nose. Other standard saddles measure as little as 19cm, and the ISM Adamo measures well less than that. So, any chart I, or another, provides will be for a particular saddle or set of saddles, and you'll have to "normalize" for your saddle. (All this will be explained in more detail in the article accompanying the forthcoming chart.)

Cockpit distance

This is a function of your torso length. In other static systems, it's measured "crotch to notch": where the inseam stops to the depression just below your adam's apple, just above your clavicle.

But we aren't measuring that way. We're taking as axiomatic a truism we've often found, that a rider of average proportion, fit properly, will have a cockpit distance equal to his saddle height. So, that distance from BB axle to the top of the saddle (halfway between the saddle's fore and aft) will equal the distance from the saddle nose to the pivot bolt on the bar end shifter.

But, on a longer torso'd person the cockpit distance would be longer than the saddle height; on a long-legged person the saddle height would be longer than the cockpit distance. We consider the saddle height as a function of your overall height, determine whether you're leg-long or torso-long, and calculate your cockpit based on saddle height and this leg:torso ratio. The equation we've derived is this:

E = .72H – .67D

E = cockpit
H = overall rider height
D = saddle height

Armrest elevation

This we've gone over before. It looks nasty, nevertheless, here is the equation:

C = .005D2 + .0075(seat°)D – .7775D

C = armrest drop
D = seat height

We're saying here that armrest drop is a function of seat height and seat angle. The taller the saddle, the more armrest elevation drop you have. The steeper the seat angle, the more drop you have.

But this is highly subjective. Most fit, trim folks find themselves in a 3cm range (+/- 1.5cm of what the equation above derives), but that's 7 or 8 out of 10. Maybe 9 out of 10, depending on just how fit and how trim an area's demographic. But there are superb pro athletes who have less drop than this equation suggests, Jurgen Zack comes to mind. I'd venture to guess Levi Leipheimer may have less elevation drop than this.

Because of these exceptions, and because riding a bike with aero bars in the position that uses those bars at their greatest efficacy requires sigificant athleticism, it's not safe to rely on the formula alone. In fact, what we teach in our workshops is that the most important determiners of armrest elevation drop are how the rider subjectively feels, and what the fitter objectively sees. If it feels good to the rider, if it looks good to the fitter, go with it.

After that, resort to your goniometer or your Retul (or other motion capture system). If the hip angle falls into an appropriate range, that's next in line in importance. Finally, if you want a further indicator that you're on the money, refer to the equation above. Note, this equation is fourth in line in importance. When you move from a dynamic fit system to a static system, what was the fourth most important indicator of armrest elevation drop moves to the first and only indicator.

Saddle to armrest

This is the final core fit coordinate, and it's the least impactful on power because it doesn't affect how you fit aboard your bike. Whether the backs of your armrests are 1cm in front of the back of the elbow, or 5cm in front of that point, your cockpit distance determines your shoulder angle.

That established, 58 percent of my cockpit distance is close to what I've noticed anecdotally.

F = .58E

F = saddle to armrest
E = cockpit

Conclusion

Everything above is a work in progress. These formulas are gauges, and some of them will certainly change in the weeks following their original publication.

A static fit system is not only inferior to a dynamic system, the gap in efficacy between static and dynamic is much wider in triathlon than it is in road or MTB.

If the reader is willing to embrace this note of caution, the static system above (with all its caveats and along with this systems adjustments in the weeks and months ahead) may serve as a starting point for the fit, trim, healthy, and athletic riders who have no bike fitter at the ready.

In this case you, once aboard your bike on the highways and laneways, provide the "dynamic" element attached to this "static" system. A proper "dynamic" fit gets you further down the road (as it were) before getting on the road. A set of fit coordinates arrived at through formulas is a hypothetical construct. Your real-world fit is something you'll discover only through pedaling your bike.