We source fit clients from all over. In January we had clients arrive from the U.K., U.S.A and various parts of Australia. Within that months total of fits, there was an issue common to five clients. All five had been “professionally fitted” (their words) and the major cause; or at least a large part of the cause for the issues that brought them here was too high a seat height. And I don’t mean a couple of mm; I mean between 12mm and 25mm too high.
Getting a rider’s seat height correct is simple stuff. If a rider sets his or her own seat height too high there is no blame attached; but if someone has paid money for a “professional” fit, there is no excuse for fitters on three continents getting it wrong, except in the rarest of circumstances. So how to reliably set seat height?
Firstly, a word about how the fitters of the five people in question had arrived at the (in)’correct’ seat height. Two had used fitters who used a motion capture system. One had used a fitter who used a video capture system. Two had used fitters who used equations (like leg x whatever number is chosen = seat height) or goniometers. To critique these methods – goniometers are joint angle measuring devices. The aim is to measure the included knee angle (angle of upper leg to lower leg behind the knee) at the bottom of the pedal stroke.
The problem is many users of goniometers blindly follow recommendations based on ‘averages’ rather than developing good observation skills. A simple version is “Correct seat height equals an included knee angle of between X and Y degrees at the bottom of the stroke.” A more sophisticated version is “Correct seat height equals an included knee angle of between A and B degrees at the bottom of the stroke if you are flexible; between C and D degrees if you are moderately tight and between E and F degrees if you are very tight.” Motion capture and video capture system recommendations are based on similar thinking with the difference being that a motion capture system measures the included angle via 3D cameras and a video capture system allows the fitter to measure the included knee angle on screen.
All have the same failing: They are based on averages. Who is to say whether you, the client, are average……………whatever average means. Averages are accurate only when applied to large groups of people and can be wildly inaccurate when applied to individuals. As an example, if we are all ‘average’ we’d all possess one testicle and one ovary. That is an accurate statement when applied to the population of Australia but not in the least bit accurate when applied to an individual. Yes, I know this is an extreme analogy but determining seat height based on averages is just plain lazy and inaccurate, no matter how much this is obscured by a technological overlay.
One recent experience will demonstrate my point. A client arrives complaining of chronic knee and low back pain when racing. He had 4 previous ‘brand name’ fittings elsewhere without resolving the issues but the most recent fitting had impressed him the most. A pressure sensitive pad had been placed on his seat which demonstrated to him that he didn’t sit with equal pressure on each side of the seat. He was told a new seat would solve the problem and purchased it. Force measuring pedals were used to demonstrate that he applied differing loads to each pedal. He was sold a pair of pedals as a ‘solution’. The cost of fitting and parts was more than $1000 and despite being impressed by the technology applied by the bike fitter, the knee and back pains persisted. I dropped his seat 4 mm, fitted appropriate arch support insoles, 1 heel wedge to one shoe and changed the angle of his bars and the placement of his brake hoods slightly. No components were changed and from my perspective it was as simple a fit as I’ve ever needed to do. The client tells me his knees and back are no longer an issue. He’s a bit perplexed that such minor changes resolved his situation. The message in this is that no amount of technology compensates for lack of basic fitting skills.
Rules of thumbs and equations have even less merit because, you the client are aside from the process, not part of it. If you want an equation to set your seat height; stay home and do it yourself rather than pay a fee for someone to ignore how you function and use a number.
All the methods above have the same fundamental failing. The seat height recommendations flowing from them are not the result of direct observation but are ‘averages’ without any regard to the idiosyncratic way that YOU relate to your bike.
How to set seat height accurately if you are a bike fitter:
Focus on the velocity of extension of the rear of the knee under significant load. That velocity should be a constant. If you see even the tiniest flicker of acceleration at the rear of the knee before the bottom of the pedal stroke, then the rider is losing control of the motion and is too high; at least on that side. If in doubt, increase the load a touch. Always check the other side under similar load and if there is a difference in fluency between sides, have a look at the pelvis from the rear for your clues as to why. What is significant load?
Significant load is enough resistance to have the rider forcing the gear a bit at 80 – 85 rpm but not so much as to sacrifice technique. This kind of load is similar to riding a hill hard while seated, in one gear harder than is comfortable. Under this load, better than 99 percent of riders will drop their heels more, and extend their legs more than they will under less load or in flat riding where momentum plays more of a part. It is this kind of load that determines seat height.
How to set your own seat height:
Find a hill that takes at least three minutes to ride up. Warm up thoroughly and then ride up that hill under significant load (see above for definition of significant load). Do you feel like you are riding a step machine or do you feel fluent through the bottom of the stroke on both legs?
If you feel like you are on a step machine or feel a bit powerless, drop your seat 3mm and repeat the hill.
If you feel equally fluent through the bottom of the pedal stroke on each side, raise your seat 3mm and repeat.
For those who have to drop their seat, repeat the hill and drop the seat 3mm per time until you feel fluent through the stroke while forcing the gear.
For those who have to raise their seat, repeat the hill and raise the seat until you feel like you are a touch less fluent on one side than the other. This is an early warning sign that you have entered challenge territory. (For more info about challenges.)
Now drop your seat 6mm. Why not just drop the seat to the last 3mm increment? Because not every day is the best day of your life.
Okay, with a bit of commonsense, you should now be able to set your own or your clients seat heights efficiently. Best of luck with it.
Postscript: One last word for fitters. The velocity of extension of the rear of the knee is the prime visual cue in determining an individually suitable seat height. However, you need to use a trainer that has enough momentum to not cause a staccato pedal stroke at significant load simply because the trainer doesn’t have enough momentum to mimic on road feel. Wind trainers are basic, reaslistic in feel and ideal. There is the additional aural cue of the fan noise. Succinctly, whirr, whirr, whirr is bad while whhhiiiiiiirrrrr is good. The only magnetic trainers that are worthwhile either have large flywheels or plenty of computer power to smooth them out. Fluid trainers are okay but as a general rule need a flywheel mass of at least 3.5kg. More is better.
I use a pair of Sidea SB4’s mag trainers with flywheel mass of, I would guess, 30 odd kg. And the Velotron is being commissioned soon when I have the time to modify it the way that I want to.
An addendum to this post is available here.
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