Anatomical Considerations in Bike Fit: Fitting the Machine to the Athlete – Part III

In my last article on Anatomical Considerations in Bike Fit, I outlined the process of measuring your body — both your dimensions and your flexibility constraints — so that you can optimally fit your bike to you.

In this article we’re going to complete the process by solving that one remaining (variable) aspect of the equation — your bike’s dimensions. In this piece I’m going to follow my philosophy that a good TT or tri position begins with a good road position by setting you up with the most comfortable, neutral road position for your body geometry. In the fourth and final piece in the series I’ll go into how to adapt that for tri/TT fits.

So, let’s dive right in with the starting point…

Seat Height (Bottom bracket to top of saddle — point where you sit)
The seat height that you select has a number of implications that can affect your comfort on a bike. The central two are hip range of motion and knee range of motion.

An optimal seat height will, for most folks, represent a balance of these two considerations. It will be low enough so that it allows the hips to tilt forwards, thus enabling the rider to be relatively aerodynamic while retaining a neutral posture by not maxing out the flexibility potential of the hamstring muscles. But it will also be high enough so that in the power phase of the pedal stroke, the knee isn’t starting from an angle that is too acute. This is why I recommend experimenting with a few different heights to find the optimal balance.

A good starting point is to shoot for an open knee angle that will allow the trunk to lean forward 45 degrees from horizontal; in other words, depending on frame geometry, an open hip angle of 95-100 degrees. Based on what I’ve seen, this will lead to frontal areas of ~0.4m^2 on the hoods for average sized athletes (180cm).

The more bendy folks will get away with this with a 30 degree knee angle. Others (like myself) will need to run a shorter seat height to get close to this. Corresponding seat heights for different limb length/knee angle combinations are shown to the right.

For the sake of simplicity, I’ll start by assuming a standard crank length of 175. For shorter or longer crank lengths, add or subtract from the seat height accordingly (add the difference for a shorter crank, subtract the difference for a longer crank).

After identifying a relatively aerodynamic starting position, you can check the resulting closed knee angle against the number you found last week to see if it is excessive. As you can see from the table, this is more likely to be a problem in shorter, less flexible riders who will often benefit from shorter cranks. For comparative purposes, the closed knee joint will open approximately 3 degrees for every 10mm difference in crank length (after seat height adjustments are made) so if your comfortable closed knee angle falls outside the projected knee angle from the appropriate seat height, this is a good way to correct it.

Closed Hip Angle
If the hip angle is closing beyond the range of motion that you identified with last week’s tests, you will find your pelvis rocking from side to side and open the possibility for hip flexor and back problems. The best fix for this is again shorter cranks (~1 degree/10mm), coupled with a shorter cockpit and less drop which allow for you to keep your hips more open at the top of the pedal stroke (explained further below).

Cockpit and Drop
Here are general thoughts on cockpit length for an athlete of average flexibility (45 deg trunk angle):

Starting from a neutral position of 50cm trunk length and 53cm arm length and a diagonal cockpit of 73cm with a zero drop, add 3.5cm of drop and 3.5cm to cockpit length for every 5cm of additional arm length. Take 3.5cm of drop away and add 3.5cm of cockpit length for every additional 5cm of trunk length.

For example, if we have an athlete with normal flexibility, a 55cm trunk length and a 65cm arm length, we would trial the following:

-3.5cm (extra 5cm of trunk length)
+7cm (extra 10cm of arm length)
+3.5cm Drop (to hoods)

+ 3.5 (extra 5cm of trunk length)
+ 7 (extra 10cm of arm length)

In other words, athletes who more closely resemble an orangutan should ride with a greater drop.

For seat angles other than 73 degrees, for every 1 degree, add 1.5cm of drop. So, a 78 degree frame should have the bars set ~7.5cm lower than a 73 degree frame for the same athlete.

For those athletes of less than average flexibility, the rules change a little. Unfortunately most race bikes are built for “athletic” folk and often challenge the flexabilities of recreational athletes. Take a look at your typical group of weekend warriors and you’ll see some funky bike positions. Sometimes this is the result of a poor shop fit but more often than not, it’s the result of Johnny Deskjob wanting to ride with the same geometry as Dave Zabriskie and the bike makers accommodating!

For those with hip angles more typical of Johnny Deskjob than Dave Zabriskie, the following rule of thumb can be applied. For every 5 degrees less than normal hip angle (95 degree for a 73 degree frame), take 6cm away from the vertical drop or bring the seat angle forward by the same angular amount, i.e. move from a 73 degree to a 78 degree frame. You can also do a combination of both (75.5 degree frame with + 3cm rise).

If you happen to fall into the above category, it can take some tweaking to come up with a comfortable position on a stock frame. Longer/more forward saddles, shorter stems, positive rise stems, addition of spacers (or custom frames!) can all help. Most of these will have a slightly negative impact on the handling of the bicycle but will greatly improve riding comfort by enabling the rider to spend many hours in the saddle in a very neutral position, where no muscle is held in a permanently shortened or lengthened position for long periods at a time.

It is worth remembering that all of these rules of thumb are merely designed to create the appropriate body angles that you discovered when doing your personal range of motion testing. For confirmation and fine tuning, the final step would be to test what your body angles look like while actually riding. Motion capture technology like the Retul system is very valuable in completing this final step in setting up the most comfortable bike position.

Of course, occasionally, the rider will want to sacrifice a little of this comfort for some more speed. We’ll get into this in the next article when I investigate modifications of the basic “all day, every day” road position for triathlons and time trials that strike the best comfort/power/aerodynamics balance.

Train Smart

Categories: Planning

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Alan Couzens

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