Thursday, January 22, 2015

Time Trial Bike Setup - Basic and Easy One

Time Trial is all about being aerodynamic. You can read elsewhere on drag co-efficient and drag resistance and why it is the single most important factor in anything in a stream of air or water. Rolling resistance is just too small a factor at higher speed.

You can also read elsewhere why we need to maintain various body angles - at knee, at hip flexor, at shoulder and elbows - to be effective and comfortable.

Here, my attempt is to come up with a quick TT bike setup where we can start as a base. This setup does not require any equipment or second person. All we need is a set of allen keys and a linear scale.

Crank Arm Length

It must be noted that having longer crank arm is detrimental to the aerodynamic position as it would force the seat to be lower to be able to reach the bottom stroke and at the same time, the knee would need to bend further at the upper stroke causing very close angle between thigh and abdomen (hip extension angle) when trying to be in aerodynamic position. Even if not for the hip extension angle, such as in a relaxed upright position, the knee flex would not be very powerful if the bend angle is very narrow. Cadence will also drop leading to inefficiency and lesser wattage. Here is a table with recommended crank arm lengths : http://bikedynamics.co.uk/FitGuidecranks.htm. In my case, I'm 165 cm tall and my correct crank arm would be 165mm.

Here is my own formula after analyzing the given figures by experts. The desired knee angle at any stroke position should range between 66 degree (can vary widely between individuals) and 154 degree (mostly consistent to everyone). To be exactly at these angles, the crank length must be within:
   Crank Length = GTL * [cos {(180 - 154) / 2} - cos{(180 - 66) / 2}] / 2
                          = GTL * (0.9744 - 0.5446) / 2
                          = GTL * 0.215
In other words, 21.5% of the GTL. Wait, I haven't told what is GTL. When I wrote this article as arm chair theory, I thought that the length from saddle top to bottom stroke pedal, when knee is fully extended, can be inseam length plus shoe sole plus thickness of the pedal. I was wrong about this. Many sites talk about greater trochanter as more relevant length when it comes to saddle height. Without going too much into the technical details, I coin GTL (not necessarily Greater Trochanter Length, because I don't want to risk an error) as the maximum length from saddle to pedal (usually at bottom stroke) when knee is fully extended. Note that, GTL has to be measured by wearing the shoe to be used and half of the thickness of the pedal so as to maintain the angles mentioned.

The base TT isosceles triangle

I want to give a simple and quick setup which we can do without protractor or trainer stand or a second person. All you need is a scale.

There are three important points we have to fix:
  1. Seat (top centre) height
  2. Elbow tip (the end of the elbow pad of the aerobar) and
  3. Bottom bracket.
With the three points above, we need to create a triangle. To be able to use a scale, we will define it using the three lengths of the triangle. It is roughly an isosceles triangle as the bottom bracket is equidistant from the other two points in the maximum efficiency - aerodynamically and bio-mechanically.

Distance from bottom bracket to saddle top

This distance is dictated by a knee bend angle of 150 to 155 degrees at the bottom of the stroke. If we keep 154 as the desired angle, the length from the saddle top (about the central point where we will mostly seat comfortably) to the bottom stoke pedal spindle position translates to 97.4% of GTL. 97.4% is cos(13), where 13 is half of the (180 - 154) degrees. To make things simple, we wanted to measure the saddle top from the bottom bracket. This will translate to 75.9% of  GTL, which is 97.4 minus crank arm length percent.

Distance from bottom bracket to elbow tip

This distance is same as the length between saddle top and bottom bracket.

Distance from seat top to elbow tip.

This distance depends on the upper torso length. But I promised to keep it simple. So, we will keep this 75% of the above length between bottom bracket and saddle top or simply 57% of the GTL.

There, we have got the perfect isosceles triangle. This triangle focuses on the bio-mechanic aspect. But we haven't exactly ensured that upper body is as horizontal as possible. We can rotate this triangle around the bottom bracket, keeping the lengths constant, thereby without changing the overall body posture.

UCI mandates the seat nose to be at a minimum distance of 5 cm behind the vertical line passing through the bottom bracket.

The above distances have been made keeping in mind of this offset. Otherwise, we could set the seat position and elbow tip even more forward releasing more hip extension angle for better hip flexor power. If you want, you could ignore this and keep the seat position almost above the bottom bracket which will cause the elbow rest to move forward, ignoring the exact measurements that we created above.

For now, we will stick to the above measurements. We will maintain the seat position at an angle of 11 degree from the vertical line passing through bottom bracket. This will also give slightly better power by using our natural downward force of body weight on the pedal.

Rotate the triangle such that the seat top centre is behind the bottom bracket vertical line by a distance of 19% of the length between bottom bracket and seat top or 14.4% of GTL. This positions the seat at an angle of 11 degree from the BB vertical line. Sin(11) = 19.

Where do I begin? Since the bottom bracket is fixed and we have a setback of the seat behind the bottom bracket vertical line, we fix the seat first. Then fix the handlebar height, elbow pads and aerobars. That is it.

Other things to remember is to keep the seat mostly horizontal for comfort. Keep the aerobar such that forearms are horizontal and elbow bend is within 90 degrees.

Do some minor tweaks by trial runs. Each person has their own body proportions and joint flexibility issues.

If you have a road bike, chances are that your saddle is much more reclined backward from the bottom bracket vertical line. Bringing back to 11 degree might require the seat post to flip 180 degree assuming the seat post has an offset. In my case, I had to file the saddle clamps a bit to make this possible.

Next, the handle bar might still be too high if the bike geometry was built for an endurance ride, keeping body at slightly upright position. In my case, it is an oversize bike requiring the seat post to be at it lowest possible. Hence bringing the handlebar to its lowest possible height is still not enough to make the above triangle. If you have a raised stem, flip it upside down and it will take the handlebar a notch lower. That is what I plan to do for myself. This is an unusual case because I bought a bigger frame in the first place.

More on the theory, I found this link very useful : http://bikedynamics.co.uk/FitGuideTT.htm

Disclaimer : I'm not a cyclist in the first place, let alone a time trialist. I haven't validated my theories on any cyclist. I wrote this as I was looking for an easy setup guide for amateurs who wouldn't want to visit a bike fit shop or may not have much patience to experiment. Use your own discretion to use my formula.

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