Phil got me thinking about my bike fit. In the last post I described the CONI method, which has limitations in that it is largely based on the dimensions of an average person. So, I downloaded the Kinovea software (Kinovea, 2013) as suggested, dusted off the old family camcorder and (hated) turbo-trainer, and made some dynamic measurements of me actually riding the Tricross. The result is shown in the video above; and what a great piece of software it is, with the facility to apply a 'bikefit' tool which overlays a skeleton to measure body angles. This being the beta version there are some glitches and it is necessary to stick pieces of white tape to your joints to help the software 'pin' the moving skeleton in place - this does not always work, as evidenced by the necessity to pin one point to the front axle and the points on the arms not being fixed. Nevertheless, it is still possible to make accurate measurements at any pojnt in the pedal stroke by taking snapshots. The advantage of making dynamic measurements (i.e. cycling rather than static) is that they are more representative of the actual angles when pedalling.
In order to make sense of this a bit more searching on the web yielded the BikeDynamics website (BikeDynamics, 2013) which has a very useful summary of the expected body angles as follows:
- Saddle height should be adjusted so that knee angle is between a minimum of 65-70 degrees and a maximum of 143-148 degrees, with the caveat that people with tight hamstrings (generally men) might prefer a maximum angle of 138 degrees and women up to 150 degrees because they tend to have looser hams. Snapshots taken of me at 12 and 6 o'clock below show that I am within these bounds.
- Saddle setback can initially be adjusted using the KOPS (knee over pedal spindle) method. BikeDynamic point out that there is no biomechanical reaon for this but it serves as a useful ballpark measurement to start. Analysis of my position on the Tricross seems to indicate that my knee joint is too far forward infront of the spindle. A slight forward position is not, of itself, a problem - it would be if the knee was behind the spindle - but it contributes to my forward position on the bike which feels cramped. The obvious solution is to set the saddle further back BUT there is no more travel on the rails of my Brooks B17 to enable this (note to Brooks to address this failing). Using a seat post with more set back might solve the problem, but since I am customising my own frame I might as well go the whole distance and decrease the ST angle to 72 degrees. With a ST design length of 53.5 cm this will have the effect of moving the saddle back by 1.3 cm and dropping it by 0.4 cm for the same ST length. This will also reduce the the minimum leg angle and increase the maximum leg angle slightly, but this can be dealt with by readjusting ST or crank length if necessary.
- Torso angle is another parameter which requires attention. BikeDynamics recommends an angle of 45 deg for touring and 34 deg for sportive/racing (measured between torso and horizontal) As you can see, I have a very upright position with an angle of 57 deg (not shown on the picture). Moving the seat back will reduce this but I clearly have much room for adjustment - providing my not very flexible lower back can cope that is! Another consequence is that the torso-upper arm angle will increase from 60 deg closer to the 75-90 deg recommended range.
- In conclusion, reducing the ST angle will: move the seat back and bring my knee over the spindle; decrease my torso angle; increase my torso-arm angle; and move my centre of gravity back a bit, so taking weight off the bars. . . that's the logic. Plus some other fiddling around with stem length, bar position and saddle height of course.
Awesome stuff! Your bike is going to be spot on.
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