When setting up the BikeDynamics business a few years ago, I was keen to establish in my own mind how to set the absolutely critical dimension of saddle height. The established formulas of 0.885 x inseam length to bottom bracket, or 1.09 x to pedal platform had not worked particularly well for me, nor did I think I could justify asking people for large sums of money just to shove a tape measure into their crotch. More on my precise technique later, but I’m pretty happy that dynamic knee angles, combined with direct observation and subjective impressions give a robust methodology.
Whilst reading some journals and articles recently it became apparent that some of the ‘big names’ in Bike Fitting were very keen on the 0.885 x inseam rule, so I became very curious to see how well it works. Fortunately, the information required is part of my every day data gathering, so it was quite easy to plot the last couple of months worth of customers inseam and final saddle heights. Note these were all roadbikes as TT or Tri saddle heights would introduce a whole new level of variability.
If you can’t see the formula on the plot, it is
Saddle Height = 0.8766 (inseam) + 9.8
with an R2 of 0.9162. To those that know their stats this is an OK correlation but not perfect. There was a very slightly better correlation with a polynomial expression, but the improvement did not warrant the added complexity of the equation.
I’m actually quite pleased with this formula because it recognises that the origin is not at zero. We all use similar shoes and pedals with an offset between foot and pedal axis. This formula suggest that offset is 9.8mm, which is probably not that far off the mark!
So what of the actual numbers. Taking a selection of inseam lengths, we can compare the saddle heights.
Inseam 0.885x 0.876x + 9.8
- 750 663.7 666.6
- 800 708.0 710.2
- 850 752.3 754.4
So I’m tending to set people up on average about 2mm higher than the established formula suggests. This could be explained by the fact that about 10% of my client base are women, whose more flexible hamstrings tend to allow slightly higher saddles for the same inseam. I’m guessing the 0.885 rule came about when less women cycled?
Initially I was not sure If I should be pleased or disappointed with this outcome. On one hand it suggests I should not have been so quick to dismiss simple formulae in the first place, but on the other, closer inspection of the data reveals how individuals could be very wrong if just doing simple sums. Looking at the data points, approx. 25% are close to the line but the majority are up to 15mm away from it! This includes myself as doing the sums suggests my saddle should be 8mm higher than I like it, but even going up 2mm starts to give me problems.
As mentioned earlier, I use dynamic knee angles, direct observation and subjective impressions to establish saddle height, and once ‘tuned in’ many customers are amazed at how even small height changes can make huge differences. Once in the right place, the power seems to flow so much easier, the upper body steadies and the turbo trainer emits a more consistent ‘hum’. I’m not going to give away all my trade secrets but observing a rapid decelleration of the knee joint at the bottom of the stroke is a strong visual indicator that the saddle is too high.