Saddle Set Back
Saddle Fore-Aft and the Knee Over Pedal Spindle (KOPS) rule
A good starting point for the fore-aft location of a saddle is the Knee Over Pedal Spindle (KOPS) rule. This has no biomechanical justification in its own right but does seem to work quite well. It works because it is a good predictor of the position of the Greater Trochanter (GT) relative to the seat tube axis (note the marker on the hip), which in turn is a good predictor of the rider centre of gravity position. With the hip in the correct location relative to the crank, the saddle will be close to horizontal and the rider well balanced and as the bike designer intended. Moving the saddle back could free up a cramped cockpit, better engage your glutes and can take weight off your hands and arms. Moving the saddle forwards will open up the hip angles and by reducing any stretch to the bars, possibly relax the arms whilst putting more weight onto them. KOPS should be regarded as a 'Happy Accident' rather than a rule.
Hip location vs KOPS
At first sight, the rider above would appear to be in a relatively good fore-aft position according to KOPS with the knee maybe a few mm in front of the pedal spindle. The hip marker by comparison is well away from the seat tube axis. This rider's position is actually very wrong as the saddle is over 20mm too high, which has the effect of bringing the knee back and pushing the rider forwards on the saddle. The hip marker is a much better predictor of the correct location and implies the saddle is far too forwards. The final recommendation during this Bike Fititng was for the saddle to go down 24mm and back 20mm. Finding the Greater Trochanter (GT) can be tricky, but aligning it with the seat-tube axis will generally be a more reliable means of establishing a good fore-aft location and saddle set-back.
Saddle Fore-Aft - Unconventional frames.
Note the KOPS rule only applies to conventional road bikes with typically 72-74° seat tube angles. With more aggressive or relaxed seat tube angles, the KOPS rule no longer applies but hip location criteria is still valid. With slacker seat tube angles, the bike designer intends you to sit further back, more upright and with less weight on your hands and arms. On TT or Triathlon bikes, the saddles will be intentionally much further forwards relative to the cranks than a road bike. This is to allow the front end of the bike to be much lower (improving aerodynamics) whilst maintaining a large enough minimum hip angle to prevent power loss.
Saddle Set Back - Power Generation
There tends to be quite a lot of misunderstanding about how the fore-aft relationship of the saddle to the cranks affects power. Many people believe moving the saddle back favours the Glutes whilst forwards the Quads. This is true, but only due to the secondary effect of the torso (and so pelvis) inclining forwards or backwards respectively. See Muscle Activation for an explanation of this behaviour. If the hip and knee angles were to remain constant then the power generating capability of any fore-aft position will be the same. Many people do not realise how much the saddle height changes when you alter a fore-aft position, which can flavour their subjective impressions when riding.
What Actually Matters!
Some cyclists appear very concerned that there knees are precisely over pedal spindles and regard it as the most important aspect of their fit. What actually matters on a road bike set up is that the rider is well balanced without excessive weight on their hands. A simple test is to try to hover your hands above the hoods (on a turbo trainer). If you can't do this without falling forwards, either your core strength is very poor or the saddle fore-aft location is wrong. Bernard Hinault suggested you should be able to play a piano keyboard on your handlebars; others talk of passing round a tea tray. This is one area where the turbo trainer does not accurately reflect the road condition. On the road at any speed above 15mph, the pressure of wind on your chest will actually help to unload your hands, so whereas assessment on a turbo can be a worst case for loads through your arms and shoulders, this may be mitigated by the fact there is no contribution from road vibration and pot holes.Neck and shoulder pain is another strong indicator that something is amiss.
DIY Dynamic Bike Fitting
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45 Page PDF download describing all the observations, simple measurements and calculations required to do your own Dynamic Bike Fits.
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