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Saddle Position.


Knee Extension Torque v Angle

Saddle Height

Optimum saddle height can be defined by a characteristic of the leg at full extension. From the knee extension curves we can see how there is still useful torque generation by the knee, even at angles over 150°. At that point though, the pedal is approaching the bottom of the stroke and further knee extension will tend to push the pedal down rather than around. The contribution of the Gluteus Maximus is also diminishing as the hip moves from extension to flexion. The Hamstrings are now becoming the dominant contributor and will be approaching a fully extended condition, especially if the cyclist is in a crouched position. Hamstring flexibility is a key factor in determining optimum saddle height with optimum knee angles being up to 147° for women and approx. 142° for men. Tight Hamstrings will inhibit knee extension, so some people are unable to extend beyond 140° before the hips start to rock. Actual saddle height as measured from saddle top to crank centre will be dependent upon knee angle and ankle posture. There is enormous diversity in the angle of the foot around the pedal stroke. Some people prefer to be very toe down, with others flat or heel down. This can create significant errors if using inseam multipliers or heel on pedal techniques to determine saddle height.

Saddle position

Saddle Fore-Aft

There tends to be quite a lot of misunderstanding about the fore-aft relationship of the saddle to the cranks. 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.

KOPS Knee over pedal spindle

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 and is an ideal starting point when setting a bike up. It works because it is a good predictor of the position of the hip relative to the seat tube axis, 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 need to be horizontal and the rider will be well balanced and as the bike designer intended. Moving the saddle back could reduce a cramped cockpit, better engage your glutes and 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 put more weight onto them.

TT Bike fore-aft.

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 using hip location criteria is still valid. With lower seat angles, the bike designer intends you to sit further back, more upright and with less weight on your hands and arms. Upright seat tubes as shown here allow the rider to be much lower whilst maintaining sensible minimum hip angles.



Centre of Gravity

Saddle Fore-Aft and Rider Centre of Gravity

Exploring the forces applied to the rider in more detail gives us another means to determine and understand an ideal fore-aft saddle position. In Muscle Activation (position 3) we showed that at around 125° from TDC, the torque at the knee joint tends to zero. This is a very useful point in the stroke as it is the only location where we can deduce the direction of the force applied at the pedal without using complex instrumentation. With zero torque at both knee joint and pedal spindle, the pedal force vector must be a line that passes through both. An extrapolation of this line would ideally pass through the rider’s Centre of Gravity. If this vector was in front or behind the CofG it will tend to pitch the rider, and require restraining muscle activity. The CofG fore-aft plane can be calculated by using weighing scales to measure axle loads, but a good approximation to CofG position can be made by using the cyclist’s belly button. Obviously this position will change dependent upon the torso angle of the rider. This theory suggests that fore-aft saddle location has more relevance to upper body smoothness than power delivery. This is a very brief summary of this theory so please contact us if you would like more detail.

 

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Page Last Updated : 17th July 2014 All Rights Reserved. BikeDynamics - Bike Fitting Specialists