BikeDynamics - Bike Fitting Specialists

BikeDynamics - Bike Fitting Specialists

Saddle Position.

Saddle Height

Optimum saddle height is always 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. Taking all these factors into account puts the ideal maximum dynamic knee angle at up to 147.5° for men and 150° for women.

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.

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. One of the reasons it works is really very simple. A diamond frame bike could theoretically have a much steeper or shallower seat-tube and be as biomechanically efficient at either extreme, but the inclination of the saddle would have to be tilted up or down and friction would be insufficient to stop you sliding off it. Similarly, the pedal forces in the KOPS position are predominantly upward with a small rearward component. In normal pedalling efforts these will be reacted by gravitational and saddle frictional forces.

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.

Saddle Fore-Aft and Pelvic Tilt

As mentioned previously, if the handlebar position were to remain unchanged, the saddle fore-aft position will affect the inclination of the pelvis. Moving forward will make you more upright, moving back more inclined. Ideally one could achieve the same effect by altering the bar position but in some cases this is not feasible. Moving the saddle back could reduce a cramped cockpit and better engage your glutes. Moving the saddle forwards will open up the hip angles and by reducing any stretch to the bars, relax the shoulders and arms. Opening up the hip angles could also be important in improving upper body stability. If the hips are tight, you often see a vertical ‘bob’ of the torso as the knee approaches the chest.

For more details please use the following links.

Muscle Activation around the Stroke	Length Tension Relationship
Foot to Pedal	Varus Valgus Angles Cleat Alignment Foot to Pedal spindle Longitudinal Arch Support Pronation and Supination Coupling of Dorsiflexion with Foot Inversion
Handlebars	Reach - Torso, Upper Arm, Elbow and Wrist angles Handlebar Width
Crank Arm Lengths	Recommendations Hip and Knee angles
Temporary Riding Positions	Climbing Ankling Honking On the Drops On the Rivet
Time Trial/Triathlon Positions	TT Geometry Biomechanics Aerodynamics Comfort
Hips, Knees and Ankles	Typical Knee Displacements Ankle Behaviours

BikeDynamics Bibliography

You may be interested in some of the contents of the BikeDynamics library.

We say : If you are going to buy one book on Bike Fitting and the implications to injury, comfort and performance - this is the one.	We say : The latest scientific knowledge in a range of subjects from Biomechanics to Nutrition.	We say : Full of formulae and graphs. One for the Engineers.
We say : Good anatomical summary but also very strong on suitable weight training workouts.	We say : Excellent training recommendations but also good sections on bike handling skills and flexibility.	We say : A celebration of cycling technology. Good for those scared off by graphs and formulae.
We say : 'The' text book on conditioning, road cycling biomechanics, nutrition, hydration and trauma.	We say : A practical guide to the human body without being too "clinical".	We say : A clear, simple and inexpensive reference.

Page Last Updated : 23rd January 2010

BikeDynamics - Bike Fitting Specialists