There are three main forces resisting motion of the bike and rider:
Rolling Resistance (i.e. friction generated by the tyres)
Aerodynamic drag (i.e. pushing air out of the way)
Gradient Resistance (i.e. gravity acting on the bike/rider)
Only one force is available to overcome these:-
Motive force generated by the rider and transmission system



When the motive force exactly equals the sum of the resistive forces the rider will travel at a constant speed, if the motive force is greater the rider will accelerate until such a point as the forces do equal out.


Rolling Resistance is a function of rider/bike weight and the specification of the tyres.  It varies very little with bike speed, and accounts for the majority of the resistance forces at slow speeds


On flat ground at speeds above 10mph aerodynamic drag dominates. Aerodynamic drag is determined by the rider/bike size and shape: smaller and more streamlined is better.


As the road begins to incline gradient resistance occurs and increases with increasing steepness until it dominates on very steep climbs.  Gradient resistance is determined by rider/bike weight



The transmission system connects the cranks to the rear wheel so that the rider's power can generate motive force to move the bike.  In order to match the riders pedalling speed to the road speed the transmission system incorporates a step-up (for high speed) or a step down (for low speed) ratio using different sized gears. 

The transmission system is not 100% efficient; some of the rider’s power is lost as heat in the transmission system due to friction.


The typical rider generates most power at a pedal speed (cadence) of between 90 and 110 rpm, but is most efficient (higher power:calorie  ratio) at pedal speeds of 60 to 70rpm, therefore gear selection should ensure that pedal speeds stay within the 60 to 110rpm range at all times if possible.

A single-speed bike can only match the rider's peak power output to one road speed, which is why it can be very difficult to climb at slow speed on a single-speed bike)

Unfortunately the human body cannot sustain high power indefinitely; A typical rider can produce 2 to 3 times as much power for 1 minute as they can sustain for an hour.


The power output of a rider is determined by:-

·         The skeletal muscle system

·         The cardiovascular system

·         Energy availability

·         Levels of fatigue

·         Hydration

·         Pedalling technique

·         Riding position

Shortcomings in any one of these areas will limit power output, regardless of how well developed the other areas are, therefore training, ride preparation and bike set-up should be designed to improve all of these areas.

The steady-state motion of a rider can be reduced to the mathematical equation shown opposite.  The coefficients of aerodynamic drag (Cd), rolling resistance (Cr) and transmission efficiency (n) can be determined from wind tunnel and dynamometer testing, combine these with the total rider and equipment mass (m), plus the course gradient (θ), the rider's speed (v) and an estimate of their frontal area (A) and we can very accurately determine the rider's power output in watts.

We can then rearrange the equation and substitute different values of m, Cd, Cr and Rider Power Output to determine the effect on the rider's speed (v).

For example a vented helmet and a teardrop helmet will have different Cd values.  By substituting these different Cd values into the equation we can determine each helmet's effect on a rider's speed.

Laboratory studies often show that the latest training scheme affects different people in different ways, or two wheels with roughly the same specification perform completely differently in practice (for example two wheels each with 50mm deep rims may have widely different drag coefficients). 

Obviously we cannot test every single item of kit on the market (although that would be fun!), and we cannot test every individual to see how specific training schemes affect them personally, so here at CYCLE-SPEED.COM we present the "typical" results you would expect from each of the different types of item tested.

e.g. if the MARGINAL GAINS CALCULATOR says you will gain 5.6 seconds from a particular item of kit, depending on the actual product you buy, the improvement may actually be somewhere between 3 and 8 seconds....



Now let’s see how we can use this science to improve one of your rides, click the "Marginal Gains Calculator" button below:-