Chain ring shape in cycling

Bikes have traditionally used circular chain rings to drive the chain and turn the rear wheel. Over the years however, there’s been a continual interest in the use of slightly non-circular chain rings (containing a very slight ellipse), which some have claimed to be better suited to the biomechanical requirements of pedalling, resulting in more force delivered to the crank for less perceived effort from the rider.

To look at this in more detail, Norwegian scientists have been studying the effects on force development and pedalling rate of using a non-circular (Biopace) chain ring compared to a conventional circular chain wheel. Ten trained cyclists (average VO2max of 61.7mls/kg/min, average age 27 years) cycled with a Biopace and a circular chain wheel at 180 watts power output at 65 and 90rpm, during which their crank torque profiles were measured. They also pedalled using both chain ring types at their freely chosen pedal rate where metabolic responses were monitored, including oxygen uptake and blood lactate concentration.

The researchers discovered that crank torque profiles were similar between the two chain wheels during cycling at the preset pedal rates of 65 and 90rpm. They also found that the freely chosen pedal rate was not dependent on chain ring type, averaging 93rpm in both cases. Interestingly however, blood lactate concentration was significantly different between the two types of chain rings being on average 0.2mmol per litre lower with the Biopace rings compared to the conventional circular rings.

The researchers then carried out a musculoskeletal simulation model. This seemed to support the idea that a contributing factor to the observed difference in blood lactate concentration could be the slightly reduced muscle activity observed where peak crank torque occurred during each crank rotation when using the Biopace rings. The researchers concluded that this slightly lower observed muscle activity might result from larger transfer of energy from the legs to the crank when the Biopace rings were used. This study then provides some evidence to support the anecdotal observations of some cyclists that pedalling using non-circular chain rings feels easier for a given pace compared to conventional rings.

J Physiol Anthropol. 2009 Nov;28(6):261-7