Which crank arm length

A good bike fitter will help you find the best crank arm length for you. If you are changing the cranks on an existing bike. Get a bike fit and then buy a cheap crank or borrow a crank with a different length. Go for a few rides and see what length is best. When you know what size you need. Go ahead and buy the crank you want. There is a common myth among cyclists that a longer crank gives you a more powerful pedal stroke.

Common sense may make you feel this true, as a longer lever allows you to use less force to generate more power than a shorter one, right? So the longest crank would be the best choice? Not so fast. It is harder to keep a higher cadence with a long crank as your feet have to move further to complete the pedal stroke. Gaining speed by spinning up from a standing start can take longer. Too long a crank can create stress on the knees at the top of the stroke.

A shorter crank makes it easier to spin at a higher cadence. The reduction of force can be eliminated by selecting a bigger gear on the rear cassette. Smaller cranks can allow you to get into a more aerodynamic position. This is because your saddle height is lower with a shorter crank, allowing you to bend more at the waist when you reach for the handlebars.

Martin and McDaniel found that muscles will naturally contract at an optimal, most efficient speed. Now since a shorter crank arm will have a smaller circumference pedal circle, for the speed to remain unaffected, cadence will increase. For most riders, this change is so natural that they are unlikely to notice a substantial difference. Sadly most OEM bike brands are not yet. That means that most non-custom triathlon bike builds will come with crank arms that are in some degree proportional to the size of the frame.

That is, most small frames will come with shorter arms and large frames with longer arms. Swapping these for short — often the shortest mm — arms is something that we do quite often as an aftermarket, bike fit service.

Want more info? In this article, Australian tech editor Matt Wikstrom takes a look at research on the influence of crank length on the performance of road cyclists, and explains that the results are actually quite clear.

The whole notion of free speed and improved efficiency is compelling in an endurance-oriented sport like cycling. Cranksets have received a fair share of this attention. Originally made from steel, they have evolved from largely utilitarian creations to become lightweight and elegant.

Aluminium alloy remains the most common construction material, however the last decade or so has seen the successful introduction of composites. The same period has also seen immense proliferation in axle and bottom bracket designs while chainrings have been getting smaller and using fewer bolts. One thing has remained constant throughout all of this refinement: the length of the crank arms. At one point, during the dominance of the English bike industry prior to World War II, an attempt was made to standardise crank length 6.

While research on this topic may have been slow to start, it has received a lot of attention over the last years and efforts are ongoing. As with any field of research, there has been some contention, and some of the results may run counter to conventional wisdom.

Any debate on the influence of crank length normally starts out by considering the problem in terms of simple physics. When viewed from this perspective, a bicycle crank is considered a lever, and hence, any increase in the length of the cranks has the potential to provide the rider with extra leverage.

While this approach does a lot to simplify the problem, it does not allow for the influence of biomechanics, which, as it turns out, is quite considerable. After all, there are three human-powered joints involved in driving each side of a crankset that require energy in extension and flexion, so there is more to the problem than simply calculating leverage. Nevertheless, the influence of crank length on leverage for the drive train can be demonstrated under a set of very specific circumstances, namely from a standing start with a fixed gear over a short distance m.

Then, longer cranks allow a rider to develop more speed than shorter cranks, even when the difference is as little as 2mm. This kind of scenario is quite removed from road cycling, since riders spend most of their time seated and have the freedom to change gear ratios as they please.

Under these circumstances, crank length has no effect on maximum power output, and indeed, near-identical results have been observed for a substantial range of crank lengths. For example, Inbar et al. If crank length has no effect on power output for a road cyclist, can a rider save energy by changing the length of the cranks? Research on this question goes back as far as when Astrand measured oxygen consumption by cyclists riding a bike on a treadmill.

Changing the crank length from mm to mm and mm had no effect on oxygen consumption whereas a change of tyres did.

Indeed, a subsequent study by McDaniel et al published in clearly demonstrated that the metabolic cost of cycling was largely dependent upon power output, cadence, and pedal speed.