"Mountain Bike" riding and racing is an increasingly popular sport. For this type of use, experience seems to indicate that a slightly longer crankset than that used for road riding is best; this may be due to the "stomping" nature of the sport, as opposed to the "spinning" needed for road use. The fact that mountain bikers normally use a more upright riding posture probably also plays a part. Therefore, once the proper road crank length is determined, adding about 5mm is suggested for selecting an off-road crankset.
Note that many MTB experts will suggest a larger difference between a road crank and an off-road crank. However, a large part of the difference they are suggesting is already built into the formula itself; the same riders that have learned to install long cranks for off-road (and are often fairly large people) would be using a longer crank on the road if they followed this formula.
Track racing typically calls for a crankset somewhat shorter than for road use. This is probably due to the extreme pedal speeds required for success, and there is also value to keeping the rider's knees down so that he can assume a lower and more aerodynamic position. Traditionally, there may also be some basis in the need for cornering ground clearance on bikes on which you cannot hold the pedal up in a turn. Whatever, the track racer would be well advised to subtract 5mm from the result of the formula -- possibly more.
As mentioned in the derivation of the formula, there are some possible shortcomings to basing the formula on the inseam measurement. If the rider is unusually proportioned, some correction may be advisable. For example, if it is evident that the lower leg is unusually long in relation to the upper leg, a few mm should be subtracted from the result given by the formula, since the upper leg length -- which is the really important value -- is somewhat shorter than the inseam measurement would imply. Also, if the rider's feet are unusually large for his size, a couple mm should be added. Remember that the formula will properly account for large feet on a large person; the adjustment should be made only for unusually sized feet in relation to the rest of the body.
There is some evidence to indicate that women can use a crankset very slightly longer than that indicated by this formula. Add no more than 5mm on account of the rider being female; better yet, simply use the formula as is and round upward to the nearest available increment in crank length.
Finally, of course there should always be room for personal preference. If the rider is the type that likes spinning, selecting a crank a few mm shorter than that called for by the formula may be wise. If the rider prefers to select a tall gear and pound, rounding up to the next longer available crankset is in order.
In the specific area of road riding/racing involved in hillclimbing, it may be found that a longer crank is of some help. However, once the proper crank length (according to the formula) is used, further lengthening is likely to be of limited benefit even on climbs, and will be seriously detrimental in other road use. Therefore, switching to a longer crank for a hillclimb event is not recommended.
In time trialing, a slightly longer crank may also be
allows optimum performance at a lower cadence, which by itself may seem
helpful. And cornering clearance is normally not an issue in the TT, so
fitting a longer crank may seem the way to go. However, it is probable
that such a benefit would actually only apply when the crank is too
to begin with; if properly sized for road use, lengthening for TT use
provide little or no benefit, and may possibly result in poorer
due to operating in a condition other than that used in training.
Finally, let me caution against the temptation to misuse
fudge factors to justify inaction. If you are presently using a
170mm crank and the formula calls for a 190mm crank, there isn't a
fudge factor here that justifies keeping that 170mm crank on your bike.
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