Bicycle Crank Length: Implications

Unless a rider has a 31" inseam, the formula probably calls for a different length crankset than is presently fitted to the bike. There are several implications to replacing the present crank with one of the correct length.

First, it should perhaps be obvious that the changes noted would be proportional to how incorrect the crank length was before. If the rider had been using a 170mm crankset and the formula calls for a 175mm, the effects of the change are likely to be minor -- positive, but minor. On the other hand, if the rider has a 36-inch inseam, the formula calls for a 198mm crank. This would be a major change for this rider, and may take him some considerable retraining to become accustomed to. The same is true for very short riders. Experience with such changes has shown that highly trained cyclists usually require several training rides with the new crank length before they surpass their previous performance, while the more casual rider takes to the correct length crank like a fish takes to water.

The biggest result to be noticed from correcting the crank length will be comfort. Tall riders, switching from a 170mm to something more their size, will immediately feel like they are turning a serious crank rather than spinning a child's bike. Short riders getting a shorter crank will be able to spin comfortably for a change, especially on the low bars. And the incidence of knee problems -- for both tall and short riders -- will be greatly reduced.

It must also be pointed out that changing the crank length affects a few other things. For example, installing a longer crank may require lowering the seat, depending on where the seat was before (when the crank is too short, riders have arrived at different conclusions about what extension should be aimed for). With the correct crank length fitted, the proper extension will be readily apparent on the first ride, since it is more critical than with a crank that's too short. Usually the seat will have to be lowered, often by nearly the same amount as the increase in crank length. In cases of considerable crank length change, this can affect the selection of frame size!

Another issue is cornering clearance, a concern when changing to a longer crankset. The pedal gets closer to the ground, and that can be a problem with the length cranks needed for very tall riders. If this crank length formula were commonly accepted, modern frame designs could easily compensate by locating the bottom bracket higher off the ground on taller frames. Many of the better framemakers do this now, but the differences are minor, only a few mm at most. Ideally, a 185mm crankset should be mounted in a BB that is 15mm higher off the ground than a 170mm -- perhaps even 16 or 17mm higher, since the taller rider is also likely to have wider feet and wider pedals. A quick review of the market will clearly indicate that frameset lines with BB heights varying adequately with frame size simply don't exist.

The value to look at in frameset geometry is called the "hanger drop", and it is the distance the BB is below the centerline of the wheels. Hence, the smaller the hanger drop, the higher the BB is.

Even if no single line of framesets offers adequate variation in hanger drop, there lies more promise in the differences between one make and another. The assortment of framesets available on the market includes a great many geometry layouts, including a wide variety of hanger drops. Therefore, it is advisable for a very tall rider to shop for the highest BB (smallest hanger drop) he can find, regardless of what that maker does with the other sizes. It is possible to find makers whose entire lines have high BB's, apparently because their designers like them.

Short riders may want to seek framesets with a lot of hanger drop, even though the concerns associated with using an average height BB -- more ground clearance than necessary -- are not as worrisome as the pedal-banging issue facing the tall riders.   You'll get more stability and smaller frontal area, as well as lower frontal area -- you can draft tall people, but they get little benefit from drafting you!

If you take the above considerations together, you realize that the proper frame needed for a tall rider getting a longer crank would generally have the top tube and the seat in the same place as before, but the bottom bracket higher. This would result in the frameset being called a shorter size, since frameset sizes are measured from the BB up.

According to Edward C. Zimmermann's Competition Road Racing Bicycle Size/ Proportions Analysis site, "In Article 49, the paragraph on Technical Specifications, of the UCI regulations the height of the "bottom bracket" is limited to the range of 24 cm minimum and 30 cm maximum."  It doesn't mention why the rulemakers would concoct such nonsense, but anyone familiar with rulemaking will not be surprised by anything.  These limits actually should not be terribly restrictive unless one is very tall or very short indeed, in which case I suggest complaining loud and long to the UCI.  You certainly cannot hold the crank length formula at fault for such idiocy.

Bill Pachoud (who is very tall indeed) points out that another implication of long cranks is that the chainstays will need to be narrow enough for the crank to clear them farther rearward than before. A simple solution would be to use a longer BB axle to position the crank arms farther outward, but this is not a pleasant alternative and may cause chain alignment and cornering clearance problems; it would be better for the framemaker to design the chainstays properly for the length crank used. On the other hand, if it can be assumed that the ideal side-to-side spacing of the pedals should be wider for a larger rider than for an average rider, maybe the longer axle is the way to go. I personally doubt this, and suspect that narrower is better for almost anyone.

The chainstays should also be long enough that the rider's right heel doesn't hit the rear derailleur. In the case of very tall riders, this may call for lengthening the wheelbase to move the rear wheel and derailleur farther away from the BB. That's not really a bad option; the very short wheelbase bicycle is overrated, especially for tall riders.

You need to watch the clearance in the other direction, too. On most bicycles, the tip of the rider's toe cannot touch the front wheel no matter what. But a longer crank combined with big feet and a short wheelbase may allow the rider's toe to foul the front wheel when the steering is turned and the pedal is forward. This is not normally a serious problem since the steering is not commonly turned that far, but it may startle a rider who isn't prepared for it during tight maneuvering. The correction, of course, is a longer top tube and longer wheelbase -- which a rider of this size probably should have anyway.

Another frame geometry issue I am often asked about is seat tube angle.  This is sort of a non-implication, really.  If a tall rider is using a crankset proportional to his size, then the proper seat tube angle for him is exactly the same angle as used by average size people using a 170mm crank.  The problem is that, up to now, tall riders haven't been using properly-sized cranksets, they've been using the same 170's that their shorter compatriots are using.  This screws up everything, since the downward pedal isn't as far forward as it should be, the ankling motion is forced rather than natural, the rider is having to maintain high cadences to avoid knee pain, etc., etc.  The result is that framemakers have been varying the seat tube angle on tall frames in an attempt to correct for these problems.  Once the use of properly-sized cranksets becomes accepted practice, this will no longer be necessary and the seat tube angle can be optimized for one and all using the same criteria -- namely, varying only according to intended application, which is generally steeper for competition and shallower for casual riding or touring.

It should also be noted that changing crank length will usually affect the gear ratios needed. When a longer crank is installed, most riders (after becoming accustomed to the new length) will either maintain the same cadence or drop it only slightly; a large drop in cadence usually indicates the new crank is too long.  However, they may put out somewhat more power, allowing them to maintain that cadence in taller gears.  Of more significance, however, is the off-peak performance; a tall rider using a short crank will need to maintain a high cadence even when taking it easy, or his knees will suffer.  Once the correct length crank is installed, he will find it much better to shift into a tall gear and pedal slowly when goofing off.  Hence, he will normally want to have a taller selection of ratios than before.

The shorter rider will find it much easier to spin with the correct length crank than with a 170, because his knees are no longer hitting his chest.  Once he finds that spinning actually works the way it should, he will usually opt for slightly lower ratios than before.

One other concern may involve frame stiffness. When a frame is too flexible or "whippy" for the rider, it can effectively defeat efforts toward more power; when the rider applies more force to the pedals, the result is a lot more frame flex and little more speed. When riding such a frame, the only way to get more speed is to spin faster -- and hence the installation of long cranks may appear to hurt performance when they otherwise would help. The best idea in this case is always to replace that frame with something stiffer, regardless of crank length concerns; sell that frame to someone lighter or less powerful.

As far as performance implications go, they are probably not what one might expect. For example, in time trialing, it is likely that correcting the crank length will not make much difference. The determining factor in a TT is the cardiovascular system, and changing the crank length will usually not have enough effect on efficiency to change TT performance much. The exception, of course, is the case where the crank was wrong by so much before as to inhibit pedalling motion or cause knee pain, in which cases the TT performance may improve considerably.

In road racing, many riders are concerned that having a long crank will adversely affect their finishing sprint. This is hogwash, plain and simple. If the crank is the right length for their legs and they have the proper gearing on the bike, their sprint will be the best it can be. More importantly, if that sprint comes after pedalling 100 miles, having the correct crank length will leave them in far better shape for that sprint.

In hill climbing, it is of utmost importance that the crankset be long enough, and too long may even have some benefit. The fact that most tall riders have always used cranks far too short for them may help explain why large riders are never expected to do well in hillclimbs. There is another reason, though: a larger organism has more trouble dissipating excess body heat than a smaller organism, and for this reason the smaller riders are always likely to excel at hillclimbs. But once the taller riders get the right cranks fitted, they certainly won't be as far behind!

There is one special case worth mentioning. The Junior and Midget racing classes have gearing limits intended to protect the knees of young competitors. These limits are a really good idea, but unfortunately they do not figure in crank length. The limit is so many inches of forward travel per turn of the crank, regardless of the length of that crank. Now, since use of a longer crank may lower cadence a little and therefore call for taller gears, tall Juniors are in something of a dilemma. If the race in question doesn't call for winding out the top gear, then applying this formula is sound. However, in any event where the competitor's pedal speed in top gear is a problem, the guys fitting the shorter cranksets are gonna do better. This is true beyond the 170mm; if the race calls for winding that top gear, going to a 165mm or even shorter may be a secret weapon! Of course, it may also screw up some knees. Hopefully, if properly sizing cranks for different size riders ever catches on, the various racing organizations will see fit to alter the gearing limits for Juniors and Midgets to reflect crank length.

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