Why custom cranks and how long to get them?
Here is the formula I recommend:
Crank length (mm) = Inseam (mm) X 0.216
Or, more conservatively for tall riders:
Crank length (mm) = Inseam (mm) X 0.21
Another formula that I like is from fit guru Bill Boston (http://www.billbostoncycles.com) and comes up with similar results. He suggests measuring your femur (thighbone) from the center of the hip joint to the end of the bone in inches. This number will be your crank length in centimeters. For instance, if you have a 20†femur, you would have a 20cm (200mm) crank.
Andy Pruitt, director of the Boulder Center for Sports Medicine and fit expert of many superstars, has a few other things to add.
“Crank length formulas using femoral length or leg length are fine,†he says. “But if your style is mashing, use longer cranks, and if you are a spinner, shorten them a bit. Mountain bike cranks should be a bit longer for that moment to get you over a rock. Use 2.5mm or 5mm longer for purely time trial usage, and vice versa for the track.†Pruitt warns that, although one study showed that everybody was faster with a super-long crank over short distances, you can hurt yourself if you do not stick to proportionality. Pruitt goes on to say that if you use cranks too long for your legs, the compressive and shear forces in the knee joints “go up exponentially.†(Compressive forces in the knee are stagnant, felt behind the knee. Shear forces are the result of fore-aft sliding of the condyles – cartilage-covered rounded femur ends – as they are rotating on the soft meniscus – cartilage pad – atop the knee platform.)
What else to change?
Before I get into the whys and wherefores of these formulas for crank length, I want to tell you what other ramifications adjusting the crank length has on your bike position. It makes sense that if you increase your crank length, you should lower your saddle and stem by the amount of the change, and vice versa for decreasing crank length. This maintains the same pedal-to-saddle reach. However, in practice with riders on stock frames and cranks, I have found that it is often preferable to leave the stem and bar where they were and only move the saddle. With tall riders, this is because they often have an exceptionally large amount of drop from their saddle to their handlebars anyway, if they have shoehorned themselves onto a bike that is too small for them. Increasing the crank length requires lowering the saddle the amount of the length change; that’s a given, assuming your seat height was correct before. But the longer crank will make the knees come up higher and may hit the chest and give an uncomfortably sharp hip angle if the large amount of drop from saddle to bar is maintained. Conversely, a short rider may have minimal drop from their saddle to bar on a stock frame, having already lowered the handlebar as far as it could go. This may be as low as they can tolerate having their handlebar anyway, since if their stock crank length may be proportionately so long for them that their knees come up very high, and they cannot drop their chest down very far without contacting them. When they switch to a proportionate-length crank, the saddle will come up the length of the crank-length decrease, giving them more saddle-to-handlebar drop, which may work out well, since their knees will not be coming up so high, and they will not be so constrained by their hip angles and chest height.
You could argue that you should also push the saddle forward and increase your stem length by the amount of the length increase as well. This adds some complexity, because, to maintain the same pedal-to-saddle distance and saddle-to-bar drop, the seat and handlebar should also go up half the distance of the forward movement as well. The inverse is true if you switch to a shorter crank – raise the saddle and bar the amount of the length change and perhaps adjust the saddle aft.
With a longer crank, your pedal clearance in a corner will be reduced, and vice versa with a shorter crank. So, ideally, the frame’s bottom bracket height should be greater with the longer crank and lower with the shorter one. And since more or less of your leg extension will be taken up in the crank if it is longer or shorter, the seat tube should be shortened or lengthened accordingly (from the bottom, by raising or lowering the bottom bracket).
Why proportionality between leg and crank length?
No other conclusion makes sense to me. Muscles and joints work most effectively when operating in a certain range of motion. Short riders should not be required to force their muscles through a greater range of motion than the person with an 80cm inseam riding a 172.5mm crank. And on the other end, 7-foot basketball players do not bend their legs any less when they jump than shorter players. So why should they use minimal knee bend and operate their muscles only through a tiny part of their range when they ride a bike?
I published some crank-length tests in VeloNews in 1995 and 1996. These tests were either inconclusive or seemed to indicate that all riders, regardless of size, put out more maximum power with super-long (220mm) cranks, and that all riders had lower heart rates at low power outputs with super-short cranks (100 to 130mm). My experimental method in these tests was lacking in those tests, but I was simply not willing to stop there, since I knew from personal experience that increasing crank length for a tall rider like myself (6’6â€Â) makes a difference. It also made sense to me that there must be a limitation dependent on rider size for how long you can go. In the late 1970s, when I went from 177.5mm to 180mm cranks, the improvement in my racing results was marked. In 1980 when I was on the national team, coach Eddie Borysewicz told me that I should be using yet longer cranks than my 180s, and longer yet for time trials and hill climbs, but I never found anything longer at that time. Since then, I have continued to experiment, beginning by using the range of cranks that Bruce Boone built for those 1996 tests (eight cranks, evenly spaced between 100mm and 220mm) and of these eight, I found that I was most happy with 202.3mm cranks. (The weird length has nothing to do with some super-accurate calculation related to my leg length; it is because the eight test cranks were evenly spaced between 100mm and 220mm, or every 17+ millimeters.) Eventually, those Boone 202.3s broke, and we started making our own (stronger) custom cranks. For at least three years now, I have been riding 205mm cranks most of the time (200mm on the mountain bike, due to clearance constraints) and find that I not only like them, but that I am considerably faster on the nearby, approximately half-hour climb up Flasgstaff mountain west of Boulder that I do frequently with a stopwatch.
Thus encouraged, I have conducted other crank studies ever since. However, in understanding what went wrong in those 1995 and 1996 tests, I developed higher standards for what constitutes a publishable test, and my subsequent tests still have not met that standard, mostly due to having too few subjects (often just me and a couple of other riders). Too bad, because I have put a lot of time and effort into a number of them! It is one thing if you are a physiology researcher trained and funded to do these sorts of studies. It is not easy to do a test in which you eliminate all other variables besides crank length. It requires lots of time, planning, willing (read, paid) subjects and equipment. It’s hardly the type of thing that is realistic to undertake with no budget in order to write one article for a cycling magazine that still expects an article from me on something else every two weeks as well. Anyway, I have conducted all of these recent tests on the road with tall riders (6’5†and over) because it was simpler and cheaper to use my personal stable of bikes than to always be switching cranks on other people’s bikes. By being willing to take my custom crank recommendations, my tall custom frame customers have also have graciously acted as test subjects. Besides having data showing people going faster and generating more power on my own personal bikes, it is hard to deny it when you have many people raving about how much more comfortable, natural and powerful they feel on cranks proportional to their leg length. On mountain bikes, tall customers report being able to smoothly power over obstacles they could not have before. They also report liking the longer, more stable stance when coasting downhill over technical obstacles. And the higher bottom bracket I build into the frame makes hitting the chainrings on logs and the like almost impossible (especially with 29-inch wheels), yet the rider’s center of gravity is no higher (since the bottom foot is still the same height above the ground due to the longer crank).
The results indicate clearly enough to me that crank length must be proportional to rider size in some way. Whether you decide it is proportional to leg length, thigh length, overall height or something else is a minor point. The same goes for what you think the constant of proportionality should be. It could be something different from 0.21 or 0.216, but whatever it is, it will indicate for a lot of people that they should be using a vastly different length than they are. That is the part that is hard to accept for a lot of people. No matter our size, we are by and large all stuck on cranks of the same length. The 3% difference between a 170mm and a 175mm hardly constitutes a length choice, and the 180mm you can find in only top-end component groups still does not broaden the range much. Accepting that cranks should be scaled up or down with rider size opens up a whole can of worms that a lot of riders, bikes-shop personnel and component companies would just as soon stayed closed. Obviously, economies of scale of producing cranks go out the window if you have to supply a range from say, 140mm to 220mm, and a bike shop’s inventory costs go way up to stock a lot more lengths as well. The same goes for bike frames; if a manufacturer increases the bottom bracket height with every increase in frame size in order to accommodate crank arms proportional to the size of the rider, its costs and complexity of frame jigs goes up, and the staff training and inventory costs for bike shops goes up as well.
There are obvious practical reasons to stick with the status quo. Those may have to do with what is best for the pocketbooks of consumers, bike shops, and manufacturers, but not what is best for the rider’s performance and comfort.
The constant of proportionality
Okay, if you have accepted the idea of a proportional relationship between leg and crank length, how would you come up with the constant of proportionality relating them? I propose that one way would be by looking at what works for a wide range of riders. For instance, the world is full of successful bike racers with 80cm (31.5â€Â) inseams. Thirty years ago, racers with inseams this length probably would have been racing on 170mm cranks. Nowadays, they would likely be on the extremely popular 172.5mm length. (In 2003, approximately 50% of the high-end carbon road cranks that FSA sold were 172.5mm, 35% were 175mm, and only 15% were 170mm. Campagnolo’s approximate 2003 sales numbers were 60% in 172.5mm, 30% in 170mm and 10% in 175mm. That is a big change from around 1970, when the vast majority of all high-end road cranks were 170mm.) If a rider has an 800mm (80cm) inseam, 170/800 = 0.2125. In other words, a 170mm crank would be 21.3% of an 80cm leg length. Furthermore, a 172.5mm would be 21.6% of it, while 165mm would be 20.6% and 175mm would be 21.9%. So, if you multiply a rider’s inseam in millimeters by 0.213 or 0.216, you will determine a crank in the same proportion as a 170mm or 172.5mm for a rider with an 80cm inseam. Both riders’ knees and hips will go through the same bending range, and their muscles will reach the same extension and contraction.
If you want to be conservative on the long end, you could go with 0.21 for the constant. This is what I have been doing for a number of years with my very tall custom frame customers. For instance, a 6’7†rider with a 100mm inseam would use a 210mm crank with a 30cm high bottom bracket. My tall and short customers opting for custom, proportional cranks almost universally love the new length. On the other hand, 0.21 gives surprising numbers on the short end, like 168mm for our rider with the 80cm inseam. So you could argue for 0.216, since that yields 172.5mm for an 80cm inseam, consistent with what we see in pro racing. The 6’7†rider’s crankarm gets 6mm longer with 0.216 than 0.21, but notice that we are now haggling over a few millimeters while being centimeters beyond where the tall rider would have been when locked into the normal crank length range.
Can you test for what is ideal for you?
Trying various cranks and seeing how you measure up against other riders with whom you are competitive or timing yourself up a climb you frequently clock is a good way. There are adjustable-length cranks available, but they are boat anchors and increase your stance width, rendering objectivity difficult. On http://www.nettally.com/palmk/Crankset.html Kirby Palm offers some ideas about crank length testing.
if you kept the pedals turning. I replaced them with 162mm trials cranks and problem solved...it also means I can keep a higher cadenece much easier :wink: 
) so I had to pay a little out of pocket, but fortunately I went with a much more bicycle knowledgeable friend of mine who steered me to these beauties!
