crank length advice

eric_draven

hi,i ride both road and mountain bikes,,and have used different lengths on mountain bike,i can feel a difference between 170 and 175,how much difference is there in a 2.5mm step,i currently ride 172.5 will there big a big jump in going to 175,as i am looking to upgrade my groupset ,I have a 32" inside leg,any advice would be welcomed

cougie

In my experience - you can't notice any difference.

cycleclinic

On my road bikes I have 170mm, 172.5mm, 175mm and 177.5mm. I am comfortable on all of them. For 32" inside leg if you have a choice then a 172.5mm would be "perfect" but a 170mm would work as would a 175mm. I would not worry about the difference, I can feel the difference but I can swap between bikes and not encounter problems.

On my MTB's I have 170 and 175mm cranks and I prefer the 170mm even though I have 34" inside leg.

bernithebiker

I find it can be as simple as this; do you want you increase your cadence? Many cyclists would benefit from upping their cadence, especially on hills, etc. Power = speed x torque, so if you can keep your force on the pedals constant, but increase cadence, you're going to go faster. Grinding up a hill at cadences less than 60 is not the most efficient way to do it.

But if you feel you already turn the pedals quite nicely, sit at 80 all day and can spin up to 100+ no problem, then maybe consider the longer cranks.

Shorter cranks = less chance of groundstrike in corners, teeny bit lighter.

Phil_D

http://www.peterwhitecycles.com/fitting.htm

Crank Length
Crank length determines the diameter of the circle that the pedals move in. The larger that circle is, the more flexion of your knee and thigh muscles will be needed to turn the cranks. Your thigh muscles cannot exert the same force throughout their range of motion. This is very easy to demonstrate. If you squat down so that your knees are fully bent and lift yourself up, say, five inches, it takes a good deal more effort than it would to squat down just five inches from standing straight and then lift yourself back up. At the full squat position, your muscles can't put out the same power as when your knees are just bent enough to drop you down five inches. So if you had to choose between a crank length that had your knees bending through their entire range of motion and a length that only required, say, 20 degrees of flexion at the knee, you would choose the shorter crank. That crank would have your muscles working through a more efficient range of motion. You would avoid having to flex your knees enough to bring you into an inefficient range of motion.

So how long should the cranks be? Well, that's a good question. I wish I had a good answer but I don't. It should be obvious that a 5' 2" rider would not want to use the same length crank arms as a 6' 7" rider unless they somehow managed to have the same leg length (highly unlikely). Some research has been done to determine the optimum percentage of leg length to crank length. I doubt that there is an optimum percentage that would apply to all people. One writer in a major magazine article quite a few years ago claimed that after considerable testing with many different riders, 18.5% of the distance from the top of the femur to the floor in bare feet should be the crank length. You can find the top of the femur pretty easily. It's 5" to 6" below your hip bone, and moves rearward when you raise your knee. After reading this I promptly changed from the industry standard 170mm cranks for road bikes to 175mm cranks. There was an immediate improvement in power and endurance. I began using this formula when recommending cranks to my customers. So far, I haven't gotten any complaints. But of course that doesn't mean my customers wouldn't be as happy or happier with some other length. And I must admit that I have never tried still longer cranks than 175mm for enough time to tell if I would be even happier with them.

The top of the femur measurement ignores differences in legs themselves. Differences in the proportion of calf length to thigh length should affect the optimum crank length. A rider with longer thighs and shorter calves would use a longer crank to get the same flexion at the knee as a rider with short thigh and long calf. Of two riders with the same body proportions, one might prefer to pedal at a faster cadence. That might favor a shorter crank length. And perhaps even two riders with identical skeletal proportions would find after testing that they required different crank lengths to achieve maximum performance due simply to differences in their muscles.

Trying different cranks to find the optimum length would be time consuming and expensive, but I believe it is the only way to determine the correct length for any individual, assuming there is a correct length. It would be nice to have a crank with many pedal threads at various lengths to test. But I know of no such thing being made and I lack the ability to make one! Of course, some riders with multiple bikes report being just as happy on one crank length as another. Go figure! So, for lack of a better system, I'm staying with the 18.5% guide for my customers until something better comes along. It hasn't failed yet.

Note: As of August, 2007, we have here at Peter White Cycles a special crank with adjustable length that we use while fitting cyclists to our bikes. And, if you would like to come by and have it installed on your bike for determining your best crank length, just call ahead for an appointment.

In the United States, it has been difficult and expensive to obtain cranks shorter than 165mm or longer than 175mm. But a French company, Specialites TA has been making high quality cranks in lengths of 155mm through 185mm for many years. In order to offer my customers better fitting bicycles, I've decided to sell these cranks. See my web page; http://www.PeterWhiteCycles.com/carmina.asp for details.