I have a P3 w/ 170mm cranks on DA10 and have recently put them on my R2.5. ( had 175mm) My Question is are 172.5 a good choice to get for the P3? I am looking at the FSA Mega exo or team issue. I am 5’ 9" 158 30in. inseam. I know the 175mm were too long. I like the 170mm but thought maby 172.5 might be a better length for TT. A little advice would be great. Thanks in advance.
I suspect that given your height, the 170 mm. cranks would be a good bet. I base my crank recommendations on a percentage of femur length. A good rule of thumb is to measure your femur in inches and use that number as the length of your cranks in centimeters. For example, a 17 inch femur would yield a 17 cm. crank (170 mm.).
For a more detailed explanation, see the crank length topic under bike fit http://www.billbostoncycles.com/bicycle_fit.htm on my web site.
Bill Boston
Just an observation in general…not specifically directed at you…
It’s strange that people give measurements significant to 25.4mm and are looking for advice on selecting cranks that vary by 2.5mm.
Anyhow, I’d make a recommendation based off your height in mm, your inseam in mm, and the measurement from your navel to the ground, standing flat-footed with your arms stretched above your head to be sure you are as upright as possible, also in mm. Obviously somebody else would have to take those measurements.
Kinda personal question if you ask me.
Bill, while i see that you are trying to provide a service and to provide some guidance, the no. of unsubstantiated assumptions about leg vs. crank length that you make in your fit website is quite vast. i could make a list of them, but i am not sure there would be a point.
it is interesting, but given the amazing lack of verifiable science in almost almost all aspects of bicycle fit and given the vastly different solutions riders of different sizes and styles use to find incredible speed, why don’t people in the industry admit at least a few things?
for starters, 99% of cranks are all extremely close to one size (around 17 cm) for one very good reason (but it’s not fit at all): it is one hell of a lot easier (read cheaper) to manufacturer standardized bike frames to fit a very narrow range of crank lengths than to manufacturer standardized bike frames to fit a wide range of cranks. sad as it may seem, that is the reason, not because of fit or anything related to it.
What cadence you prefer to ride at is just about as important as any biomorphic issues… If you’re a real spinner, go shorter. A real masher, go longer.
Greg,
I agree that the prevalence of 170 mm cranks is all about profit for the manufacturers and has nothing to do with what is best for the riders in terms of fit or efficiency. if you are lucky enough to be in the 50th percentile (about 68 - 70 inches tall) that 170 mm cranks actually work for, then there is no issue. If on the other hand, you fall outside of this height range, you might want to consider exploring other crank length options.
I feel that my 15 years as a custom frame builder and over 30 years devoted to the improving frame fit allows me some latitude in making the “unsubstantiated assumptions” that you refer to.
Actually, I am doing more than “trying” to provide a service. Accufit was developed for my use as a custom frame builder to help me provide better fitting and performing custom frames for my customers. Several years ago, I decided to offer Personal Accufit for $29.95 to help individual riders determine a good starting position and provide help in setting up their bikes to match that position. This decision was prompted by the industries lack of understanding (or at least interest) about fit. $29.95 is a lot cheaper than using the trial and error method of finding the right length stem.
Since I first offered Personal Accufit, I have sold hundreds of copies and offer personal assistance to these users. In that time, I have had virtually no complaints from users and receive e-mails from many thanking me for providing the software.
Accufit is slowly gaining popularity among coaches, professional bike shops and frame builders looking for consistent and reliable fit information.
Bill Boston
Ok, Bill, if you don’t mind a little inquiry, I am quite curious to ask you this: what does your 15/30 years devoted to framebuilding and fit make you feel about these interesting and controversial assumptions (below)?
http://www.nettally.com/palmk/crankset.html
http://www.insidetri.com/train/bike/articles/1875.0.html
Both links have some very intriguing formulas and assumptions regarding crank length vs leg length, of which inseam is a rough measure. They seem to imply that a lot of people even within the 50th percentile are riding on cranks too long or too short for them, and by a good measure.
In the second link, you have to go down the page just a bit to come to Mr. Zinn’s formula. My questions are this–what do you think of these formulas and if you put your own inseam into the formulas, how close are the results to the crank length you are riding now?
Greg,
While I had visited both of these sites in the past, your post prompted me to visit them again.
I agree with most of what Palm Kirby has to say regarding crank length. In fact, the only thing that I disagree with is that it is easier and more repeatable to measure crotch height than femur length. Actually, up to about two yeaqrs ago I would have agreed with that too if I had not been plagued with widely varing crotch heights that gave me difficulties in providing accurate computer models of riders. The latest version of of Personal Accufit contains a new measuring method (Accufit Links) that does not require a crotch height measurement and provides more accurate and repeatable measurements. To see hoe this measuring system works, I would suggest that you download Personal Accufit and read the help files and try it out with my measurements (demo mode).
It is interesting to note that PK prefers femur length as a basis for calculating crank length.
Using the PK calculations, I would require a 175 mm crank. Accufit recommends a crank lenght of 165 - 172 mm. I have always used 165 mm on a fixed gear and 170 mm for the road. The reason for the difference in the Accufit and the PK crank lengths is that I have femurs that are a bit on the short side compared to my total inseam. The results using Lennard Zinns formula was quite similar as one would expect.
I think that the biggest difference between myself, LZ and PK is that they have been primarily concerned with the plight of tall riders while I have been campaining to get the word out to the smaller riders (under 5’6") that make up well over 50% of the adult population.
I feel that smaller riders should be concerned about using excessively long cranks because of the excessive knee lift that in many cases causes the pelvis to tilt to a more upright position making them less able to tolerate longer top tubes. In an industry where manufacturers seem unwilling to actually produce top tubes that are actually short enough for the sub 5’4" riders, the use of 170 mm cranks is usually enough to make the difference between an enjoyable form of transportation and exercise and pain and discomfort.
I find nothing controversial about any of these assumptions. Our methods may vary slightly, but our goals are all the same, to provide riders with imformation that will make them more efficient and comfortable.
Bill Boston