Just read something on the net at www.nettally.com/palmk/bikefit.html
that a engineer raise about multipy 5.48 times your inseam and this give you your crank size. Pretty interesting read. Bases it on pretty interesting facts.
The biggest point, and what I’m wondering is who came up with the formula for cranks. If you put a 170 on the left and a 172.5 on the right you probably will not know the differnece. Because the difference is so minute. How do we trust the formulas that have always been used and reserch a new and better one if one exists.
I have used the Lemond formula’s and have discovered them to put me on a bike that is too small. A seat height that is to short. I have the computrainer to prove that. More power with a 1 1/4 higher that what Lemond would have put me at.
How do say that a road bike fit and a tri bike fit is the same. Or even a mountain bike frame. Different crank sizes and especially a different seat tube degree. Will these frame and seat heights not be radically different, especially since the riding and the terrain is so different?
Just curious–with a 32.5in inseam acccording to the formula I would be using 178 size cranks. Radically different than the LBS, Lemond, Peter White formulas of 170172.5 cranks. Who’s right!?
Thanks for reading, Matt
(1) If you put a 170 on the left and a 172.5 on the right you probably will not know the differnece. Because the difference is so minute. How do we trust the formulas that have always been used and reserch a new and better one if one exists.
(2) More power with a 1 1/4 higher that what Lemond would have put me at.
 How do say that a road bike fit and a tri bike fit is the same. Or even a mountain bike frame.
 Just curious–with a 32.5in inseam acccording to the formula I would be using 178 size cranks. Radically different than the LBS, Lemond, Peter White formulas of 170172.5 cranks. Who’s right!?
Thanks for reading, Matt

if you put a 170 on the left and a 172.5 on the right, you will feel it. Guaranteed. (I know from personal experience, putting the bike together late at night, grabbed the wrong crankarm.)

1 1/4 what? inches, cms, mms. Remember all that any formula gives you is a rough guide, to be fine tuned by YOU.

They’re not. esp not the mtn bike, I need about 1cm lower on that. horses for courses.

Who’s right? All of them. No one. Who knows. Borrow a set of 177.5 mm cranks and find out if they work for you. I have almost the same leg length as you, if I put on 175’s my spin goes to pot, so I really doubt that I could use 177.5 's.
Again, this one is a bugger. The best crank literature I’ve seen has been Lennard Zinn’s columns in VeloNews and Inside Triathlon fro some years ago and also in Bernard Hinault and Claude Genzling’s book “Bicycle Road Racing” which is no longer in print in English. I tried your formula just for amusement and got some pretty wacky results. There are enough factors involved that I haven’t yet seen a “formula” where you could plug one dimension in and then get a crank length out. We look at shoe size, leg length (overall), femur length as compared to leg length and pedalling style as well as body type. Remember though, the difference between 172.5 and 175 cranks is not 2.5mm, it’s 5mm or 2X2.5mm. BTW, I could never see a situation where I would put a customer on two different crank lengths Holy back problems Batman!
I did a search a view weeks ago on the net for information about crank length. There certainly are a bunch of theories. Most of it somewhat contradictory and nearly all of it ignored by the bike companies when they spec bikes. And frankly, I am not sure how much it matters.
I have seen these theories put into practice in the case of leg length differences.
I was pretty interested in the theory of longer cranks for mountain bikes. Given that most of the time on mtb the consistant spin is not as inportant as the ability to turn power on and off. I asked some former bmx rats and they confirmed that the added leverage of longer cranks was helpful.
Do bike companies really investigate this problem or answer this question–you never see literature by bike companies on crank arm length. Do they care? They probably don’t because the supply is not great for their demand. If you only care about money and selling bikes, they will put anything on a bike and sell it. Is R&D put into every aspect of the bike. No but we now have so great XTR shifters that cost a arm and a leg but really don’t give a whole lot of improvement to the old but we can sell them as new and make money.
The change is 1 1/4 inch higher than Lemond.