I have always used a 172.5 crank. I sent out my body measurments to a shop to put together a build for me. They were suggesting I go to a 170mm crank arm size. I have been on 172.5mm Crank arms forever. Would I notice a difference? Is it worth changing, I mean is there any benefits to this for spinning or anything? How does one calculate teh correct crank arm size? I went with a size down from my 175mm MT. Bike crank arms when I got my first road bike. Are there any good fitting threads on RBR?

thanks for your help... :thumbsup:

I doubt you'll notice any difference. The human body is pretty accommodating, otherwise there would be 15+ different crank sizes.

That was my thought too but I am thinking I should just stick to 172.5 since that is what I have used always and not go with the shops size recommendation.

Don't forget, just coz you've always done something 1 way, or always used a certain size product, doesn't mean its the correct one.

Why not try it out?

Doesn't matter but cranks can be expensive to change. Go with what you want because, even though there may be no discernable difference, you will think there's one and it will bug you. I just bought a bike with 170's and I've always ridden 172.5. I don't notice a difference but I do think about it. I have a crank and BB I could change out (172.5) but I don't want to go through the work because I know there won't be a difference. If I spec'ed the bike before I bought it there would be a 172.5 crank on it.

You don't mention how tall you are.
If you are 5'6" or below, you might get away with it.

I run 170mm cranks... A couple of years ago, I mistakingly replaced a non driveside arm with 172.5mm. I rode 6 months before realizing my mistake..

Second Grump's post. Pip squeeks, or 5'6" or shorter usually go with 170's, right?
You'll notice a difference. Shorter= easier spinning.

.... Wouldn't it be related to femur length?

I ride 170mm on road and 175mm on the MTB. I had some XTR 172.5 cranks on the MTB and I knda prefered it. I switched because the "gurus" said I want more leverage so go 175mm. Did I notice it? Not really maybe only when going over some thing. But 175mm feel comfortable.

When getting sized for the road bike, it was reccomend that I use 170mm for a very good bike fitter. I feel fine. Maybe someday I will try 172.5 on the road bike. But again why?

The tought is smaller cranks will help your spin and longer for leverage.

I am 5' 7" with avg leg length and a 31.6 cycling inseam.

Think of it this way. When you are fitted for seat height the rule of thumb is a 15 degree bend at your knee when your at the bottom of your pedal stroke. If you switch from 172.5 to 170 you should raise your saddle about 2.5 millimeters, combine that with shorter cranks and the top of the stroke is 5mm lower. Think of it this way, It's like doing squats and not having to go down as far.

I went back to 170's from 175's and feel a lot better. I've also set new PR's for everything since switching back. People only focus on the power phase and forget about everything else. I feel better not having to bring my leg up high like I used to. People said I was going to go slower, but this has been my best year to date.

I don't know a lot about this subject but I did read somewhere that an incorrect crank lenght could lead to knee problems for some people. I guess this would depend on how incorrect the length is and probably on various differences in how your legs and joints are aligned.

I don't know a lot about this subject but I did read somewhere that an incorrect crank lenght could lead to knee problems for some people. I guess this would depend on how incorrect the length is and probably on various differences in how your legs and joints are aligned.I've heard that and I think a lot of this has to do with flexabilty and your range of motion. I don't think it's a simple as taking an inseam measurement and putting it in a formula.

You hear about doing squats in off season training that your thigh shouldn't go below parallel to the floor, but look at all the Olympic lifts that people butts are also to the floor.

I have 172.5's on all of my bikes. As Glen Beck would say, "this I know", if only the three common sizes are considered then the worst case scenario is that I am off by 2.5mm. If I switch then I hit just right, left where I should be or I am 5.0mm off. Please don't call me Mr. Obvious. Just finished a ride with big hills and I put on a 27 tooth cassette to make it easier, but it stands to reason that more leverage would be beneficial on brutal climbs, hence the 175 is optimum on the climbs, but being short fat and lazy do I really want to spend my day forcing my feet to travel that much further. There is only one way to find out and that is to experiment. Buy some bare crankarms on the bay and see what you like, assuming you have an older crankset or mucho dinero.

http://books.google.com/books?id=0JJo6DlF9iMC&pg=PA90&lpg=PA90&dq=longer+cranks&source=web&ots=Tq0AGSasj6&sig=4D_Zy3kI3R1363b8nvoT0rYOFF8#PPA91,M1

Michele Ferrari
http://www.53x12.com/do/show?page=article&id=59

Peter White
http://www.53x12.com/do/show?page=article&id=59

Imho
It doesn't make any difference at all and I'd stick with the 172.5. 2.5mm, a quarter of a cm is nothing. I'm just under 5'10, and 85 cm inseam, 45 to 46 shoe size, 11.5 to 12, and went from a 172.5 to 175 crank and I didn't notice any difference except that I went from an old Centaur bb and crank to a new Record square taper crank and bb so it seemed a little smoother. My cadence stays from 85 to 105 about 95% of the time. You won't notice the difference and in the Peter White link, he writes he went from a 170 to 175 with improved results.

Back in the day the second mountain bike I ever bought was too big for me. As a result, the cranks were 175mm. When I got back into cycling, three years ago, I bought a Cannondale full suspension mountain bike. I wanted a new set of cranks so I bought a set in 175mm because that's what I thought I had on the 'Dale. Surprise, surprise. When I rode it the cranks felt like they were longer than I was used to. I checked the cranks I pulled off and found that they were 170mm. Oh, well. I moved on to a road bike and naturally ordered 170mm cranks for it. On my second build I bought a build kit and it only came with 172.5mm cranks.

I installed a computer on all of the bikes with cadence measuring capabilities and noted the following, with regards to the cadence and different crank lengths: with 170mm cranks, I can spin up to 120 RPM and 105 RPM feels right for just cruising along at 75% output or so; with 172.5mm cranks, I can spin up to 110 RPM and 95 RPM feels right; with 175mm cranks I can't spin any faster than about 100 RPM and 85 RPM feels right for cruising. I can tell a difference when riding the bike with 170mm cranks after putting in some miles on the 'Dale with the 175s and vice-versa, but when I ride the bike with the 172.5mm cranks I can't tell either way. In other words, I can feel the 5mm difference but not the 2.5mm difference.

I'm 5'7" and I'm supposed to ride 170mm cranks. I like the way my bikes are set up so I will not be changing anything on them.