Road Cycling - crank length

Hi guys,

A quick query on crank length - sorry if it's been done to death before.

I'm 173cm (5'8) tall and have a 83cm (33") leg length - from the stand against the wall and wedge book in crotch method. I think that means I have long legs.

My cranks are 170mm and lately I've been feeling like they're too short - as in when I'm spinning, my legs feel kind of constrained. Should I get longer cranks? I'm tempted to try some, but I find it hard to believe that a 2.5 or 5mm difference is really noticeable, after all 2.5mm is only 1.5%.

Anyone here got this really sussed out and does 2.5mm of crank length really make a difference?

I've already considered the idea of foot speed being the important factor and relating that to gearing, but haven't reached any great conclusions yet. At the moment with a 40/20 low gear, I feel that my foot speed is too low when I'm going slow up hills. Should I expect my new bike - 2 weeks away - with a 39/23 and still with 170mm cranks to feel considerably different or would longer cranks still be worthwhile?

I am almost the same height and the cranks on the bicycle I ride are 175 mm. Which feels just fine.
The increase in 5mm on one arm, gives you a whole sentimetere increasement in diameter around which pedals travel. That will affcre the lenth of the circuit through formula, what was it, 2pR? Anyway, it means you will have to turn the front chain rings slower to have the same rotation speed at the rear wheel. I think this is very important, as on the flat roads, when you have enough power and strenght, this will help you keep a higher speed.
Same applies to going down the hill.

I would definitely try 175mm crank arms.

Yeah, I think you should have 175mm cranks. I was always told that 170mm cranks were for pretty short people (5'6" and below??). If you have long legs for your size, then this would also call for a longer-than-usual crank.

If you're in that grey area between sizes, I think the general rule is to go with the longest size you can get away with. Longer cranks = more torque.

Colorado Cyclist has great bike fit info on their website (under Resources). For cranks; they recommend 170mm cranks on 54cm C-T road frames and smaller, 170–172.5mm cranks on 55–61cm frames, and 172.5–175mm cranks on 62cm frames and larger. They say mountain bikes generally get cranks 2.5–5mm longer than road bikes; that is, you might want a 175mm crank on your mountain bike if you’re set up with a 172.5 on your road bike.

Thanks chaps, I think I'll give some 175s a go. Before I do that though, I'll do some cadence/foot speed/bike speed/torque calculations just to entertain myself and so that I fully understand the differences.

The only trouble with changing cranks now of course is hassling the shop to do it for me before I pick up the bike.

I find crank length changes very noticable. Going from 170mm to 172.5 seemed huge.

Perhaps this is too late to influence your opinion, but I'd opt for 172.5s, and possibly move up, rather than jumping all the way to 175s. It really makes quite a bit of difference in bike setup... lower saddle, changes KOP, more knee bend at the top of the stroke.

I'm 171cm tall with an inseam of 81cm and I use 172.5mm cranks.
IMO 175's will be too long for you and you'll find yourself cranking a lot.

Is this for a road or mtn bike?

If it is road, you should try 172.5 and 175 for mtn.

I rode on 172.5 arms on my road bike for about eight years. Last year I upgraded to Dura ace and got some 180 arms ( I am 6' 5" (200.5 cm) with a 36" (91.4 cm) inseam).

For me the change was well worth it. It feels much more natural and I actually increased my rpm average (I now spin in the mid 90's). I really notice the inprovement on the climbs.

I put the 172.5's on my fixed gear. This seems to work also.


The only downdfall is you can not pedal through alot of the corners. So I coast, then I have to pedal a little more to catch up to people.

I have to be very careful on my fixed gear in corners.

Thanks everyone for the constructive replies. I think I'll give the shop a call and see if they've got some 172.5 cranks before they finish building.

Amusingly, I found a web site yesterday that told me I should use 180s... the URL is http://www.nettally.com/palmk/crankset.html if anyone's interested.

There's a lot of information out there about this but it seems to be just as contested as the knee over pedal spindle issue. I'd love to fully understand it, I guess the only way is to try things out and see what happens.

(Oh yeah, it's a road bike.)