Choosing Crank length

Hey guys, I just need some really good advises on choosing the right crank length for my bike.

I have been riding with a 165 mm crankset with the seat post raised quite a bit below the ‘maximum’ mark on it and it has been really good. I have just been given a 170 mm crankset to try out and have lowered the seat post to the position that falls below the ‘maximum’ mark a cm or two. Both of the time I adjusted the seat post according to ‘The Heel method’.

How do you choose whether to go for 165, 167.5, or 170 mm? I do know about the fact that with longer crank length it sure gives more push power, but because it is longer you loose the quickness of pedaling when the crank is on the upright position; unlike shorter crank where the revolution is shorter.

I need your help all you wise men!

Cheers!!!

There’s a few things to consider… Sheldon Brown had a great article but I had a quick look and couldn’t find it.

From what I can remember he suggested that because you can choose your cog the crank length becomes less important, but in general larger cranks suit longer legs and shorter cranks fo shorter legs.

There’s also (as you mentioned) the fact that shorter cranks suit a higher rpm which in my opinion is more important than mashing on a fixed gear. Shorter cranks also give more ground clearance so if you’re not on a track geometry frame that might be important so you don’t snag going round a corner

hope that helps… Anyone feel free to correct me if I’m wrong!

165 was/is the norm for track bikes- mainly for clearance, nowadays a lot of riders use 170.
This is possible because of the lower profile pedals used. try it yourself- put on a fully caged pedal such as a campag pista and strap in a shoe, lean the bike over - then try the same thing with a low profile ,single sided modern “clipless” pedal- you can lean over a lot further with the same length crank.
It is the same reason I believe that on road cranks 172.5 is the norm rather than 170mm of the days of pedals and toeclips.
The difference in power is negligible- over a 5 mm difference, over a 2.5mm - just makes cornering that bit more exciting !

Oh and you either need to get a longer seatpost or a bigger frame - simple maths states if you lengthen the cranks by 5 mm then you drop the saddle by 5mm ???

Yeah I agree… I can only really tell teh difference if I try to - if its a fixie go shorter, if its a mtb go the right length for your height

and heres the link to that article now that i’m not on my phone. As always Sheldon has the answer:

Bicycle Cranks

Digging up an old thread here - I’m trying to make up my mind about this right now.

I’m re-arranging the parts between my track bike and my fixed-road-training-bike. I have two sets of old Superbe Pro cranks to put to use, one 170mm and one 165. I’m 178cm tall, so either size fits my legs fine.

This is what I’m thinking so far:

For the track:

  • 170mm gives better leverage for standing starts, and seems smoother for the kilo, motorpace etc. (I’m not a sprinter)
  • 165mm is safer on the banking. But then lots of people use 170. My BB height and pedals are normal enough.

For the road

  • 170 gives better leverage for smashing hillzzz. I like hilllz.
  • 165 safer on the corners, but again you’ve really got to lean it right over. This is training, not crit racing.
  • 165 is easier to spin fast on, so better on the road keeping up with the roadie bunch.

On average I think I spin faster on the road than on the track, due to the lower ratio, so I’m leaning towards the 165s for the road.

Anyone got an opinion?

And on a side note, there’s a mistake in Sheldon’s article on crank length:

“I had a pair of TA 150 cranks … so I’ve put these on the Trek… For a given speed, my pedal rpm is higher (though my pedal speed is the same) but the short cranks make it easy to spin much faster than I normally would.”

I’m sure that should read:

“I had a pair of TA 150 cranks … so I’ve put these on the Trek… For a given speed, my pedal speed is lower (though my pedal rpm is the same) but the short cranks make it easy to spin much faster than I normally would.”

At the same rpm, the pedals on short cranks are doing smaller circles, ie: not travelling as far as they would be on longer cranks, in the same amount of time.

Done a fair bit with kid’s bikes since my last post on this thread- would say that 165 road cranks are a great idea for anyone with really short legs, but on a geared bike- no advantage on the road, I ride 167.5 on the track and 170 on the road, all calculations say I should ride 172.5 or even 175,tried both lengths but in 35 years stuck with the 170’s
I have tried 170 on the track, and oculd not notice much difference- mechanically there is very very little in the numbers- your rpm with 165 and 175 cranks for a given gear will be the same.

PW - I agree.

I did a bit more reading, and I think the ergonomics of riding shorter cranks (smaller leg circles) and more important than the other factors. When down in the drops, your knees come closer to hitting your chest with long cranks.

Ground clearance isn’t an issue unless going very slowly on the track (match sprint) or in a crit.

I can definitely spin faster/more smoothly with shorter cranks - I know this from riding my polo bike down the highway on 51 gear inches, and testing 170, 165 and 160mm cranks.

I’m going with 165 on the road and 170 on the track for the above reasons.

The most concise explanation I found was this one:
Crank Length Suggestions- Roger Marquis