Crank length

secretsam

Have ordered a Spesh Tarmac in 54 but it comes with 172.5mm cranks; my current road bike uses 170mm. I've inquired about swapping them but do I need to? Am concerned as I only have short legs - 80cm inseam.

imposter2.0

I switched bikes a few months and in doing so went from 170 to 172.5. Felt different at first, but feels completely normal now. As long as you compensate your saddle height you shouldn't have any issues..

lesfirth

secretsam said:

Have ordered a Spesh Tarmac in 54 but it comes with 172.5mm cranks; my current road bike uses 170mm. I've inquired about swapping them but do I need to? Am concerned as I only have short legs - 80cm inseam.

My legs are shorter than yours! Your legs are not short by my standards.You won't have a problem.
I have bikes with cranks from 165mm to 172.5mm and I can't tell the difference. However you have to remember I am the guy who can't tell if my wheels spin up quick or roll well or if my bottom bracket is stiff.

StillGoing

I have short legs and use 165mm cranks on my main bike. Of my other bikes, two have 170mm and one has 172.5mm I can tell the difference. How do I tell? By the room I have coming over the top on the pedal stroke and my cadence is slightly slower. I'd ideally like 160mm on the TT bike.

thecycleclinic

You can use what ever crank length tou like. In principles shorter legs means shorter cranks but there are studies that show we should use child length cranks really. So it all a pile of jumbo jumbo. Use whatever you like. Track racers use 165mm yet the riders on the road use long cranks. It turns out there no right answer.

rick_chasey

As a fellow I found the switch to smaller cranks to be mildly helpful in keeping the hip angle open.

It’s definitely a little more spinny

StillGoing

thecycleclinic said:

You can use what ever crank length tou like. In principles shorter legs means shorter cranks but there are studies that show we should use child length cranks really. So it all a pile of jumbo jumbo. Use whatever you like. Track racers use 165mm yet the riders on the road use long cranks. It turns out there no right answer.

Bikes tend to come with the standard 172.5mm for cost saving purposes as the average across the board. It's maybe also another money spinner for the manufacturers knowing customers with shorter or longer legs will want to change.

A lot depends on what type of rider you are. If you like grinding gears out with a slow cadence, then maybe a long crank and the extra torque suits you. But if you like a high cadence, a shorter crank will suit. The hip angle at the top of the stroke can be too tight for riders like myself, with no way to open the angle up other than dropping the saddle onto the top tube (yuk) or by using shorter cranks. A tight hip angle makes it harder and more tiring to get on the power stroke. The shorter crank arm means the saddle can be raised creating the extra gap at the top.

Plenty pooh-pooh the idea of shorter cranks claiming 2.5mm difference going from 172.5 to 170 is not noticeable. Except its actually 5mm difference at the top. Going from 172.5 down to 165 is a massive difference of 15mm at the top. I know my legs are fresher for longer using the 165mm than the 170mm.

imposter2.0

StillGoing said:

Plenty pooh-pooh the idea of shorter cranks claiming 2.5mm difference going from 172.5 to 170 is not noticeable. Except its actually 5mm difference at the top.

It isn't 5mm at the top. It's 2.5mm at the top and 2.5mm at the bottom, so 5mm in total.

New_Old_Stock

I don't notice the difference between 170 and 175. Haven't tried 172.5 so I can't be sure.

amrushton

you are describing a smaller circle (pi x d) so a shorter crank is a smaller diameter. Malcolm at Cycle Clinic has it right and some TT riders prefer them as it can stop your knees hitting your chest. Phil Burt has 3 mins on You Tube on shorter cranks

imposter2.0

thecycleclinic said:

Track racers use 165mm yet the riders on the road use long cranks. It turns out there no right answer.

Crank length varies on the track as well, there's no convention on using 165. 170 is also pretty common.

rick_chasey

imposter2.0 said:

StillGoing said:

Plenty pooh-pooh the idea of shorter cranks claiming 2.5mm difference going from 172.5 to 170 is not noticeable. Except its actually 5mm difference at the top.

It isn't 5mm at the top. It's 2.5mm at the top and 2.5mm at the bottom, so 5mm in total.

Sure but most people set their saddle so the leg extension is the same at the bottom of the pedal stroke.

StillGoing

imposter2.0 said:

StillGoing said:

Plenty pooh-pooh the idea of shorter cranks claiming 2.5mm difference going from 172.5 to 170 is not noticeable. Except its actually 5mm difference at the top.

It isn't 5mm at the top. It's 2.5mm at the top and 2.5mm at the bottom, so 5mm in total.

Instead of being pedantic, you'll see that is exactly what I put. When you set the pedal at the bottom of the stroke for the leg extension, you create 5mm at the top.

fenix

It's a tiny difference. Look at how long your legs and foot are.

I'd definitely try what you've bought before buying anything else.

thecycleclinic

Biomechanically we should be using 155mm cranks or shorter. I think 145mm came out of one paper.

Shorter is on trend as well.

As for shorter being more spinner - your gear ratio has not changed. In fact to get the same torque you have to put more force at the pedals slightly.

StillGoing

Torque and cadence are different things. The shorter crank will mean less torque.