Crank set length?

cogs19

Likes Bikes
Hiya,

Just looking a little advice on the various benefits of short/long cranksets. I'm interested in knowing what kind of rider benefits from a longer crank and what rider benefits from a shorter crank. Or, does the length of crank suit a style of riding (DH, Trail, XC), or even terrain, perhaps? I'm looking at XT cranks and have options for 165mm 170mm 175mm 180mm. I mainly do XC and I'm a smaller rider.

Thanks,
cogs.
 

DJR

Likes Dirt
When you say smaller do you mean , 5 foot tall, 5foot6, 6 foot? If you're shorter than 5 foot 8 i'd go the 165mm.

I'm bang on 6 foot tall and run 165 cranks on the DH bike for ground clearance and better stance (feels better having feet closer together when cranks are flat) and 175mm on everything else ( BMX, commuter and AM bike )

I did a lot of reading on the effect of cranklength and the advice basically was if you have gears the power difference doesn't really matter (as you can change gears to change any difference in leverage), it's more about comfort and ground clearance. There is a very mild power difference. BMX in particular and singlespeeds are effected by cranklength as there's no gears so the leverage matters.

Wouldn't stress about it that much on a geared MTB... so probably stick with a 165 or 170
 
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175mm seems fairly standard these days for XC riding. However if you are under 5ft6 then you will benefit in a shorter crank.
 

redbruce

Eats Squid
I'm around 5'6" or 5'7".

So I think 165mm is the go.

Thanks for the feedback.
More related to preferred style of riding (eg spinner or slogger and DH, XC, etc), ground clearance and in any case leg length rather than overall height.

The biomechanics of the latter is readily available on the interweb.
 

Daniel Hale

She fid, he fid, I fidn't
yes go with the 165, i think attitudes are slowly coming around to shorter crank lengths, slight increase in power output, better clearance, more aero [nil apply to mtb], the longer cranks do apply a very small gear adv with the same size chain rings, but thats what your back cluster is for anyway -it applies more to road where you need to keep in a pack at 50kmph
 

ianganderton

Likes Dirt
When you say smaller do you mean , 5 foot tall, 5foot6, 6 foot? If you're shorter than 5 foot 8 i'd go the 165mm.

I'm bang on 6 foot tall and run 165 cranks on the DH bike for ground clearance and better stance (feels better having feet closer together when cranks are flat) and 175mm on everything else ( BMX, commuter and AM bike )

I did a lot of reading on the effect of cranklength and the advice basically was if you have gears the power difference doesn't really matter (as you can change gears to change any difference in leverage), it's more about comfort and ground clearance. There is a very mild power difference. BMX in particular and singlespeeds are effected by cranklength as there's no gears so the leverage matters.

Wouldn't stress about it that much on a geared MTB... so probably stick with a 165 or 170
I red something from Chris Boardman on this. Basically he said it made no difference to power or energy output. He was talking about it though because he was thinking that because it made no difference to power/energy it made sense to do it for other reasons e.g. Time trialers could use shorter cranks as in enabled a more aero position (knees don't come up as far)

Best bet would be to buy the cheapest ones
 

Flow-Rider

Burner
I went to shorter 165 crank arms a while back, the only advantage I found was less pedal strikes. When I went back to 175 I gained a lot more power on the climbs. I already used my lowest gears on climbs so there is no lower gears to use. I found if you run a small frame, most of them are designed for 170 and smaller, your knees almost hit you in the chest and it feels a bit unco. I'm about 5foot 5 and my torso is longish but shortish legs.
 

schred

Likes Bikes and Dirt
My prev exp with lengths 180/177.5/175/172.5/170 and 165 was that for lots of pedalling biometrically (?) the difference in 2.5mm wasn't that noticeable, the 5mm was quite noticeable, and 10mm difference was uncomfortably noticeable. Ideally I'd run 172.5 but its pretty uncommon spec and I'm too tight to justify it. I'm 5 9 btw, but suspect its about inseam vs overall height. I'd think long and hard before putting 165 on an adults bike who's main ride is xc, but try it out, before you buy.
 

slippy

Likes Bikes and Dirt
It has been shown (wish I could remember where) that crank length has no effect on power output, despite rider perception.

I'm tall and run 180mm cranks. I've had many pedal strikes and one that almost literally killed me. I'd recommend going for the crank length suggested for your height or one shorter for the increased clearance. DHers tend to run shorter cranks, partly for this reason.
 

schred

Likes Bikes and Dirt
Im generally open minded, and having a quick read indicates there may be other benefits to shorter cranks. As discussed length diffs can be accounted for by gearing, the real question for me remains what therefore is the ideal length. I'll keep reading.
 

brutasauras

Likes Dirt
Myth and Science in cycling by James C Martin is the name of the paper that dispels the myth about crank length and power output. Good read, and let other important factors such as ground clearance and personal preference, cause it really doesn't matter.
 

Flow-Rider

Burner
Myth and Science in cycling by James C Martin is the name of the paper that dispels the myth about crank length and power output. Good read, and let other important factors such as ground clearance and personal preference, cause it really doesn't matter.
Looks like they are chasing maximum watts on a road bike, a bit different to loads of mountain biking. You need more torque than speed in most hill climbs. I would love to see a test for a max torque duration at lower speeds.
 

brutasauras

Likes Dirt
Looks like they are chasing maximum watts on a road bike, a bit different to loads of mountain biking. You need more torque than speed in most hill climbs. I would love to see a test for a max torque duration at lower speeds.
And how are you going to measure that torque with a time component?
 

The Duckmeister

Has a juicy midrange
Long cranks allow you to produce more torque, but your cadence drops. Short cranks lose a little bit of torque, but are easier to maintain a higher cadence, which can be beneficial particularly for XC where you're on & off the gas a lot, so they're more responsive.

The main thing though is physiological; the shorter your legs, the greater the relative range of motion on any given crank length. Long cranks/large range of motion can increase therisk of knee injuries, especially if there are already underlying issues. Shorter cranks help alleviate these potential problems.
 

cogs19

Likes Bikes
Thanks for the advice and the range of opinions. There's plenty to think about but it seems on balance that a shorter crank length will suit me. Coincidentally, I had a number of solid pedal strikes on the weekend which shook me up a bit.
 
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