crank lengths

i am looking to swap my current fsa afterburner 175mm cranks for a double & bash setup.

i have the option of 170mm or 175mm.
would 170mm drastically alter the feel of the bike or not.

im only 5ft 8 & im thinking the slightly shorter cranks would be a benifit or is there a hugh drawback.

imn looking at the slx cranks if that helps.much appreciated .

The consensus is that most people actually ride with cranks too long for them in the first place. A 175mm crank should really only be used by taller blokes, say 6ft or over (though it is of course dictated by the leg length rather than the total height of the person), at your height/leg length, you should probably be using 170mm cranks ideally anyway.

Why do bicycle manufacturers still supply bikes with cranks that are too long? I know people under 5ft6 and with short legs riding 175mm cranks! Well, it’s probably cos a 175mm crank gives you a little extra leverage, making it “seem” easier to get up that big hill! The fact is, cranks that are too long knacker your knees. I’m 5ft11 with a 33″ inside leg (so hardly short) and I don’t like 175mm cranks, they make my knees hurt! I use 170’s exclusively. Shorter cranks promote a better “spinning” technique, they’re kinder on the knees and you get a touch more ground clearance. The only drawback is that most cranks are 175, so finding 170’s at as good a price as 175’s can be a bit tougher, but what price a new pair of knees!

This goes some way to explaining the crank length question too.

Regarding SLX cranks, BIG thumbs up from me! Have a set on one of my bikes, they’re only about 20g heavier than XT, look cooler IMO, just as stiff, chainring shifting is superb (on my triple set anyway, not tried the double setup), basically I highly reccomend them. Certainly they’re a big step up in quality from the last LX cranks.

i’ve 170, 172.5 and 175mm cranks on different bikes

i don’t really notice any difference

I have 170 one side and 175 the other. Can’t feel a difference, unless I really concentrate when on a flat road, when the longer one feels shorter…

thats great guys cheers.would i be ok still using my x-9 front mech & shifter with this crank.

hats great guys cheers.would i be ok still using my x-9 front mech & shifter with this crank.

unless it’s a funny shape and they hit each other ?

I have seen equations for this in the past, and remember ‘should be’ lengths are shorter than we usually have. The italians and French are much hotter on this than us if you look at their crank sets they come in stacks of sizes starting down at 150mm to 185mm in 2.5mm jumps. But reckon this makes real sense on a road bike or if you do miles sat down on tracks. On a mountain bike we stand up more, move about more and have much greater need for instant leverage, hills, bumps in the track, front wheel lifts etc… We have quite a few bikes here used in all sorts of places and this is what we have ended up with: Sprog James has 165mm on his road bike, 155mm on his XC and 160mm on his DH. Jane has 167.6mm on her XC, I have 175mm on my hardtail, 170mm on my full sus and 180mm on my single speed (the only one I notice the leverage on -but short chain need to keep moving might be more significant). I notice pedal strikes and different stiffnesses in chainsets more. SLX seem a really good buy.
cheers R

again thankyou especially mboy for that comprehensive reply.

and have much greater need for instant leverage

a 5mm change in crank length is equivalent to one tooth out of a 34 tooth cassette ring ie not much. There no special effect due to extra leverage specifically at the crank – it could be anywhere in the transmission. Also shorter cranks could be lighter and require smaller rings at the back for the same overall gearing 🙂

The consensus is that most people actually ride with cranks too long for them in the first place

Consensus? Are you sure? After all, even Sheldon thinks one shouldn’t obsess too much about it in the article you linked.

I run them with a length in relation to my BB height. Riding 2 bikes with low BB’s I run 165’s. I can’t really notice any pedalling abiliy differences when swapping them though (other than a few less pedal strikes).

knott4me – Member

again thankyou especially mboy for that comprehensive reply.

No worries, glad to be of help

nickc – Member

The consensus is that most people actually ride with cranks too long for them in the first place

Consensus? Are you sure? After all, even Sheldon thinks one shouldn’t obsess too much about it in the article you linked.

Maybe not worth obsessing about indeed, but it’s worthwhile knowing the effects of using cranks that aren’t of the optimum length of the individual. Not everybody is going to have knees that are as sensitive as mine are/were when using longer cranks of course. Not everyone notices the difference in length, some can notice it very obviously.

In fairness though, Sheldon did contradict himself a little bit though, as only a few sentences in he makes note of how he learnt about knee pain due to crank length the hard way when he bought a 2nd hand bike with 180mm cranks on.

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.

This is perhaps the best argument if you don’t have problems with your knees, or ground clearance issues. Spinning the cranks faster is better/more efficient than honking a big gear. Some people are unashamed pedal mashers though, so they may well want longer cranks for extra leverage!

I swapped from some 175mm Spesh Strongarm cranks for 170mm XT cranks. The only way I can tell the difference is that I hardly ever ground my pedals whereas I used to do that quite a lot. Can’t tell the difference in terms of pedalling.
I think the BB on my Stumpjumper FSR is pretty low and my erm, back end is quite saggy.