crank arm lengths

[139NI]

hi

i am 5'9" tall, my inside leg is c.30"

i am currently using a 175mm length crank but have read that my measurements should require a 170mm crank

5mm is so small yet some people feel it makes a difference

my bike is a 18" mtb montague paratrooper frame.

does crank length difference of 5mm in my case matter??

Cheers


w

 

[Smokin Joe]

You'd be unlikely to notice any difference.

 

[ColinJ]

I'm not convinced either that it makes a huge difference. A mate reckons that crank length is blindingly obvious to him, but I put a spare 170 mm crank on to replace a cracked 175 mm one and have done thousands of miles with a mismatched pair and not noticed anything untoward! ***



*** Mind you - I have mismatched legs, so perhaps that is something to do with it! (The shorter crank is on the side of the shorter leg.)

 

[numbnuts]

I have heard it said that shorter cranks are gentler on the knees, but I can't see it myself.

 

[Deleted member 23692]

I've got 170mm on my road bike and 175mm on my MTB, and they from the same manufacturer for the the same height person.

Both seem fine to me

 

[cosmicbike]

I'm not sure, so all mine are 172.5mm

 

[jowwy]

My 175's on my 29er seem huge compared to the 172.5's on my tarmac.

Tarmac is now getting 170's as thats what my measurements recommend. Will find out once i ride it sunday with the new cranks

 

[boydj]

It's unlikely you'll notice the difference when riding. However, your saddle will be 5mm higher and the range of movement at your knees will be reduced by 10mm per revolution, which will be just a little kinder to your knees over time. I've got 165mm cranks on my two main bikes.

 

[139NI]

It's unlikely you'll notice the difference when riding. However, your saddle will be 5mm higher and the range of movement at your knees will be reduced by 10mm per revolution, which will be just a little kinder to your knees over time. I've got 165mm cranks on my two main bikes.

Umm, you and numbnuts are are reporting the notion that trauma on knees over time will be reduced if a shorter crank is used. This is obviously good, so is it the case that cyclists should use a crank as short as possible to prevent injury later in life, given their physical measurements??

I previously thought that the longer the crank the better as the extra length allows extra mechanical advantage when driving the cranks hard to achieve more speed.

I think i favour being kinder to my knees, but by having shorter cranks, would i not then need to put greater physical effort on driving the crank to achieve movement.... it then follows, would i then not still be causing trauma to the knees but in a different way?

 

[Smokin Joe]

Cycling does not cause trauma to the knees, whatever the crank length or how high a gear you push. Otherwise those of us over a certain age would be crippled by now. Cycling is very kind to the joints because your weight is supported and you are not putting shock loads on them. Walking up and down stairs a dozen times a day probably strains the knees far more than a 100 mile bike ride.

If runners, footballers and tennis players can in the main survive knee damage we have no problems.

 

[Profpointy]

Cycling does not cause trauma to the knees, whatever the crank length or how high a gear you push. Otherwise those of us over a certain age would be crippled by now. Cycling is very kind to the joints because your weight is supported and you are not putting shock loads on them. Walking up and down stairs a dozen times a day probably strains the knees far more than a 100 mile bike ride.

If runners, footballers and tennis players can in the main survive knee damage we have no problems.

don't entirely agree with that. I'm reasonably convinced one of my knees twinges from doing hill starts on my fixie; basically straining it a bit too much.

 

[Smokin Joe]

don't entirely agree with that. I'm reasonably convinced one of my knees twinges doing hill starts on my fixie.

That's assuming the twinge is in the first place caused by riding a bike. Having known countless elderly ladies and gents who've cycled all their lives I doubt it.

 

[numbnuts]

I really think that if you sit and spin it is kinder to your knees, than getting out of the saddle and pushing in high gears does all the damage.

 

[Profpointy]

That's assuming the twinge is in the first place caused by riding a bike. Having known countless elderly ladies and gents who've cycled all their lives I doubt it.

Well I didn't have it before, and it's subsided now I'm more carefully on the specific hill start. I had similar problem 30 years ago after straining on a weight lifting machine, and after climbing a very steep hill carrying an excessively heavy rucksack - same symptoms, same cause - though only one was the bike - or more to the point grunting up in too high a gear. All my own fault

Bikes in general being broadly good for knees I'm not disputing.

 

[139NI]

I have now swapped the 175mm for the 170mm for the time being just see how it sits with me.

I do feel a number of differences:-
(i) that the peddling 'rotation' feels smaller [or could it be that i am conscious a shorter crank has just been fitted].
(ii) that acceleration from a standing start is less labour intensive with the shorter crank. ie its more responsive and easier.
(iii) that when climbing hills/slopes, the peddling action is actually easier with the shorter crank.
(iv) that when trying to achieve more speed on the flat surfaces, i need to spin the cranks a bit faster to attain the same mph.

so if i was to conclude: shorter cranks are better than longer cranks for acceleration and uphill. whilst longer cranks are better for flat long-distance speed.

just wondering does anyone else have similar experiences when downsizing [or indeed upsizing] cranks.

The bit about being better for the knees is a massive bonus [which is true apparently form an article i read earlier].