Crank lengths

[n/a]

One of the problems with havinga good bike is that you become even more aware of possibilities for improvement!!  I have had my Raven Tour for about 6 weeks now, delighted with it, deceptively fast, like the Rohloff etc.

Before I ordered this, I had no idea that cranks came in different lengths, and mine is fitted with 170 mm.  I have now seen an article with a formula wihich suggests that my inside leg (29.5 ins / 75.5cm) warrants a crank length of about 160 mm.  Can anyone tell me what difference it makes??  I tend to ride in teh saddle, rarely out of the saddle, and am a plodder rather than a hard pusher . . .

Ideas would be welcome - mind you, if people start saying mine are wrong I will begin to want to do something about it . . . . .

Lewis

[graham]

170mm does seem to be the 'standard' crank length, in that more bikes seem to be offered for sale with them than any other length.
On the other hand, Shimano don't seem to offer any of their better cranks in shorter lengths than 165mm. So regarding the formula, there seems to be a discrepancy between what you have worked out and what a lot people actually do.
What does 'plodder' mean in terms of cadence?
It seems to me that if you are trying to spin the cranks at 100rpm, having them the 'optimum' length might be a good idea.
If on the other hand 75rpm is more your thing, effectively you are pushing a bigger gear and more leverage via a longer crank might be a good thing.
The formula says I should use 165mm cranks and the wife probably a bit under 160mm. But I use 170mm and the wife 165mm (because that's the shortest XT cranks you can get), and we both seem fine with it.

[avdave]

Lewis try this link http://www.highpath.co.uk/cycles/notes/03.html
Hopefully this will help. Like you I've always used 170mm cranks but my Catalyst is coming (soon I hope) with 165mm cranks.
I've had no problems using 170's but I thought as Thorn do a full range of cranks I'd use what are supposedly the correct ones for me.
Like you I rarley get out of the saddle(as I've been riding exclusively full suspension for the last 8 years) and prefer to spin the cranks rather than push them round.

[n/a]

Thanks for that!!

Lewis

[stutho]

quote:If on the other hand 75rpm is more your thing, effectively you are pushing a bigger gear and more leverage via a longer crank might be a good thing.

I have hear this said before - I am not sure I agree.  Yes you will have more 'leverage' with a larger crank; however the only mechanical advantage that matters is the difference between the force on the pedal and the tyre to the road. This can be achieved by changing you gear ratio.

You knee works best when almost straight. Reducing the crank length will help reduce the angle you knee joint makes.  (If the reduction in leverage is a problem then a smaller chain ring is also required)

I may well be wrong!  I am not a sports scientist

[graham]

On the assumption that torque is force times distance, then more distance i.e a longer crank should mean more torque. More power for a given cadence.
I suppose a longer crank means the pedal describes a bigger circle, so the body is also doing more work per revolution.

I'd imagine you're right, a sports scientist might be better placed to know whats best. I assume the calculation for crank length is based on some sports science, including the most efficient cadence, which seems to generally accepted as being 90 to 100 rpm.

If you're not aerobically fit enough to keep that up continously, and a lot of non-dedicated cyclists don't seem to be, then a slower cadence might allow comfortable use of a longer crank.

Of course, I may well be completely wrong...

[stutho]

Hi Graham.  

Firstly let me say that I agree with everything you said in your last post.  Rereading my post I realised that I hadn't expressed my self very well.  

I'll try again!

While I believe a longer crank will improve the power transfer, I also believe that knee pain will also increase as the cyclist will have to develop force through the knee joint over a greater range of angle.  

I realise there is an argument that says that longer cranks do not require so much force to turn, because of the increased leverage, however a smaller chain ring will accomplish the same thing.  

(Effectively this argument says that people with short legs will never cycle as fast as people with long legs!)

I've have seen this discussion on crank length run on and on before now so I will say no more and let you have the last word.  

At the end of the day everyone needs to make a choice that they themselves are happy with.  

[avdave]

Well I've only done 25 miles on the new Catalyst with 165 cranks but it does seem better than the 170's on the mountain bike when using it for commuting on the road where I have a 5 mile stretch which requires no stopping or slowing. That would sometimes leave my right knee feeling "out of line" not painful , just like it needed to click into place. I've moved the pedals over so it's not that making a difference but I suppose the Q-factor may be different. I'm not going to rush out and by new cranks for the mtb though as off road with all the movement it's never a problem.