I need shorter cranks, but by how much?? (155mm vs 160mm)

[Ugo51]

Hi guys,

I finally decided to invest in new cranks. I currently run 170mm, which are way too long for me.
Various formulas found online and several measurements of my legs, tibia, and femurs, all point to 160mm as my ideal crank length.

However, I would like to reduce to a minimum the stress on the knees, so I may even benefit from an even shorter crank length. There is no way of knowing without trying, but for sake of argument, do you think it's better to go for 160 and then in case swtich to 155, or the other way around?

It would seem logical to start with 155, the lower limit, so if I don't feel comfortable I only have one direction to go: longer cranks.
If I try 160, even though it's the "ideal" length, it will leave unanswered ther question "how would it feel to have 155mm?"

Not sure if it makes sense to non-overthinking mind...

 

[hamster]

If you are trying to save your knees, then the most important thing in addition is to avoid crunching big gears. Swap to smaller chainrings at the same time.

 

[Ugo51]

Oh yes, definitely.
By a rough estimate, I think that I would end up using the inner chainring in place of the outer.
I will play around with cassettes too.

 

[pigman]

Try and buy second hand or budget versions before you commit to a permanent decision.
I reduced to 170s based on what the magazines and forum advice and whilst it worked on the flat, I was climbing slower and less efficiently and now have a drawer full of redundant cranks and what was quite an expensive experiment

 

[hamster]

Interesting - I swapped to 170 from 175 on my singlespeed and immediately it was more comfortable, I spun faster and didn't feel a difference climbing, even though it was effectively a higher gear in terms of effort. I think a neglected (or at least hideously complicated) factor is the ratio of shin to thigh length in all this. My wife has short shins relatively and seems to prefer longer cranks than her leg length would suggest.
Which in effect validates Pigman's approach of trying with cheap stuff first.

 

[Ugo51]

The problem is that they are unusual sizes to find.
SpaCycles makes cheap cranks. I doubt I can find a better deal than £38 for a complete chainset.

 

[Ugo51]

Interesting - I swapped to 170 from 175 on my singlespeed and immediately it was more comfortable, I spun faster and didn't feel a difference climbing, even though it was effectively a higher gear in terms of effort. I think a neglected (or at least hideously complicated) factor is the ratio of shin to thigh length in all this. My wife has short shins relatively and seems to prefer longer cranks than her leg length would suggest.
Which in effect validates Pigman's approach of trying with cheap stuff first.

I'm basiclly the opposite. I have short femurs (short legs in general, but also high tibia/femur ratio)

 

[hamster]

And Spa's are really decent cranks too!

 

[2manyoranges]

This is an interesting area, and reality in the field is different to a lot of the theory. There's some very good commentary on this online, which it sounds as though you have accessed, and one piece (which I don't have immediately to hand I am afraid) emphasises that, counterintuitively, those with longer legs are less tolerant of oversized cranks than those with shorter legs - due to the amount of mass they need to rotate and flex. Many studies show no huge variation in power output related to notionally oversized or undersized cranks. The wisdom is that time triallists should use short cranks (to enable a very low tuck) and children should have short cranks, and that having too long cranks on the road means you get dropped on hills, while too short means you get dropped on the flats. But in the rough and battering of off-road, things begin to feel very different in practice.

I am 5-7, reasonably proportioned, and have a mix of 165 and 170 cranks. I have used 175 and have appreciated the additional leverage, which of course reduces pressure on knees but increases the diameter of rotation. But I now tend to use 170 - and 165 on bikes which are prone to ground strikes due to low BB. Saddle height and saddle position on rails/seat tube angle makes a hell of a difference. Get that wrong - too low, and saddle too far back - and that increases pressure on the knees a great deal. The objective and easy bit is that shorter cranks increase pressure on the knees through reducing leverage. This can be remedied by using lower gears and spinning more. Which is fine but needs to be done deliberately. I found that going to 165 was challenging. Hamster appears to have had the same experience. I needed to change my pedalling style A LOT- otherwise my knees really grumbled. If I failed to up cadence and downshift, I found that my knees hurt a lot after riding. It took maybe 8 months to get it right on the shorter cranks. It's all fine now, and I honked up a 200m impossible climb yesterday, without trashing my knees. I would go down 5mm at a time, rather than do a huge jump in sizing, and really make sure that saddle height is right and saddle is way forward on the rails. I use an inline post on all my bikes, with the saddle quite forward on the rails. This is contrary to 1990s thinking, but entirely consistent with hill-climbing bikes of the 1950s and the current thinking about very steep seat angles.

I'd say get your saddle height and position on the rails sorted first - that costs nothing - and then work on the cranks.

 

[hamster]

I have dodgy knees, but climbing out of the saddle is fine. It's grinding on the road in a too-big gear that does it for me, typically at about the 75 mile mark. I've now switched to 48T outers which helps limit self-destructive tendencies.