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Mismatched crank lengths (probably very stupid)

If I could pick your excellent brains, I've been offered a left-side Dura Ace crank with powermeter at a bargain price, it's 175mm and my current Ultegra cranks are 172.5mm (which suit me very well). I assume it would be daft to ride with mismatched cranks, even though it's only 2.5mm?

Thanks!

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17 comments

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Balthazar | 2 years ago
1 like

Long ago, when I was briefly a bike courier, I rode for months with mismatched cranks without realising. After a left crank failure I whacked on the nearest replacement, which was an old 175 XT crank, 5mm longer than the previous 170. 

I only noticed when I cleaned the bike once, which would have been thousands of miles later. Young tolerant knees may have helped, and  I have a fairly robust attitude to imperfections like this, but no biggie.

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Recoveryride | 2 years ago
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After the fact, of course, but you could have shimmed the cleat by 2.5mm on the shorter side...

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Welsh boy replied to Recoveryride | 2 years ago
2 likes

But that doesn't affect the length of a crank, your foot is 2.5mm higher at the bottom of the stroke and 2.5mm higher at the top of the stroke too so the crank length is exactly the same with or without the shim.

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Recoveryride replied to Welsh boy | 2 years ago
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Hang on, if I have a 175 crank, and place a 2.5mm shim under my cleat, then while the crank is higher/lower, my foot is surely in the same place as a 172.5 crank, no? This is the whole thinking behind bike fitters shimming cleats - to compensate for different leg lengths. And it's much cheaper than changing cranks. 

In this case, though we have someone who has the same length legs, but different length cranks, the logic is the same. 

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Sriracha replied to Recoveryride | 2 years ago
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Shimming makes your leg longer. It doesn't make the crank shorter. Try thinking about it as wearing a built up shoe instead.

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mdavidford replied to Recoveryride | 2 years ago
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Recoveryride wrote:

if I have a 175 crank, and place a 2.5mm shim under my cleat, then while the crank is higher/lower, my foot is surely in the same place as a 172.5 crank, no?

Only at the bottom of the pedalstroke. At the top it's 5mm higher - 2.5mm extra crank length plus 2.5mm shim.

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Rendel Harris | 2 years ago
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Thanks for all the replies, most helpful. I did decide to buy it, ride it for a little while and resell if it was a problem, but when I got back to the seller just now found he'd sold it to someone else, so I'll keep hanging on for a good deal on a 172.5.

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Drinfinity | 2 years ago
3 likes

The Ultegra crank will come apart at the bonding soon enough, then you can replace it with one that matches the power meter. 

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Judge dreadful | 2 years ago
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172.5mm to 175mm will almost certainly not be noticeable. The length of the cranks is insignificant compared to the length of the levers you're using to push them round ( your legs ) it's possible that you could get more toe overlap from the longer crank, and that could be a problem, but if you've got no overlap issues, I'd say you'll probably be fine with that much mismatch on the cranks. 175mm to 170 / 169mm ( 5 or 6 mm difference) might be noticeable, but 2.5mm difference probably wouldn't be.

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jollygoodvelo replied to Judge dreadful | 2 years ago
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You say that, but I definitely noticed  the difference in feel when I switched from 172.5 to 170mm cranks (needed a new crankset, same size was out of stock). 1.5% difference in lever length, it's not much but it's something. 

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Dnnnnnn replied to jollygoodvelo | 2 years ago
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I wonder how much of that is in your head?

That's not a dig at you - we're all liable to noticing things that aren't really significant but we anticipate that they might be. I suppose it's a sort of placebo effect.

Alternatively, there may have been other minor - but unstated - differences which might have contributed singly or in combination to a different feel (e.g. the Q factor)?

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Gimpl replied to jollygoodvelo | 2 years ago
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But you knew that had happened so it was always a 'chimp' in your brain. 

I couldn't source a same length crankset for my son's bike and fitted one 2.5 mm longer - he's never noticed and I haven't told him. 

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ktache | 2 years ago
1 like

I have used the same 2.5 mm difference many years ago, didn't seem to make much of a difference.

I'm pressuming that you are right handed and that your right leg might be the more powerful, the slightly extra length on the left crank would equalise this.

Check to see if it's your left leg that is slightly longer, one leg is generally so, if it is the extra length on the bottom of the stroke will make less problem.

Only my completely unknowledgeable opinion of course.

And it all depends how much of a bargain you are getting...

And you could always talk to a proper bike fitting specialist.

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Welsh boy | 2 years ago
5 likes

I rode for about 5 months on different length cranks, 165 and 170 mm without realising (I do have slightly different length legs) and without any adverse effects, no aches or pains and I certainly didn't sprint around in circles so it might be worth giving it a try 

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Whiii | 2 years ago
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No excellent brains here but I'd say you're correct. If my maths is right that 2.5mm translates to around 16mm of extra travel relative to the other leg for every pedal stroke, or 1.6m for every minute you spend at 100rpm. I suspect what you've saved on the power meter, you'll be spending on a physio several times over! And I've never tried it, but sprinting in a straight line might be tricky?

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Rendel Harris replied to Whiii | 2 years ago
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Thanks for such a clear explanation, I hadn't thought of it in those terms! Right, no power meter for me then (yet...).

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Captain Badger replied to Whiii | 2 years ago
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Whiii wrote:

No excellent brains here but I'd say you're correct. If my maths is right that 2.5mm translates to around 16mm of extra travel relative to the other leg for every pedal stroke, or 1.6m for every minute you spend at 100rpm. I suspect what you've saved on the power meter, you'll be spending on a physio several times over! And I've never tried it, but sprinting in a straight line might be tricky?

Your overall change in length of pedal stroke would be 5mm, as the force is largely applied to the vertical vector, the horizontal "float" is incidental I would have thought.

The difference in length of path that your foot travels would be 16mm as you say, however only half of that would be applying pressure, the other half relaxed on the return stroke.

If you are wearing cleats it might also be worth considering the precision to which you have adjusted the cleat position - eg is it within a tolerence of 2.5mm?

Considering the variance that people generally have in their bodies I might hazard that 2.5mm is negligible and unlikely to make any noticable difference. discrepencies in limb length 10 times that aren't considered to be a clinical condition.

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