I am 5’6″ with about a 31.5 inch inside leg. I usually ride with 175mm length cranks. Dont have any problems with them, although I may clip the pedals occasionally on technical sections. I ride a small size frame. I am in the process of buying some new cranks and wonder whether I should go for 170mm instead of 175mm? Any advice would be appreciated.

Thanks

the only real answer is try it and see. Some people physically don’t notice the change some do.

I’m 5ft 7 with a 31 inch inside leg and run 170mm cranks.

I found that I got a pain at the back of my knee’s when riding on 175mm cranks on 4hr + rides.

I think it’s a very personal thing though.

I’d like to try it, just dont want to spend £80 if its not right…

I had 175’s on all my previous bikes and used to think that I just had dodgy knee’s.

It was only when I bought a bike that by chance happened to come 170’s fitted, that I discovered that they were better for me.

Its unlikely to be ‘wrong’….even if you notice it at all.Certainly by the 3-4th ride you’ll will have adapted to it.

Unless you run ss.

Shorter cranks transformed my riding, so I’d say go even shorter than 170cm, if you can

I think it’s a very personal thing, I have always had 175’s and found that any knee pain or dodgy feeling had to do with the height of my saddle, if I was doing long XC rides.

170’s will give you marginally better ground clearence, maybe they would make sence on a smaller frame?

Like mrmo said the only way is to do it and see. If you don’t feel that £80 quid is worth seeing then stick to 175.

Maybe these links will help you decide?

http://forums.mtbr.com/showthread.php?p=4138148

http://www.singletrackworld.com/forum/topic/cranks-170mm-or-175mm

http://www.cyclingforums.com/forum/thread/98669/pros-and-cons-of-crank-length-170-vs-175

http://www.bikeradar.com/forum/viewtopic.php?p=15442281&sid=1897e982cd2fc19abb0e674295049466

Hope this helps

Changed from 175 to 165 a few months back. Main difference I’ve noticed is pedal/cranks clipping the ground a lot less.

Thanks for your replies. Cheers Moonhead for the links.

jimjam – Member
Changed from 175 to 165 a few months back. Main difference I’ve noticed is pedal/cranks clipping the ground a lot less

Me too, a year or more now (got cranks because of pedal strikes with 175)

(I’m about 6ft 2, if it matters)

Shortening the crank length effectively alters the gearing. A shorter crank for the same gear will mean you spin more revs over a given distance.
I’m 5’7 with a 30″ inseam and I’ve always ridden 175mm cranks. They give you a lower centre of gravity in the corners when you drop the outside foot, and mean you spin fewer revs if you run double and bash.
Shorter cranks are good for getting the power down, and that’s why I used them for trials.
I guess you’ve got to find a balance between comfort, handling, power and sustainable speed.

Shorter cranks make sense for shorter people. In fact, they make sense for a lot of average height people, like me, too!

Most bike manufacturers are lazy and spec 175mm cranks across their bike ranges and sizes. This is easy for them, as it only means stocking one size. Some bike manufacturers are wise to this though, and fit different length cranks on different size bikes. Personally, I get problems if I ride a bike with 175mm cranks for any time over a couple of hours, I get bad knee pains. I’m 5ft11 (or near as dammit) and have a 33″ inside leg, and always use 170’s.

See if you can try a pair of 170’s first before committing, might be a good way to decide if they’re for you or not.

Skyline-GTR – Member
Shortening the crank length effectively alters the gearing. A shorter crank for the same gear will mean you spin more revs over a given distance.

No it doesn’t! 💡
The only thing that changes the gearing on a bike is the gears.

Regardless of crank length, the ratios between the chainset and the cassette stay the same; so one revolution of the crank still gives x number of revolutions of the rear wheel, just as it did before.

What it will change is the effort required to turn the cranks (although whether you’d notice it or not is another matter).
A shorter crank gives you less turning moment, so you are applying less torque at the crank spindle for the same pedal stroke force.

175mm to 170mm cranks is about a 3% reduction in length, so you’d probably barely even notice.

As others have said, try different legnths if you can and see what is most comfortable for you.

Shortening the crank length effectively alters the gearing. A shorter crank for the same gear will mean you spin more revs over a given distance.

It will effect the distance your foot has to travel, but the number of revolutions it makes will be exactly the same. Perhaps you are getting confused with wheel size?

I really noticed the extra effort required going from 175 to 165s and didn’t notice any difference in ground clearance – the only time I get pedal strikes is when I’m not careful when I pedal. So I stick to 175s. That said, I’m 6’1, shorter cranks might make some sense if you’re shorter legged but IMO it makes sod all difference to pedal strikes. It’s 5mm per side – 1/4 of the width of your fingers – what percentage of strikes that you have are slight scuffs? If it’s much more than a slight scuff I’d say 170’s are still going to clonk.

Sheldon brown says (and he is always right)

Different cyclists have different leg lengths. It seems obvious that crank length should be proportional, so long legged cyclists should have long cranks, short-legged cyclists should have short cranks….and yet, 99.9% of adult bicycles have crank lengths between 165 and 175 mm. Have the bicycle manufacturers joined in a great conspiracy to force everybody to ride the same length cranks, regardless of their needs?

This is a common misunderstanding. The “leverage” of a bicycle drive train, also known as “gain ratio” depends on the crank length, wheel diameter and the sizes of both sprockets.

Yes, if you go to longer cranks without changing any of the other variables, you will have more “leverage”, which is another way of saying you’ll have a lower effective gear…but on a multi-speed bike, you can change gears at will!

Ay, there’s the rub! Assuming you adjust your gearing appropriately, crank length has no effect on leverage, it just has to do with the range of motion of the knee and hip joints.

Too long cranks cause excessive knee flex, and can cause pain/injury if it causes your knee to flex more than it is used to.

I’m 5ft 7 and 31″ leg and run 170mm cranks.

Works for me – if they’d been available at the same price I’d have got 165mm as I like to spin quite a high cadence and it seems to help.

Thanks everyone for your advice. I have decided to buy the 175’s again as I have had no issues in the past. 170mm may suit me better, but I guess its impossible to know without trying, and I cant afford to buy something that ends up being a mistake.

Cheers

172.5 ftw 😉

It sounds like the decision has been made but MachineHead Crank Calculator is a good way of taking the decision making out of your own hands!

This would have recommended 165mm cranks for OP. The calculation was devised for road biking where a good cadence style would be assumed. I think for mountain biking it helps to have slightly longer cranks for greater leverage to allow more explosive responses. This then needs to be tempered with a decision about ground clearance.

coffeeking

the only time I get pedal strikes is when I’m not careful when I pedal.

Careful when you pedal? Is that just a fancy way of saying mincing? 😆

coffeeking
IMO it makes sod all difference to pedal strikes.

In my experience it does. The difference is plain to see in the 175 cranks I took off as opposed to my 165’s. The end of the longer crank arms were trashed, the shorter ones, despite the same treatment are relatively clean. Same for the pedal bodies which were replaced at the same time.

Random’s generator doesn’t get near 175mm until 34.7in inside leg. How tall would you be with that inside leg? 7 foot?

Shortening the crank length effectively alters the gearing. A shorter crank for the same gear will mean you spin more revs over a given distance.

It will effect the distance your foot has to travel, but the number of revolutions it makes will be exactly the same. Perhaps you are getting confused with wheel size?

It won’t change the gearing, but you will need to use a lower gear to get the same power to the wheel compared to your personal power output, and hence you’ll travel at a lower speed for the same effort (Principle of moments)