Crank Length Compatibility

[nlmkiii]

I need to switch out my 175mm cranks for 170mm, but it's a triple, so finding a direct match to change the chainset is proving impossible, so I'm wondering what workarounds there may be?

It's a 175mm FSA Gossamer Triple, with a BB30 bottom bracket. Part number ck-0s6020 triple.

Can I just purchase cranks for them? If so what do I need to keep an eye out for with regards to compatibility?

If more info is needed please do let me know and I'll happily go find out.

 

[boydj]

Is there a reason for this change other than you having relatively short legs? The reason I ask is that this is a change where you will struggle to notice the difference when cycling. I'm 5'6" and have 165mm cranks on my good bike, but 170 on my winter bike. I don't notice the difference swapping between the two.

I know a bike fit might recommend shorter cranks in some cases and ideally that's what you would use if you were starting from scratch. But it's an expensive change for little noticeable benefit, since you would probably have to change the whole chainset.

 

[midlife]

BITD I swapped cranks / bikes / regularly and never noticed. Very likely ran with cranks of different sizes left and right and never noticed lol.

 

[nlmkiii]

Is there a reason for this change other than you having relatively short legs? The reason I ask is that this is a change where you will struggle to notice the difference when cycling. I'm 5'6" and have 165mm cranks on my good bike, but 170 on my winter bike. I don't notice the difference swapping between the two.

I know a bike fit might recommend shorter cranks in some cases and ideally that's what you would use if you were starting from scratch. But it's an expensive change for little noticeable benefit, since you would probably have to change the whole chainset.

I have longer legs for my height, but I'm only 5'8"!

It's trying to get the right bike fit for me to ensure longer comfort. It's more relevant this year as I'm going to be taking on some 10+ hour rides.

In particular, I tend to get knee pain, and everything I've looked at, and experts spoken to suggest that crank length could help it. I've had a bike fit which has already helped it a bit, but this is the last piece of the puzzle.

I think that's the main thing, it may not even feel noticably different, but minor changes can make enough of a difference that knee pain won't appear, instead of coming up at around 2hours in to a ride.

 

[faster]

I don't really have an answer for your original question, but what I would say is don't let people put you off trying shorter cranks just to save a few quid.

I made the change from 175mm to 170mm on my wet/commuting bike for a couple of reasons - firstly because at the time I had a hip issue which I thought may have been caused by my position on the bike, and secondly because I had another bike with 172.5mm cranks which just felt nicer to pedal - especially at high cadences.

Did it help with my hip? Difficult to say really. Am I quicker with short cranks? That's difficult to answer too.

What I will say is, in contrast with what others have said above, is that 5mm off the cranks is a big change and massively noticeable for me. It just feels nicer, I pedal at a higher cadence (it was already high anyway) and roughly a gear lower everywhere.

I think your cranks are either too long, or they are not too long if you know what I mean. If your position means that your knees/hips are flexing to an angle that they cannot cope with, short cranks will help. If you have a 'dead spot' at top dead centre that you struggle to get over, again, short cranks may help, but I don't believe for a moment that you won't notice the difference.

I don't think leg length has that much to do with it - I'm 6'4 and have long legs for my height.

Don't forget to put your seat up 5mm and good luck with the knee.

 

[Nigelnightmare]

Is this any good?
Amazon product ASIN B07JPJN15HView: https://www.amazon.co.uk/FSA-Gossamer-Crank-170mm-COMPACT/dp/B07JPJN15H/ref=asc_df_B07JPJN15H/?tag=bingshoppinga-21&linkCode=df0&hvadid=&hvpos=&hvnetw=o&hvrand=&hvpone=&hvptwo=&hvqmt=e&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=&hvtargid=pla-4584001421057956&psc=1

 

[Nigelnightmare]

Or this one?
https://www.ebay.co.uk/itm/FSA-Road...086785&hash=item2ad6b643ce:g:lT8AAOSwy2BdvZZH

That took less than 2 minutes to find both.

Good luck.

 

[nlmkiii]

Or this one?
https://www.ebay.co.uk/itm/FSA-Road...086785&hash=item2ad6b643ce:g:lT8AAOSwy2BdvZZH

That took less than 2 minutes to find both.

Good luck.

That is absolutely barmy. They must have just listed that minutes after I posted this haha! I've been looking all week and that is the only one to have popped up!!!

The Amazon one is the double version I think (as BCD is 110 rather than 130).

Thanks a lot, you're a star

 

[Ajax Bay]

NB: from China

 

[nlmkiii]

Well, it's on the way! Not too thrilled about it possibly taking 2 months to arrive... but it's better than never... which is what I've been facing up until this point!

 

[boydj]

I have longer legs for my height, but I'm only 5'8"!

It's trying to get the right bike fit for me to ensure longer comfort. It's more relevant this year as I'm going to be taking on some 10+ hour rides.

In particular, I tend to get knee pain, and everything I've looked at, and experts spoken to suggest that crank length could help it. I've had a bike fit which has already helped it a bit, but this is the last piece of the puzzle.

I think that's the main thing, it may not even feel noticably different, but minor changes can make enough of a difference that knee pain won't appear, instead of coming up at around 2hours in to a ride.

I have to agree that you should find the shorter cranks slightly more comfortable on longer rides. You should also check that saddle height and for/aft positioning is correct and that your cleats are set properly. Cleat position is particularly important for knee comfort in having your feet at the correct natural angle. Personally, I use Speedplay pedals with lots of float, but I know others prefer pedals which lock your feet in to a relatively fixed position.