Changing length of cranks

Welshkev

I currently have the Shimano Ultegra 6800 (53/39) on my bike, with 172.5 mm cranks. I'd like to change these to 165mm cranks - is it possible to change these out without changing the entire crankset? i.e. just the crank arms? And if so, where could I get them from?
Cheers

niceonetom

No, you need to change the entire crankset.

Seweryn

Kevinleeowen wrote: »

I currently have the Shimano Ultegra 6800 (53/39) on my bike, with 172.5 mm cranks. I'd like to change these to 165mm cranks - is it possible to change these out without changing the entire crankset? i.e. just the crank arms?

Sure, it is possible. Just remove the chainrings and swap the cranks.

Kevinleeowen wrote: »

And if so, where could I get them from?
Cheers

Not sure if a new set of cranks are available without the chainrings. Maybe try second hand or alternatively buy a new complete crankset with 165mm cranks and sell your own one. That would be a better idea, and I would go with the compact set instead of standard, especially with shorter cranks (I now also use a 165mm set). Unless you only cycle on flat roads...
Anyway, something like 36/52T or 36/50T or 34/50T would be more usable, in my opinion .

mp31

Why the change to shorter cranks OP?

Welshkev

Seweryn wrote: »

Sure, it is possible. Just remove the chainrings and swap the cranks.


Not sure if a new set of cranks are available without the chainrings. Maybe try second hand or alternatively buy a new complete crankset with 165mm cranks and sell your own one. That would be a better idea, and I would go with the compact set instead of standard, especially with shorter cranks (I now also use a 165mm set). Unless you only cycle on flat roads...
Anyway, something like 36/52T or 36/50T or 34/50T would be more usable, in my opinion .

Thanks - yeah, that was my next thought depending on cost. Usually with things like this it's cheaper to buy the entire set rather than indivdual compnents, but I can't find anywhere so I can at least compare.
Use the bike for racing, so want to stick with a standard 53/39.
Did you change over to 165's? And did they make any difference? I just think the 172.5's are too long for me (being about 5'7")

Welshkev

mp31 wrote: »

Why the change to shorter cranks OP?

I was using 170's with a compact, then changed to a standard, which has the 172.5's. I just think they're too long for me. And from what I read, the shorter one's would be more efficient. If it's not an expensive job I'd like to change them out, but I can't find anywhere to see prices, other than buying a complete new crankset.

magicbastarder

i've read that longer cranks are tougher on the knees, but surely a longer crank means better leverage, so less strength required to push on the cranks?

Seweryn

Kevinleeowen wrote: »

Thanks - yeah, that was my next thought depending on cost. Usually with things like this it's cheaper to buy the entire set rather than indivdual compnents, but I can't find anywhere so I can at least compare.
Use the bike for racing, so want to stick with a standard 53/39.
Did you change over to 165's? And did they make any difference? I just think the 172.5's are too long for me (being about 5'7")

Yes, I changed the cranks from 170 to 165mm early this year. Sure, they made a difference and I decided to stick with the 165mm. For reference, I am a little taller than you with over 84cm inseam in the legs, so if your legs are shorter, you will find it even more beneficial, especially if you are coming from 172.5mm cranks, as they are too long for you to fit on the bike comfortably in my opinion.

I do not race, but if I was you, I would still go for a compact set. You can put, say 39/53T 110mm BCD chainrings on if you want, but personally I believe you will find the smaller chainrings and lower gears in general more efficient and more usable. There are two reasons for this – you will find that your cadence will increase, as you will spin easier and faster with the new cranks, and secondly the overall gear ratios would go up with the shorter cranks, so it makes sense to reduce the chainring sizes by 1-3 tooth.

But first please remember to give your legs a couple of weeks to adapt to the new cranks. And make sure to re-adjust your saddle up and backward, or better go for a bike fit. You may also need to slightly raise your handlebars and to install a 5-10mm shorter stem to compensate the reach to the handlebars.

Seweryn

magicbastarder wrote: »

i've read that longer cranks are tougher on the knees

True .

magicbastarder wrote: »

but surely a longer crank means better leverage, so less strength required to push on the cranks?

That is a lot more complex issue, but generally, that is not true.

Jack Joll

Making a change like that is big, you'd definetly require a bike fit again. Or one for the first time, if you haven't done it before. You may find when fitting the bike to suit you and the 165mm, you may then find your reach is too long and your streched out across the bike and resultant will create pain in your shoulders and traps. To counteract this, you would probably change the stem, but a shorter stem leads to a giddy front end.
Moral is bikes are usually constructed within common averages, which allow for minor adjustments, If you have to do a major adjustment like change crank length, stem length, the likelihood is the bike wasn't sized properly in the first place.

Seweryn

Jack Joll wrote: »

If you have to do a major adjustment like change crank length, stem length, the likelihood is the bike wasn't sized properly in the first place.

Jack, the crank arm length change by 7.5mm is not a major change that a properly sized bike frame cannot accommodate. If Kevin is currently on a 100mm stem, he will need to replace it with a 90mm stem, which is one size down and I can see nothing wrong with it. It doesn't mean that the bike frame size is incorrect. That is what the bike fit is for - to fit the bike to your body, not the other way round. There are a lot of people with disproportionally long torsos or long legs (like me) that require specific bike fit and correctly sized bike components.
And in fairness, if the bike came with 172.5mm cranks originally for five foot seven inches Kevin (assuming he bought the correct size bike), it means that the installed crank arms were too long in the first place...

Jack Joll

Seweryn wrote: »

Jack, the crank arm length change by 7.5mm is not a major change that a properly sized bike frame cannot accommodate. If Kevin is currently on a 100mm stem, he will need to replace it with a 90mm stem, which is one size down and I can see nothing wrong with it. It doesn't mean that the bike frame size is incorrect. That is what the bike fit is for - to fit the bike to your body, not the other way round. There are a lot of people with disproportionally long torsos or long legs (like me) that require specific bike fit and correctly sized bike components.
And in fairness, if the bike came with 172.5mm cranks originally for five foot seven inches Kevin (assuming he bought the correct size bike), it means that the installed crank arms were too long in the first place...

Seweryn,
Have you not already contradicted my point, bike shouldn't have had a longer crank if it was sized properly in the first place??
I was merely highlighted the importance of getting re-fitted, and 7.5mm in my book is a big step, you'd know it if one leg is 7.5mm longer than the other.
I've changed from a 175mm crank to a 172.5mm, due to regulating for a PM crank across all my bikes, re-fitted and trust me it does make a big difference. Unless I was at the limit of the frame size in the first place.
Not taking from Kevin, but just trying to highlight the importance of getting the bike right, particularly when his legs will possibly be doing the same repetative movement between 5000 times per hour.

Welshkev

Thanks for the advice guys. I've actually had 2 bike fits done. What came out of the second one was I could (ideally) go to a longer stem - from my 100mm one up by 10mm, so I think if I changed the cranks the current stem would suffice. I'd defiintely look at increasing the saddle height though, more than likely by approx. the 7.5mm difference. I'd use this as the starting point, then make any minor adjustments from there. Failing that, would look in to a follow up to my last bike fit.
I really can't find anywhere that sells just the cranks though, so it looks like it's going have to be a complete new crankset. I'm thinkin of putting an ad on here, but suspect 165mm cranks are few and far between, especially second hand.

Seweryn

Jack Joll wrote: »

Seweryn,
Have you not already contradicted my point, bike shouldn't have had a longer crank if it was sized properly in the first place??

What I believe Jack, the bikes are generally equipped with cranksets being on the long side by the manufacturer when new. This is the new incorrect "standard" these days. I think the same happened with Kevin's bike - too long cranks on a new bike.
Just look at the tiny range of crank arm lengths available versus people's leg inseam lengths. Why? Because manufacturers don't care and it is easier for them not to offer more than 3-5 sizes that are very close to each other.

Jack Joll wrote: »

I was merely highlighted the importance of getting re-fitted, and 7.5mm in my book is a big step, you'd know it if one leg is 7.5mm longer than the other.
I've changed from a 175mm crank to a 172.5mm, due to regulating for a PM crank across all my bikes, re-fitted and trust me it does make a big difference. Unless I was at the limit of the frame size in the first place.
Not taking from Kevin, but just trying to highlight the importance of getting the bike right, particularly when his legs will possibly be doing the same repetative movement between 5000 times per hour.

I know how big difference the crank arm length change makes, I went through this myself (see my earlier posts). 5-7.5mm change down is a lot to get used to, that is for sure, but not that much in terms of bike fitting changes. You need to make sure the frame size is right in the first place, but then you only need to change the saddle and handlebar position. A new stem most likely, but that's about it.

Seweryn

Kevinleeowen wrote: »

Thanks for the advice guys. I've actually had 2 bike fits done. What came out of the second one was I could (ideally) go to a longer stem - from my 100mm one up by 10mm, so I think if I changed the cranks the current stem would suffice. I'd definitely look at increasing the saddle height though, more than likely by approx. the 7.5mm difference. I'd use this as the starting point, then make any minor adjustments from there. Failing that, would look in to a follow up to my last bike fit.

If your stem was 10mm too short at the last bike fit, it means you should be OK with the existing stem and the new cranks.
When doing the changes yourself, mark your current saddle height on the post with a piece of tape and increase the height by about 5mm to start with, as your knee angle at the top 12 o'clock position will be greatly increased anyway, which is positive. Then try to go up a little, if you feel comfortable.
What I am trying to say is that I would not go any higher with the saddle than the whole 7.5mm, but quite possibly a tiny bit less than that.

Kevinleeowen wrote: »

I really can't find anywhere that sells just the cranks though, so it looks like it's going have to be a complete new crankset. I'm thinkin of putting an ad on here, but suspect 165mm cranks are few and far between, especially second hand.

You can try eBay and other bike forums, but be patient. Otherwise, just get a new one and sell your current set.