Changing crank length
londoncommuter
Posts: 1,550
Bit of a noddy question this but I've just swapped a 172.5mm chainset onto a bike that previously had a 170mm and was wondering which way to move the saddle to compensate.
OK, it's only 2.5mm but I had a bike fit a while ago so I may as well try to maintain the same knee angles etc.
(This all of ff course that ignores the fact I'm about three years older and probably less flexible, have changed shoes and have less padded shorts and the bike fitter really wouldn't have been working to that kind of tolerance.....).
OK, it's only 2.5mm but I had a bike fit a while ago so I may as well try to maintain the same knee angles etc.
(This all of ff course that ignores the fact I'm about three years older and probably less flexible, have changed shoes and have less padded shorts and the bike fitter really wouldn't have been working to that kind of tolerance.....).
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Shouldn't make any difference whatsoever - I run 172.5, 175 and 170 on different bikes and can't feel any difference at all.
Some people say adjust the saddle though and others don't: frequently asked question (generally in Workshop I think) so worth a search on there.Postby team47b » Sun Jun 28, 2015 11:53 am
De Sisti wrote:
This is one of the silliest threads I've come across.
Recognition at last Matthew, well done!, a justified honoursmithy21 wrote:
He's right you know.0 -
If you are going to move the saddle it would be 2.5mm down as the leg at the bottom of the stroke is what is used to determine saddle height.0
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You should move the saddle down 2.5mm if you wish to maintain the same leg angle at the bottom of the stroke.
Since the new cranks are 2.5mm longer, the distance from the saddle to the pedal is 2.5mm longer at the furthest point and 2.5mm nearer at the nearest point if you don't make any saddle adjustment. Generally saddle position is set so that the knee is still 25 to 30 degrees off straight at maximum extension. Raising the saddle 2.5mm will maintain that angle but will also raise the pedal at the top of the stroke by 5mm.
2.5mm is not a big difference in saddle height but it's enough that many riders would notice it.0 -
Thanks everyone. I couldn't remember whether the angle when the pedal is horizontal mattered but think I'm just being dozy this morning.0
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So, the padding on a new pair of bibs could well be a couple of millimetres thicker than an old pair, should you adjust your saddle height every time you buy new bibs? ;-)0
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So, the padding on a new pair of bibs could well be a couple of millimetres thicker than an old pair, should you adjust your saddle height every time you buy new bibs? ;-)
No, but if you know for a fact that you are fitting cranks which will extend your leg reach by a known amount, it would be silly not to anticipate that difference by adjusting your saddle accordingly.
For everything else, you can just make changes as you go along...0 -
I can't tell the difference between 170 and 172.5, that's for sure.Shouldn't make any difference whatsoever - I run 172.5, 175 and 170 on different bikes and can't feel any difference at all.
Some people say adjust the saddle though and others don't: frequently asked question (generally in Workshop I think) so worth a search on there.
I'd say give it a try first, because for the most part our setups aren't that precise. But be wary of lower back issues and/or knee hyperextension issues. They are unlikely, but at least possible, so don't head out on a 100 mile ride in the first instance.
Any adjustment of the saddle would be 2.5mm down though, as others have said.0 -
It definitely does make a difference , but most noticeably if you get it wrong. With the longer cranks, it just feels like a longer lever, and my cadence goes down. Mainstream professional opinion currently seems to be that there is no real difference in power output between 165-180mm cranks all other things being equal, but also that a large sector of the population would be better off on cranks shorter than 170mm.
I'm 6ft with long legs, and many like me use 177.5mm, but the longest I have is 175mm, I have 170mm on my road bike, and 172.5mm on my TT bike. Thinking of following the current trend towards shorter cranks, and going for 165mm on the TT bike.0 -
If you're on the bike all the time and that's the only change you've made, then you will notice the difference. It sounds barmy, but it is so. You may even feel some discomfort in one or another joint in your leg when first riding on longer cranks.
However, that may not be a reason to change back to 170mm.
If you like the longer cranks, stay with them. You may want to move your saddle, but see how it feels first. On a bicycle, every adjustment has some impact on every other adjustment... so make tiny changes if you do make any - and log them so you can go back if it doesn't work.
Lots of people use different geometries and different crank lengths on different types of bicycle.
If it works, it works. If it doesn't, adjust or revert.0
