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I am looking at a 39/53 crankset with the option of a 170 or a 175 mm length.
Is size in relation to bike fit the only difference between the two? In other words personal fit and/or preference? Or is there a mechanical advantage to be had by using the slightly greater length?
Think it boils down to whether you prefer grinding big gears or spinning, longer cranks will help you grind the bigger gears but penalise you when you want to spin, and vice versa.
Think shorter cranks are supposed to be more knee friendly, that's why I went 170mm even though I'm 5'11.
My singlespeed came with 170mm cranks as stock which I was surprised by, but it's OK as I wussed out and lowered the gear anyway : )
I inadvertently had 175 on one side and 170 on the other for around 9 months 🤣🤣
Haven’t noticed any difference since I rectified it 🤷🏻♂️🤷🏻♂️
As a very rough guide, height in cm = crank length in mm. I find that a slightly shorter crank helps 'spin-up' the gears plus I can maintain a slightly higher cadence on a shorter crank. On the occasions I've tried a longer crank, particularly on an MTB grinding the gears slowly did create some knee discomfort.
I inadvertently had 175 on one side and 170 on the other for around 9 months
I got dropped off at the bike shop to ride my new road bike ~20 miles home. I got to the first hill before I thought wtf is going on here? It felt really weird so I rode back to the shop. Turned out it had 170 on one side and 175 on the other. There was a mirror image of the crankset back on the shelf for sale - they sorted that out PDQ!
OP: some small gearing effect, but personal preference. Mine is for 165 on all my bikes 'cos it keeps my knees happier ( I'm 5'6").
As a very rough guide, height in cm = crank length in mm.
Very rough indeed! Otherwise I'd be riding 180mm cranks when I actually ride 172.5mm on both road, cross & track.
One of Paton's links up there suggests going even shorter! Would be tempted to try 165mm if it didn't cost a new set of cranks, anything to reduce twingy knees!
After a bike fit I now have 165mm on road and gravel bikes. It made such a difference for me.
What height are you McNulty?
Makes **** all difference.
If it mattered it would be easy to prove.
I've got a mix across bikes and it's not something I've ever considered when I've bought complete bikes. When buying groupsets I've gone down the standard 172.5 route for no real reason. Last bike I bought was a TT bike with oval rotor Q rings and I can't even tell I'm using them except when spinning the little ring so I have concluded I'm just not very sensitive to this stuff. Others may or may claim to be!
I can’t imagine how height has got anything to do with it, leg length yes, but not height. I’m a spinner and can spin just as well with 175’s as with 165’s. I found I hadve no sensitivity between 170 and 175 - I literally can’t tell a difference, but recently put 165’s on my MTB and can notice it. Puts more pressure onto my lower quads as my knee angle feels a bit more acute at the 3 o’clock position (crank out front) and the saddle is slid all the way back. I noticed some minor knee aching the first coulple of rides, but am getting used to it. Doesn’t seem to have affected my performance in any way.., i’m just as crap as I was on 175’s. I have a 31” inside leg. I guess if I was buying cranks again i’d settle on 170’s as a minimum length, but illegal see how I get on with the 165’s, so far so good despite the differences - i’ll adapt i’m sure.
I've 165 on one bike and everything else is 170 but I'd have 165 by choice. I've also had 175 before on a mountain bike. It was fine off road but my knees didn't like it for prolonged in the saddle riding. From that I concluded that for myself crank length was more important on a road bike than a mountain bike. Never heard of anyone having knee issues from too short a crank.
Honestly, it’s not a big deal. In time trialling, Shorter are becoming more favoured for aero reasons. But I ride 165 fixed, 170 and 172.5. Don’t notice a change.
@13thfloormonk I’m 177cm (about 5’10” I think that works out to) but with a short inside leg (usually 29” in trousers).
When I went for my bike fit spinning at 90rpm once the cranks has been adjusted from 172.5 (mimicking the stock cranks on my bike) down to 165mm I was able to increase the resistance a few notches maintainingbthat cadence and effort. It was a really simple demonstration that the shorter cranks allowed me to get more power down.
I was also getting some slight knee pain on longer rides, this has now gone (but the bike fit altered a few things so I don’t know if this is just down to crank length).
I'm 5'5" and ride 165 or 170.
If you're short, you can feel the difference with shorter cranks - smoother, less strain on the knees, easier to spin and maintain cadence.
+1 for buying a bike with different crank lengths. And yes, to me it felt awful.
They say that we are in the habit of buying long cranks. It's coming out more now that we should at the very least try 165. If you notice the comments by people with 165 length it's positive. Main issue is trying them.
Forget sizing the conventional way as this is part of the issue.
Cyclist magazine suggested that 170mm cranks were even too long for anyone 5ft 10 or shorter. It said that shorter cranks give less hip closure which was obviously desirable.
Makes **** all difference.
if it mattered it would be easy to prove.
Annecdotally I can definitely feel the improvement in terms of no knee pain using 170mm vs 175... (5'10", 30" inseam, moderate ape index).
And of course in rather simple biomechanical terms; forcing someone with shorter legs to make the same size circles as someone with longer legs is always going to require them to use a greater range of motion and hence increase the chances of musculoskeletal strain and injury...
Then of course there's responses like this:
When I went for my bike fit spinning at 90rpm once the cranks has been adjusted from 172.5 (mimicking the stock cranks on my bike) down to 165mm I was able to increase the resistance a few notches maintainingbthat cadence and effort. It was a really simple demonstration that the shorter cranks allowed me to get more power down.
To my mind that's proof, better comfort, plus being able to put more power through the bike...
In basic terms a bike is just a collection of support points and levers, the better you can adjust those features to suit the specific anthropometrics of the user the better it works for them, adopt a "one size fits all" approach and only a few people really benefit, there are different crank length options for the same reason there are different frame sizes, bar and stem sizes and control positions are adjustable, no two people are ever quite the same.
Do people consider stack height of shoes, cleats & pedals? As there can be variances greater than the 2.5mm difference in crank length
Do people consider stack height of shoes, cleats & pedals?
It's not relevant - they're the same top and bottom of the stroke and can be compensated for by adjusting saddle height. Your foot travels up and down the same amount no matter how thick your shoes / pedals. Change the crank length and the travel changes: 2.5mm on crank length = 5mm more uppy-downy.
There's at least one study (maybe linked to above) that shows crank length makes next to bugger-all difference to power output over a very wide range of lengths. It has made a big difference to my knees though.
Do people consider stack height of shoes, cleats & pedals? As there can be variances greater than the 2.5mm difference in crank length
People do tweak cleat position and put wedges in their shoes to try and even up support and keep their feet straight through the stroke. The cleat/shoe stack height is sort of "self canceling" at the top/bottom of the pedal stroke, so such things are considered...
Of course the difference in crank length is doubled (there's two crank arms) so the circle might shrink by 5/10/20mm (not 2.5mm) depending on the change in crank length, which might not seem much, but people can get feel the benefit of similarly 'small' adjustments in stem length, bar height or saddle position on most bikes...
I suppose you don't have to belive any of it makes a difference, just ride your bike as the shop set it up and you might be just fine, but as you get older, and maybe less flexible, knee/hip/back/shoulder/neck pain becomes more noticeable (IME). So if for no other reason than your own comfort it's worth thinking about fit (including crank length)...
I think current idea from sport science is that one should not expect performance gain from change in crank length but it can make significant change in comfort, especially for people with knee issues.