Anyone around 172cm (5’7.5”ft) tall using 165mm cranks?

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Lewn777
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by Lewn777

I'm sorry but I really couldn't go as short as 165cm or less, and it's silly to advocate short cranks for all as that surely is fashion. What's hidden and underlies wattage is torque, and for me 170mm or less would simply rob me of too much torque and make me slower. Although for the OP at 5ft7inches I would look at 170mm and definitely try 165mm or less.

If you are a high cadence rider, have injuries short cranks might be a great choice, but not for me and many others. Having said that I would never go over 172.5 and would remove and sell 175mm or longer cranks if they came on a bike I'd bought but would be fine with keeping and using 170mm cranks. You can certainly go far further wrong with cranks that are too long rather than too short.

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Hexsense
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by Hexsense

power = torque x angular velocity (cadence in rpm).
Shorter crank reduce torque but naturally increase rpm without you trying. You don't spin faster intentionally because you have reduced torque, you automatically spin faster because the circle is smaller. Same cadence with longer crank = faster foot speed than same cadence on shorter crank. Most people have their rhythm sync to linear foot speed, rather than cadence number. If you naturally spin at 60rpm with longer crank, change to shorter crank, your natural cadence automatically become 64rpm, for example.

That said, 175 to 165 is 6% change. You lose 6% of torque and naturally spin 6% faster cadence. That align well with one gear change from 16t to 17t which is also 6% change. Changing gear to 6% easier one reduce the torque needed by 6%. Now, see the equalization?
175mm crank + 16t at 60rpm
and
165mm crank +17t at 64rpm
is the same speed and power, and require the same forces at the pedals. They even have the same linear foot speed (but reported as different cadence). That said, muscle memory of years using longer crank can get in the way of adopting shorter crank though.

Edit: mistake corrected.
Last edited by Hexsense on Tue Nov 23, 2021 5:50 pm, edited 2 times in total.

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pdlpsher1
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by pdlpsher1

A very good analysis but I think one of your sentences has an error. 175mm crank + 16t at 60rpm and 165mm crank +17t at 64rpm is the same speed and power, and require the same torque should read 175mm crank + 16t at 60rpm and 165mm crank +17t at 64rpm is the same speed and power, and require the same forces on the pedals. If both have the same speed and power, obviously they can't have the same torque since the 165mm crank is spinning faster. So the 165mm crank has less torque but it's compensated by a higher rpm.

So in a nutshell there's really no mechanical advantage from a shorter crank assuming you have identical linear foot speed and pedaling force whether you're on a 175mm or 165mm crank. Both cranks produce identical amount of power. But the shorter crank will require you to have an additional lower gear when on a steep climb. If you lack that extra low gear you will have to produce a higher pedaling force to make up the loss in torque, thus causing premature fatigue.

ghostinthemachine
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by ghostinthemachine

pdlpsher1 wrote:
Tue Nov 23, 2021 9:01 am
So in a nutshell there's really no mechanical advantage from a shorter crank assuming you have identical linear foot speed and pedaling force whether you're on a 175mm or 165mm crank. Both cranks produce identical amount of power. But the shorter crank will require you to have an additional lower gear when on a steep climb. If you lack that extra low gear you will have to produce a higher pedaling force to make up the loss in torque, thus causing premature fatigue.
Noticable biomechanical advantages though... And with modern 11/12/13 ratio systems, all of which seem to be what I'd call wide range, an extra large sprocket is a small price to pay.

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by CustomMetal

I'm 6ft3 and on 170mm cranks.. if i was your height i would be on 160 or less.. shorter is the way ;)
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Hexsense
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by Hexsense

pdlpsher1 wrote:
Tue Nov 23, 2021 9:01 am
A very good analysis but I think one of your sentences has an error.
Oops, right on. I meant perceived force from the foot but my brain fart type same torque.
Thank you for correction.
ghostinthemachine wrote:
Tue Nov 23, 2021 9:48 am
pdlpsher1 wrote:
Tue Nov 23, 2021 9:01 am
So in a nutshell there's really no mechanical advantage from a shorter crank assuming you have identical linear foot speed and pedaling force whether you're on a 175mm or 165mm crank. Both cranks produce identical amount of power. But the shorter crank will require you to have an additional lower gear when on a steep climb. If you lack that extra low gear you will have to produce a higher pedaling force to make up the loss in torque, thus causing premature fatigue.
Noticable biomechanical advantages though... And with modern 11/12/13 ratio systems, all of which seem to be what I'd call wide range, an extra large sprocket is a small price to pay.
Also, the need for high gear is reduced as the cadence automatically raised. 52x11 down to 50x11 is 4% down in gear ratio. 36 to 34 is 5.8%. Both are less than 6% from the change from 175 to 165. TBH, just 6% change here or 4% there are less significant each time I look at it. I don't know why people speak like 6% decrease in torque and compensate with 6% higher natural cadence is going to be earth shatteringly unrideable.

ghostinthemachine
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by ghostinthemachine

Hexsense wrote:
Tue Nov 23, 2021 5:42 pm
I don't know why people speak like 6% decrease in torque and compensate with 6% higher natural cadence is going to be earth shatteringly unrideable.
Because they are more interested in the emotions and "common sense" around the issue, not the biomechanics, physiology and the physics.

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Lewn777
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by Lewn777

Yes but older people seem to not lose their torque so fast as they grow older, whereas they lose the ability to produce high cadence watts earlier, so there is more logic in staying with longer cranks if you are older.

Maybe in the future people will start off with much shorter cranks as juniors, maybe 155cm and go longer as needed/desired but go back to shorter again in case of injury.

Hexsense
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by Hexsense

Lewn777 wrote:
Wed Nov 24, 2021 1:06 am
Yes but older people seem to not lose their torque so fast as they grow older, whereas they lose the ability to produce high cadence watts earlier, so there is more logic in staying with longer cranks if you are older.
decrypting the high cadence thing.
If you think 90rpm is right where it's too fast on long crank.
In that case, 90rpm is not too fast on shorting crank. Rather, say 90rpm is high cadence on 175mm crank. Then high cadence for 165mm crank is 95rpm.
90rpm on long crank and 95rpm on short crank has the same linear foot speed and the same perceived spinny feel.
95rpm on long crank is perceived as way FASTER than 95rpm on shorter crank.
Even 90rpm on long crank has higher linear foot speed than 94rpm on short crank... So, yeah. I'm saying 90rpm can feel faster than 94rpm if crank length are not equal.

robertbb
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by robertbb

Lewn777 wrote:
Sun Nov 21, 2021 4:48 pm
I'm sorry but I really couldn't go as short as 165cm or less, and it's silly to advocate short cranks for all as that surely is fashion. What's hidden and underlies wattage is torque, and for me 170mm or less would simply rob me of too much torque and make me slower. Although for the OP at 5ft7inches I would look at 170mm and definitely try 165mm or less.

If you are a high cadence rider, have injuries short cranks might be a great choice, but not for me and many others. Having said that I would never go over 172.5 and would remove and sell 175mm or longer cranks if they came on a bike I'd bought but would be fine with keeping and using 170mm cranks. You can certainly go far further wrong with cranks that are too long rather than too short.
Yeah I'd agree with this.

Cross-post from another thread:

Shorter cranks are almost always better, right up until they aren't :lol:

My advice would be to try in increments, and keep going shorter until it feels "off". I realise this may be costly, however.

This is based on my experience as follows:

I went from 172.5 to 170 (having read and watched all about this trend) and most certainly did feel a difference. Loved it instantly. Tried going back to 172.5 and felt like I was on a penny farthing. And this is after 15 years riding on 172.5's because, for the most part, that's just "what most people ride".

After the 170's I then tried 165's as I thought it would be worthwhile to find out if further improvements to feel could be had. My cadence did go up, and notably, my heart rate went down for a given sustained power. There really is a metabolic advantage to shorter cranks in endurance efforts that's for sure. However, I noticed it didn't feel as fluid as the 170's. It was like running with my shoelaces tied together. I never really looked at the data to ascertain if there was a sprint (dis)advantage so I can't offer anything there.

I'd have loved to try 167.5's and that may well have been a good compromise but that length is quirky and not as widely available so I've settled on 170's

For reference I'm 178cm with an 84cm inseam.

Brokenladder
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by Brokenladder

Just another data point... I have been playing around with shorter cranksets for the last few years.

172cm. Age > 50. Riding seriously since my teen years....

XC full-suspension mtb - 165mm
Hardtail mtb - 170mm
OPEN UP gravel bike - 170mm
OPEN MIND road bike - 165mm

All are Rotor/oval. Generally, I'll say that I like the shorter cranksets for two [perceived] reasons: less stress on my knees and fewer pedal strikes offroad. My typical ride is one to two hours--but today I rode for four hours since I had the time. Terrain is rolling where I live. My XC full-suspension bike is currently my favorite to ride.

Again, I would just say that I feel like my knees take a bit less of a beating than when I was on longer cranks. It's not very scientific; I fully appreciate that.

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Lewn777
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by Lewn777

Brokenladder wrote:
Thu Nov 25, 2021 5:30 am
Just another data point... I have been playing around with shorter cranksets for the last few years.

172cm. Age > 50. Riding seriously since my teen years....

XC full-suspension mtb - 165mm
Hardtail mtb - 170mm
OPEN UP gravel bike - 170mm
OPEN MIND road bike - 165mm

All are Rotor/oval. Generally, I'll say that I like the shorter cranksets for two [perceived] reasons: less stress on my knees and fewer pedal strikes offroad. My typical ride is one to two hours--but today I rode for four hours since I had the time. Terrain is rolling where I live. My XC full-suspension bike is currently my favorite to ride.

Again, I would just say that I feel like my knees take a bit less of a beating than when I was on longer cranks. It's not very scientific; I fully appreciate that.
Anecdotal, but still useful.
My view ideally I would like to build on these thoughts, I have tried various lengths. and have my own opinion.

1. Most pros with the same height and leg length as me use 172.5mm cranks so I'm with good company. Maybe start with short cranks and go longer as you feel comfortable with. It seems fairly easy to identify what's too long.
2. Always go default to shorter cranks, as you would to a smaller frame if between sizes. But this fad for short cranks might go too far and we'll start seeing riders that are 6ft5 on 160mm cranks, which will be silly unless you've really tested the other options or are prone to injury.
3. Ideally use a variety of crank lengths, this might be possible on an indoor trainer and look at the power numbers but remember that outside of testable conditions on real life climbs that torque might be useful.

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