Crank arm length

flegomenos

Hi all,

I was wondering, since I'm going to buy a new crank set, whether I should get the 170mm or 175mm option (Sora R3000 chain set is not available in 172.5mm). My current FSA Vero crank set is 172.5mm long.

I am 182 cm tall and my inseam is 85 cm.

170 or 175?

Thanks!!

LimitedGarry

While I'm aware that you can base the decision on your size, just like the frame for example, I personally have found it to be more of a preference. On the road bike, I previously had 172,5. Got a 170 crank and I plain and simply prefer it.

lesfirth

I am 168mm tall/short and ride two bikes regularly,one with 165 cranks and one with 172.5 cranks. In a blind test I could not tell the difference. This could be explained by me being a bit dim. Some people on here can tell that wheels 100gms lighter spin up quicker and roll better. I cant . I cant even tell if I have emptied my drinks bottles and made my bike 1.5 kgs lighter. I wish I was clever.

Garry H

lesfirth wrote:

I am 168mm tall/short and ride two bikes regularly,one with 165 cranks and one with 172.5 cranks. In a blind test I could not tell the difference. This could be explained by me being a bit dim. Some people on here can tell that wheels 100gms lighter spin up quicker and roll better. I cant . I cant even tell if I have emptied my drinks bottles and made my bike 1.5 kgs lighter. I wish I was clever.

Take it you're 168 cms! Similar to me. Have used 170 and 175 and like you, couldn't really tell the difference.

lesfirth

Garry H wrote:

lesfirth wrote:

I am 168mm tall/short and ride two bikes regularly,one with 165 cranks and one with 172.5 cranks. In a blind test I could not tell the difference. This could be explained by me being a bit dim. Some people on here can tell that wheels 100gms lighter spin up quicker and roll better. I cant . I cant even tell if I have emptied my drinks bottles and made my bike 1.5 kgs lighter. I wish I was clever.

Take it you're 168 cms! Similar to me. Have used 170 and 175 and like you, couldn't really tell the difference.

I did say i was a bit dim!

trivial_poursuivant

lesfirth wrote:

Garry H wrote:

lesfirth wrote:

I am 168mm tall/short and ride two bikes regularly,one with 165 cranks and one with 172.5 cranks. In a blind test I could not tell the difference. This could be explained by me being a bit dim. Some people on here can tell that wheels 100gms lighter spin up quicker and roll better. I cant . I cant even tell if I have emptied my drinks bottles and made my bike 1.5 kgs lighter. I wish I was clever.

Take it you're 168 cms! Similar to me. Have used 170 and 175 and like you, couldn't really tell the difference.

I did say i was a bit dim!

Same height. Tried 165s on my TT bike. No discernible difference noticed. Stuck with 170 now.

cycleclinic

I have a 35 inch inside leg. I have bike with cranks from 165mm to 177.5mm. Crank length makes no bloody difference, in fact the shorter ones are better for me as my left knee bows out less at the top of the pedal stroke. Just alter your seat height if you change length

mcstumpy

i currently ride 2 different bikes, one with 172.5 and the other with 170. I can't tell the difference

flegomenos

Thank you all for the replies, much appreciated!

Since it makes no noticeable difference, I will go with the 170mm option and adjust the saddle height if needed, like thecycleclinic suggested.

StillGoing

flegomenos wrote:

Thank you all for the replies, much appreciated!

Since it makes no noticeable difference, I will go with the 170mm option and adjust the saddle height if needed, like thecycleclinic suggested.

I suggest you read some rather more informed opinions from actual qualified fitters such as Mike at Bike Dynamics than posters on here. One has even contradicted themselves in saying there is no difference yet the shorter cranks mean their knee doesn't bow out?

A lot will depend on whether you grind gears or like to spin a high cadence. It takes less time to rotate a small circumference circle than it does a bigger one. if you've long crank arms, it will inevitably take longer to rotate the full turn of the cranks compared to shorter crank arms at the same cadence. If you like a high cadence, you'll have to spin harder to achieve the same rate with long crank arms.

The next argument will be that 5mm makes no difference, except that it isn't 5mm. It's 10mm across the full diameter, so your seat can be raised 5mm to allow for the reduced distance to the pedal on the shorter crank arm at the bottom of the stroke, but it will give you an increased clearance of 10mm at the top of the pedal stroke enabling you to get over the top far easier and unrestricted.

http://bikedynamics.co.uk/FitGuidecranks.htm

milemuncher1

I'm about the same size as you, I used to ride with 172.5 or 175 mm cranks. I recently upgraded my latest bike purchase with a 105 crankset, and opted for 170mm cranks as an experiment. They are a bit of a revelation to be fair. I find it much easier to maintain a steady ( and higher than I normally would use) cadence for a given gear, I've also found I can lean over much more in the corners, without risking grounding out the pedals. So I would personally go for the 170mm cranks if it was my money.

flegomenos

Milemuncher1 wrote:

I've also found I can lean over much more in the corners, without risking grounding out the pedals.

Good point.

philthy3 wrote:

I suggest you read some rather more informed opinions from actual qualified fitters such as Mike at Bike Dynamics than posters on here. One has even contradicted themselves in saying there is no difference yet the shorter cranks mean their knee doesn't bow out?

This is why I'll be choosing the 170mm crank. When I measured mine (before I noticed that the size is actually written on the back side of the crank arm :shock:) I couldn't really tell if it was 170 or 175mm; it was 172.5mm. And I thought, if I'm between the two I'll go with the smaller as I've already regretted not getting a 54 size frame, instead of the 56 that the charts suggested was suitable for me. I got to understand that it's better to be able to 'stretch' and adjust things as a whole than not having the ability to shrink them...

philthy3 wrote:

The next argument will be that 5mm makes no difference, except that it isn't 5mm. It's 10mm across the full diameter, so your seat can be raised 5mm to allow for the reduced distance to the pedal on the shorter crank arm at the bottom of the stroke, but it will give you an increased clearance of 10mm at the top of the pedal stroke enabling you to get over the top far easier and unrestricted.

http://bikedynamics.co.uk/FitGuidecranks.htm

I had never considered this to be honest, also good point. Will certainly keep it in mind if one day I decide to go for a bigger change.

svetty

philthy3 wrote:

.......if you've long crank arms, it will inevitably take longer to rotate the full turn of the cranks compared to shorter crank arms at the same cadence.

:roll: :roll: :roll:

rick_chasey

Surprised you not don't notice since I really do.

Alex99

35" inside leg here. I ride 170 mm. Tried 175 mm for about 5 months a few years ago, really didn't get on with it and sold cranks on ebay. I can feel the difference of 5 mm, delicate flower that I am.

You'll get the jist by now that people are really different when it comes to crank length preference.

lesfirth

Svetty wrote:

philthy3 wrote:

.......if you've long crank arms, it will inevitably take longer to rotate the full turn of the cranks compared to shorter crank arms at the same cadence.

:roll: :roll: :roll:

I had read the post from Philthy3 but not bothered to take in just what he was saying. Now I have I can not stop laughing.

Alex99

philthy3 wrote:

flegomenos wrote:

Thank you all for the replies, much appreciated!

Since it makes no noticeable difference, I will go with the 170mm option and adjust the saddle height if needed, like thecycleclinic suggested.

A lot will depend on whether you grind gears or like to spin a high cadence. It takes less time to rotate a small circumference circle than it does a bigger one. if you've long crank arms, it will inevitably take longer to rotate the full turn of the cranks compared to shorter crank arms at the same cadence. If you like a high cadence, you'll have to spin harder to achieve the same rate with long crank arms.

Do people with longer cranks actually have lower cadence in practice? Any data to back up the idea that spinners use short cranks and grinders use long cranks? Long crank arms will take the same time for a full rotation with the same angular velocity.

drlodge

I went from 170mm to 172.5mm and found the difference to be marginal. A bit extra leverage on steep hills, that's about it.

rick_chasey

Alex99 wrote:

philthy3 wrote:

flegomenos wrote:

Thank you all for the replies, much appreciated!

Since it makes no noticeable difference, I will go with the 170mm option and adjust the saddle height if needed, like thecycleclinic suggested.

A lot will depend on whether you grind gears or like to spin a high cadence. It takes less time to rotate a small circumference circle than it does a bigger one. if you've long crank arms, it will inevitably take longer to rotate the full turn of the cranks compared to shorter crank arms at the same cadence. If you like a high cadence, you'll have to spin harder to achieve the same rate with long crank arms.

Do people with longer cranks actually have lower cadence in practice? Any data to back up the idea that spinners use short cranks and grinders use long cranks? Long crank arms will take the same time for a full rotation with the same angular velocity.

Anecdotal, but yes I did.

imposter2.0

This is worth a read - it dispels many of the myths and misconceptions around crank length and cadence...

http://myworldfromabicycle.blogspot.co. ... -just.html

rick_chasey

Imposter wrote:

This is worth a read - it dispels many of the myths and misconceptions around crank length and cadence...

http://myworldfromabicycle.blogspot.co. ... -just.html

Misses the reason I changed - it hurts my hip flexors a lot less not having to lift my leg as far up.

Opens up the hip angle a fraction for the same position.

lostboysaint

lesfirth wrote:

Svetty wrote:

philthy3 wrote:

.......if you've long crank arms, it will inevitably take longer to rotate the full turn of the cranks compared to shorter crank arms at the same cadence.

:roll: :roll: :roll:

I had read the post from Philthy3 but not bothered to take in just what he was saying. Now I have I can not stop laughing.

Ditto! Post of the day.

fat daddy

I dunno .. you laugh, but has any one actually tried it .. or are you going on theory ?

if I had 165mm cranks and pedalled at 60rpm that would be 1 rotation a second
but if I had 175mm cranks and pedalled at 60 rpm .... can anyone prove that it wouldn't sake 1 second to do a rotation ????

Alex99

fat daddy wrote:

I dunno .. you laugh, but has any one actually tried it .. or are you going on theory ?

if I had 165mm cranks and pedalled at 60rpm that would be 1 rotation a second
but if I had 175mm cranks and pedalled at 60 rpm .... can anyone prove that it wouldn't sake 1 second to do a rotation ????

It would inevitably lead to 1 rotation taking more than a second. It's a consequence of Special Relativity and greater pedal velocity.

fat daddy

so does it take the entire crank the same time to go around .... or does the part nearest the BB go around quicker or slower than the end nearest the pedal ?

surely at the point 165mm from the bb the crank is still doing 1 rotation per second and its only the nest 5mm that's going quicker and thusly getting round quicker

lostboysaint

This is a wind-up isn't it.

The clue is in the "revolutions per minute" (or "cadence" as some like to call it). It wouldn't matter if it was 1m long or 1 cm long, 60rpm is the same. The distance that the pedal will travel and thus the tip velocity of the crank will be greater as it's covering a longer distance to do the 1 rpm but it will still only be turning 1 rpm.

rick_chasey

fat daddy wrote:

so does it take the entire crank the same time to go around .... or does the part nearest the BB go around quicker or slower than the end nearest the pedal ?

surely at the point 165mm from the bb the crank is still doing 1 rotation per second and its only the nest 5mm that's going quicker and thusly getting round quicker

With shorter cranks, the pedal will travel less distance to complete the same revolution.

So.

If we take this as read:

Power, as a function of time, is the rate at which work is done, so can be expressed by this equation:



and work is force*distance we can make this



Now if we assume for a minute that the power stays the same.

By reducing the distance the pedal travels (d) (because the radius of the circle is now smaller), the force (f) applied needs to be increased to compensate.

So in essence, for the same power you have to push harder.

Longer cranks will make you feel like you're putting less force into the pedal - but you're having to pedal it around a longer distance to compensate. That's how levers work, ultimately.

rick_chasey

What in reality happens is a little more complicated when you start adding gears into the mix.

Alex99

fat daddy wrote:

so does it take the entire crank the same time to go around .... or does the part nearest the BB go around quicker or slower than the end nearest the pedal ?

surely at the point 165mm from the bb the crank is still doing 1 rotation per second and its only the nest 5mm that's going quicker and thusly getting round quicker

If you attached an accurate clock to the centre of the crank, and one to the pedal, then went for a ride. The timer on the pedal would come in very slightly behind in time compared to the crank centre. Therefore, I don't know what to conclude, and yes, this is a wind up.

lesfirth

Still laughing but now I can't work out who is serious and who is taking the p1ss. I think there are some who I think really must be taking the p1ss but in fact are serious!

StillGoing

lesfirth wrote:

Still laughing but now I can't work out who is serious and who is taking the p1ss. I think there are some who I think really must be taking the p1ss but in fact are serious!

Or maybe you're simply being a jerk and don't understand simple gearing? A larger wheel will take longer to rotate than a small wheel, which is effectively what you have with shorter crank arms. to turn the larger wheel at the same rpm, you'd need to put a hell of a lot more effort in to achieve it.

https://pptcrafter.wordpress.com/2013/0 ... owerpoint/