How Critical Is Crank Length?

furiousox

Just wondering how critical crank length is?
I'm 174cm (just under 5'9") and I'm using 172.5mm cranks, have been for years.
Apparently if you're 5'10" or under you should be using 170mm cranks.
So should I bother changing to 170mm, would I notice a discernible difference?

Lumen

No. If it aint broke don't fix it.

bcmf

some folk say its critical but srhorter cranks are supposedly better for those who spin and longer (as well as those with longer legs) ate better for mashers. But as Lumen says if you are not haveing any difficulties then stay with what you have

Murph100

+1 on what the lads have said.

If you search the web you'll find proponents of crazy long cranks and silly short cranks battling it out with their various conflicting theories :rolleyes: ... but you'd be better off spending those wasted hours training instead.

Paul Kiernan

I'm building up a bike for my wife at the moment and unthinkingly ordered a 172.5 crank instead of a 170 (she's 5'5"). Am I going to change it? Not a chance in hell.

Of more concern is that she's used to a triple and I got her a compact and there's going to be hell to pay when she hits that first big hill and goes looking for her granny .................

anoble66

i moved from 172 to 170 crank arms, and I cannot tell any difference.

ednwireland

anoble66 wrote: »

i moved from 172 to 170 crank arms, and I cannot tell any difference.

+1
gone from 170 to 172.5 (on different bikes)

switch between them cant tell the difference

Inquitus

Paul Kiernan wrote: »

Of more concern is that she's used to a triple and I got her a compact and there's going to be hell to pay when she hits that first big hill and goes looking for her granny .................

Depends what cassette you put on the back, with a 12/25 she might not be impressed!

Quigs Snr

furiousox wrote: »

Just wondering how critical crank length is?
QUOTE]

Apparently it's the girth that counts, or at least that's what I read in the missus's cosmo magazine.

No seriously, very little difference between 170 and 172.5, it's very much a case of personal preference. I am 170cm and ride a 170mm crank, but I have ridden a 172.5 and felt no difference, my numbers (recorded between the two bikes over a period of months on an Imagic Turbo Trainer) show no difference at all in terms of power or speed, the only difference was that under pressure - HR over 160 - my cadence increased by an average of 1 rpm on the 170mm cranks, at a regular cruise steady there was no cadence difference. I did fatigue a little in the hip joint over long distances at a low threshold with the 172.5 but that could be unrelated to the crank length as it didn't happen enough for me to categorically state that was the reason.

Most riders around your height would ride a 172.5 (notable exceptions are Sastre and Cavendish who ride 170).

Paul Kiernan

Inquitus wrote: »

Depends what cassette you put on the back, with a 12/25 she might not be impressed!

That's not a bad idea. I might whack on an 11-23 and take her for a little loop around Slieve Maan, Shay Elliot and Sally Gap. Then when I change it to an 11-28 she'll be over the moon:pac:.

velogirl

Very recently changed bikes and changed from a 172.5 to 170 cranks (quite by accident) and WOW what a difference it has made. Cadence went way up - has changed my pedalling style from a big gear pusher to a pedaller/spinner. Speed has increased also and climbing - although I pedal more is easier.

Thought it was the new bike but now am of the opinion that it is the crank lenght as the new bike is very similar to the old bike. By the way I'm 5ft 7

Paul Kiernan

velogirl wrote: »

Very recently changed bikes and changed from a 172.5 to 170 cranks (quite by accident) and WOW what a difference it has made. Cadence went way up - has changed my pedalling style from a big gear pusher to a pedaller/spinner. Speed has increased also and climbing - although I pedal more is easier.

Thought it was the new bike but now am of the opinion that it is the crank lenght as the new bike is very similar to the old bike. By the way I'm 5ft 7

You are taking the piss, aren't you?

feck sake lads

just drop or raise your saddle by same amount ,can't think who told me that;)
leave well enough alone you'll be fine honest.

DonalK1981

I was looking for a new crankset, and if any of you guys are changing let me know. Looking for ultegra, just to match you know!

velogirl

No I'm not taking the piss.............. I am deadly serious. It made a huge difference

Paul Kiernan

velogirl wrote: »

No I'm not taking the piss.............. I am deadly serious. It made a huge difference

It's a difference of 2.5mm, a tenth of an inch! Now, I don't have much knowledge of these things but if you have any interesting stories on what a difference a tenth of an inch can make I'd just love to hear them.

velogirl

I'm not a mathematician - perhaps someone else would be more qualified to enlighten you on the technicalities of it. I just know what a difference it made.

http://www.machinehead-software.co.uk/bike/cranks/cyclist_crank_length_calculator.html

The above calculator might help someone.

Idleater

velogirl wrote: »

I just know what a difference it made.

I'm going to agree with this, I've gone from a 175 to a 172.5 after a bike fit and I much prefer the new size. When I go back to the old one it tends to niggle my bad knee a bit.

Obviously there are two reasonably different bikes and groupsets involved, as well as the new one being the correct frame size which might have more influence, but it's the crank rotation that I seem to notice most. Both have the correct seat post measurement, the only difference being the length to handlebars being 10mm too long on the old.

C3PO

velogirl wrote: »

No I'm not taking the piss.............. I am deadly serious. It made a huge difference

I think it's only a very serious cyclist who could distinguish between 170mm and 172.5mm .... 2.5mm is really a tiny difference and certainly it couldn't change your whole riding style!
I've got 170mm on my road bike, 172.5mm on my cross/winter trainer and 175mm on my mountain bikes and I can't feel any difference between them!

bcmf

About 18mnths I changed from 170 to 175. Yes it made a difference. I did have to adjust seat position a tad but found less cadence and found it better. Everyone is different to what kinda cadence they like and how their legs/knees react.

fat bloke

RPL1 wrote: »

I think it's only a very serious cyclist who could distinguish between 170mm and 172.5mm .... 2.5mm is really a tiny difference and certainly it couldn't change your whole riding style!
I've got 170mm on my road bike, 172.5mm on my cross/winter trainer and 175mm on my mountain bikes and I can't feel any difference between them!

If it was a negligible difference, then crank manufacturers wouldn't make them with those 2.5 mm stepped intervals.

Put a carpet knife gently against the side of your face. Now push it in 2.5mm and tell me it's negligible.

It's all relative bud. If the girl found an appreciable difference, who are you to tell her otherwise?:rolleyes:

user111

Paul Kiernan wrote: »

I'm building up a bike for my wife at the moment and unthinkingly ordered a 172.5 crank instead of a 170 (she's 5'5"). Am I going to change it? Not a chance in hell.

Of more concern is that she's used to a triple and I got her a compact and there's going to be hell to pay when she hits that first big hill and goes looking for her granny .................

longer crank arm will give you better force, so it will be easier to push. small saddle height adjustments will be recommended.
What fr.derailleur&r.derailleur&cassette do you have atm ?

Lumen

Crank length obviously matters to some degree since it has an effect on gearing and joint angles.

2.5mm doesn't sound much that but for a given effective saddle height an extra 2.5mm on the cranks will raise your knee by 5mm at the top of the stroke.

People with knee and back problems will fuss over 5mm of saddle height so I don't think it's unreasonable to be fussy about 2.5mm of crank length, if you're a princess-and-the-pea kind of person.

I still wouldn't change it unless I had a reason to, or if I was well outside normal recommendations for my leg length.

Hermy

fat bloke wrote: »

Put a carpet knife gently against the side of your face. Now push it in 2.5mm and tell me it's negligible.

But according to Lumen you would have to push it in 5mm to get the full effect.:)

kumate_champ07

feck sake lads wrote: »

just drop or raise your saddle by same amount ,can't think who told me that;)
leave well enough alone you'll be fine honest.

the pedals move in a circular motion so it would be like moving your saddle back 2.5mm also, but forward 2.5mm at the opposite part of the crank revolution. With 2.5mm more, youre making a bigger circle

kumate_champ07

Ive heard of people doing TT's pusing a big gear with 180mm cranks, the extra leverage apparently helps.

C3PO

Lumen wrote: »

2.5mm doesn't sound much that but for a given effective saddle height an extra 2.5mm on the cranks will raise your knee by 5mm at the top of the stroke.

I'm probably going to regret this but please explain that to me - I can't see how 2.5mm on the crank raises your knee by 5mm? I can see that the circumference of the circle increases by 5mm but the radius also?

Beasty

kumate_champ07 wrote: »

Ive heard of people doing TT's pusing a big gear with 180mm cranks, the extra leverage apparently helps.

Indurain used 190mm for his hour record, although the geometry on TT bikes will be significantly different anyway

You normally are restricted on the track - 165mm in Manchester, which is 10mm shorter than my road bike, although I never notice (the fact I am riding fixie I guess overshadows something like that)

Lumen

RPL1 wrote: »

I'm probably going to regret this but please explain that to me - I can't see how 2.5mm on the crank raises your knee by 5mm? I can see that the circumference of the circle increases by 5mm but the radius also?

It seemed plausible when I wrote it.

C3PO

Lumen wrote: »

It seemed plausible when I wrote it.

I'll get you to explain it to me on Friday at the kids spin!!

TinyExplosions

Lumen wrote: »

It seemed plausible when I wrote it.

I think it's a little off, as the centre of the circles should be in the same place, meaning that a 2.5mm difference in crank length = a 2.5mm shortening at the top of the stroke (unless my idea of math is very wrong)

leftism

We have done several studies in the TCD lab on this question. No differences in cycling economy were detected when changing crank length between 165, 170 and 175 cm. Minor differences were detected in EMG recorded from the knee extensors and knee flexors (only when comparing 165 and 175cm) but these differences had no effect on metabolic cost or performance. When questioned after the test, all subjects reported that the crank length they felt most comfortable with ended up being the crank length they had been using all along anyway (i.e the guys using 175 felt most comfortable at 175. The guys using 170 preferred 170). So the conclusion were that changing crank length didn't make much difference to overall cycling performance or metabolic cost.

Interestingly, we saw more notable differences when seat tube angle was adjusted. Maybe we should be looking more closely at saddle fore-aft position then worrying about crank lengths.

niceonetom

This thread is in its infancy but the warning signs are there. Discussion of crank length is banned on some TTing forums so contentious it is.

@Tiny - presuming you adjust saddle height to make the leg extension identical after changing crank length the centre of the circle would have moved relative to the rider - this accounts for the idea that the knees must now rise 5mm to account for a 2.5mm change in circle radius. The BB is a poor datum here, better to use the bottom of the stroke or the saddle.

@Lumen - assuming I've understood your diagram (unlikely) the proposal that L1 = L2 is only necessarily true for people with no ankles. I moved from 172.5 to 175 cranks for the RAI and am fairly convinced that my knees were rising no higher than they had been. Instead, my ankle was bending more at the top of the stroke, stretching my achilles more, and ensuring the future prosperity of my physiotherapist.

I agree with your first post.

Lumen

niceonetom wrote: »

@Lumen - assuming I've understood your diagram (unlikely) the proposal that L1 = L2 is only necessarily true for people with no ankles. I moved from 172.5 to 175 cranks for the RAI and am fairly convinced that my knees were rising no higher than they had been. Instead, my ankle was bending more at the top of the stroke, stretching my achilles more, and ensuring the future prosperity of my physiotherapist.

I don't dispute your empirical evidence, I think ankling tends to compensate when your other joints are prevented from doing to by ROM limits. You can also raise your seatpost and retain the same leg extension by ankling more - I saw this on video capture during a bike fit session.

Are you back on 172.5mm now?

Notwithstanding the "don't screw with stuff which works" rule, the "shorter is safer" approach makes sense to me. I'd rather use a fat cassette and spin more than tough it out for the benefit of more "leverage". Leverage is what breaks stuff.

niceonetom

Yeah, back on 172.5 now and intending to stay with that from now on, even for TTing.

Ankling is, IMO, bad. It's the only place in the pedalling motion that can, as you say, compensate for problems elsewhere. Over ankling (heel too low) at the top of the stroke was a way of maintaining more open hip angles while on longer cranks and in a low position on TT extensions. My achillles was, I think, still a little more extended than usual when I came to the power phase of the stroke. The result was badness.

cdaly_

Lumen wrote: »

2.5mm doesn't sound much that but for a given effective saddle height an extra 2.5mm on the cranks will raise your knee by 5mm at the top of the stroke.

kumate_champ07 wrote: »

Ive heard of people doing TT's pusing a big gear with 180mm cranks, the extra leverage apparently helps.

TinyExplosions wrote: »

Lumen wrote: »

It seemed plausible when I wrote it.

I think it's a little off, as the centre of the circles should be in the same place, meaning that a 2.5mm difference in crank length = a 2.5mm shortening at the top of the stroke (unless my idea of math is very wrong)

No, it's a valid diagram. If you assume that the saddle is adjusted such that leg extension is the same for either crank length, then you can place the bottom of each circle on the same line. Now the top of a 172.5 crank circle is 5mm higher than the top of a 170 crank circle. It therefore follows that your knee is raised 5mm higher by a 172.5 crank than a 170.

kumate_champ07 wrote: »

Ive heard of people doing TT's pusing a big gear with 180mm cranks, the extra leverage apparently helps.

It should do. It effectively lowers the gearing a touch.

Beasty

cdaly_ wrote: »

It should do. It effectively lowers the gearing a touch.

Moving from 170 to 180 would be approximately equivalent to moving from a 53 to a 50. However if that was the only effect, I would question why not simply do it this way (ie change chainring size). I guess there must be wider issues when TT'ing which makes a larger crank more efficient.

Quigs Snr

leftism wrote: »

We have done several studies in the TCD lab on this question. No differences in cycling economy were detected when changing crank length between 165, 170 and 175 cm. Minor differences were detected in EMG recorded from the knee extensors and knee flexors (only when comparing 165 and 175cm) but these differences had no effect on metabolic cost or performance. When questioned after the test, all subjects reported that the crank length they felt most comfortable with ended up being the crank length they had been using all along anyway (i.e the guys using 175 felt most comfortable at 175. The guys using 170 preferred 170). So the conclusion were that changing crank length didn't make much difference to overall cycling performance or metabolic cost.

Interestingly, we saw more notable differences when seat tube angle was adjusted. Maybe we should be looking more closely at saddle fore-aft position then worrying about crank lengths.

Thanks for that, it backs up what I was saying earlier about how I didn't notice any difference in my numbers with slightly different lengths. What is interesting is the fore - aft debate. I am mediocre on a road bike, but a hell of a lot more powerful with the saddle forward hence I can be beaten horribly on a road bike by a good rider, but can do the same do that rider if we both happen to be on TT bikes. I am thinking it's perhaps because I have a rowing background and strong gluts to match, so the forward saddle position takes advantage of that more.