Effects of crank length on bike fit

markhewitt1978

What effect does crank length have on an overall bike fit?

While bit fit will look at the saddle heigh and fore/aft position, bar height, stem length etc, what about the length of the cranks?

I ask because at 5'6" and apparently short legs for my height I'm pretty sure my 170mm cranks are too long for me, so what would happen if I switched to 165mm?

Is it just a matter that my saddle would need to go up by 5mm or are there more changes to my position that would need to be accommodated?

keezx

Saddleheight is obvious, but IMO you can ignore the rest, 5 mm isn't much for reach and bar height.

simon_masterson

Saddle height is indeed the main one. The emphasis of this debate often does centre around people with shorter legs ending up with cranks that are too long, but if you are serious about this, I would try one of the sizing systems and see what you get - it might be shorter than 165mm. In that instance you would probably want to have a standard crankset shortened, possibly the one you have now.

markhewitt1978

Simon Masterson wrote:

Saddle height is indeed the main one. The emphasis of this debate often does centre around people with shorter legs ending up with cranks that are too long, but if you are serious about this, I would try one of the sizing systems and see what you get - it might be shorter than 165mm. In that instance you would probably want to have a standard crankset shortened, possibly the one you have now.

Putting in numbers into online calculators seems to suggest an ideal length of between 160-165mm, which would suggest 165mm is worth getting, but perhaps not worth going to the trouble of trying to source shorter than that?

smoggysteve

My main bike came with a compact crank with 172.5mm crank arms. I later upgraded to a standard double with 165mm arms and the difference was noticeably better. I have since gone back to 170mm on my current crank and it feels no different from the shorter arms but I did have to readjust my saddle forward to compensate which had an overall effect on my position which I had to correct with a longer stem.

styxd

Saddle height will probably do it. Don't go 5mm though, try 3mm to start with. Or even just see how they feel without moving your saddle up at all.

simon_masterson

markhewitt1978 wrote:

Simon Masterson wrote:

Saddle height is indeed the main one. The emphasis of this debate often does centre around people with shorter legs ending up with cranks that are too long, but if you are serious about this, I would try one of the sizing systems and see what you get - it might be shorter than 165mm. In that instance you would probably want to have a standard crankset shortened, possibly the one you have now.

Putting in numbers into online calculators seems to suggest an ideal length of between 160-165mm, which would suggest 165mm is worth getting, but perhaps not worth going to the trouble of trying to source shorter than that?

That sounds about right. I guess it would depend on your riding style and the condition of your knees - if you prefer high cadence and/or get any knee trouble, the shorter ones might suit you better.

Stedman

markhewitt1978 wrote:

What effect does crank length have on an overall bike fit?

While bit fit will look at the saddle heigh and fore/aft position, bar height, stem length etc, what about the length of the cranks?

I ask because at 5'6" and apparently short legs for my height I'm pretty sure my 170mm cranks are too long for me, so what would happen if I switched to 165mm?

Is it just a matter that my saddle would need to go up by 5mm or are there more changes to my position that would need to be accommodated?

I am the same height as you and I run with 165 cranks on three of my bikes. With these my peddling cycle and style is much smoother with the ability to comfortably up my cadence. Although the cranks are shorter, I am also able to put in more power. I also do a lot of long distance rides and I find that there is less stress on my knees and hips with these cranks.

165mm cranks are very rare in the UK and difficult to source despite more than 10% of the adult male and more than 50% of the average female falling within the scope of these cranks. Hopefully with bike fitting this will put pressure on the manufacturers to widen the scope of their range and put correct length cranks on smaller framed bikes!

markhewitt1978

Stedman wrote:

165mm cranks are very rare in the UK and difficult to source despite more than 10% of the adult male and more than 50% of the average female falling within the scope of these cranks. Hopefully with bike fitting this will put pressure on the manufacturers to widen the scope of their range and put correct length cranks on smaller framed bikes!

Are they difficult to source? For 105 or Ultegra I can get them in 165mm from any number of online stores, although many are sold out of 6800 just now. Admittedly this is after market not new bikes.

I did have issues with my knees but lowering the saddle after a bike fit solved this, but seemed to create problems with my lower back, so I'm going back to see the bike fitter again this weekend and I was thinking of suggesting shorter cranks as part of an overall package to make the bike better suitable for me, as I'm riding a 52cm frame when I should really be riding 50cm.

I wouldn't expect shorter cranks to solve everything but I was thinking perhaps as part of an overall package of small changes it could improve my comfort on the bike and help my lower back issues.

imposter2.0

styxd wrote:

Saddle height will probably do it. Don't go 5mm though, try 3mm to start with. Or even just see how they feel without moving your saddle up at all.

I don't get it. If we know that the difference in crank length is going to equate to a 5mm difference in effective saddle height - why only adjust the saddle by 3mm..??

Similarly, suggesting that the saddle is not adjusted for compensation would be akin to telling someone at random to adjust their saddle height by 5mm without good reason. I don't get that either...

If you know the difference is 5mm, then adjust by 5mm.

styxd

Imposter wrote:

styxd wrote:

Saddle height will probably do it. Don't go 5mm though, try 3mm to start with. Or even just see how they feel without moving your saddle up at all.

I don't get it. If we know that the difference in crank length is going to equate to a 5mm difference in effective saddle height - why only adjust the saddle by 3mm..??

Similarly, suggesting that the saddle is not adjusted for compensation would be akin to telling someone at random to adjust their saddle height by 5mm without good reason. I don't get that either...

If you know the difference is 5mm, then adjust by 5mm.

What's to say the saddle height is correct to start with? WE also don't know whether the change in crank length will affect the OPs pedal stroke such that a change i saddle height isn't needed.

simon_masterson

styxd wrote:

Imposter wrote:

styxd wrote:

Saddle height will probably do it. Don't go 5mm though, try 3mm to start with. Or even just see how they feel without moving your saddle up at all.

I don't get it. If we know that the difference in crank length is going to equate to a 5mm difference in effective saddle height - why only adjust the saddle by 3mm..??

Similarly, suggesting that the saddle is not adjusted for compensation would be akin to telling someone at random to adjust their saddle height by 5mm without good reason. I don't get that either...

If you know the difference is 5mm, then adjust by 5mm.

What's to say the saddle height is correct to start with? WE also don't know whether the change in crank length will affect the OPs pedal stroke such that a change i saddle height isn't needed.

That was my experience - I went from 170 to 175 and didn't change it straight away. I wasn't particularly conscious of having had my saddle too high previously.

imposter2.0

styxd wrote:

What's to say the saddle height is correct to start with? WE also don't know whether the change in crank length will affect the OPs pedal stroke such that a change i saddle height isn't needed.

That's obviously an unknown quantity - the OP's saddle may indeed be too high, or too low, or it might be spot on. But not knowing the answer to that is no reason to suggest leaving the saddle height un-touched when you know the crank length is changing. You have to change the saddle height in order for the OP's setup (regardless of whether right or wrong) to remain consistent.

type:epyt

I am the same height as the OP and have always ridden 170mm cranks, if for no other reason than that is what comes on small or 50cm sized bikes. Never thought or bothered to change as the only difference I can think is going to occur at the very bottom of the pedal stroke (which is the point of least pressure in your stroke) which can easily be adjusted for with saddle height and fore/aft saddle position.

Obviously everyone is pysiologically different and may carry injuries/limitations but for me, I could think of better ways to spend £200 on the bike which would give more benefit.

paul_onabike

A lot of riders underestimate the benefits of using the correct crank length.
Manufacturers very rarely spec the correct length of crank for the size of frame/rider.

I spent most of my cycling life on 170 cranks, suffering with lower back and knee pain. Also getting hot foot sole pain in the summer.
It got to the stage that the back pain was turning into sciatica, and I had to find the root of the problem, otherwise I would have had to stop riding.
I videoed myself on the turbo from several angles and viewed the footage enlarged on the TV. I then measured my knee angles at the top and bottom of the pedal stroke. It was soon obvious that my crank length was too long as I could not get within the knee angle range that is recommended by bike fitters, especially at the top of the stroke, no matter how I adjusted the saddle position.

Anyway, now I ride 165 cranks, and have adjusted my saddle position accordingly. The saddle height has actually increased by 20mm, as I am now able to extend my leg more effectively do to much reduced back pain. The knee and foot pain have disappeared altogether. Unfortunately 30+ years of riding with incorrect crank length has left me with some longer lasting lower back problems, but still working on that.

People worry about loss of leg power with shorter cranks, I can report that this is a myth as far as I am concerned. There was an initial drop in power for the first couple of months while my legs adapted to the shorter cranks. My cadence increased slightly at first as I tried to maintain the same power numbers. After this initial period my legs strengthened and my cadence returned to the same as before, with the same power output as before with the 170 cranks.
The biggest change was the the increase in comfort, and the ability to get more aero when riding because of this. resulting in more speed for the same power.
Also with the absence of the hot foot, and knee pain, the ability to train harder for longer has brought fitness improvements.

I am 5'7" by the way.
I really could do with 160 cranks, but is are a lot more difficult to find some that fit my Power2Max power meters, so I had to settle for 165. These still only put me on the limit as far as knee angle is concerned.

markhewitt1978

That's interesting Paul. I guess the consensus is that being 5'6" that all things being equal I should be running cranks shorter than my current 170mm. That being 165mm as these are the shortest widely available.

imposter2.0

155mm is available on a square taper fitment...

Stedman

markhewitt1978 wrote:

Stedman wrote:

165mm cranks are very rare in the UK and difficult to source despite more than 10% of the adult male and more than 50% of the average female falling within the scope of these cranks. Hopefully with bike fitting this will put pressure on the manufacturers to widen the scope of their range and put correct length cranks on smaller framed bikes!

Are they difficult to source? For 105 or Ultegra I can get them in 165mm from any number of online stores, although many are sold out of 6800 just now. Admittedly this is after market not new bikes.

I did have issues with my knees but lowering the saddle after a bike fit solved this, but seemed to create problems with my lower back, so I'm going back to see the bike fitter again this weekend and I was thinking of suggesting shorter cranks as part of an overall package to make the bike better suitable for me, as I'm riding a 52cm frame when I should really be riding 50cm.

I wouldn't expect shorter cranks to solve everything but I was thinking perhaps as part of an overall package of small changes it could improve my comfort on the bike and help my lower back issues.

I bought an Ultegra 6800 equipped bike this year and although it was from a UK bike builder, they could not purchase this item through the official trade supplier. In the end I had to purchase this from Wiggle and get this delivered to the bike builder.

MichaelW

markhewitt1978 wrote:

I guess the consensus is that being 5'6" that all things being equal I should be running cranks shorter than my current 170mm.

OP notes his proportionately short legs. For average build 5'6", the need for <170mm is not so important.
Many medium-sized riders fret about a few mm here and there. It is the short and tall riders who are poorly served.

W12_Lad

I have a 6800 165mm compact crankset, boxed, that has done less than 50 miles.
PM me if you're interested.

djp66

Another vote for 165mm from me. I'm slightly taller at 5ft 7 (hey it's not much but an inch is important to us shorties ).

I had ridden on 170mm but during my last re-buld and having had some lower back pain I looked into part selection carefully. The calculators told me that 165mm would suit and it's been all good so far.

simon_masterson

Speaking from the 'other side' (long legged six footer, like 175mm, would probably get on with 177.5), it is laughable to me that it's easier to get a 180mm crankset than 160 or less. It's bizarre - a bit like if the shops only had trousers with 33"+ inside leg, and shoes 11 and up. Normally it feels more like the world is biased against tall people.

ForumNewbie

I'm 5'8" and just realised that my cranks on my Audax bike are too long for me at 175mm. I was going to change my chainset anyway from a Tiagra 52/42/30 to 50/39/30 but there only seems to be 2 options of crank length 175 or 170:
http://www.wiggle.co.uk/shimano-tiagra-4603-hollowtech-ii-10-spd-triple-chainset/
The 170s will probably be okay for me, but it seems strange that these are the only 2 choices.

markhewitt1978

ForumNewbie wrote:

I'm 5'8" and just realised that my cranks on my Audax bike are too long for me at 175mm. I was going to change my chainset anyway from a Tiagra 52/42/30 to 50/39/30 but there only seems to be 2 options of crank length 175 or 170:
http://www.wiggle.co.uk/shimano-tiagra-4603-hollowtech-ii-10-spd-triple-chainset/
The 170s will probably be okay for me, but it seems strange that these are the only 2 choices.

Shimano site claims "Crank Arm Length 165, 170, 172.5, 175mm"

dj58

markhewitt1978 wrote:

What effect does crank length have on an overall bike fit.

I ask because at 5'6" and apparently short legs for my height I'm pretty sure my 170mm cranks are too long for me, so what would happen if I switched to 165mm?

Is it just a matter that my saddle would need to go up by 5mm or are there more changes to my position that would need to be accommodated?

Mark, have you seen this before? http://bikedynamics.co.uk/FitGuidecranks.htm

StillGoing

type:epyt wrote:

I am the same height as the OP and have always ridden 170mm cranks, if for no other reason than that is what comes on small or 50cm sized bikes. Never thought or bothered to change as the only difference I can think is going to occur at the very bottom of the pedal stroke (which is the point of least pressure in your stroke) which can easily be adjusted for with saddle height and fore/aft saddle position.

Obviously everyone is pysiologically different and may carry injuries/limitations but for me, I could think of better ways to spend £200 on the bike which would give more benefit.

No the difference will be at the top and bottom of the pedal stroke. If the leg is over compressed at the top (which it inevitably will be to avoid overextending at the bottom of the stroke) it will reduce your ability to get the power down quickly.

type:epyt

^ I get what you're saying ... Pretty much the same effect as bringing your knees too far in on the leg press at the gym, although to a much lesser extent.

Although, in return you get the added leverage at the 3 o'clock position with a larger crank arm ... but then your circumference is greater so rpm is lower.

It's all minuté in reality, as any change in shorts/shoes etc. can cause the same changes to pedal action.

ai_1

type:epyt wrote:

^ I get what you're saying ... Pretty much the same effect as bringing your knees too far in on the leg press at the gym, although to a much lesser extent.

Although, in return you get the added leverage at the 3 o'clock position with a larger crank arm ... but then your circumference is greater so rpm is lower.

It's all minuté in reality, as any change in shorts/shoes etc. can cause the same changes to pedal action.

Changes in shoes/shorts may effect the required saddle height but not the height of stroke from top to bottom. This is dictated solely by the cranks. So the range of motion between the greatest and least angle at the hip is also dictated by the crank length.
I'm 1.78m (5'10") and about 835mm inseam (33") and used to use 175mm cranks until recently. I've now bought a new bike with 172.5mm cranks and have replaced the heavily worn chainset on my older bike including new 170mm cranks. I can't say the difference is obvious! However all of these are probably close to the predicted range for my dimensions. Maybe this is the wrong place to mention it but I think many triathletes are now switching to smaller crank lengths so they can maintain a more open hip angle which supposedly keeps the legs in better shape for the run. So there are 6' athletes using 165mm cranks with no apparent difficulty.

markhewitt1978

DJ58 wrote:

markhewitt1978 wrote:

What effect does crank length have on an overall bike fit.

I ask because at 5'6" and apparently short legs for my height I'm pretty sure my 170mm cranks are too long for me, so what would happen if I switched to 165mm?

Is it just a matter that my saddle would need to go up by 5mm or are there more changes to my position that would need to be accommodated?

Mark, have you seen this before? http://bikedynamics.co.uk/FitGuidecranks.htm

Yes that article is what made me thing 165mm might be right for me.

markhewitt1978

So I've switched from 170mm to 165mm cranks.

It's hard to make a proper comparison as my bike was off the road for repairs for the best part of a month and with Christmas inbetween. But it does feel better, like I'm spinning the pedals properly and my legs aren't coming too far up as they were before. My cadence is still 90rpm which it was before.

StillGoing

markhewitt1978 wrote:

So I've switched from 170mm to 165mm cranks.

It's hard to make a proper comparison as my bike was off the road for repairs for the best part of a month and with Christmas inbetween. But it does feel better, like I'm spinning the pedals properly and my legs aren't coming too far up as they were before. My cadence is still 90rpm which it was before.

Make sure you've adjusted the saddle accordingly rather than just leave it as was. With the smaller turning circle, your cadence should speed up slightly with less distance to travel for the full rotation. Don't take average cadence as the figure either. We all have times during a ride when we are below our targeted cadence and the more times you dip, the lower the average is compared to the target. What cadence were you doing with your old cranks on a flat road with no head or tail wind? Compare that with what you can do with the shorter cranks bearing in mind the winter weather will have an effect.