Crank length or frame size to put saddle in right place?

dhobiwallah

For very dull reasons (mostly ignorance on sizing when I was a bit younger during purchase) I have 3 bikes and they are all different sizes!

One is a mid-90s Proflex in Large (19"), one is a Merlin Malt hardtail in Med (17") and one is an Iron Horse full suss in small (15" I think).

I get by on them all - replacing the enormous 90s steering tiller with a shorter stem fits the Proflex (but standover is a bit scary), and a Comedy 400mm seatpost takes care of the small bike.

Thing is my saddle is as far forward on the rails as it will go on all 3 bikes and I still end up sitting more on the nose than on the 'meat' of the saddle. Also when trying to fit the cockpit with the old plumb line off the knee and through the pedal axle it is nowhere near - my foot is way too far forward.

I have found (some) info on crank lengths on MTBs but most of it is contradictory or inconclusive other than to say that most people have cranks 5mm longer than they would on an equivalent road bike.

I am 5'9" with a 30" leg length (book in crotch to ground in bare feet) which suggests either a 165 or 170mm crank depending on which calculator you use.

Would shelling out on shorter cranks make any difference to my position / comfort / efficiency or would it just be a waste of cash?

miss notax

Due to a bit of an obsessive Ebay bike shopping habit I also have three bikes in three different sizes :oops:

My Orange is a 14" frame, my Kona is a 15" frame and my Santa Cruz is (apparently) a 15" frame which they label as 'extra small' for people 5ft and under - I am 5 ft 4" :roll:

I have 170 cranks on the Orange, 175 on the Santa Cruz and god knows what they are on the Kona (the only bike not custom built!!) - but they all fit me, and to be honest I can't tell the difference between them all!

As long as they feel right to you, I have concluded that all the helpful guides are twaddle and you are really the best judge of what's right and wrong to put on it. I genuinely can't tell the difference between the cranks though so I wouldn't bother with that.

nicklouse

dhobiwallah wrote:

For very dull reasons (mostly ignorance on sizing when I was a bit younger during purchase) I have 3 bikes and they are all different sizes!

One is a mid-90s Proflex in Large (19"), one is a Merlin Malt hardtail in Med (17") and one is an Iron Horse full suss in small (15" I think).

I get by on them all - replacing the enormous 90s steering tiller with a shorter stem fits the Proflex (but standover is a bit scary), and a Comedy 400mm seatpost takes care of the small bike.

Thing is my saddle is as far forward on the rails as it will go on all 3 bikes and I still end up sitting more on the nose than on the 'meat' of the saddle. Also when trying to fit the cockpit with the old plumb line off the knee and through the pedal axle it is nowhere near - my foot is way too far forward.

I have found (some) info on crank lengths on MTBs but most of it is contradictory or inconclusive other than to say that most people have cranks 5mm longer than they would on an equivalent road bike.

I am 5'9" with a 30" leg length (book in crotch to ground in bare feet) which suggests either a 165 or 170mm crank depending on which calculator you use.

Would shelling out on shorter cranks make any difference to my position / comfort / efficiency or would it just be a waste of cash?

forget the plumlinecrap as you are not a road bike rider you do not sit in the same position for hours on end.

crank lengtn is not the way to go.

175mm will do you fine and is what 99% of MTBs come with.

diy

I'd suggest going in to a shop and paying to get measured and then translate the sizing.

from what you say - it sounds like you have a long body and short legs.

craigw99

im the same long body short legs - try dropping the nose of your seat a little it may help you sit more on the meat of the seat :-) or get an inline seat post

YeehaaMcgee

IF there is a problem at all, then maybe your saddle is too wide, making you sit on the nose. Or maybe it's tilted too far forwards.

dhobiwallah

Most MTBs do indeed come with 175mm cranks so that's what everyone cracks on with. Doesn't mean they fit though! It just means manufacturers spend less on OEM fits due economies of scale by putting the same cranks on every bike. In the same way as few manufacturers vary the length of the rear triangle for differing frame sizes but some do...

It makes perfect sense that people with different length Femurs would want to have different length cranks or else your pedalling mechanics would be different due to your height - someone must have the sweet spot! As to whether that 5mm change would make any noticeable difference is another matter entirely.

I have different saddles on the 3 bikes - none are particularly comfy or uncomfy, narrow or wide (one is a hand-me-down from a mate who complained it was far too narrow - but one suggestion above for tipping it back and one for tipping it forward - guess I'll give both go! Just can't help feeling that the saddle shouldn't have to be fully forward (and still not going far enough) on a S,M & L frame unless something else is the issue...

On the other threads I have found some do mention knee pain when using cranks that are too long - I do have knee issues, but I can't say they are any worse on the bike then while running so no help there. Don't know which discipline caused it though.

I realise I'm not going to be roadie spinning - but on the longer seated sections I can't help notice how far in front of me my feet seem to be and presumably I am losing efficiency. I don't go out for longer than 2 hours at the mo (mostly due to riding buddy limitations ) but may well feel it if I go for longer.

Will prob try a set of £5 trashed used crank arms from eBay and see if there is any difference. But given how MTB reviewers talk about taking hours to dial suspension and I have set 25% sag, rebound in the middle and left it for 3 years I can't see me detecting any subtle differences! :oops:

nicklouse

cranks will not change a thing with what you are saying is your problem.

but on the longer seated sections I can't help notice how far in front of me my feet seem to be and presumably I am losing efficiency.

do you run a layback seat post?

YeehaaMcgee

dhobiwallah wrote:

As to whether that 5mm change would make any noticeable difference is another matter entirely.

It's often mentioned that even top road and track cyclists, with precision, custom-fit works bikes can't actually notice a difference in crank length.

diy

for 5'9" with 30" legs that is the legs of a small frame and the body of at least a large frame on most bikes.

dhobiwallah

diy wrote:

for 5'9" with 30" legs that is the legs of a small frame and the body of at least a large frame on most bikes.

Might explain why I'm having issues :shock:

supersonic

You are three inches shorter than me - but I have 5 inch longer legs. I ride 17, 17, 18, and 20.5 inch frames. Measurements here mean nothing when comparing brand to brand, as is just one out of several. You sound pretty normal to me! Just need to experiment.

Forget the crank length, forget all this silly roadie formulae that have no bearing in the MTB world. Try sliding your saddle back again to increase your reach. The experiment with height, and bar rotation. If you prefer the saddle further forward for a steeper effective seat angle, try running a longer stem again.

dhobiwallah

nicklouse wrote:

do you run a layback seat post?

Well I suppose they aren't inline, but just the ones that came stock - (except the longer post on the S bike). The clamp is to the rear of the post - not specifically set back. Inline posts may be the answer....

diy

Can I just check that inside leg measurement again 30" crotch to ground?

In clothing which is crotch to ankle bone:
short 30-31"
regular is 31 - 32
Long 32 - 34

Given your ankle bone is probably 2 -3" off the ground, and you are almost spot on avg. height. I'd say your legs are 3-4" shorter than the typical person of your height. Or did I miss something in the measurements?

dhobiwallah

Re-measured using a more solid book to mark the wall (I used a magazine before), and did it twice and came up with 31" to floor (28" to ankle bone). And I'm 174cm tall which is actually just a smidge over 5 feet 8and a half inches

But I wear 30" trousers ('short') because any that stop at your ankle bone are best teamed up with stripy socks, NHS specs and a tank-top for that truly 'special' look!

The crank length calcs I have found include

L(mm) = 5.48 x I(in)
..........
And from another source (referencing your inseam in inches):
inseam < 29 inches - 165 mm crank
inseam 29 - 32 inches - 170 mm crank
inseam 32 - 34 inches - 172.5 mm crank
inseam > 34 inches - 175 mm crank
Track riders generally choose crankarms up to 5 mm shorter and mountain bikers up to 5 mm longer than the above recommendations
............
http://www.machinehead-software.co.uk/bike/cranks/cyclist_crank_length_calculator.html

Given the advice that MTBers generally add 5mm to crank length compared to roadies the 3 sources above recommend
173mm / 175mm / 168mm :roll:


General discussion on crank length
http://www.billbostoncycles.com/crank_length.htm

supersonic

MTBers do not generally add 5mm. 99% ride whatever is on sale or come with the bike and can not tell any difference. In fact I'd say 99.99%.

styxd

Have a read of Steve Hoggs bike fitting website. It explains everything with regards to seat position, seat height, crank length, cleat/foot position etc. etc.

Why is your saddle so far forward though? Are you struggling to reach the bars? Or perhaps your saddle is to high? Try moving it down a bit

cooldad

Er 165 plus 5 = 170 anyway, not 168? Etc.

nicklouse

and all that is a load of bollocks.

crank length has nothing to do with it and will not provide you with your fix but please go and waste your cash if you want.

YeehaaMcgee

styxd wrote:

Have a read of Steve Hoggs bike fitting website. It explains everything with regards to seat position, seat height, crank length, cleat/foot position etc. etc.

All of which (yes, I said ALL) is largely irrelevant for mountain bikes.

nicklouse wrote:

crank length has nothing to do with it and will not provide you with your fix but please go and waste your cash if you want.

Dunno about you, but it depresses me how many people on this forum insist on throwing money at a problem to try and "fix" anything.

nicklouse

YeehaaMcgee wrote:

styxd wrote:

Have a read of Steve Hoggs bike fitting website. It explains everything with regards to seat position, seat height, crank length, cleat/foot position etc. etc.

All of which (yes, I said ALL) is largely irrelevant for mountain bikes.

nicklouse wrote:

crank length has nothing to do with it and will not provide you with your fix but please go and waste your cash if you want.

Dunno about you, but it depresses me how many people on this forum insist on throwing money at a problem to try and "fix" anything.

i did not throw money at stuff I just used stuff i had and 165 is a pain. 170 OK for the DH bike but 175 on 7 other bikes..... and 175cm and the frames have a number of seat posts to get things just right when i need to be seated.

styxd

All of which (yes, I said ALL) is largely irrelevant for mountain bikes.

Fucking hell, bike fit is largely irrelevant for mountain bikes. You spend so much time moving about on the bike it doesnt really matter at all.

However, if hes struggling with seat position and crank length issues then Id say that website is a good place to start.

bails87

But he hasn't got crank length issues. He's got some issue around the saddle, and is thinking that a change of cranks will solve it, which it won't!

styxd

So rightly or wrongly, he's got issues with the length of his cranks then.

YeehaaMcgee

styxd wrote:

So rightly or wrongly, he's got issues with the length of his cranks then.

No, he's got issues with saddle positioning.

bails87

YeehaaMcgee wrote:

styxd wrote:

So rightly or wrongly, he's got issues with the length of his cranks then.

No, he's got issues with saddle positioning.

Exactly.

It's like if I said "my lower back aches on long rides, I think it's because my helmet doesn't have enough vents", and then someone linked to a website that had an analysis of all the different vent combinations. And then a load of people who know what they're talking about told me the helmet had nothing to do with my back, it was probably a 'reach' related problem. But I ignored them and carried on asking about different helmet vents to solve my back ache, encouraged by the poster who linked to the vent analysis site......

styxd

Whats your definition of "issue"?

http://dictionary.reference.com/browse/issue

bails87

FFS.... :roll:

OP: don't change your cranks, they're not the cause of your problem.

nicklouse

as we have been saying all along but he does not want to listen.

dhobiwallah

nicklouse wrote:

as we have been saying all along but he does not want to listen.

Not at all - I'm just quite amazed by how strongly people seem to feel about some things that aren't causing them problems.

After trying to explore why the standard off-the-shelf setups don't seem to work for me I've basically come to the conclusion I probably need an inline seat post so I don't spend all my seated time on the nose of the saddle. (Although it still seems odd that I would have to have my saddle forward on that setup - but it would be nice to have it in the right place!) Since my issue appears to be short legs I thought I might need short cranks - especially as most sites seem to say that if you shorten your cranks you need to move your seat up and back - which was pretty much my problem. If I sit on the saddle my feet are far too far out in front of me - if I just pedal I find I have moved forward on my saddle till I am sitting more on the nose - even on the flat; I would imagine for longer rides this will get uncomfortable...

My links to calculators basically showed that 3 different sites came up with 3 different lengths proving it is a bit bobbins! (31" leg length was rounded up from 30.8" which gave 170, 172.5 & 175mm cranks!)

From what I gather plenty MTBers do add 5mm to cranks - or roadies subtract 5mm if you prefer; just because the manufacturer does it for you doesn't mean it isn't the case....

But I do find it interesting that some of you seem to think that crank length is almost entirely irrelevant. I can't believe that someone who is 5'4" gets the same range of motion as someone who is 6'4" out of the same cranks. Surely one of them is going to have a more efficient stroke than the other? (whether spinning up a fireroad or mashing up a short section). Given bikes have top tubes that can vary in length by only 2cm before you customise it further with cockpit setup presumably a circling diameter varying by 1cm must make some sort of difference?

YeehaaMcgee

dhobiwallah wrote:

I'm just quite amazed by how strongly people seem to feel about some things that aren't causing them problems.

We just don't want to see you spend money on something that will not solve the issue. It's far too common on BikeRadar forums to end up with an echochamber of "spend spend spend".

Since my issue appears to be short legs I thought I might need short cranks ...
If I sit on the saddle my feet are far too far out in front of me

If you had short legs, then the saddle would be lower down the frame, and on all mountain bikes, this means the saddle moves forwards, towards the bars, due to the angle of the seatpost.
So taller riders will be further behind the bottom bracket than you are. Shorter riders would be more forward.

Forget the bike-fit nonsense, it really really doesn't apply to MTBs. And changing cranks won't solve your issue.

In future though, it may be advantageous when purchasing a bike or frame, to consider a more racey, XC frame, since these generally have a long reach, so you could still have a medium or small frame, but plenty of length for the upper body.