Chainset arm lengths

soulbrother74

I currently use 175mm and have had my bike set up for that length of arm however I've been offered a nice Ultegra with 170mm arms. Now, could i just raise my seat post by 5mm to compensate or would this not work at all?

Thanks

thiscocks

to 170 id raise the saddle by 2-3mm and see how it feels. I have three bikes all on different size arms (170, 172.5 and 175). The seat height (bottom of saddle to center of BB) varies by about 5-8mm between them all.

Buckie2k5

the difference in arm length is 5mm which is nothing, some people claim to feel it and others dont. If anything shorter arms are easier to spin higher cadence and longer arms allow more push for climbing. Raising the saddle 5mm is fine but personally going by the fact you have 175 cranks means you have a large bike i would stick with 175mm

me-109

Buckie2k5 wrote:

personally going by the fact you have 175 cranks means you have a large bike i would stick with 175mm

Unless he got 175s because they were cheap/he thought they'd be 'better' and doesn't really need them that long after all.

soulbrother74

Me-109 wrote:

Buckie2k5 wrote:

personally going by the fact you have 175 cranks means you have a large bike i would stick with 175mm

Unless he got 175s because they were cheap/he thought they'd be 'better' and doesn't really need them that long after all.

Got them as they came as standard on the SRAM Rival chainset. Never changed them. However I'm tempted to swap them out and give it a go. Hey, if it doesn't work I've a nice Ultegra 170mm chainset to knock on

mfin

The relevant point here is how tall is the OP, how long are his legs. If he's 6ft2 or something he won't want to be using 170s

soulbrother74

mfin wrote:

The relevant point here is how tall is the OP, how long are his legs. If he's 6ft2 or something he won't want to be using 170s

5'10" and leg measurement of 32".

Inside leg I couldn't tell.

Monty Dog

Crude measure = height in cm = crank length in mm, so 5ft10" = 175cm. That said, track riders of all sizes cope with 165mm so it's really down to whether you want to spin or mash? I have bikes with both 170mm and 165mm - all I really notice is that I can spin the shorter cranks a bit quicker if needed.

drlodge

soulbrother74 wrote:

mfin wrote:

The relevant point here is how tall is the OP, how long are his legs. If he's 6ft2 or something he won't want to be using 170s

5'10" and leg measurement of 32".

Inside leg I couldn't tell.

About the same as me then. I was on 170mm now 172.5mm. Can't tell much difference, may be the longer cranks feel a little better on steep hills due to better leverage but that's about it.

soulbrother74

All food for thought so thank you for taking time to help me. I'll have a chat with my bike fitter at weekend and see what he says too!

Going on gut feeling and what's been said here, no major reason to swap over as no discernible gain.

paulbox

thiscocks wrote:

to 170 id raise the saddle by 2-3mm and see how it feels. I have three bikes all on different size arms (170, 172.5 and 175). The seat height (bottom of saddle to center of BB) varies by about 5-8mm between them all.

Sorry if I'm being thick... But why would you raise your seat if you are shortening your crank arms?

I realise that at the bottom of the stroke your pedal will be a couple of mm closer to you @rse, but at the top of the stroke it will be a couple of mm further away. Surely you're best leaving your seat where it is.

drlodge

PaulBox wrote:

thiscocks wrote:

to 170 id raise the saddle by 2-3mm and see how it feels. I have three bikes all on different size arms (170, 172.5 and 175). The seat height (bottom of saddle to center of BB) varies by about 5-8mm between them all.

Sorry if I'm being thick... But why would you raise your seat if you are shortening your crank arms?

I realise that at the bottom of the stroke your pedal will be a couple of mm closer to you @rse, but at the top of the stroke it will be a couple of mm further away. Surely you're best leaving your seat where it is.

Because you want the distance bewteen the saddle and the pedal at the bottom of the stroke to remain the same irrespective of crank arm length. It also means of course that the pedal spindle at the 3 o'clock postion will be 5mm further back, so you might also need to move the saddle rearwards a bit...

A 5mm shorter crank arm means raising the seat by 5mm, and possibly moving it a back a smidge too (raising the saddle will move it back a bit too)

bernithebiker

Monty Dog wrote:

Crude measure = height in cm = crank length in mm, so 5ft10" = 175cm. That said, track riders of all sizes cope with 165mm so it's really down to whether you want to spin or mash? I have bikes with both 170mm and 165mm - all I really notice is that I can spin the shorter cranks a bit quicker if needed.

Your height is not very useful - it's your inside leg you want to measure.

Mine is short for my height, so I use 170's.

IMO spinning is winning, and if you can't sustain 100 cadence+ over a decent period, then you might want to consider shorter cranks (and training!).

paulbox

drlodge wrote:

Because you want the distance bewteen the saddle and the pedal at the bottom of the stroke to remain the same irrespective of crank arm length.

This bit makes sense.

drlodge wrote:

A 5mm shorter crank arm means lowering the seat by 5mm,

But did you mean raising the seat by 5mm here?

drlodge

Oh yes, sorry :oops:

paulbox

drlodge wrote:

Oh yes, sorry :oops:

No worries, thanks for answering my question.