Crank Length

mattmaximus

Hi couldn't find an answer to this using the search, please redirect me if this has been done to death!

I'm thinking of changing my crankset (teeth starting to look uneven, it's a double but i would like a compact, just fancy buying something shiny ) and am wondering whether to stick with what seems 'normal' at 175mm or to go to 180. I read a guide to crankarm length that suggests that 18.5% of the length from top of femur to floor is a good guide, which gives me an answer of 179mm.

Also, and I hope this doesn't muddy the waters, my knee sits just in front of the pedal axle at horizontal, which I'd like to correct and it would be useful (in terms of drop to the handlebars) to lower the seatpost by 5mm. Finding that a longer crankarm was 'recommended' was a bonus!

If anyone has opinions on the calculation referred to, or on trading up to 180 cranks, I'd be very grateful.

thiscocks

180 cranks arn't normal unless you have pretty long legs compared to the rest of your body. Unless you are built like this I would stick with 175.

thescouselander

It might be worth a go. Crank length does make a difference in my opinion. I find longer cranks better - I went from 172.5mm cranks on my old bike to 175mm on my current bike and it seemed to help a lot even though the usual guidance suggests I should have stuck with the 172.5 mm cranks. Everyone is different though, some prefer shorter cranks.

andrewjoseph

Also, longer cranks may not necessarily mean seat height would be better. Yes, your foot would be lower at the bottom of the stroke, but it would be correspondingly higher at the top of the stroke and may cause knee problems.

if your knee is giving you problems now with it being slightly forward of pedal axle, then move your saddle back. This may impact your reach to the bars so you may need a shorter stem.

if you don't have any problems, leave well alone.

are there any spacers under the stem your can swap around to drop bars down?

And you can just buy replacement rings, no need to get a new crankset.

But new and shiny is good! :-D

smidsy

Also may affect your toe /wheel overlap if you go longer.

mattmaximus

thiscocks wrote:

180 cranks arn't normal unless you have pretty long legs compared to the rest of your body. Unless you are built like this I would stick with 175.

Well I thought I was normally proportioned but after competitivecyclist recommended me a seat tube of 600 and TT of 550 (I'm 6'4) I'm starting to wonder :shock:

mattmaximus

thescouselander wrote:

It might be worth a go. Crank length does make a difference in my opinion. I find longer cranks better - I went from 172.5mm cranks on my old bike to 175mm on my current bike and it seemed to help a lot even though the usual guidance suggests I should have stuck with the 172.5 mm cranks. Everyone is different though, some prefer shorter cranks.

thanks, good to know.

mattmaximus

@andrewjoseph

Good points, thanks. If it weren't for the fact that it was recommended by an unsubstantiated source off the internet written by a bloke I've never met I wouldn't be considering it.

I'm moving to a shorter stem as it is, but wanted to get the set up at the rear of the bike correct independently of the front. I don't have any knee pain, just trying to find the point of maximum power transfer.

MichaelW

The calculated crank length of 180mm is about right for a tall 6'4" guy (not commonplace or normal, just right). Any crank you can buy is going to be on the short side for your long legs. The fact that your knees will bend through a wider range on longer cranks is true but that probably means they are constricted on your short 175mm cranks.

175mm cranks work out about normal for a 6 footer and 170mm is about normal for a 5'8-6' ish average height.

There are issue fitting longer cranks to frames, as noted, bottom bracket height and cornering pedal clearance, toe-clip interference with the front tyre.

You may have to move your saddle forward and down to compensate for the 180mm crank. See here for fitting guide for any size rider.

The usual source for long cranks is Specialities TA.

Tom Dean

andrewjoseph wrote:

Also, longer cranks may not necessarily mean seat height would be better. Yes, your foot would be lower at the bottom of the stroke, but it would be correspondingly higher at the top of the stroke and may cause knee problems.

No reason it should cause knee problems if crank length:leg length is in a normal range.

Longer cranks do mean that the hip angle is tighter though so you may need to raise your bars or put the saddle forward and use a longer stem to compensate.

I have used the calculators and come up with 190-200mm. I use 180s at the moment but I would probably stick to 175 in future, I don't really see any benefit and the hip angle thing is a negative.

Go and get a proper bike fit:)

mattmaximus

I will, I promise!

I just wanted to get the benefit of the wisdom / experience of the real cyclists here to complement (rather than replace) the scientific modelling and hypothetical ideal.

Thanks again for your help.

andrewjoseph

Tom Dean wrote:

andrewjoseph wrote:

Also, longer cranks may not necessarily mean seat height would be better. Yes, your foot would be lower at the bottom of the stroke, but it would be correspondingly higher at the top of the stroke and may cause knee problems.

No reason it should cause knee problems if crank length:leg length is in a normal range.

...

Agreed that for a tall person, longer cranks on their own shouldn't cause knee problems, but the OP was talking about putting his saddle lower and getting longer cranks. This would mean a greater bend in the knee at the top of the stroke which could cause problems.

beams87

I know this is a little out of date, but I have just purchased a 180mm DA 7900 Double and intend on putting it on over the coming week. I am 6ft 7in, so for those vertically unchallenged who are interested, I will post my feedback when I give them a thorough test.