There’s only one size of whee……hang on ,wait a minute.

There’s only one standard response to mention of wheel sizes

STATO – Member
The problem of course with running very short cranks is your seat height is higher (presuming xc/road riding obv) so reaching the ground is near impossible. Ive seen people solve this by smaller wheel size, to bring the pedal-ground distance back to what it was with the designed cranks.

I honestly think it’s cool that you’ve considered the implications.

But it’s not as big a problem as you fear.

My wife has 140mm cranks on her hard tail (we had some sram s600’s shortened by high path). They’re only 25mm shorter than 165’s which means her saddle is only 25mm higher. This even comes with 2 benefits: 1) it brings her saddle UP to the level of her bars, and 2) it exposes enough seat post to fit a dropper.

But here’s the problem: they’re square-taper, which won’t fit in her new bb90 frame – she’s stuck with the 165’s that came with the bike (The S600’s are also very heavy). I’d need to try 220mm cranks to see how bad her 165’s must feel.

(When we ordered the bike, we were told it had a bb30, which CAN be adapted to accept her heavy square taper cranks)

Usually, crank-length isn’t a problem for the extended leg : you can just adjust the saddle height. The impact of crank length is felt by the bent leg – where the effect is doubled: 165’s aren’t 25mm too long for my wife, they’re 50mm too long.

So yes. You can get 140mm cranks for your mtb, but you have to get them custom made. And you’ll struggle to fit them to a new bike.

I know I’m boring you all, but it just seems daft, every other component is widely available in a huge range of sizes. If nothing else, think of the weight savings!

Marin also do a low rise bar with a narrow grip section, I bought one for my wife’s bike in my lbs, it comes with thinner grips and plastic shims for brakes and shifters.

awhiles

In summary the problem is bottom bracket standard not crank lengths. Not sure why you didn’t return the bike?

Mrs Gasket uses 160mm Rotor cranks, that are HT2 so will slot into BB86/90.

She previously ran a set of square taper TA Carminas (in 160) but when we bought a new Giant frame they wouldn’t fit the BB86, as you say, so we bought the Rotor cranks.

ampthill – Member
awhiles

In summary the problem is bottom bracket standard not crank lengths.

no. We had to get her square-taper cranks made. They didn’t exist until highpath made them for us.

…Not sure why you didn’t return the bike?

because I was sure there would be some way of bodging it to accept her square-taper cranks, And then I worked out how to modify some deore ht2 cranks. I just think it’s daft that Xs cranks weren’t fitted to an Xs bike. It’s even more daft that we can’t buy Xs cranks separately.

We don’t expect people who want short stems to have to make them from long stems. But that’s what I’m faced with trying to get cranks for my wife’s mtb. We’re not talking about making special modifications to adapt a bike for an individual with disabilities, she’s 5’1″. In the world of women on bikes, that’s one size smaller than average.

(My wife does have those Sjs thorn cranks on her road bike, 140mm, they’re a suitable length, but heavy, and won’t work on her mtb)

While logic would tend to suggest that smaller riders need shorter cranks, most studies on the area suggest that even wide variances in crank length have a marginal effect on power output. Therefore it doesn’t make sense for manufacturers to produce a huge array of crank lengths when the extreme sizes are only going to have limited sales.

As for chain pitch Shimano did try 10mm pitch for a while…

Awhiles

But here’s the problem: they’re square-taper, which won’t fit in her new bb90 frame – she’s stuck with the 165’s that came with the bike (The S600’s are also very heavy). I’d need to try 220mm cranks to see how bad her 165’s must feel.

You can see why I thought you hadn’t solved the problem. I’m glad that you have.

Its an interesting question as to why bikes don’t come with much shorter cranks on smaller frames. If its deigned in from the start the with extra BB drop you would just end up with the saddle at the same height and the same pedal clearance.

While logic would tend to suggest that smaller riders need shorter cranks, most studies on the area suggest that even wide variances in crank length have a marginal effect on power output. Therefore it doesn’t make sense for manufacturers to produce a huge array of crank lengths when the extreme sizes are only going to have limited sales.

Mike Burrows (Chris Boardman’s aero Lotus bike guru and all-round crazy inventor chap) has a theory that we should all be using much shorter cranks for a couple of reasons, one of which is that it would actually increase efficiency quite dramatically. He runs shorter cranks on his aero recumbents and gets good results, I believe. I think he reckoned that as low as 140mm or so was the optimum length for most riders (obviously you change gearing to adjust for the different leverage).

Studies carried out with riders close to average size AFAIK – and mostly with cranks shorter than “normal” rather than longer. Which would tend to suggest that it would be reasonable for all bikes to be supplied with shorter cranks than they are currently. Because for shorter riders having cranks too long results in biomechanical issues other than maximum power output.

I have a variety of crank lengths from 125 up to 175 and whilst I couldn’t say whether I produce as much power on the shorter cranks or whether I’m as efficient, I’m comfortable using them and I’m not sure I would be with significantly longer cranks.

Sam – Member
While logic would tend to suggest that smaller riders need shorter cranks, most studies on the area suggest that even wide variances in crank length have a marginal effect on power output.

Firstly, there really aren’t many ‘studies’, and those that have been done are scientifically very poor/limited, With no attempt to examine the impact of rider height on the results.

i.e. It seems no short people were used in the tests.

it does seem that there are few problems experienced by people using shorter-than-expected cranks. And few benefits of long cranks.

Edit: what aracer said.

ampthill – Member

Its an interesting question as to why bikes don’t come with much shorter cranks on smaller frames. If its deigned in from the start the with extra BB drop you would just end up with the saddle at the same height and the same pedal clearance.

My wife loves the extra clearance offered by her 140’s, the extra saddle height comes in handy too…