I run a 2011 Epic with 175mm Double chainset and looking for advice on two matters changing – Length of Crank arms & chain ring teeth.
I have quite short legs 29″ and I have always run the standard 175mm with triple chainsets on the beleif that Mountain biking requires variable power delivery to the chainset and the leverage of a longer crank is better suited for obstacle clearing like steep rocks roots etc.
I have had my Epic for 3 months and have taken extreemly well to the double chainset and gearing yet still find myself grinding up the climbs and wonder if a shorter crank, similar to my road bike of 170mm would assist in uping my cadence?

Also would I notice a signifcant difference in gearing if I change from 38 x 24 teeth to 42 x 28 T ??? as CRC have the Sram X9 in 170mm for a bargain price –
Advice appreciated

I changed to 170 and it helped me loads.No more knee pain.
I have glass knees though and prefer to spin than grind.

42/28 chainset would be about 10% and 15% respectively higher gearing than 38/24 if you keep the same cassette so I’d guess it’ll feel a bit harder all round – if you go to a bigger sprocket range at the back then that’ll even things back out

There’ll be a bit less difference between the 2 chainrings in terms of overall ratios (ie “little” will be less of a bailout ring)

170 marginally less leverage than 175 but also marginally easier to spin – I doubt you’ll notice this above the altered ratios as it’s way more subtle IME

170 cranks saved my bad knees years ago. And I’ve got a 33″ inside leg!

Way too many mtb’s come with too long cranks cos it’s easier to stock them IMO. That said, some people do grind rather than spin! I learnt to spin years ago though and i prefer it.

If you’re going for shorter cranks, I’d keep the same gearing. It may only be a 5mm crank length difference, but you’re effectively making it slightly harder to pedal by shortening the crank. You will suddenly be able to spin easier though, meaning you will be able to make each gear last slightly longer.

I’d say a 170 is right for you anyway. I changed from 175 to 170 when I started getting knee probs many years ago (175 wasn’t the cause btw). I was getting the pain more at the top of the stroke and the shorter crank did help (not cure) this. I still run 170’s on all my bikes except a recently acquired retro bike, I hardly use the latter so no real issues there but I can notice the difference.

No idea my leg length, not long at 5’8″!

Oh and I’m more of a spinner than grinder anyway. 🙂

Are you seriously asking if increasin chainring size will increase your cadence?

I am looking at the crank length for an increase in cadence, but as the 170 Cranks at CRC come with a teeth increase of +4T, apart from requiring an increase in chain length, I wanted to find out how significant the gearing increase is as I have never used the lowest 11T x 24T and quite possibly the lowest gears I use is 2 up from the Granny. I know that it defeats the point of tyring to increase cadence but how significant is it?

I reckon you go for shorter cranks but have a ponder about that gearing.
I changed to 170 cranks to help with knee pain, and it definitely made some difference. I’m 5’10” btw. My mate who is 6’5″ also uses 170 cranks (and he’s seriously quick!)
You will lose some leverage but that is minimal, but the smaller circles are more efficient imo.

The gearing you suggested sounds a bit high to me. Depends where you ride though. Have a look at an on-line gear inch calculator. (Sheldon Brown). That will give you a better idea.

Thanks Stanley, I will have a look at gear inch calculator.
As I’m a vertically challenged “stumpy time bandit” at 5’6″, you have swayed me to definatly give the 170 cranks lengths ago.
Cheers Chris

Agree with Stanley, change the cranks but keep to the 175 gearing is what I would do.