Too much crank

RutlandGav

Not a doping thread, unfortunately.

I suspect I have some bike fit issues. Started commuting 100 miles a week on a Carrera One 650b rigid MTB.

I bought the largest frame size since my last bike was too small and felt really hunched over the bars.

I started getting knee trouble however. To begin with, front and back and sides, but now only at the back.

I suspect the 180mm cranks are just too long for my 5'9 body.

According to this site,

http://bikedynamics.co.uk/FitGuidecranks.htm

The knee angle at the top of the stroke should be at least 70 deg. I'm seeing 66.

At the bottom of the stroke, the leg angle should be no straighter than 140 - i'm getting about 151.

If i raise the seat to fix the problem with an excessively bent knee at the top of the stroke, then i'll make the overextension prob at the bottom even worse.

In fact i might lower the seat a little anyway, since i spin high cadences and the prob is now at the back of my knee, not the front, and i've got very tight hamstrings (flexibility is not my strong suit).

But i need to fix this. Last time i built up my own chainset was in 2005, I spent a week's holiday downloading PDFs, scratching my head, trying to figure out what would go with what etc. Ended up going for some fairly pricey components since the cheaper ones didn't come with enough info. Everything did in fact fit but i lost a lot of brain cells and gained many grey hairs, not sure i fancy repeating the experience.*

Shall i just take it to the NBS (nearest bike shop, i'm out in the boonies) and pay them to do it?

How much is a reasonable amount for them to charge me for this, over and above the cost of just ordering it from Wiggle?


*I suppose i had some wierd requirements back in the day, causing me to order each chainring separately.

I wanted a custom set of ratios, since i commuted through town and my number 1 priority was max acceleration from a set of red traffic lights. MTB inner chainrings are meant for winching up impossible slopes and are too short, the middle one is a bit too long for a quick getaway without destroying my knees. I created something with an unusually long inner chainring gearing and a narrow gap between each chainring. So I'd blaze off on the inner chainring on the largest cog on my rear casette, firing an upshift every 1.25 seconds till i was on the small one at the back, then shift up to an unsually short geared middle chainring, which was a similar step size from 7th to 8th gear on the cassette - keeping me in my "power band" till at 18mph, by which point i'd have safely vacated the junction.

It custom chainring worked well, unfortunately the new shifters were crap and my new 8 spd cassette had a very slow upshift compared with my old 7. Holiday was over at that point, never did find the time to get the thing working properly...

CYCLESPORT1

Get a Road Bike

The Rookie

RutlandGav wrote:

At the bottom of the stroke, the leg angle should be no straighter than 140 - i'm getting about 151.

I know a lot of people who run there legs much straighter than that without issues,
I'd be raising the saddle to reduce the bend at the top of the stroke.

RutlandGav

pain is behind the knee though. i'm finding this a tricky bike to stop at junctions - hard time mounting and dismounting and touching the floor. that's probably just the height of the bottom bracket to ground though, which isn't relevant. need to learn to trackstand. can do it if i've got a wind to lean into but otherwise have to creep fwd v slow

desweller

180mm is a big crank. I am 5'10" with 32" inseam and get knee pain on 175s.

RutlandGav

I can get a Shimano 170mm 9 spd crank for £50.

165mm are rare , I only saw a 10 spd for £100.

My old stuff is showing some signs of wear so replacing the chain and cassette at the same time is not a bad idea, but the 10 speed components are more expensive making the whole thing cost twice as much overall.

desweller

You don't have to get a 10-speed. 9-speed will be fine.

markhewitt1978

180mm cranks are insanely long. It's like they've figured what's the longest cranks even the tallest person will need and fitted them.

172.5 or 175 is more normal for a mountain bike.

For your size I would reckon 170 is going to be a better fit.

I'm 5'6" and on my road bike I used 170 for ages but switched to 165 at the start of this year and it's much nicer to ride.

As above the 'speed' of the crankset doesn't matter. I'm running an 11 chainset on my 10 speed bike, for example.

Monty Dog

Pain behind knee = hyper-extension. Lower your saddle - probably not helped by 180mm cranks. Sound like bike is simply too big.

RutlandGav

I'm now wearing a Shimano Deore chainset M590 - 22/32/44 with 170mm cranks.

Went for a 4 hour ride. Halfway round i put the seat up 1 centimetre. I get a wierd feeling that my feet slide around on the pedals (flat pedals, no clips or cleats) which i've never had before on any bike. On the first ride, the insides of my heels would catch the crank arms.

Took a day off and went 6 and a half hours yesterday. The slipping, wobbling, heel striking were less. I think I'm just getting used to the shorter stroke, i had ridden 3 months with cranks that were much too big so my pelvis and ankles were over articulating as a matter of course.

The good news is that even after that 6 and a half hour ride, no knee pain !