Crank length ideas?

mattsccm

What do you lot reckon?
Since I bought my first TT bike 30 years ago I have always used 170mm cranks. Well thats what most road bikes come with. My MTB has 175 - ditto. Funnliy enough over the last few years my knackered knee has always hurt more on the MTb. I blamed the totally different postion, being sat far mor towards the rear on the MTB. Just wonder if the crank length contributes. Just to add to it spinning hurts more that pushing a big gear!
Various on line crank length calculators suggest 160, 162.5! and 165mm cranks for my 78cm inside leg. ( From ground, measured with a book firmly in my crotch). It all points to digging out some 165's but what do you lot have anything else to throw into the ring?

Steveorow

175 does seem a tad long for a inside leg of 78cm ( 31 inches approx in old money ) .
How do you measure your seat height ie are you setting your seat height the same on the MTB and the Road bike ? If so how do you measure it ?
If you are setting the seat height by measureing from the centre of the bottom bracket to the top of the seat then you will have a 5mm difference to the bottom of the pedal stroke .
Its not an issue for me but I can tell the difference if I measure BB to seat height ( I have 170mm on bike parked in the turbo trainer and 175mm on the summer bike so the seat height on the sumer bike is a few mm lower ; but the same if I measure pedal spindle to seat top ) but if you have a dodgy knee then you may feel it .

mattsccm

Seat height set on MTb according to where I am riding. Its a variable to some extent but generally its the same length from pedal to seat top. its the leverage and angle that differ. Not worried at this stage by the MTB. its been relegated to about 4th place by the Cx bike, fixed and road bike. All of those run 170's which i reckon may be too long

Bobbinogs

http://www.bikeradar.com/forums/viewtop ... t=12715710

hotnajers

Ive ordered crank set should be here this morn, for the fist time have ordered 170 crank length from 175 just to see if there is any difference im 5. 7.5ft tall inside leg of 30, although never had any probs with 175 thought id see if theres any diff. I read somewhere that a shorter crank length is supposedly better for mtb due to less need to keep up a cadence, than road work, hope i havent wasted money!

ireland57

I've recently changed my roadie cranks from 172.5 (53/39) to 165 (52/42/30) cranks because my knees were causing me big problems. My knees have a long history of bothers well before I started bike riding.
I was unable to complete some road rides on flat ground with the 172.5 cranks; my knees were painful at the pace we were doing and I'd only go about 12 kms solo warmup then 6 kms serious then pull out and ride home.
I could climb some serious hills but only standing up. It was mostly pushing into wind that hurts the knees even spinning a lower gear at around 95 – 105 rpm.

My mtn bike has 175 cranks. I pulled out of an endurance race on Sat nt due to knee pain.
I noticed that since I got the shorter cranks on the roadie the mtn bike cranks are causing me more knee bother than a few months ago. I also notice the high pedal position at the top of the stroke.
I've been fitted professionally to both bikes (before the crank change).

Since I changed to 165 cranks I’ve ridden just over 100 kms at a speed I couldn’t carry for 10 – 12 kms before.
I now keep up on the early morning rides and sprint with the leaders when they go. I had a max of 42 – 43kmh a month ago and have now touched 56kmh. There’s more to come yet.

In short it made a huge positive difference to what I can do now.
I will have to change the mtn bike cranks as well.
I’m thinking of gear combos to run 165 cranks but due to us having a lot of steep hills here I may need to run a 36 cassette (running 34 now).

I noticed it took a lot more strength to push the shorter cranks. I couldn’t care less what the experts say about there being no or little difference. They are harder to push and it’s noticeable to me.

Someone with strong legs may not notice it.
I can’t wait to sort out something for the mtn bike as well; if I can push the gearing the 165 cranks will be good I think.

cougie

I really dont think theres much difference. I've ridden all sorts of crank lengths and I couldnt tell you what was what without looking at the numbers on the cranks. 5mm or so is bugger all difference if you look at the leg length as a whole.

Monty Dog

If you have got knee trouble, then riding a shorter crank, spinning and sitting in the saddle all lessen the strain on the knees - you can put more leverage through a 175mm crank particularly if the pedal spindle is forward of your knee on the downstroke - creating additional strain - the knee goes through a larger range of motion with a longer crank too

MichaelW

When you switch to smaller cranks you need to alter your cadence/gearing.
Assuming that your power output and pedalling force are unchanged, a smaller crank generates less torque so you need to lower your cog ratio (drop down a gear). Then you spin a little faster.

mattsccm

It is beginning to look as if I am sticking with theoretically too long cranks ie 170's. a) I cannot spin. Is it going everywhere as a teenager on my racing wheels with a 13-17 5 speed block and a 42/52 chainset. Even touring. well tubs were far nicer than 27 1/4's.
b)Also spinning seems to hurt my knees. I have no issues with my 42/18 on my SS CX bike even in the hilly Forest of Dean until I have to spin for a bit yet stick me on a mountain bike which I have to drag up hill needing low gears and my knee gives me hell. Thansk a lot anyway. Maybe I'll give the 165's a try.

Garz

I know your saying 5mm makes no difference but that's all it can take to make your seat uncomfortable or out of position. If the knees stop being aggravated from the change then it has to be a good change IMO.

danlikesbikes

Ride 172.5 on both rode bikes but have ridden 170's in the past and cant tell the difference. Guess its more about the whole position and how it affects you, especially if your having problems in the knee department.

I used to suffer big time whilst riding SPD's and changed over to Look and my knee's got a lot better, that was until I changed my bike set up position on the recommendation of some people in my local club. 1 hour later and after a trip to my LBS and no change to cleat position but seat hight increased and moved forward, shorter stem length and minor adjustment to bar angle. Couldn't believe the difference and my 30 minute ride home became an hour and half of pure joy and can believe the difference in feeling in both my knees but my whole seating position.

Could be a a combination or any one of the above that might do it for you - go luck with what ever you opt to go with.

Kirky

The general rule of thumb is that you can't have cranks that are too short - but can have them too long!! If you look at some of the top riders now (especially in the triathlon scene) they are running shorter cranks.

The length of your cranks is determined by the closed hip angle at the top of the pedal stroke - basically if the cranks are too long, the angle will close up too much (i.e. your thigh will come up too close to your chest) and you won't be able to produce maximum power output as you start the pedal stroke.

Without seeing you on your bike, I'd say err of the side of caution and go for shorter cranks - like I say, I don't think you can go too short!! But sticking with 170's or dropping to 165's would sound about right.

KIRKY

floosy

I have been using 165m cranks for about 18 months now and was still getting a bit of knee pain when pushing hard on the cranks for extended periods of time. Have now changed to 155mm and now dont have that pain...(but it does feel like there`s a lot more drag in the system.. like going up hill constantly). Im going to persevere though

Oh i have 735mm inseem..

Dave.

Berk Bonebonce

Only experience of riding different length cranks will help you to select a preference. Check out products fron Stronglight that allow you to change crank length between the two standards - adjustable length cranks.

FergieTT

This is quite a good summary of the crank length connundrum.

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

I was fitted at BikeDynamics and they told me that at 5'11" I would not notice much difference between 170,172.5 or 175mm cranks. Much shorter people need smaller cranks though or their knees are shoved too far up into their chests and the kneecap is pulled onto the joint.