Cranks Length - How to work out what you need ?

drjones · Post by **drjones** » 8 Jan 2007, 5:28pm

Does anyone have a good way of determining what cranks are suitable for a given leg length? So many opinions and forumlae out there, all giving different advice.

Generally speaking, does one want a road-bike length or a MTB length for touring? Why are MTB cranks longer than Road cranks anyway? Arhh!!
Many thanks!

Terry T · Post by **Terry T** » 8 Jan 2007, 6:17pm

Measure your height without shoes, then measure your height whilst sitting on the floor. Subtract the sitting measurement from the standing measurement and divide by 5. This is best done using the metric system as crank length is given in millimetres.

reohn2 · Post by **reohn2** » 8 Jan 2007, 9:20pm

I've always ridden 170mm cranks although Iam 6ft tall and have long legs.Last year we bought a new tandem and it to have 175mm cranks on the front,after riding it for a few miles I soon got used to the exrta length.I've been told that the 175's are the correct length for me but I feel Ok with either.

PS i've just done Terry T's formula and come up with 296mm!!! ie total height 1810mm- sitting height 920mm=296mm

Post by **Mick F** » 8 Jan 2007, 10:17pm

R2,

What sort of shape are you?

Do you have strange legs? 296mm?

mmm ....

Wigan?

Mick F. Cornwall

reohn2 · Post by **reohn2** » 8 Jan 2007, 10:32pm

sorry forgot to put in the "divide by 3" oops. the answer's right though

reohn2 · Post by **reohn2** » 8 Jan 2007, 10:34pm

Mick, I've got Wigginers legs

thirdcrank · Post by **thirdcrank** » 9 Jan 2007, 2:52pm

Basically, if you feel the urge to fit anything much outside the range of 170 - 175mm you need a lot of money.

daveawood · Post by **daveawood** » 15 Jan 2007, 5:00pm

I can only help with the last part of the question.
MTB cranks are longer for more leverage to cope with the stop/start nature of mountain biking such as bieng able to power up a short steep rise with no run up.
Road cycling usually allows for a constant highish cadence which needs less leverage and this is more comfortable with the smaller pedalling circle of short cranks.
dave

CJ · Post by CJ » 16 Jan 2007, 3:04pm

Not divide by 3 Terry T, divide by 5 more like!

Thus if we work the formula on the measurements supplied by rehon2: (total height 1810 - sitting height 920) / 5 = a reasonable crank length of 178mm.

So the usual 175mm is not too wide of the mark and a bit on the short side will do no harm anyway.

reohn2 · Post by **reohn2** » 16 Jan 2007, 7:36pm

CJ wrote:Not divide by 3 Terry T, divide by 5 more like!

Thus if we work the formula on the measurements supplied by rehon2: (total height 1810 - sitting height 920) / 5 = a reasonable crank length of 178mm.

So the usual 175mm is not too wide of the mark and a bit on the short side will do no harm anyway.

Thanks for that Chris, me knees have been hitting me under the chin for the last two weeks.

PS anyone want buy a pair of 296mm cranks(little use)

Terry T · Post by **Terry T** » 16 Jan 2007, 7:55pm

It was a typing error, OK?
I got the info from the CTC site.

thirdcrank · Post by **thirdcrank** » 17 Jan 2007, 9:18am

Whatever the formula and whatever the answer it gives, the fact is that you either settle for 170-175 mm or dig deep in your pockets for TA or similar. Stronglight have introduced an adjustable crank with a reversible pedal hole in a slot but the variation provided is 170-175 mm so no help here. If you are very strong and famous like Big Mig, Campag will make long cranks 'just for you'. Highpath will shorten standard ones for you, and there is always StJSC. Altering your legs to fit the cranks is probably nearly as easy as trying the other way round.

Terry T · Post by **Terry T** » 17 Jan 2007, 11:21am

I managed to get a set of 150mm Ofmega cranks (NOS) off Ebay for £14.
They fit Campag bottom brackets and have saved me a fortune!

Post by **Mick F** » 17 Jan 2007, 12:23pm

thirdcrank wrote: Altering your legs to fit the cranks is probably nearly as easy as trying the other way round.

Well, come on then R2, get those thighs shortened! From what I remember of you lot up there, you're all too tall anyway. I met a chap from Abram the other day, he was a strange shape too!

Mick F. Cornwall (Mr Average 5ft 10in - 170mm cranks)

Terry T · Post by **Terry T** » 17 Jan 2007, 3:58pm

I hasten to add that the cranks were for my wife who is only 5'2".
I'm also Mr. Average, 5'10".