correct crank length

[ians]

Hi

Could someone please tell me how to calculate the correct crank length?

Thanks

Ian



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[Pete Biggs]

ians wrote:
> Could someone please tell me how to calculate the correct crank
> length?

There are various formulae to work it out but they tend to give different
results - some of them very stupid! I think it's best to use personal
experience, preference, intuition and common sense instead.

170mm is generally considered suitable for average inside leg (average
British adult) for normal road cycling. New MTB's tend to come with
175mm.

~PB

[Henry Braun]

On Wed, 23 Jul 2003, Pete Biggs wrote:

> There are various formulae to work it out but they tend to give different
> results - some of them very stupid! I think it's best to use personal
> experience, preference, intuition and common sense instead.
>
> 170mm is generally considered suitable for average inside leg (average
> British adult) for normal road cycling. New MTB's tend to come with
> 175mm.

Perhaps because the average British adult has been gaining two inches in
height per generation, of late, and the MTB standard has been developed
more recently than the road bike? Or just because MTB frames are designed
with higher bottom brackets, so early MTB riders could put longer cranks
on without trouble?

Does *anybody* sell bicycles with 185-190mm cranks off the shelf?

[Brizza]

It depends what you are riding and why. There are different lengths.
Longer cranks allow you to use more power in each stroke, while shorter
cranks make each stoke easier and allow you to 'spin' more easily. Track
bikes use shorter cranks to aviod the crank hitting the raised bank at
low speed. They are measured from the middle of the pedal to the middel
of the BB, and are usually 170 or 175mm.

I hope that helps.

Brian



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[ians]

Thanks guys. I find this whole thing very confusing. I'm a
recreational/touring rider. I read somewhere that crank length is a
function of inside leg measurement, and if it's not right can cause
knee problems.

I've just done a search in Google for 'crank length' and the the first 3
hits give me results varying from 165mm to 175mm.

I guess it's toss a coin time.

Ian



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[Biktor]

ians wrote:
> Hi
> Could someone please tell me how to calculate the correct crank length?
> Thanks
> Ian



Ian, unfortunatelly I do not know how to calculate the right crank
lenght, but I can share my experience with you hoping this would help
you decide if you need to change your current length. I had a 175 mm in
my original crank arm when I bought my bike, then in order to gain some
power I decided to go big with the enormuos 180 mm from Shimano
Dura-Ace. After three or four months of using it I realized that I have
not really make a significant improvement in speed or power but instead
a sudden and rare pain started to build up in my butt and hamstring of
my right leg. I am not really sure as the increase in cranck arm lenght
contributed or started up my pain. At the end my advice is: consider
that if you increase the arm length you are also increasing the diameter
of your spinning and all the muscles in your legs will work much harder
in a different way, but if you decide to go larger anyway give it at
least 1 month to pedal really easy and don't go more than 30 Mi a day.
Whatever you do, don't stop riding!.



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[Edwin]

ians wrote:
> Hi
> Could someone please tell me how to calculate the correct crank length?
> Thanks
> Ian


An good reference on the subject is Road Racing: Technique and Training
by Bernard Hinault. He devotes 4 pages to crank length. The book is a
bit dated, but his discussion of the bike and your position on it is
very good.



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[James Annan]

Henry Braun wrote:

>
> Perhaps because the average British adult has been gaining two inches in
> height per generation, of late, and the MTB standard has been developed
> more recently than the road bike? Or just because MTB frames are designed
> with higher bottom brackets, so early MTB riders could put longer cranks
> on without trouble?
>
> Does *anybody* sell bicycles with 185-190mm cranks off the shelf?

I don't think anyone even _makes_ 190mm cranks as standard production (a
few people may make them to order).

After rowing for a few years, I started cycling more seriously and soon
changed to 185mm cranks. It felt silly on the 170mm standard twiddling
along with my feet spinning away and my thighs hardly moving. It's hard
to see how 185mm can be 'long enough' by any reasonable measure but they
are the longest that are reasonably easy to get hold of.

James

[Dorwinion]

Is the Inside Leg measurement the measurement from the top of the pedal
to the top of the saddle, measuring along the axis of the seat tube?

The measurement from the top of the pedal to the top of the saddle on my
bicycles are set to 34.25 inches.

I am 6'2" and 20 years ago, I was set up with a bicycle having crank arm
length of 172.5. This bicycle is a 24 inch or 61cm frame from center of
BB to top of seat tube. I have a newer bicycle with 175 crank arms. I
seem to be able to climb better with the 175 crank arms, but I am faster
overall with the bicycle with the 172.5 crank arms. Of course it could
be other differences in the bicycles.

What are other riders of my size using?



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[Simon Brooke]

Dorwinion > writes:

> Is the Inside Leg measurement the measurement from the top of the pedal
> to the top of the saddle, measuring along the axis of the seat tube?
>
> The measurement from the top of the pedal to the top of the saddle on my
> bicycles are set to 34.25 inches.
>
> I am 6'2" and 20 years ago, I was set up with a bicycle having crank arm

I'm trying to picture your bike, and failing. I'm also about 6'2", and
I'm rather longer in the back than average. My road bike frame is
650mm or 25.5". The distance from the centre of the bottom bracket to
the top of by saddle (parallel with the seat tube) is 780mm or 30.75
inches; the distance from the top of the lower pedal with the crank
aligned with the seat tube to the top of the seat (parallel with the
seat tube) is 940mm or 37".

> length of 172.5. This bicycle is a 24 inch or 61cm frame from center of
> BB to top of seat tube. I have a newer bicycle with 175 crank arms. I
> seem to be able to climb better with the 175 crank arms, but I am faster
> overall with the bicycle with the 172.5 crank arms. Of course it could
> be other differences in the bicycles.

FWIW the cranks on my hill bike appear to be 175mm centre to centre,
and those on my road bike appear to be 170mm centre to centre. I can't
say I notice this difference in practice.

--
(Simon Brooke) http://www.jasmine.org.uk/~simon/

There are no messages. The above is just a random stream of
bytes. Any opinion or meaning you find in it is your own creation.

[Helen Deborah Vecht]

Dorwinion >typed


> Is the Inside Leg measurement the measurement from the top of the pedal
> to the top of the saddle, measuring along the axis of the seat tube?

No, it's the measurement from *your* crotch bone (pubis) to the floor
(without high heels, shoes etc!).

> The measurement from the top of the pedal to the top of the saddle on my
> bicycles are set to 34.25 inches.

That sounds a little low for someone of your height. It's about right
for me - 5'6", 31" from pubis to floor. Your feet add some effective leg
length too, don't forget.

> I am 6'2" and 20 years ago, I was set up with a bicycle having crank arm
> length of 172.5. This bicycle is a 24 inch or 61cm frame from center of
> BB to top of seat tube. I have a newer bicycle with 175 crank arms. I
> seem to be able to climb better with the 175 crank arms, but I am faster
> overall with the bicycle with the 172.5 crank arms. Of course it could
> be other differences in the bicycles.

> What are other riders of my size using?

Something similar, what ever they can get, probably! Shin, thigh and
foot lengths all vary and affect how some sizes fit different people
better.

--
Helen D. Vecht:
Edgware.

[Niv]

> I seem to be able to climb better with the 175 crank arms, but I am faster
> overall with the bicycle with the 172.5 crank arms. >

You (obviously) have a bit more leverage with longer cranks, so easier to
climb.
But you have to move your legs in bigger circles also, maybe a disadvantage
at any sort of speed. Mind you, I'm no expert, and only 67.8" tall.
(Important that 0.8"!).

Niv.

[James Hodson]

On Sat, 8 Nov 2003 16:20:29 -0000, "Niv" > wrote:

>> I seem to be able to climb better with the 175 crank arms, but I am faster
>> overall with the bicycle with the 172.5 crank arms. >
>
>You (obviously) have a bit more leverage with longer cranks, so easier to
>climb.
>

Hi Niv

I'd say harder, not easier. With a given wheel diameter and the same
number of sprocket and chainwheel teeth - ie the same gear size -
more effort is required with longer cranks, IMO. OK, you'll travel
further with each rotation using longer cranks but life will be far
easier with shorter cranks.

I'm assuming here that the bike is correctly sized and everything else
is equal.

James

--
"Sorry mate, I didn't see you" is not a satisfactory excuse.

[VERNON LEVY]

> Hi Niv
>
> I'd say harder, not easier. With a given wheel diameter and the same
> number of sprocket and chainwheel teeth - ie the same gear size -
> more effort is required with longer cranks, IMO. OK, you'll travel
> further with each rotation using longer cranks but life will be far
> easier with shorter cranks.

I'd love to know the physics behind your assertions!!

Same number of teeth and chain wheel leads to the same distance travelled
per rotation of the chain wheel irrespective of the crank length.
Furthermore a longer crank reduces the effort needed not increases it.

Must have had a brain fart :-)

[Niv]

"VERNON LEVY" > wrote in message
...
>
> > Hi Niv
> >
> > I'd say harder, not easier. With a given wheel diameter and the same
> > number of sprocket and chainwheel teeth - ie the same gear size -
> > more effort is required with longer cranks, IMO. OK, you'll travel
> > further with each rotation using longer cranks but life will be far
> > easier with shorter cranks.
>
> I'd love to know the physics behind your assertions!!
>
> Same number of teeth and chain wheel leads to the same distance travelled
> per rotation of the chain wheel irrespective of the crank length.
> Furthermore a longer crank reduces the effort needed not increases it.
>
> Must have had a brain fart :-)
>
>
>
Thanks for backing me up Vernon, I thought it was obvious myself.

Niv.

[Dave Kahn]

On Sun, 09 Nov 2003 03:03:17 +0000, James Hodson
> wrote:

>I'd say harder, not easier. With a given wheel diameter and the same
>number of sprocket and chainwheel teeth - ie the same gear size -
>more effort is required with longer cranks, IMO.

Less effort - longer levers.

> OK, you'll travel further with each rotation using longer cranks

Crank length has no effect on the distance travelled on each rotation
of the cranks. This is determined solely by the wheel size, the number
of teeth on the chainwheel and the number of teeth on the sprocket.

--
Dave...

[James Hodson]

On Sun, 9 Nov 2003 08:54:31 -0000, "VERNON LEVY"
> wrote:

>Must have had a brain fart :-)
>

Agreed. Fart.

James

--
"Sorry mate, I didn't see you" is not a satisfactory excuse.

[Dave Kahn]

pig pog > wrote in message >...

> isn't pedaling force something that you address with your gears though.
> Crank length is to do with the optimal alignment/movement of the leg
> joints surely?

Yes and yes. But remember that crank length is actually part of the
gearing. As previously mentioned, changing the crank length does not
alter the distance travelled per crank rotation. However, it does
affect the effort required and the distance travelled by the foot. It
therefore follows that a longer crank equates to a lower gear.
Conventional gear tables do not take crank length into account. Canny
juniors can exploit this in gear-limited races by using shorter
cranks. This is not really a good idea as the gear limit is supposed
to be there for their protection.

Sheldon Brown has a discussion of this on his website, and also a
proposed alternative system for measuring gear size that takes crank
length into account. Governing bodies should switch to the Sheldon
method when setting gear limits.

--
Dave...