I know! I'm trying to help you, you dumbarse!

Oli wrote:I know! I'm trying to help you, you dumbarse!

In plain English, longer femur = longer cranks, but not always. HTH(I'm a twerp)

anyone got any tips on what length cranks I should run?

some general rules? maybe some formulas?

Wobbler wrote:anyone got any tips on what length cranks I should run?

155mm

HTH(I'm a twerp)

factoid : longer cranks for the same cadence means faster pedal speed (circumferential).

this may or may not shed light on the subject
also, why BMX 180mm cranks and Track Sprint 165mm same need for acceleration?


below not my thoughts i dont have any



Lennard Zinn on Big Cranks for Big Folks.

One of the things I discovered when I went to a longer crank (180mm vs 175mm) length was that I was able to utilize more of my natural power climbing. It also enhanced my psuedo-sprinter style in that I could turn more gear at a slightly slower pace. It may not seem like 5mm could make such a big difference, but for me it really did. I probably could go even longer but so far 180mm cranks have been berry, berry guud, ta meh!. ;-)

William

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Why custom cranks and how long to get them? Here is the formula I recommend:
Crank length (mm) = Inseam (mm) X 0.216

Or, more conservatively for tall riders:
Crank length (mm) = Inseam (mm) X 0.21



Another formula that I like is from fit guru Bill Boston (http://www.billbostoncycles.com) and comes up with similar results. He suggests measuring your femur (thighbone) from the center of the hip joint to the end of the bone in inches. This number will be your crank length in centimeters. For instance, if you have a 20” femur, you would have a 20cm (200mm) crank.

SamKnight wrote:if your having trouble getting one made PM me, i'll talk to our machinists and see if i can get something turned down for you out of ali or steel (whichever you prefer)

Thanks Sam, I'm talking to an engineering friend later on today, Will let you know if I need a hand getting a spacer made up.

Wobbler wrote:some general rules? maybe some formulas?

R/T+(√((π±3L^3)/(165-√20Q)) X 100M)

Where T=The cost of your crankset
R=Is the value of a card drawn at random from a deck of playing cards, face cards are worth 42 and the ace of spades is worth 10,000
L= distance between your eyes in feet
Q= the combined total of all your posts on internet forums
M=the length of your chain in centimetres.

JohnnyC wrote:

Wobbler wrote:some general rules? maybe some formulas?

R/T+(√((π±3L^3)/(165-√20Q)) X 100M)

Where T=The cost of your crankset
R=Is the value of a card drawn at random from a deck of playing cards, face cards are worth 42 and the ace of spades is worth 10,000
L= distance between your eyes in feet
Q= the combined total of all your posts on internet forums
M=the length of your chain in centimetres.

now where is my deck of cards..

JohnnyC wrote:

Wobbler wrote:some general rules? maybe some formulas?

R/T+(√((π±3L^3)/(165-√20Q)) X 100M)

Where T=The cost of your crankset
R=Is the value of a card drawn at random from a deck of playing cards, face cards are worth 42 and the ace of spades is worth 10,000
L= distance between your eyes in feet
Q= the combined total of all your posts on internet forums
M=the length of your chain in centimetres.

You also need to take into account that it might take off on a treadmill

So by Lennard Zinn's conservative formula I should have 196mm cranks.

johnL wrote: this may or may not shed light on the subject
also, why BMX 180mm cranks and Track Sprint 165mm same need for acceleration?

I would say because track sprinters are always pedalling, the shorter cranks are easier to spin once they get up to speed.
In bmx, the gate start is everything and after that they get a lot of the speed from pumping so racers are willing to sacrifice being able to spin at a higher cadence for extra acceleration out of the gate.

Mr_Bob wrote:

SamKnight wrote:if your having trouble getting one made PM me, i'll talk to our machinists and see if i can get something turned down for you out of ali or steel (whichever you prefer)

Thanks Sam, I'm talking to an engineering friend later on today, Will let you know if I need a hand getting a spacer made up.

Sweet as