## Bicycle Crank Length: A Formula

This page, along with the associated pages, provides a
method of estimating the correct length for a crankset for a given
cyclist. Crank length is the
distance from the center of the crank axle to the center of the pedal
axle,
and is only one of dozens of variables involved in adjusting a bicycle
to
"fit" a rider properly. As you will learn here, it is an often
overlooked and misunderstood variable.

However, right here I must make one thing very clear: The
formula presented here is *not* a generally accepted rule of
thumb among the bicycle industry
nor the cycling crowd. In fact, you will not likely hear of it anywhere
but
here. For these reasons, I do not present the formula alone; I present
the
entire reasoning behind it, its implications, some references,
some examples and testimonials,
as well as some insights into why the rest of
the cycling world isn't using the same analysis. The cyclist, thus
informed, can decide for himself whether to trust my guidance or what
the bike shop is telling him.

For those simply interested in the formula itself, it is
simple enough. First, the rider must measure his inseam. It is not
recommended that the pants
size be used as this may not be accurate enough, an inch one way or the
other
makes a pretty big difference. Stand barefoot on a hard floor,
straight,
feet fairly close together, back against a wall, and have an assistant
insert
a clipboard between your legs. Lift gently, sliding the clipboard as
high
as it will comfortably go while keeping it firmly against the wall,
thereby
assuring it isn't tilted. While holding the clipboard this way, measure
the
distance from the top edge of the clipboard to the floor, in inches.
Don't
round to the nearest inch; measure as accurately as possible.

# L(mm) = 5.48 x I(in)

Now, multiply the inseam measurement (in inches) by 5.48.
This provides a
good estimate of proper crank length, in millimeters, for general road
cycling
or racing.
For those who prefer metric units, simply convert the
equation by 2.54 cm/in.

# L(mm) = 2.16 x I(cm)

Now, many of you, especially those who are significantly
taller or shorter than average, have just decided that I must be an
idiot -- this can't possibly be right! For you, I suggest reading all
the attached discussions linked above.
You may still decide I'm an idiot, but I think you're going to have a
hard
time supporting your contention. Before you warm up the flamethrowers
and
start bombing my e-mail box, however, I suggest you read this.
If you find this formula believeable but keep getting
conflicting input from others, I counter some of the lies, misinformation and old wive's tales.

For those of you who *are* convinced: before you go
and spend money on a new crankset, you should read about some fudge factors.

Finally, once you decide what crank length you need,
finding it is another issue. I can offer a few tips
here.

Return to Kirby Palm's home page.

Of course, if you have questions or comments, you are
welcome to send e-mail
to me at "palmk at nettally dot com".