Crank Arm Length
Jeff Enderwick
How does one know what crank-arm length is "optimal" ? Is there any
formula based on inseam or whatever ?
Jeff Enderwick
je...@otis.sps.mot.com
Steven Eugene Sergeant
>How does one know what crank-arm length is "optimal" ? Is there any
>formula based on inseam or whatever ?
In the August 1991 issue of BikeCentennial's BikeReport, John Schubert
writes in his column "Cycle Sense: 10 Myths you need never repeat":
MYTH #8: Crank length should vary in proportion with leg length.
REALITY: It shouldn't, and it doesn't. Within the range of 'normal'
adults (say, 5 feet tall to 6 feet 4 inches), optimum crank lengths
range from 165mm to 180mm. Percentagewise, this change in crank
length is about 1/3 the variation in their body sizes.
(Okay, some people at the extreme end of this height range might
prefer 160s and 185s if they can find such rare specialty items,
but they are still not changing crank length in proportion to their
body size.)
Why? Here's the technical explanation: the question of leverage and
muscle work load and range of motion all come out in the wash. If you
view the entire leg, foot and crank as a giant interconnected system,
the leg muscles are happier with a standard length crank, modified only
slightly to accomodate leg length. Several rigorous physiological
studies have demonstrated that the rider is better off with these middle
sized cranks. You don't believe me? Go buy 150s (made in tiny quantities
by France's T.A. for young racers). They're like running with your knees
tied together, even if you're only 4'11".
###
(A hand scanner is really handy for forwarding hard-copy stuff like
this...)
--Steve Sergeant
"You don't have to disagree with me to be wrong."
Stev...@CUP.Portal.com {The PORTAL(tm) System}
GEnie: STEVSERGEANT
CIS: 71620,272
Allan Armstrong
I've always ridden with 170 cranks. I'm 5'11" and have short legs and a
long torso. Then people started telling me I'd do better with 172.5s or
175s. So when I finally had to replace my Super Record crank with a
Dura-Ace on one bike, I got a 172.5. I'm not sure I can even tell the
difference. I still have 170s on my favorite bike. When I was racing, I
justified using cranks that were on the small side of the recommended
range for my body because I felt that I didn't need more climbing power
because I climbed well and spun but definitely needed more leg speed in
the sprints. I now race crits with a bike with a low BB and 170 cranks
and can pedal through every corner safely, and I think that leg speed is
so important that I won't give anything to sacrifice it.
Personally, I think the differences between crank sizes are not worth
worrying about unless you have very unusual body proportions.
Allan
Tom Reingold
was of the opposite opinion. I said that I did not see how 5 mm could
make a noticeable difference. Just about everyone who had tried
switching crankarm lengths said that I should shut up because it's
definitely extremely noticeable.
What might account for the change in consensus?
(In case you're worried, I'm amused, not insulted.)
--
Tom Reingold
t...@samadams.princeton.edu OR ...!princeton!samadams!tr
"Warning: Do not drive with Auto-Shade in place. Remove
from windshield before starting ignition."
Jim Muller
>Wow. I am surprised. Last time this discussion came up, the consensus
>was] definitely extremely noticeable.
I too was surprised when I read that posting. When we built my wife's bike
we went through this issue. In fact, a crank 10 mm longer means your knee
rises *20* mm higher from lowest point to highest. My wife is 5'2". Her
bike frame is unusual, with a shorter-than-expected top tube and a seat tube
length of about 18-1/8 inches (if I remember right). When riding properly-
adjusted store-bought bikes (even with small frames) with more "conventional"
cranks, her knees would come up so high that her thighs were raised up above
horizontal and her knee joint angle was less than 90 degrees. The top of
her stroke started from this ridiculus position and even the power portion
(when the crank was near horizontal) had her "pushing" with her knee flexed
more than would be advisable for most everyone else. It was as if her seat
was too low, giving that "novice" knees-in-the-air look, but we couldn't
raise the seat any more since her legs did have to reach the pedals at the
bottom of the stroke. A shorter crank helped all this, plus gave the added
benefit that the toe clips didn't hit the front tire as readily. It certainly
madea difference for her.
A quick look at the geometry suggests that you'd really like to adjust your
seat height for optimum power with the cranks near horizontal. Unfortunately,
conventional wisdom has us adjust the seat for the *bottom* of the pedal
stroke. What the seat height really affects is the range of angle the knee
experiences over one stroke, from nearly straight at the bottom to nearly
90 degrees at the top. Since the leg can exert more force when it is nearly
straight than mostly bent, it is almost always best to have the leg operating
over a range with the knee straighter. Thus it makes sense to raise the seat
as much as possible (as long as you can still reach the pedals). If your leg
is shorter than average, your knee will cover a greater range of extension,
meaning you'll be spending more of your stroke in a less-powerful range. In
physics terms, the integral of your force over the total stroke will be less.
A shorter crank reduces the angle of knee flex at the top while the knee flex
at the bottom stays the same, i.e. nearly straight. Hence the integral of the
angle over the stroke is less, giving more force, especially at the critical
power portion.
There are limits to shorter crank length. (There *must* be some or we'd all
be running shorter cranks already.) A shorter crank gives a proportionally
lower torque on the crank spindle, effectively raising the gearing, but this
can be countered by running lower gears and so turns out to be insignificant.
More importantly, there are also limits to the legs' ability to exert both
power and force, and to move at speed regardless of the force being exerted.
(If we had no limits to our legs' speed, we`d all be running super-low gears,
spinning like mad even on flat terrain.) While the legs' ability to exert
force increases as the knee becomes nearly straight, its ability to exert
power does not, since its displacement potential is limited. Inotherwords,
you can overdo the short-crank approach, with the limit being the trivial case
of no cranks at all! The conventional crank length evolved for average legs,
but if you are significantly different from average, it can make sense to
scale the crank too.
The question then is how much. A first guess would be to make it proportional
to the rest of the frame. A second guess says:
1. we don't really scale the *entire* bike to our bodies anyway;
2. our bodies are not perfectly scaled example that just differ in total size;
3. even if our bodies were scaled the same, displacement is proportional to
size, strength is proportional to square of size, and mass (weight) is
proportional to the cube of size, so our capacities wouldn't scale even
if our bodies were built that way;
4. most importantly, crank length (i.e. leg displacement vertically) is not a
fixed dimension that must match some static frame component; rather, it is
a distance over which something moves, and thus can accomodate a range of
possibilities.
For these reasons, the crank length need not necessarily be scaled with the
rest of the bike. "Average" is plenty good enough for *almost* everyone. The
difference between optimal and not-so-optimal is not very great, certainly not
as great as with, say, a poorly-fitting frame. It becomes an issue only if
you are built significantly different from average. So don't worry about it
unless you are really different, but if you are, consider the crank length.
--
__~o\____ ------------|\-----|
Jim Muller ~ '-0-----0-` --------|~--o|\|---:| { repeat as }
jmu...@stardent.com -~-~----~-----~~------------o|-----o|----| { necessary }
David States
|> Wow. I am surprised. Last time this discussion came up, the consensus
|> was of the opposite opinion. I said that I did not see how 5 mm could
|> make a noticeable difference. Just about everyone who had tried
|> switching crankarm lengths said that I should shut up because it's
|> definitely extremely noticeable.
It's OK, don't shut up, but I do think 5mm is a noticeable difference.
IMHO 170mm cranks are a throw back to the days when the average rider
was 5' 6". For a 6' rider, I would argue that 175-180mm would be
more appropriate. Also, the more fat you have on your leg, the more
a high cadence stroke is going to cost you for viscous drag in the flab.
Ie. the fact that professional riders with thinned down legs can
efficiently turn a 170mm crank at 110 rpm may not be real applicable to
your average American semi-recreational amateur.
Note: part of the reason that long cranks have a bad rep for knees may
be because 5mm is noticeable. I found it took a long time to get my
stroke coordination and relaxed after changing crank length. While
you are getting used to a new stroke geometry, your knees are definitely
more stressed.
|> What might account for the change in consensus?
Fatigue and apathy. :-)
|> Tom Reingold
David States
Cyndi Smith
In article <45...@cup.portal.com> Stev...@cup.portal.com (Steven Eugene
Sergeant) writes:
> . Several rigorous physiological
> studies have demonstrated that the rider is better off with these middle
> sized cranks. You don't believe me? Go buy 150s (made in tiny
quantities
> by France's T.A. for young racers). They're like running with your knees
> tied together, even if you're only 4'11".
Until you shrink to 4'11", don't assume!
I AM 4'11" and find that cranks longer that 155 are uncomfortable and hard
to ride. The rental bikes at our velodrome all have 165's so I have lots
of experience in riding longer cranks. They are very hard for me to spin.
My road bike, on the other hand, has 155's (had 152.5's until I cracked
one!) and I can spin pretty nicely.
Other disadvantage of long cranks: when I am down on the drops ("aero
position"), my knee hits my chest with the 165's but not with the 155's!
Cyndi Smith
NOTE NEW ADDRESS --> c...@odin.mda.uth.tmc.edu
My opinions are MINE, I tell you, MINE, MINE, MINE, MINE, MINE!
(Unless you wish to rent them for an appropriate fee...)
Bill Pachoud
This is not a myth. While the differences in absolute power output
may be small, they are noticable. I would like to see the references
on the papers, when I researched this question several years ago in
the kineseology literature, the overall concensus was the crank length
SHOULD vary in proportion to leg length. Other factors such as riding
style, event, etc. also play a role in selection of crank length.
For recreational riders who are close to average size, crank length
probably won't be an issue. For top pro's and people of unusual size
big or small, it is definitly noticible and important.
I am 6'8" tall and have a 39.25" inseam when measured by the method
specified in "G. LeMonds' Complete Book of Cycling". I have riden
crank lengths of 170mm, 180mm and currently ride a very custom
Columbine w/ a crank length of 215mm. It makes a large subjective
difference and a small objective one.
I wrote all this up in a note to the net a while back. If anyone would
like to see it email me. I researched the question very throughly before
building the bike and manufacturing the crankset, it took 2 years, 4 peoples'
effort and cost $5K. I have been very happy with the longer cranks, which
are proportional to my leg length and can recommend this approach to bike
fit.
Bill Pachoud
bi...@rootgroup.com
KPUR...@liverpool.ac.uk
Tony Oliver is a British frame/bike builder living in Wales. He has some
very strong opinions on how very good touring bikes should be made and he
lays out his case here. An excellent book, full of sensible ideas. I think
I found his approach refreshing because he was a physicist, so he like to
know what's happening, rather than following the latest fad. He also likes
to maximise performance/price ratio (i.e he's a skinflint like me!).
The opening chapter is on designing a frame, and getting the measurements
right. As he says, designing the frame is an art not a science, compromise
and iterative design being most important.
He believes that getting the right top tube length is most important,
and offers a lot of advice on using body measurements to get frame. He
poo-poos the the CONI approch (seat-to-pedal = 1.09 x inside leg) as resulting
from measurements for racing machines and resulting in 1970's Italian style
racing frames.
He uses two measurements:
1. The inside leg (crotch to ground in bare feet)
2. Torso length (crotch to top of the sternum) + arm length (wrist to
acromnium*).
* the acromnium is the place where the arm bends at the shoulder. The easiest
way to find it is: put a finger on the outside of your arm, press in and move
it up, till you hit the bony projection on the shoulder blade/shoulder. Its
easier if you have a friend to help out with the measurements.
He then gives the following tables:
Inside Leg (inches) Frame size (inches) Crank Length (mm)
------------------- ------------------- -----------------
28 19 162.5
29 19 3/4 165.0
30 20 3/4 165.0
31 21 1/2 167.5
32 22 1/2 170.0
33 23 1/4 170.0
34 24 1/4 172.5
35 25 175.0
36 26 175.0
37 27 177.5
38 28 177.5
Torso + arm length (inches) Top tube length (inches) Stem extension (mm)
--------------------------- ------------------------ -------------------
39 20.0 60
40 20.2 70
41 20.4 75
42 20.6 80
43 20.8 90
44 21.0 90
45 21.3 95
46 21.6 95
47 21.9 100
48 22.2 100
49 22.5 105
50 22.9 105
51 23.4 110
His opinion is the majority of bikes have to long a top tube, which
he believes to be the most important dimension (and I agree if your
not comfortable on the handlebars how can you ride any distance.
His opinions on crank length run to two pages! The upshot: match the
leg length to the crank length. After all most peoples legs are different
lengths, but the ration of inside length to calf length is very nearly
constant.
I strongly recommend this book (even if you disagree with some of the
opinions!).
BTW, I don't know Tony Oliver and I am only a satisfied reader of the book!
Comments?
Kevin Purcell | kpur...@liverpool.ac.uk
Surface Science |
Liverpool University |
Liverpool L69 3BX | "It's not a bug; it's a VW."
Michael Boom
looking through the Bullseye literature I got a couple of years ago, and
it turns out that Bullseye makes cranks that go up to 222 mm long. Just
make sure your bottom bracket has lots of ground clearance and that your
front wheel isn't packed too closely into the rest of your frame....
Mike Boom
Toxic Screen Dumps, Inc.