Crank arm length question

The root question:

How do I determine the appropriate starting-point for crank-arm
length?


The background:

I have posted in the past about a minor soreness across the top of my
kneecap which shows up several miles into a ride, and which always goes
away by the next day. I have raised my seat as much as I can without
rocking my hips, and the last adjustment of 3mm or so a month ago helped
significantly, but not completely. Some other reading I've done has
indicated that a too-long crank arm can contribute to knee soreness,
which of course brings up the question of how to determine the correct
length. My current ones are 175mm, and I wear pants with a 32 inch
inseam (I haven't measured my crotch-to-ground height yet). Is it
likely that a shorter crank might help? If so, how short?


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On Mon, 17 May 2004 10:06:31 -0400, David Kerber
wrote:

The root question:

How do I determine the appropriate starting-point for crank-arm
length?



Find a helpful bike shop that will let you try different lengths?


The background:

I have posted in the past about a minor soreness across the top of my
kneecap which shows up several miles into a ride, and which always goes
away by the next day. I have raised my seat as much as I can without
rocking my hips, and the last adjustment of 3mm or so a month ago helped
significantly, but not completely. Some other reading I've done has
indicated that a too-long crank arm can contribute to knee soreness,
which of course brings up the question of how to determine the correct
length. My current ones are 175mm, and I wear pants with a 32 inch
inseam (I haven't measured my crotch-to-ground height yet). Is it
likely that a shorter crank might help? If so, how short?


Have you tried moving the seat forward? That took away my upper
kneecap aches. I've even lowered the saddle down a bit, having raised
it again and again while the seat was back, and things are fine.

A discussion of crank length, with links to other articles-

http://www.myra-simon.com/bike/cranks.html

In article ,
says...
On Mon, 17 May 2004 10:06:31 -0400, David Kerber
wrote:

The root question:

How do I determine the appropriate starting-point for crank-arm
length?



Find a helpful bike shop that will let you try different lengths?


The background:

I have posted in the past about a minor soreness across the top of my
kneecap which shows up several miles into a ride, and which always goes
away by the next day. I have raised my seat as much as I can without
rocking my hips, and the last adjustment of 3mm or so a month ago helped
significantly, but not completely. Some other reading I've done has
indicated that a too-long crank arm can contribute to knee soreness,
which of course brings up the question of how to determine the correct
length. My current ones are 175mm, and I wear pants with a 32 inch
inseam (I haven't measured my crotch-to-ground height yet). Is it
likely that a shorter crank might help? If so, how short?


Have you tried moving the seat forward? That took away my upper
kneecap aches. I've even lowered the saddle down a bit, having raised
it again and again while the seat was back, and things are fine.

The seat's already close to as far forward as it can go, but I'll see if
I've got any room left at all.


A discussion of crank length, with links to other articles-

http://www.myra-simon.com/bike/cranks.html

Thanks for that.


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Remove the ns_ from if replying by e-mail (but keep posts in the
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David Kerber wrote:
The root question:

How do I determine the appropriate starting-point for crank-arm
length?


snip

Seems to be a rather hearty investigation of the subject he

http://www.cranklength.info/index.htm

- khill

On Mon, 17 May 2004 13:50:13 -0400, David Kerber
wrote:
The seat's already close to as far forward as it can go, but I'll see if
I've got any room left at all.

Some (most?) seatposts have the clamp slightly offset instead of
centered over the tube; turn the seatpost around, remount the
saddle, and make sure there's still enough tilt available (not
always).
--
Rick Onanian

On Mon, 17 May 2004 10:06:31 -0400, David Kerber
wrote:
My current ones are 175mm, and I wear pants with a 32 inch
inseam (I haven't measured my crotch-to-ground height yet). Is it
likely that a shorter crank might help? If so, how short?

175 is pretty long. I used 172.5 with my 30 inch inseam, but went
down to 165 and am surprisingly happy; I expected not to like them.
--
Rick Onanian

In article ,
says...
On Mon, 17 May 2004 13:50:13 -0400, David Kerber
wrote:
The seat's already close to as far forward as it can go, but I'll see if
I've got any room left at all.

Some (most?) seatposts have the clamp slightly offset instead of
centered over the tube; turn the seatpost around, remount the
saddle, and make sure there's still enough tilt available (not
always).

Yeah, I'm still playing with things as I get more miles in. I tweaked
the tilt by 1/2 a turn last night, so I'll wait a bit before adjusting
anything else, but I've still got about .5 cm of room left to go forward
with it if I decide to do so.


--
Remove the ns_ from if replying by e-mail (but keep posts in the
newsgroups if possible).

On Mon, 17 May 2004 10:06:31 -0400, David Kerber
wrote:
My current ones are 175mm, and I wear pants with a 32 inch
inseam (I haven't measured my crotch-to-ground height yet). Is it
likely that a shorter crank might help? If so, how short?

175 is pretty long. I used 172.5 with my 30 inch inseam, but went
down to 165 and am surprisingly happy; I expected not to like them.

I'm not so sure about that.

My Levi's are a 32 inch inseam and I run 175's on my mountain bike and
tandem, and 172.5's on my road bike.

My wife has a 28 inch inseam and rides 165's.

YMMV.


Chris Neary


"Science, freedom, beauty, adventu what more could
you ask of life? Bicycling combined all the elements I
loved" - Adapted from a quotation by Charles Lindbergh

I wrote:
175 is pretty long. I used 172.5 with my 30 inch inseam, but went
down to 165 and am surprisingly happy; I expected not to like them.
On Wed, 19 May 2004 01:23:52 GMT, Chris Neary
wrote:
I'm not so sure about that.

My Levi's are a 32 inch inseam and I run 175's on my mountain bike and
tandem, and 172.5's on my road bike.

Mountain bikes have very different requirements for crank length. I
don't know anything about tandems.

My wife has a 28 inch inseam and rides 165's.

Make sure you're measuring both your inseams accurately; for biking
purposes, it's not which pants are comfortable, but rather the exact
height from the floor to your crotch. One method I read somewhere is
to stand with your feet flat and your back (and legs) against a
wall, and hold a book with one edge against the wall and one edge up
to your leg joint (as high as it can), and measure from the book's
crotch edge to the floor.

Of course, having said that, I don't remember if my inseam was 30
inches when I measured that way.

YMMV.

Agreed...In fact, that's the most important rule about crank length.
--
Rick Onanian

My Levi's are a 32 inch inseam and I run 175's on my mountain bike and
tandem, and 172.5's on my road bike.

Mountain bikes have very different requirements for crank length. I
don't know anything about tandems.

Mountain bikes, tandems, and time trial bikes tend to have slightly longer
cranks since cadence on these bikes is usually lower but torque is somewhat
higher.

Fixed gear bikes, OTOH, tend to run shorter cranks for the opposite reason,
as well to avoid pedal strikes in some instances. My fixie has 170mm cranks.

My wife has a 28 inch inseam and rides 165's.

Make sure you're measuring both your inseams accurately; for biking
purposes, it's not which pants are comfortable, but rather the exact
height from the floor to your crotch. One method I read somewhere is
to stand with your feet flat and your back (and legs) against a
wall, and hold a book with one edge against the wall and one edge up
to your leg joint (as high as it can), and measure from the book's
crotch edge to the floor.

Of course, having said that, I don't remember if my inseam was 30
inches when I measured that way.

I answered the question as I did since the original poster only supplied his
pants inseam. What size pants do you wear?



Chris Neary


"Science, freedom, beauty, adventu what more could
you ask of life? Bicycling combined all the elements I
loved" - Adapted from a quotation by Charles Lindbergh