Crank arm length question

[David Kerber]

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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[Dan Daniel]

On Mon, 17 May 2004 10:06:31 -0400, David Kerber
<ns_dkerber@ns_ids.net> 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

 

[David Kerber]

In article <[email protected]>,
[email protected] says...
> On Mon, 17 May 2004 10:06:31 -0400, David Kerber
> <ns_dkerber@ns_ids.net> 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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[Khill]

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 here:

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

- khill

 

[Rick Onanian]

On Mon, 17 May 2004 13:50:13 -0400, David Kerber
<ns_dkerber@ns_ids.net> 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

 

[Rick Onanian]

On Mon, 17 May 2004 10:06:31 -0400, David Kerber
<ns_dkerber@ns_ids.net> 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

 

[David Kerber]

In article <[email protected]>,
[email protected] says...
> On Mon, 17 May 2004 13:50:13 -0400, David Kerber
> <ns_dkerber@ns_ids.net> 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.

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[Chris Neary]

>On Mon, 17 May 2004 10:06:31 -0400, David Kerber
><ns_dkerber@ns_ids.net> 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 [email protected]

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

 

[Rick Onanian]

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
<[email protected] > 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

 

[Chris Neary]

>>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 [email protected]

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

 

[Ryan Cousineau]

In article <[email protected]>,
Chris Neary <[email protected] > wrote:

> >>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.

Why do mountain bikes run long cranks? Torque can be
adjusted with lower gearing to make the gain ratio
<http://www.sheldonbrown.com/gain.html> come out the same,
and pedal strikes are a huge problem for mountain bikes even
on straight stretches.

I intend to put this theory to the test as soon as I find
some 165 mm cranks that fit within my budget.

--
Ryan Cousineau, [email protected]
http://www.sfu.ca/~rcousine/wiredcola/ President, Fabrizio
Mazzoleni Fan Club

 

[Rick Onanian]

I wrote:
>>Of course, having said that, I don't remember if my inseam
>>was 30 inches when I measured that way.
On Wed, 19 May 2004 23:50:51 GMT, Chris Neary
<[email protected] > wrote:
>I answered the question as I did since the original
>poster only supplied his pants inseam. What size pants do
>you wear?

Inseam 30, or 29 if I can find it, but my pants don't relate
particularly well to my actual physical inseam. It seems the
only choices in pants are to have the belt up around your
neck, or to have the crotch near your knees (and the legs
way too long) (like that silly urban style seen among
today's youths).

If I created a new line of jeans, they'd be made at least
partially out of coolmax, and the belt-to-crotch height
would be very short, allowing guys with aerobellies to be
comfortable both at the waist AND at the crotch.

Then, my inseam would probably be 30 or maybe even a little
more, without having to struggle with the damned hem...and I
wouldn't have to alternate between looking like an old fart
and looking like a young fool.
--
Rick Onanian

 

[Rick Onanian]

On Wed, 19 May 2004 18:14:06 -0700, Ryan Cousineau <[email protected]>
wrote:
>Why do mountain bikes run long cranks? Torque can be
>adjusted with lower gearing to make the gain ratio
><http://www.sheldonbrown.com/gain.html> come out the same,
>and pedal strikes are a huge problem for mountain bikes
>even on straight stretches.

While pedal strikes are a big issue, a few millimeters won't
help. Off roading includes very random quick bits of
different grades and unexpected slowdowns from terrain. Gear
choice is neither an art nor a science, and often it is a
terrible time to change gears right when you need to. As a
result, it's helpful to have whatever leverage you can
available to shove a buttload of torque into whatever gear
you happen to be in.

This, of course, has been pulled entirely from the MYASS
system: http://www.xs4all.nl/~kluter/funnies/millenia.html
--
Rick Onanian

 

[S O R N I]

Rick Onanian wrote:

> Inseam 30, or 29 if I can find it

tail.php?imagename=littlepecker.jpg&category=Toys&date=2004-
05-20

Bill "sorry, Rick -- too easy to pass up " S.