Crank arm length question



D

David Kerber

Guest
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?


--
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
<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
 
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.


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


--
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
><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
 
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
 
>>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
 
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
 
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
 
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