Crank Creep
fd...@ix.netcom.com
cranks. It was said that the crank bolt becomes loose because the crank
"creeps" up the spindle a bit.
Forgive any engineering ignorance that I may betray, but...
Does that mean that the crank actually becomes tighter on the spindle?
How can any harm befall the crank if it is retightened, no matter how often,
as long as the torque spec is never exceeded?
Frank DiDomenico
Selkirk Ny
jobst....@stanfordalumni.org
> I have read with interest advice against retightening properly
> installed cranks. It was said that the crank bolt becomes loose
> because the crank "creeps" up the spindle a bit. Does that mean
> that the crank actually becomes tighter on the spindle?
If you read the piece on that subject, then you saw the answer. Yes
it gets tighter and if you persist in re-tightening the bolt, you'll
split the crank as in the seventh picture at:
http://pardo.net/pardo/bike/pic/fail/FAIL-001.html
> How can any harm befall the crank if it is re-tightened, no matter
> how often, as long as the torque spec is never exceeded?
Re-read the item at:
http://draco.acs.uci.edu/rbfaq/FAQ/125.html
> Selkirk Ny
Say what?
Jobst Brandt <jobst....@stanfordalumni.org>
fd...@ix.netcom.com
jobst....@stanfordalumni.org at jobst....@stanfordalumni.org wrote on
10/14/01 5:40 PM:
> Frank DiDomenico wrote:
>
>> I have read with interest advice against retightening properly
>> installed cranks. It was said that the crank bolt becomes loose
>> because the crank "creeps" up the spindle a bit. Does that mean
>> that the crank actually becomes tighter on the spindle?
>
Jobst Brandt replied:
> If you read the piece on that subject, then you saw the answer. Yes
> it gets tighter and if you persist in re-tightening the bolt, you'll
> split the crank as in the seventh picture at:
>
> http://pardo.net/pardo/bike/pic/fail/FAIL-001.html
>
>> How can any harm befall the crank if it is re-tightened, no matter
>> how often, as long as the torque spec is never exceeded?
>
> Re-read the item at:
>
> http://draco.acs.uci.edu/rbfaq/FAQ/125.html
>
>> Selkirk Ny
>
> Say what?
>
> Jobst Brandt <jobst....@stanfordalumni.org>
>
Thank you. Here is what I don't get:
If the crank creeping up the spindle results in it becoming tighter than it
was after proper initial installation and the crank bolt is retightened to
the proper torque, then the "retightening" of the bolt will not really
result in the crank being pushed any farther up the spindle, because it's
already on tighter than the torque applied during initial installation and
subsequently reapplied during "retightening." Is this wrong headed?
Not trying to be contrary here, just trying to understand.
and, oh Selkirk Ny is Selkirk NY -- my home.
I looked at the crank failure photos. I might quit riding!
Mark Hickey
>Thank you. Here is what I don't get:
>
>If the crank creeping up the spindle results in it becoming tighter than it
>was after proper initial installation and the crank bolt is retightened to
>the proper torque, then the "retightening" of the bolt will not really
>result in the crank being pushed any farther up the spindle, because it's
>already on tighter than the torque applied during initial installation and
>subsequently reapplied during "retightening." Is this wrong headed?
Yep. ;-)
Think of it this way.
When you first tighten down a crank, it comes to an equilibrium, with
equal force from the bolt trying to push the crank up the spindle, and
from the spindle, forcing the crank into the bolt (not proper physics,
but you get the picture).
When you ride, the crank will "wobble" ever so slightly since it's no
longer loaded equally on all four faces of the BB spindle. When it
does this, the crank can't move toward the bolt, but it CAN move
slightly up the spindle's taper. So it does.
Soon it reaches a "new equilibrium" (the one the crank is really
designed to work at), where the forces each direction are once again
equal, but slightly lower.
But if you re-toque the crank bolt, you start the process all over
again, and the crank once again "squirms up the taper" until
equilibrium is once again restored. Until you tighten the bolt
again... ad infinitum. Busted crank.
Mark Hickey
Habanero Cycles
http://www.habcycles.com
Home of the $695 ti frame
jobst....@stanfordalumni.org
>> If you read the piece on that subject, then you saw the answer.
>> Yes it gets tighter and if you persist in re-tightening the bolt,
>> you'll split the crank as in the seventh picture at:
>> http://pardo.net/pardo/bike/pic/fail/FAIL-001.html
>>> How can any harm befall the crank if it is re-tightened, no matter
>>> how often, as long as the torque spec is never exceeded?
>> Re-read the item at:
>> http://draco.acs.uci.edu/rbfaq/FAQ/125.html
> Thank you. Here is what I don't get:
> If the crank creeping up the spindle results in it becoming tighter
> than it was after proper initial installation and the crank bolt is
> retightened to the proper torque, then the "retightening" of the
> bolt will not really result in the crank being pushed any farther up
> the spindle, because it's already on tighter than the torque applied
> during initial installation and subsequently reapplied during
> "retightening."
As is explained in the FAQ item, the face the crank distorts under
pedaling loads against the retaining bolt with high and low areas that
represent pressure and tension relative to its press fit on the square
taper. With preload and torque applied, the face of the crank
generates a wavy surface, some of which is higher than at rest and
part that is lower (net zero displacement). The high part exerts a
force strong enough to "elbow" the crank away from the retaining bolt
ever so slightly. It is this displacement that is detectable as
"looseness" that can be taken up. When re-tightening, once up against
the crank you cannot easily press the crank any further. However, it
will mover on its own again under torque. If you repeat this exercise
the crank will split as shown in the picture. That's how it was done
for that crank in the picture.
> I looked at the crank failure photos. I might quit riding!
I broke dozens of them but learned to inspect for cracks.
Jobst Brandt <jobst....@stanfordalumni.org>
jobst....@stanfordalumni.org
>> If you read the piece on that subject, then you saw the answer.
>> Yes it gets tighter and if you persist in re-tightening the bolt,
>> you'll split the crank as in the seventh picture at:
>> http://pardo.net/pardo/bike/pic/fail/FAIL-001.html
>>> How can any harm befall the crank if it is re-tightened, no matter
>>> how often, as long as the torque spec is never exceeded?
>> Re-read the item at:
>> http://draco.acs.uci.edu/rbfaq/FAQ/125.html
> Thank you. Here is what I don't get:
> If the crank creeping up the spindle results in it becoming tighter
> than it was after proper initial installation and the crank bolt is
> retightened to the proper torque, then the "retightening" of the
> bolt will not really result in the crank being pushed any farther up
> the spindle, because it's already on tighter than the torque applied
> during initial installation and subsequently reapplied during
> "retightening."
As is explained in the FAQ item, the face the crank distorts under
pedaling loads against the retaining bolt with high and low areas that
represent pressure and tension relative to its press fit on the square
taper. With preload and torque applied, the face of the crank
generates a wavy surface, some of which is higher than at rest and
part that is lower (net zero displacement). The high part exerts a
force strong enough to "elbow" the crank away from the retaining bolt
ever so slightly. It is this displacement that is detectable as
"looseness" that can be taken up. When re-tightening, once up against
the crank you cannot easily press the crank any further. However, it
will move on its own again under torque. If you repeat this exercise
the crank will split as shown in the picture. That's how it was done
for that crank in the picture.
> I looked at the crank failure photos. I might quit riding!
I broke dozens of them but learned to inspect for cracks.
Jobst Brandt <jobst....@stanfordalumni.org>
Kiira Triea
: I have read with interest advice against retightening properly installed
: cranks. It was said that the crank bolt becomes loose because the crank
: "creeps" up the spindle a bit.
Listen Frank... if your crank arm gets loose on the spindle then get
out your thin wall 15mmm socket and tight that sucker up. If you don't
then the arm will start wobbling around on the spindle and you will
need to buy another crank.
+------
| Forgive any engineering ignorance that I may betray, but...
|
| Does that mean that the crank actually becomes tighter on the spindle?
+------
I can't take seriously any explanation that believes that things "get
tighter' when they reach equilibrium.
+------
| How can any harm befall the crank if it is retightened, no matter how often,
| as long as the torque spec is never exceeded?
+------
None. I still have the NR crank that I used during the 1972 in Europe
season on my Poglihagi - I used it on a messenger bike for three years
and whenever it loosened I tightened it up. Ican't even count the hard
miles it has on it. Don't put grease on the spindle.
Kiira
:
R. Himm
<tr...@eclipse.qis.net> wrote:
> Listen Frank... if your crank arm gets loose on the spindle then get
> out your thin wall 15mmm socket and tight that sucker up. If you don't
> then the arm will start wobbling around on the spindle and you will
> need to buy another crank.
You ought to be a little clearer. If the arm is indeed loose on the
spindle, what does that mean other than that the arm can wobble around on
the spindle, i.e. exhibit noticeable play with the slightest pressure? On
the other hand the explanation in the FAQ does not refer to the arm being
"loose" on the spindle, it refers to the bolt no longer exerting the same
pressure on the arm, as a result of some quasi-infinitesimal displacement,
i.e. there is now room to tighten the bolt more. However the arm is still
tight on the spindle, i.e. it does not wobble.
> I can't take seriously any explanation that believes that things "get
> tighter' when they reach equilibrium.
Of course not. A future course of action cannot happen in the present. The
idea is, they "get tighter" (more exactly, move up the spindle) after the
original equilibrium was reached, because of the application of new forces
that occurred during pedalling which were not present when the bolt was
first being tightened in the workstand.
> and whenever it loosened I tightened it up.
How often, or more to the point, how many times, was this? And how did you
know it was loose, if you only tightened it when that happened, and not at
some predetermined check point, like the first 100 miles or whatever?
Mark Hickey
>+------
>| How can any harm befall the crank if it is retightened, no matter how often,
>| as long as the torque spec is never exceeded?
>+------
>
>None. I still have the NR crank that I used during the 1972 in Europe
>season on my Poglihagi - I used it on a messenger bike for three years
>and whenever it loosened I tightened it up. Ican't even count the hard
>miles it has on it. Don't put grease on the spindle.
It is quite easy to split a crank with repeated tightenings, for the
reasons brought up previously in this thread. The fact that you
haven't managed to do it yet doesn't mean that your advice is valid.
A properly torqued crank bolt takes the crank to the proper
equilibrium the first time, and should never need any adjustment.
terry morse
>
> I still have the NR crank that I used during the 1972 in Europe
> season on my Poglihagi - I used it on a messenger bike for three years
> and whenever it loosened I tightened it up. Ican't even count the hard
> miles it has on it. Don't put grease on the spindle.
Let's see if we can use the facts to draw a conclusion...
1. No grease on the spindle.
2. Crank keeps coming loose, requiring re-tightening.
Cause and effect.
--
terry morse
Palo Alto, CA
http://www.terrymorse.com/bike/
Gary Jacobson
affect this problem.
I would think that they prevent creep from happening.
I ask because I have a jumpy feeling every pedal rotation, and began to
think it was a loose crank. Probably something else.
<jobst....@stanfordalumni.org> wrote in message
news:UCny7.5768$no1....@typhoon.sonic.net...
Trevor Jeffrey
>I have read with interest advice against retightening properly installed
>cranks. It was said that the crank bolt becomes loose because the crank
>"creeps" up the spindle a bit.
>
>Forgive any engineering ignorance that I may betray, but...
>
>Does that mean that the crank actually becomes tighter on the spindle?
>
Bolt tension lessens. Yes the crank/spindle interface may tighten.
>How can any harm befall the crank if it is retightened, no matter how often,
>as long as the torque spec is never exceeded?
Depends on spindle. Crank may settle at shoulder on spindle. Weight pared
cranks may split.
--
Trevor M Jeffrey
Enlightenment only occurs if one is willing.
Eat your greens before email.
------