episodic

Hi, I've determined I need to replace the left crank arm on my son's bike. Even after tightening to correct tightness, it keeps coming loose - so rounded off.

On his crank arm it is stamped LASCO. There are some numbers stamped near pedal area - and the last three are 165 - would that indicate the crankarm length?

Thanks!

Retro Grouch 

Sounds about right. To double check you can measure in millimeters from the center of the BB to the center of the pedal hole. Another thing to check on is the orientation of the square hole relative to the crank arm. Some are squares and some are diamonds.

sreten

Hi,

A couple things you can try :
(I had a very toasted * left crank) :

1) The tapers will show lifts where the BB has dug in,
preventing the square taper ever seating flat again,
file them back to flat, that should work I think.

(Anyone who is remotely au fait with using a file properly will
easily remove the ridges before remotely affecting the taper.)

2) As well as the above and oiling /greasing parts
add some Loctite (I used 2 part epoxy) into the mix.
Get it also onto the the locking nut textured ridge.

My very toasted * left crank (I let it get very loose because
I thought it was poor pedalling and hadn't ridden for many
years) is now fine, I used 1) and 2), I didn't try 1) only.

rgds, sreten.

Its very highly likely 165 on a crank means 165mm length.

* Simply wouldn't stay tight at all, loose after a few miles.
(Precession undoes any left crank nut/bolt if the joint is
at all loose, it is not reverse threaded like the left pedal.)
2 months of loose abuse. It has been fine for the last
4 months, and I completely expect it to stay that way.

onespeedbiker

This is most present in bottom brackets and pedals, but has no affect on a crank bolt as there is no secondary round part that rotates; the square taper joint is held together with plastic deformation with no secondary movement to unscrew the bolt.

To the OP, as said earlier, yes the ctank arm is a 165mm. also square and diamond end left cranks (both are square it's just the orientation of the square on the crank arm; replacements are very common and run about $10. Save your son and yourself some grief and just order a replacement crank.

dddd

I've had good luck retaining distorted crankarms by using Loctite on the bolts or nuts (spindles with male-threaded ends).

Don't use too strong a grade of (or too much) Loctite on bolts which thread into the bb spindle, or they may not be removeable.

Homebrew01

I had a left crank arm that kept coming loose due to a crack in the arm. Might be the same problem.

sreten

Hi,

Simply put you are wrong.

What do you suggest actually unscrews the bolt / nut ?

rgds, sreten.

The nut / bolt can rotate on its spindle / in its thread.
The precession of a loose left crank undoes it, unlike the right.

dbg

I had a similar issue with a neighbor's bike. I tried a new crank arm and it came loose again. Had to change the bottom bracket to get it to stop coming loose.

cny-bikeman

Onespeed is correct. What "actually unscrews the bolt" is that the crank arm is allowed to move/flex under stress because the bolt is not tight enough. Any bolt that is subject to vibration or rhythmic stress and that does not have enough torque to resist movement will loosen eventually, unless affected by precession in the opposite direction. The reason left cranks loosen more in my opinion is due to any of several factors. The left arm is reinforced typically a bit less than the right; many people mount/dismount every time by tromping down on the left pedal and then eventually swinging their leg over, which eventually puts more stress on that pedal; the left pedal more often strikes the pavement when one is not careful and rides pedal down around a turn; and the right crank arm has somewhat different stress on it, as force on the pedal is transmitted to the chainwheel more than to the spindle. .

As for other causes (crack in crank arm or spindle problem) they are rare. Almost every time it is a worn crank spindle mounting hole.

Finally, "make-do" repairs do not always work, and take much more time and trouble than merely replacing a crank arm. If the repair fails then one may be stuck away from home and then will have to replace the crank anyway. If you are really so tight on budget that buying a crank arm is too much then go ahead and give it a shot.

sreten

Hi,

The simple reason I did not simply replace my toasted
left crank is a) why not have a shot at repairing it, and
b) no matching crank to the the drive side was available.

YMMV but I like fixing stuff rather than replacing.
More grief, but you end up learning a lot more.

rgds, sreten.

cny-bikeman

Fine, but this has nothing to do with what one likes to do. I like doing repairs too but prefer solutions that are reliable when advising others.

onespeedbiker

As I said before mechanical precession is the result of two circular rotating parts rotating in opposite directions; this usually involves roller or ball bearings creating a force that is opposite the direction that it's turning. In the case of a left crank bolt, first your are mistaken that the "The nut / bolt can rotate on its spindle / in its thread", the crank arm and axle bolted or nutted together create a solid piece that have no rotation relative to each other. Even if the crank and spindle were affected, the mechanical precession causes torque in the reverse of the rotation just as the BB, which is subjected to mechanical precession, the reverse threading would be on the drive side, not the left; in other words, even if the crank and spindle were affected by mechanical precession, the left side crank bolt/ nut threading is the correct direction to counter mechanical precession.

dddd

The left arm's connection to the spindle is stressed differently from the right arm's connection.

On the left side, the maximum spindle torque occurs when the left arm is delivering it.

On the right side, the max spindle torque occurs when the right arm is 180 degrees away from the position where the right arm delivers it's highest torque (directly to the chainring).

An analysis of how the twisting (and bending outward from pedal offset) conspires to loosen a crankarm bolt is complicated, but intuitively there is some loosening torque being periodically to the bolt which is not quite the same in both rotating directions, and not quite the same between the left and right crankarm bolts.

sreten

Hi,

Your just being pompous, and are still wrong.

Don't try to tell me what precession is and isn't.

You still haven't described why a toasted left crank won't stay
tight and what mechanism undoes it according to your viewpoint.

I'm happy having a very good idea what is actually going on.

And don't like dealing with clueless contradiction that is wrong.

rgds, sreten.

The left pedal is reverse threaded due to precession and
anyone sensible can work out the rest if it from that fact ....

onespeedbiker

Wow

cny-bikeman

Ironic the post should start out talking about someone else being pompous. https://www.bikeforums.net/misc.php?do=showrules

gyozadude

I'm a bit surprised precession got into the discussion. The crank arm attachment using a tapered spindle isn't supposed to have clearance or any rotation relative to the spindle. Precession only comes into play when you have bearings and clearance between the two axis of rotating parts that the bearings separate. These conditions do NOT apply to the crank arm attachment.

The reason the left crank arm bolt loosens is because the crank is loose on the spindle and rocks back and forth. The taper encourages the crank to continue to slide outward. Downward pedaling force adds lateral outward pressure as well on the loose crank so it contacts the nut/bolt face more so the torque on the nut/bolt face tends to loosen on the left side, thus allowing greater rocking back and forth, and the bolt eventually loosens. On the right side, outward force on the face of the nut/bolt generates a torque on the face that favours rotation that tightens the bolt. So it tends not to loosen. But this isn't precession in terms of a counter-rotation due to bearing clearance.

As for Lasco crank arms, Huskybicycles.com used to carry Lasco cranksets. Don't recall them carrying 165mm cranks. Only 170 and 175mm. You might call them to see if they have any. Price for left side arm was just $8.90 plus shipping.

sreten

Hi,

I have every faith the repair I did to my toasted left
crank is more reliable than the state it turned up new.

rgds, sreten.

Its not like this is a safety issue, I toasted my crank
by riding it around loose for a month or so, being
stupid thinking it was pedaling technique.

I've fixed it. It is fine and I fully expect it to stay that
way. There is nothing IMO unreliable about the repair.

sreten

Hi,

Or the left pedal, but that is reverse threaded because of precession.

Why can't you people work out the precession applicable to bike parts
has bugger all to do with a tedious definition of precession or the way
a spinning coin finally ends up rolling around its rim due to precession.

rgds, sreten.

The precession for bike parts is the hand of the rotating torque.
It will tend to tighten or loosen any applicable bolt or nut.
It tightens both pedals and is in the opposite direction
(as are pedal threads) to the torque of a very stiff pedal.
A very stiff pedal will undo, not tighten, but if you build
it so a stiff pedal tightens, a normal pedal won't stay tight.

cny-bikeman

When giving advice as to the best course of action one has to assume the person being helped has only average mechanical skills at best. Therefore the more reliable course for that person is the more standard repair procedure. My comment had nothing to do with your abilities. You may want to learn to take things less personally.

gyozadude

Sreten:

The left pedal has two sets of bearings precessing on the main pedal spindle. This causes the left pedal to experience a moment that is counter clockwise when viewed from the non-drive side.

Your use of the pedal and spindle having precession cannot compare because the pedal is rotating relative to the shaft in question and thus exerting a moment on it due to precession.

Precession of the bearings in the BB do NOT exert moment force on the crank arm nut. The BB spindle is fixed relative to crank arm and the two rotate together. Precession may occur between bearings at the BB cups. But not at the contact point where crank arm is fixed to a tapered JIS spindle end. What exerts the moment from spinning? Certainly, there is no moment due to precession exerted (if properly attached) on the 14mm head bolt that keeps the crank arm attached to the spindle. What piece is rotating RELATIVE to the other?

sreten

Hi,

I just outlined what you can do to fix it, and don't regard what I said as unreliable.

rgds, sreten.

sreten

Hi,

From my perspective it makes no difference. The pedals apply a rotating torque
to the cranks at their juncture, and the cranks apply a rotating torque to the
BB and chainwheels at their juncture, its the same - think about it.

The pedal spindles don't move on the cranks, the cranks don't move on the BB.

The precession is the torque fed through both non-moving (usually)
parts. It rotates at both (usually) non-moving junctions.

Pedals are different threaded because the rotating torque goes
with the pedal (spindle) tightening direction. Loose cranks will
generally tend to tighten the right and loosen the left.

So left crank problems are far more common.

rgds, sreten.

onespeedbiker

I'll try one more time, but I don't think you like me very much. The juncture of the pedal to the crank is a rotating juncture, the pedal rotating around the spindle attached to the crank arm. The bottom bracket is another rotating juncture where the BB cups are attached to the bike frame through the shell with the axle rotates inside the cups. In both these cases there is a counter rotation caused by the bearings in between the 1) pedal and spindle 2) in between the cups and axle. When you look at the juncture of the axle and crank arm, there is no rotation and there are no bearings. Below is a well known video (if I do this right) showing procession (imagine bearings in between the stationery red circle red and moving blue circle) is not a transferred power, but a specific phenomena resulted from rotation and counter rotation. There is certainly something that tends to loosen the left crank bolt, some explanations have been offered above; it's just not mechanical precession. I am not sanctimonious and I make a lot of mistakes, but this one I'm pretty sure about...


BTW, have you ever noticed that the reverse thread is on the right with BB cups and left with pedals. The reason is not because the precession follows the direction of the rotation of the pedals, because precession is always opposite the rotation, it's because the spindles turn in opposite directions. On the left side the BB axle turns counter clockwise, so precession turns the bearings clockwise in the cup, so the cup has standard clockwise threads to follow clockwise precession. With the pedal, the action of keeping the pedals level while pedaling clockwise, causes the axle/ spindle (attached to the crank) to turn clockwise, so to the follow the precession (always opposite the rotation) the threads are reverse, or counter clockwise.

sreten

Hi,

IMO you are applying the wrong 2D analysis to the real 3D problem.


I am talking about the effective rotation of the highest pressure
contact point between a loose pedal and a the nut / bolt face
tying to hold it in and the effect of that precession.

That animation shows if you were patient, and the thread has surface
friction but is not tight just my applying a rotating torque to say a pedal
spindle you could slowly screw it in, no pedal or pedal bearings are involved.

Say you deliberately loosen a pedal (with good bearings).

Red is the crank thread, blue is the slightly loose pedal thread,
green is the rotating torque from pedaling. With no friction from the
pedal bearing, (if they where tight the pedal would simply unscrew *),
the pedal will slowly tighten itself from pedalling, you don't seem to
get that that is the precession. The precession in the pedal has next
to no effect on the reality of what is going on, it cannot apply torque.

Same thing applies to BBs, the precession occurs between the threads.

rgds, sreten.

* A cause of much confusion to some, surely that must be wrong ....
(They assume a stiff pedal should tighten the pedal, it doesn't .... )