BB and pedal thread direction - WHY?

Quoted message said:

I've always wondered.. why are the pedal and BB thread
direction

such that

Quoted message said:

forward pedalling direction would be loosening them?

Because when nothing goes wrong the cups and pedals could
loosen in normal use. The bearings invert the direction of
the force, so when you pedle in a clockwise motion the
bearings change the direction of this force to a
counterclockwise force. Thereby it's best to have the cups
loosen in the same direction as you pedle.

"Mike Beauchamp" <[email hidden]> wrote in
message "]news:[email hidden]...

Quoted message said:

Heyo,

I've always wondered.. why are the pedal and BB thread
direction

such that

Quoted message said:

forward pedalling direction would be loosening them?

If a pedal seized somehow (or a shoelace got wrapped
around the

spindle -

Quoted message said:

happened to me) then the pedal pops off. If something goes
wrong in

the BB,

Quoted message said:

and it seizes or something, then it unthreads itself with
pedalling.

I'd assume that these threads would be opposite, so the
pedaling

direction

Quoted message said:

would keep everything tight?

Like most bike traditions, I'm hoping there's an
interesting story

involved

Quoted message said:

in this?

Or maybe it's that way, so if something bad does
happen in the

bearings,

Quoted message said:

your pedalling won't tighten them so much that you can't
get them

apart

Quoted message said:

again?

--
Mike Beauchamp therevox.comtherevox.comOpen ↗ - custom electro-
theremins and stuff. mikebeauchamp.commikebeauchamp.comOpen ↗ - mike's
personal site.

Ronald said:

Quoted message said:

I've always wondered.. why are the pedal and BB thread
direction such that forward pedalling direction would be
loosening them?

Because when nothing goes wrong the cups and pedals could
loosen in normal use. The bearings invert the direction of
the force, so when you pedle in a clockwise motion the
bearings change the direction of this force to a
counterclockwise force. Thereby it's best to have the cups
loosen in the same direction as you pedle.

Bearings don't reverse anything.

What happens is that the radius of the thing that screws in
is necessarily less than the radius of the thing it screws
into. So if it loosens slightly, one rolls around inside the
other, and since the circumferences don't match, the one
comes out slightly ahead of the other each revolution. The
threading is selected so that this rolling retightens the
arrangement. Bearings aren't even involved.
--
Ron Hardin [email hidden]

On the internet, nobody knows you're a jerk.

Quoted message said:

Bearings don't reverse anything.

What happens is that the radius of the thing that
screws in is

necessarily less

Quoted message said:

than the radius of the thing it screws into. So if
it loosens

slightly, one rolls

Quoted message said:

around inside the other, and since the circumferences
don't match,

the one comes out

Quoted message said:

slightly ahead of the other each revolution. The
threading is

selected so that

Quoted message said:

this rolling retightens the arrangement. Bearings
aren't even

involved.

After reading a bit more about it i believe you're right.
Don't remember where i picked up the bearing theory but it
sounded plausable. It was easy to memorize because two
touching wheels turn in opposite direction so spindle
touching bearing touching cup would have the cup turning in
the same direction as the spindle if it was the same shape.
But because the bearing is touching the inside of the cup
instead of the outside it would turn in the opposite
direction.

"Ron Hardin" <[email hidden]> wrote in message
"]news:[email hidden]...

Quoted message said:

Ronald said:

Quoted message said:

I've always wondered.. why are the pedal and BB thread
direction

such that

Quoted message said:

Quoted message said:

Quoted message said:

forward pedalling direction would be loosening them?

Because when nothing goes wrong the cups and pedals
could loosen

in

Quoted message said:

Quoted message said:

normal use. The bearings invert the direction of the
force, so

when

Quoted message said:

Quoted message said:

you pedle in a clockwise motion the bearings change the
direction

of

Quoted message said:

Quoted message said:

this force to a counterclockwise force. Thereby it's
best to have

the

Quoted message said:

Quoted message said:

cups loosen in the same direction as you pedle.

Bearings don't reverse anything.

What happens is that the radius of the thing that
screws in is

necessarily less

Quoted message said:

than the radius of the thing it screws into. So if
it loosens

slightly, one rolls

Quoted message said:

around inside the other, and since the circumferences
don't match,

the one comes out

Quoted message said:

slightly ahead of the other each revolution. The
threading is

selected so that

Quoted message said:

this rolling retightens the arrangement. Bearings
aren't even

involved.

Quoted message said:

--
Ron Hardin [email hidden]

On the internet, nobody knows you're a jerk.

Ronald said:

Quoted message said:

Bearings don't reverse anything.

What happens is that the radius of the thing that
screws in is

necessarily less

Quoted message said:

than the radius of the thing it screws into. So if
it loosens

slightly, one rolls

Quoted message said:

around inside the other, and since the circumferences
don't match,

the one comes out

Quoted message said:

slightly ahead of the other each revolution. The
threading is

selected so that

Quoted message said:

this rolling retightens the arrangement. Bearings
aren't even

involved.

After reading a bit more about it i believe you're right.
Don't remember where i picked up the bearing theory but it
sounded plausable.

<snip>

what, mis-information on r.b.t??? horror!!!

Ronald who? writes anonymously as he probably should with such tales:

Quoted message said:

Quoted message said:

I've always wondered.. why are the pedal and BB thread
direction such that forward pedaling direction would be
loosening them?

Quoted message said:

Because when nothing goes wrong the cups and pedals could
loosen in normal use. The bearings invert the direction of
the force, so when you pedal in a clockwise motion the
bearings change the direction of this force to a
counterclockwise force. Thereby it's best to have the cups
loosen in the same direction as you pedal.

Wow! Pleas explain how "bearings invert" rotational forces
and how this tends to unscrew these components. I suppose
you have missed all the old wive's tales that have been
posted here on this subject.

Unscrewing occurs from precession, something that can be
modeled manually by rotating the forefinger of one hand in a
circle formed by thumb and forefinger of the other hand. As
the finger rolls around the "O" in one direction, it turns
in the opposite direction. This is how rotating loads cause
even press fitted components to precess.

Pedals wouldn't need left hand treads if they had a tapered
shoulder on their shafts similar to lug nuts on cars.
Formerly, lug nuts had left hand threads on the right side
before the conical lug nut was introduced. This has nothing
to do with ball or roller bearings, or their seizing up and
the likes.

Jobst Brandt [email hidden]

Look up precession. Phil Brown

"Mike Beauchamp" <[email hidden]> wrote in message news:<[email hidden]>...

Quoted message said:

Heyo,

I've always wondered.. why are the pedal and BB thread
direction such that forward pedalling direction would be
loosening them?

If a pedal seized somehow (or a shoelace got wrapped
around the spindle - happened to me) then the pedal pops
off. If something goes wrong in the BB, and it seizes or
something, then it unthreads itself with pedalling.

I'd assume that these threads would be opposite, so the
pedaling direction would keep everything tight?

Like most bike traditions, I'm hoping there's an
interesting story involved in this?

Or maybe it's that way, so if something bad does happen in
the bearings, your pedalling won't tighten them so much
that you can't get them apart again?

The pedalling action DOES tend to tighten the pedal, but not
due to bearing drag which, as you observe, applies a force
in the wrong direction. It is beceause the pedal spindle is
supported at only one end so, as you pedal the applied force
tends to make the pedal precess around its axis, applying a
tightening force as it does do.

See sheldonbrown.comgloss p.htmlOpen ↗ as search for
pedal, or search in rbt for "preccession pedal" as this
topic comes up fairly regularly.

Andrew Webster

In article <[email hidden]>,
[email hidden] says...

Quoted message said:

Pedals wouldn't need left hand treads if they had a tapered
shoulder on their shafts similar to lug nuts on cars.
Formerly, lug nuts had left hand threads on the right side
before the conical lug nut was introduced. This has nothing
to do with ball or roller bearings, or their seizing up and
the likes.

this is such a simple fix. I wonder why it isn't done.
--------------
Alex

Andrew Webster said:

Quoted message said:

I've always wondered.. why are the pedal and BB thread
direction such that forward pedalling direction would be
loosening them?

Quoted message said:

Quoted message said:

If a pedal seized somehow (or a shoelace got wrapped
around the spindle - happened to me) then the pedal pops
off. If something goes wrong in the BB, and it seizes or
something, then it unthreads itself with pedalling.

Quoted message said:

Quoted message said:

I'd assume that these threads would be opposite, so the
pedaling direction would keep everything tight?

Quoted message said:

Quoted message said:

Like most bike traditions, I'm hoping there's an
interesting story involved in this?

Quoted message said:

Quoted message said:

Or maybe it's that way, so if something bad does happen
in the bearings, your pedalling won't tighten them so
much that you can't get them apart again?

Quoted message said:

The pedalling action DOES tend to tighten the pedal, but
not due to bearing drag which, as you observe, applies a
force in the wrong direction. It is beceause the pedal
spindle is supported at only one end so, as you pedal the
applied force tends to make the pedal precess around its
axis, applying a tightening force as it does do.

That the pedal is an overhung load is not the reason for
precession. That would occur even better if the load were
not outboard.

Quoted message said:

See sheldonbrown.comgloss p.htmlOpen ↗ as search for
pedal, or search in rbt for "preccession pedal" as this
topic comes up fairly regularly.

======================================================================

Subject: 8i.9 Left hand threads From: Jobst Brandt
<[email hidden]> Date: Wed, 28 Apr 2004
16:14:11 PST

Left hand threads are used mainly in two places on bicycles,
on the left pedal and right bottom bracket (BB) bearing cup,
and for the same reason, that is not to prevent unscrewing
if the bearing were to seize, forcing it to unscrew. The
more insidious force is that of precession, in which a
circular object rolling in a circular ring in one direction
turns in the opposite direction. This effect is what causes
such assemblies to unscrew under a rotating load.

The rotating load on a pedal arises form the downward force
of the foot on a spindle that rotates with the crank. The
predominantly downward force effectively rotates about the
pedal spindle. What may be less evident is that even tightly
fitting parts have relative clearance due to their
elasticity, metals not being rigid materials as is evident
from steel springs. Under load, micro deformations, enough
to cause motion, occur in such joints. This can be seen from
wear marks on the faces of cranks where pedal spindles seat.

Precession of right side BB cups is less obvious because the
rotating load is only partial. The largest load being chain
tension that together with the moderately large downward
force on the right crank and the smaller upward force from
pushing down on th left crank make up 3/4 of a fully
rotating load. That is why some right BB cups have used
right hand threads and why some with left hand threads
loosen. The left BB cup has no significant rotating load and
does not unscrew.

Precession forces are large enough that no manner of thread
locking glues, short of welding, will arrest them.
Mechanical fretting, the micro-motion of tightly fitting
parts moving against one another, is the mechanism of this
problem. Elastic motions in these joints cause visible
fretting rouge, red iron oxide, on the shoulder of the BB
cup on the face of the pedal spindle.

No left hand thread would be required on left pedals if the
automotive solution were used. Before the advent of conical
lug nuts, left hand threads were used on left side wheels. A
conical face solved this problem on wheels as it could on
pedal spindles for bicycles.

However, unscrewing is not the main problem for pedals, but
rather crank failure caused by fretting erosion of the pedal
eye. This wear initiates cracks that can cause sudden and
unsuspected pedal separation when the eye of the crank
breaks. This occurs equally with right and left cranks and
is the more important reason for using a conical spindle
face and crank socket. The method has been tested.

---------------------

Jobst Brandt [email hidden]

Alex Rodriguez said:

Quoted message said:

Pedals wouldn't need left hand treads if they had a
tapered shoulder on their shafts similar to lug nuts on
cars. Formerly, lug nuts had left hand threads on the
right side before the conical lug nut was introduced.
This has nothing to do with ball or roller bearings, or
their seizing up and the likes.

Quoted message said:

this is such a simple fix. I wonder why it isn't done.

From my discussions with manufacturers I get the impression
it is partly NIH (not invented here) and partly like the
disc brake problem. No one wants to be the first to admit
there is a problem. I pursued it mainly because I broke so
many cranks and was concerned for my safety. I didn't care
whether there was a left hand thread or not. My cranks still
have a left hand thread on the left but they no longer break
off every 10,000 miles or less.

Jobst Brandt [email hidden]

In article <[email hidden]>,

Quoted message said:

Alex Rodriguez said:

Quoted message said:

Pedals wouldn't need left hand treads if they had a
tapered shoulder on their shafts similar to lug nuts on
cars. Formerly, lug nuts had left hand threads on the
right side before the conical lug nut was introduced.
This has nothing to do with ball or roller bearings, or
their seizing up and the likes.

Quoted message said:

this is such a simple fix. I wonder why it isn't done.

From my discussions with manufacturers I get the
impression it is partly NIH (not invented here) and partly
like the disc brake problem. No one wants to be the first
to admit there is a problem.

And of course, nobody wants to stick their neck out and try
to foist a new standard on pedals when they have one that
has existed for what, maybe 100 years in some form now?

Pedals are one of the most standardized fittings on a bike.
There are only two common standards: 9/16" and 1/2" (the
latter is used on one-piece cranks, which means mainly cheap
bikes, kids' bikes, and BMX bikes). According to Sheldon's
pages, only two other standards have ever even been tried: a
14mm French size, now rare, and Shimano's Dyna Drive
oversized thread, obsolete but you can still find the
adapters so these cranks can take 9/16" pedals.

So manufacturers have the choice of selling pedals which
will fit almost every bicycle ever made, or selling the
public on a new standard which would be semi-compatible at
best (I can envision a stopgap using a tapered shoulder, but
that also came with a converter washer allowing the pedal to
be used on conventional cranks).

And of course, the pedal makers would have to get the crank
makers to join in.

ISIS drive, driven by what amounts to a commercial
imperative for crank and BB makers (couldn't use Shimano's
spline-drive system, but the freeriders were breaking cranks
like crazy), still had an uphill battle. It's established
now, but will probably be superseded by the trend towards
integrated two-piece BB/crank designs.

The thing about standards is there's so many to choose from,
--
Ryan Cousineau, [email hidden]
sfu.cawiredcolaOpen ↗ President, Fabrizio
Mazzoleni Fan Club

Ryan Cousineau said:

Quoted message said:

From my discussions with manufacturers I get the
impression it is partly NIH (not invented here) and
partly like the disc brake problem. No one wants to be
the first to admit there is a problem.

And of course, nobody wants to stick their neck out and
try to foist a new standard on pedals when they have
one that has existed for what, maybe 100 years in some
form now?

I agree with you, I don't see any "problem" with left-handed
threading on the left pedal. The collet/cone may allow a right-
handed threaded left pedal to be installed, and still not
unscrew, but so what? Just taking away a _specific_
requirement doesn't mean it should be replaced by some other
arbitrary style. Colleting it and simply leaving the left-
handed threads would be fine too. Moreover, if the whole
colleted crank arm thing came to be, but the threading was
left alone, after-market cones (split cones?) would allow
people to keep using the existing huge stock of pedals.

The collet is the right thing to do because the cranks can
break in the current design. On the other hand, changing the
threading is jerking around. I'm willing to pay for the
collet and two oppositely threaded spindles. Besides, it's
not PC, unless your Mark Hickey, to pick on the leftees.
What makes them any worse than the rightees? Maybe leftees
should have their day too! Really, who gives a [censored] about
the threads as long as you can screw your pedals in and they
don't come loose? I'm not bi, but I can screw either way.

"gwhite" <[email hidden]> wrote in message
"]news:[email hidden]...

Quoted message said:

Ryan Cousineau said:

Quoted message said:

From my discussions with manufacturers I get the
impression it is partly NIH (not invented here) and
partly like the disc brake problem. No one wants to be
the first to admit there is a problem.

And of course, nobody wants to stick their neck out and
try to foist a new standard on pedals when they have
one that has existed for what, maybe 100 years in some
form now?

I agree with you, I don't see any "problem" with left-
handed threading on the left pedal.

Well, I'm one for consistency.

Greg

George White said:

Quoted message said:

Quoted message said:

From my discussions with manufacturers I get the
impression it is partly NIH (not invented here) and
partly like the disc brake problem. No one wants to be
the first to admit there is a problem.

Quoted message said:

Quoted message said:

And of course, nobody wants to stick their neck out and
try to foist a new standard on pedals when they have
one that has existed for what, maybe 100 years in some
form now?

Quoted message said:

I agree with you, I don't see any "problem" with left-
handed threading on the left pedal. The collet/cone may
allow a right-handed threaded left pedal to be installed,
and still not unscrew, but so what? Just taking away a
_specific_ requirement doesn't mean it should be replaced
by some other arbitrary style. Colleting it and simply
leaving the left-handed threads would be fine too.
Moreover, if the whole colleted crank arm thing came to
be, but the threading was left alone, after-market cones
(split cones?) would allow people to keep using the
existing huge stock of pedals.

No one can offer a collet for existing pedals because the
thread relief on those spindles are undefined. The width and
diameter of that feature (the cylindrical part between
shoulder and thread) must fit snugly into a collet. THat is
why this takes a drastic departure of a new pedal standard
like that of Shimano, that I believe also was a response to
the crank failure problem but didn't address the basic
fretting problem and left hand thread.

Quoted message said:

The collet is the right thing to do because the cranks can
break in the current design. On the other hand, changing
the threading is jerking around. I'm willing to pay for
the collet and two oppositely threaded spindles.

The difference is that manufacturers would not need to have
left hand thread tooling and could make one kind of pedal
spindle that would work as well on tandems that presently
use left cranks on the right with right hand thread and the
converse. Tandem folks can speak for that problem.

Jobst Brandt [email hidden]

In article <[email hidden]>,
[email hidden] wrote: [skip]

Quoted message said:

Subject: 8i.9 Left hand threads From: Jobst Brandt
<[email hidden]> Date: Wed, 28 Apr 2004
16:14:11 PST

Left hand threads are used mainly in two places on
bicycles, on the left pedal and right bottom bracket (BB)
bearing cup, and for the same reason, that is not to
prevent unscrewing if the bearing were to seize, forcing
it to unscrew. The more insidious force is that of
precession, in which a circular object rolling in a
circular ring in one direction turns in the opposite
direction. This effect is what causes such assemblies to
unscrew under a rotating load.

[skip]

Thanks for this explanation.

Here is another one:

When the toe clips were invented, cases when the pedal
bearing suddenly locked led to terrible traumas. The threads
on the pedals were done in such a way that the pedal with
locked bearing would unscrew out from the crank, presumably
saving the clipped foot.

Sounds suspicious that a foot can turn away the pedal
(when a rider is not expecting to do it, at least); also,
I can not imagine "sudden locking of a bearing". Your
word on this?

Konstantin.

Konstantin Shemyak said:

Quoted message said:

Left hand threads are used mainly in two places on
bicycles, on the left pedal and right bottom bracket (BB)
bearing cup, and for the same reason, that is not to
prevent unscrewing if the bearing were to seize, forcing
it to unscrew. The more insidious force is that of
precession, in which a circular object rolling in a
circular ring in one direction turns in the opposite
direction. This effect is what causes such assemblies to
unscrew under a rotating load.

Quoted message said:

Thanks for this explanation.

Quoted message said:

Here is another one:

Quoted message said:

When the toe clips were invented, cases when the pedal
bearing suddenly locked led to terrible traumas. The
threads on the pedals were done in such a way that the
pedal with locked bearing would unscrew out from the
crank, presumably saving the clipped foot.

Quoted message said:

Sounds suspicious that a foot can turn away the pedal
(when a rider is not expecting to do it, at least); also,
I can not imagine "sudden locking of a bearing". Your word
on this?

This is an old wive's tale with no merit whatsoever since
the torque a foot will exert on a pedal is not enough to
begin loosening the thread while much less than that would
cause ankle injury. Besides, try to imagine what would make
a pedal thread suddenly lock?

Don't repeat such tales, even in jest or as a straw man for
discussion. Every repetition reinforces belief in
unbelievable things... see religions if you don't think so.
That is, for all others except mine.

Jobst Brandt [email hidden]

Quoted message said:

From my discussions with manufacturers I get the impression
it is partly NIH (not invented here) and partly like the
disc brake problem. No one wants to be the first to admit
there is a problem.

You could try suggesting it to Highpath Engineering, who
already make fresh holes in cranks in order to shorten them.
They might also be up to making aftermarket pedal spindles
for common pedal types...
--
David Damerell <[email hidden]>
Distortion Field!

David Damerell said:

Quoted message said:

From my discussions with manufacturers I get the
impression it is partly NIH (not invented here) and
partly like the disc brake problem. No one wants to be
the first to admit there is a problem.

Quoted message said:

You could try suggesting it to Highpath Engineering, who
already make fresh holes in cranks in order to shorten
them. They might also be up to making aftermarket pedal
spindles for common pedal types...

You might forward the essential parts of this discussion on
this wreck.bike thread to them since you seem to know
something about this company. I don't.

Jobst Brandt [email hidden]

Quoted message said:

David Damerell writes:

[conical pedal threads]

Quoted message said:

Quoted message said:

You could try suggesting it to Highpath Engineering, who
already make fresh holes in cranks in order to shorten
them. They might also be up to making aftermarket pedal
spindles for common pedal types...

You might forward the essential parts of this discussion on
this wreck.bike thread to them since you seem to know
something about this company. I don't.

I will mention it to them next time I speak to them, and
report back here if they seem at all amenable. It's a thin
chance, but it can't hurt to ask, I suppose.
--
David Damerell <[email hidden]>
Distortion Field!