Cotter pins: Why? Why not?

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Brucey
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Re: Cotter pins: Why? Why not?

Postby Brucey » 15 Jun 2020, 10:30pm

once a crank has been run with the cotter pin slightly loose, the BB spindle bore in the crank wears (by fretting) as well as the cotter pin itself. If this wear gets bad enough (which is 'not very' IME) it doesn't matter how well installed the next cotter pin is, that too will work loose as well, because the crank bore wears to something approximating to an hourglass shape, length wise.

They need to be facing the same way vs rotation (else the cranks won't be in line with one another); I'm not sure how cottered cranks could be made so the pins could only ever be installed one way only, they seem to be made symmetric to me. 'Nuts up with pedal forwards' (same as 'pin leading' (PL)) is the usually accepted way, but even on a standard bicycle the loading in the cottered joints is not uniform, leave alone a tandem crossover drive. When torque is transmitted across a cottered joint it either compresses the flat part of the joint near the head end (HE) of the cotter pin or near the nut end (NE).

So with a PL installation on a standard bike, pressure on the left pedal produces HE loading on the left cotter but NE loading on the right cotter. When the right pedal is loaded neither cotter sees a (driving) torque-induced load. Because the pedal load is cantilevered, both cottered joints see a twisting load around an axis orthogonal to both the BB spindle and the cotter pin when the respective pedal is loaded. In addition the chain tension produces an alternate (and opposing) thrust load on the RH joint.

By far the most common fault is the LH cottered joint working loose, and (IMHO) this is nothing much to do with the orientation of the cotter pin, and everything to do with the fact that the BB spindle bore through the LH crank is a lot shorter than the BB spindle bore through the RH crank, so the joint is more susceptible to loosening, initially via the twisting load mentioned above.

A good portion of my cycling youth was wasted filing cotter pins in vain attempts to make worn (mostly LH) cranks work for more than five minutes. I tried shims in the joint (partially successful) and countless other things but nothing beats a nice new LH crank that is a snug fit on the BB spindle.

FWIW once you start filing cotter pins to fit, it is best to file them both at the same time, keeping careful track of the angle on each pin. If the angles are not kept exactly the same then the cranks won't be in line. Occasionally you will encounter a BB spindle which has been manufactured with flats that are not parallel to one another; in this case you will have to file the cotter pins so that they are not quite the same if you want the cranks to be in line.

Cotter pins; good thing or bad thing? Pfft… just a thing really. IMHO the single thing that would most improve them would be a special punch/pin combination whereby you can screw the punch over the threaded end of the pin so that the hammer blow is borne by a shoulder of suitable size on the cotter pin itself.

Modern cotter pins are of noticeably poorer quality than those found in older bikes (from the 1950s and earlier). Modern ones are pretty soft and have an M6 thread on them. Older ones are made of harder steel and have a 1/4" thread on them. It doesn't sound like it ought to make much difference but IME you are about twice as likely to wreck a modern pin getting it out.

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

colin54
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Re: Cotter pins: Why? Why not?

Postby colin54 » 15 Jun 2020, 11:00pm

Last month's cotter pin thread..
viewtopic.php?f=5&t=136562

MarcusT
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Joined: 31 Jan 2017, 10:33am

Re: Cotter pins: Why? Why not?

Postby MarcusT » 16 Jun 2020, 5:07am

I detest them, even back when they were the only game in town.
I wish it were as simple as riding a bike

iandusud
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Joined: 26 Mar 2018, 1:35pm

Re: Cotter pins: Why? Why not?

Postby iandusud » 16 Jun 2020, 7:52am

As I remember I used to stock two sizes of cotter pin: UK and Continental. Also, certainly for the UK ones, in a range of millings of the faces.

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531colin
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Re: Cotter pins: Why? Why not?

Postby 531colin » 16 Jun 2020, 5:41pm

Brucey wrote:………. I'm not sure how cottered cranks could be made so the pins could only ever be installed one way only, they seem to be made symmetric to me. ……..


Its in my mind that the cotter pin hole in the crank was tapered so that you could only put the pin in from the correct side....but I'm wondering now if it was all a dream? I dug out these old Cyclo self-extracting cotter pins (hands up all those who thought self-extracting was a new thing :mrgreen: )

Image
IMG_5141 by 531colin, on Flickr

….and the Cyclo cotter pins aren't tapered at all, apart from the flat, obviously.
However, I also found one cotter pin where I have been filing the circumference of the pin at the thread end, and I haven't filed the flat at all, the milling marks are still there.
So I don't know....was it all a dream?

Brucey
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Re: Cotter pins: Why? Why not?

Postby Brucey » 16 Jun 2020, 5:56pm

well I have never seen a self-extracting cotter pin before! That looks a like a good idea, apart from the possible trouser-cuff snagging issues....? I guess BITD many hubs would have had 3/8" x 26TPI rear axle threads so a rear tracknut could be used as an extractor?

I can't claim to have seen everything cotter-pin related by any means but all the cranks I have seen appear to have been made with a plain drilled hole for the cotter pin. I can see how the face that normally has a nut bearing on it could end up with a burr on it after a while though (e.g. from the final hammer blow that releases a cotter pin); if that is bad enough the hole won't accept a cotter pin from both ends any more; might that explain it?

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

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531colin
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Re: Cotter pins: Why? Why not?

Postby 531colin » 16 Jun 2020, 6:35pm

The burred hole on the nut end was the only explanation I could come up with, apart from senility.
The cyclo self-extracting cotter pins were supplied with nuts and washers both ends, and that's how I used them. A bit like self extracting cotterless sets, they weren't an un-qualified success, but if you backed off the retaining nut, tightened up the removing nut, and gave it a crack with a hammer, they usually came out in reasonable condition.

Brucey
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Re: Cotter pins: Why? Why not?

Postby Brucey » 16 Jun 2020, 9:01pm

re the filing on the side of the cotter pin; it occurs to me that if the crank has been moving around it can cast a burr on the knife-edge where the BB spindle bore and the cotter pin drilling intersect. This burr can already protrude into the cotter pin bore and/or a small burr can get pushed into the cotter pin drilling as the BB spindle is removed and then folded further into the cotter pin drilling when a new cotter pin is inserted, making a cotter pin a tight fit. Without a full assortment of files, reamers etc, on hand I'd be tempted to use the flat file I had in my hand anyway to dress the cotter pin until it was an easier fit. I've also had to -ahem- dress the sides of cotter pins where I've gripped them too tightly in a vice with textured jaws.. :oops: ..

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

mattsccm
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Re: Cotter pins: Why? Why not?

Postby mattsccm » 17 Jun 2020, 6:40am

As many a child from the 70's (when beaten up old roadsters were the bike of choice for bashing round the woods) the best way to fit cotter pins is beat them very very hard. If they still move beat a nail in next to them. Stuff engineering principles, force and ignorance works much more cheaply and effectively.

mattsccm
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Re: Cotter pins: Why? Why not?

Postby mattsccm » 17 Jun 2020, 6:42am

If selling a bike engineered as above to someone who wants it for the same purpose, try to use a bit of lead in the construction. It feels less rough and shuts things up. By the time it has worn lose its none of your business.

jb
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Re: Cotter pins: Why? Why not?

Postby jb » 17 Jun 2020, 1:27pm

Making a tapered or stepped hole for the cotter pin would add unnecessary expense for no good reason. As it is you just need to drill a hole through the crank for the cotter then drill the hole for the axle and that's it, neither needs to be especially tight tolerance as the wedge action of the cotter will drive both items to the back of their respective holes.
Cheap and cheerful. I'm only surprised that the cotters were never hardened as they didn't need to take any tension to speak of and it would have saved a lot of mushroomed threads.
Cheers
J Bro

Carlton green
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Re: Cotter pins: Why? Why not?

Postby Carlton green » 17 Jun 2020, 9:30pm

My thanks to the OP for starting this thread, it is one that I have found to be interesting,

It’s many years since I last used cotter pins but whilst old tech they still have a place for use at an appropriate time. For sure the idea might not be the best way of doing things but cotter pins served our fathers and grandfathers generations well so they work.

I get that there’s a right and wrong direction to install pins and that they must be installed to give the cranks 180 degrees of opposition. I now also get that for best wear and loading the wide part of the cotter pin’s flat should selectively act onto the axle - one side/edge of each flat on the shaft is loaded by torque and the other is not. The left hand side crank puts torque into the axle and the right hand side takes torque from the axle and puts it into the chainwheel attached to the right hand side crank. How one puts that descriptively into leading and trailing positions for the cotter pin head and nut I do not know (and would like to).

I have always wondered how best to file cotter pins such that the pair are filed with the same angle on their tapers (flats), are perfectly flat and have sufficient metal removed to properly engage with the axle and crank arm. What are the better methods?

jb
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Re: Cotter pins: Why? Why not?

Postby jb » 17 Jun 2020, 10:10pm

To file a small object flat is not easy, it's far better to clamp the file in a vice and then draw the cotter across it pressing down firmly . The file needs to be fine and sharp, filings need to cleared away regularly.
Some times it helps if you put a slight hollow in the middle with a half round file, this stops rocking and causing a rounded face. Finish off by strapping emmery strip around the file and continue drawing across it.
There are ways to check flatness but this requires a known flat surface and anyway the flat on the axle isn't guaranteed to be flat if it's seen a lot of use.
Cheers

J Bro

Brucey
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Joined: 4 Jan 2012, 6:25pm

Re: Cotter pins: Why? Why not?

Postby Brucey » 17 Jun 2020, 10:17pm

Carlton green wrote:
I get that there’s a right and wrong direction to install pins and that they must be installed to give the cranks 180 degrees of opposition. I now also get that for best wear and loading the wide part of the cotter pin’s flat should selectively act onto the axle - one side/edge of each flat on the shaft is loaded by torque and the other is not. The left hand side crank puts torque into the axle and the right hand side takes torque from the axle and puts it into the chainwheel attached to the right hand side crank. How one puts that descriptively into leading and trailing positions for the cotter pin head and nut I do not know (and would like to).


I've explained that upthread. With the cotter pins in any orientation that keeps the cranks in line, the loading on the cotter pins is not consistent.

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

Carlton green
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Re: Cotter pins: Why? Why not?

Postby Carlton green » 17 Jun 2020, 11:02pm

Brucey wrote:
Carlton green wrote:
I get that there’s a right and wrong direction to install pins and that they must be installed to give the cranks 180 degrees of opposition. I now also get that for best wear and loading the wide part of the cotter pin’s flat should selectively act onto the axle - one side/edge of each flat on the shaft is loaded by torque and the other is not. The left hand side crank puts torque into the axle and the right hand side takes torque from the axle and puts it into the chainwheel attached to the right hand side crank. How one puts that descriptively into leading and trailing positions for the cotter pin head and nut I do not know (and would like to).


I've explained that upthread. With the cotter pins in any orientation that keeps the cranks in line, the loading on the cotter pins is not consistent.

cheers


Yep, to keep the arms at 180 degrees to each other the cotter pins have to be installed such that both the heads do not face the same way and in doing so one or other crank will have the pin loaded on the smaller part (width) of its taper. Well, if I’ve got my head around it correctly then that will be the case. If that’s the case then I’ve missed why there’s a preferred direction for cotters to be installed.

Any tips on filing cotters?