What causes cotterred cranks to wobble?

Hello, I have a spare uni that has cotterred cranks.

I have the cranks on the hub, and the cotter pins and nuts tightened to what seems ‘good’

However, if I put the wheel on the ground, and reef up and down on the cranks, they change positions (no longer 12 and 6, or 3 and 9, more like 11 through 1 to 5 through 7)

The cranks don’t go beyond ‘too crazy’ but are able to change enough to be not good for riding.

How do you get a cottered set of cranks to ‘stay put’ ?

the problem with cotter cranks is that they dont actualy “fit” on the axle it seems that they always have about 0.5 mm gap,
and the cotter pin does its half assed job of squishing itself to make them snug.

i suppose you could cover them in locktite or something to make them a tight fit on the axle. perhaps even a bit of colke can would make them a tight fit.

or you could try the traditional way, just weld the little bastards onto the axle.

or carry a little hammer and a spanner in your pocket everywhere you go…

p.s. you are whacking them on with a hammer aren’t you?

According to a bike shop mechanic from the late 70’s, the trick is to have two hammers of equal weight, one to hit the cotter in, one on the bottom to absorb the blow. and then just small the cotter in as much as you can. only THEN do you tighten the nut to keep the pin in.

Here’s what I did:

I pounded the crank on until it was flush with the spindle.

Then I pounded the cotter pin in until I could fit a nut on it to tighten. Apparently this was not the best method.

Cotters are such a pain.

Sheldon Brown has some good info on how to install and remove cotter pins.
Cottered Cranks
Tool Tips for Cottered Cranks

Thanks for the link John,

Cotters are indeed a pain, but as it is a unicycle that was donated to our club, I am trying to make due. Although at our last meet, the newbies couldn’t even ride it as it was fine at the beginning of the meet but within 10 mins was messed up.

i have cottered cranx on my UW.make sure you have the right sized pin.there are about 4 differant sizes and use a cotter pin press if you can.Var makes one.

thats what i would do if you dont intend on bending cranks any time soon

You don’t need to use a press on a unicycle. If you hammer a cotter in on a bike then you wind up ruining the bottom bracket bearings. Unicycles spindles are fixed to the wheel so you’re pretty much ok.

Sounds like the cotter faces are rockered. The cotter is only contacting the spindle in the middle of it’s flat. That happens if you do anything other than just pedal on them - they’re not capable of taking jumps. Even strong hopping is probably too much.

The fix is easy. Drive the cotters out and file the flats flat again. Set the crank on a block of wood or other solid object to hammer so that the force of the blow doesn’t ruin anything. (DON’T just turn the unicycle upsidedown and set it on it’s seat, then you’ve got a similar situation to the bike crankset and you’ll ruin the bearings.)

Then when you put the cotters in, jiggle everything around until until the cotter flat is exactly on the spindle flat. (The threads should already be sticking out the other side. If not then you’ve got the wrong size pins.) Then tap those puppies home. If you mushroom the top you’ve hit them just a little bit too hard.

The crank is now completely on; that nut is there only to keep the pins from coming out again. It doesn’t have enough pull to seat the cotters by itself. You can carry a wrench with you if you like, but if the cranks come loose on a ride you’re better off with a rock.

Good luck.


Why are you quoting me? did you think i saying everyone with cotter pin crank should stick their uni in a vise? oh no no noh

look at what Sheldon says the 1st option should be

the cotter pin press is a dying tool but nontheless a real one…


Cotterpins are hideous instruments of torture! I hate 'em.

I reckon they come loose because they’re designed for bicycle use, ie to be ridden forwards only. When you hop or idle or (heaven forbid) ride backwards, there’s very little to resist the crank moving on the axle and hence they quickly develope wobble.

My recent experience of riding a 29er with cottered cranks bore this out. It was fine until I did some hopping on the spot for a photo op, then I had to tighten the cranks. Riding forwards after tightening was fine. I had to be careful not to roll back when mounting, otherwise this would cause wobble in the cranks. Idling caused the cranks to loosen. Pretty soon (after using stone age crank tools three times) I decided that my wheel was to be ridden forward only. This was a good philosophy until I overtightened the cotterpin nut and broke it off completely. Then the crank fell off entirely and the long walk home began.

Never again will I ride cottered cranks. I’m getting my 29er wheel rebuilt with a cotterless hub.


Re: What causes cotterred cranks to wobble?

On Sat, 31 Jan 2004 23:25:50 -0600, cyberbellum
<cyberbellum@NoEmail.Message.Poster.at.Unicyclist.com> wrote:

>The fix is easy. Drive the cotters out and file the flats flat again.

I guess they’d have to be rather perfectly flat. But filing rather
perfectly flat is not that easy (for me).

Klaas Bil - Newsgroup Addict

I have a feeling you might need two points of contact with the ground for such a thing to work? Or at least training wheels on the front and rear. - John Foss commenting on a picture of a one-wheeled vehicle he saw on RSU.

Re: Re: What causes cotterred cranks to wobble?

I wonder if they could be ground nicely flat (or even slightly concave, which might even be better) on your basic bend grinder.