I've been running 52X19 since I set up my Peugeot over a year ago. Today I was riding just south of Dupont Circle in DC, I was going over some bumps, and my cog broke. This of course caused the wheel to lock up as the chain wrapped around it. It sent me into a pretty awesome skid, but I maintained balance.


Here is the piece that broke off

While I've had the set up for a long time, I've been out of the country of 8 months, and thus the cog has pretty little wear. My chain tension was good... What could have caused this?

btw, I had noticed some slight warping in the chain-ring, it's the original peugeot, and kinda wimpy... could the slight side to side motion of the chain have caused the cog to fracture?

And I know it has been a year, but should I send this back to EAI?

-ty

this is the 3rd report I've read of an exploding EAI cog.

Mine did the same thing. EAI 16t. Broke into 3 pieces.

they are heat treated. if one gets too much heat, it becomes brittle. It's their quality control problem, so send it back and get it replaced.

Are these 3/32" cogs or 1/8" cogs? Or is it a mix?

Its shame to hear that EAI have some quality control issues, cause their cogs are normally bomb-proof.

another question: are these exploding cogs alloy or steel?

I guess it's steel from the photo.
Will at www.63xc.com has done a good article about cog choice, and explains these sudden failures quite well here.

excerpt
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Secondly, consider carefully the implications of heat treatment.

Materials don't cost much. The steel in your cog was probably cheaper than the packaging in which you bought it. The reason why an ounce or two of metal ended up costing as much as a pair of good jeans was that the manufacturer subjected it to various post-machining treatments to make it wear better.

Durability in a component is a complex phenomenon. Consider a samurai sword.

At its edge, the sword needs to be very hard, to take and hold a keen edge. If the metal at the edge of the sword was soft, the sword would blunt too quickly to be of use.

At its centre, the sword needs to be pliable. This allows it to bend and absorb impact. If the metal in the centre of the sword was hard, the sword would likely shatter against other metal weapons.

The skill of the swordsmith lies in judging hardness and softness and how to combine them.

In just the same way, a sprocket needs hard teeth to withstand wear, but must retain enough pliability to cope with road shocks, sudden changes of speed, and so on.

Case hardening, the deliberate creation of a hard crystalline 'skin' on the outside of a metal component, requires very precise control. Since it isn't economic to process sprockets one by one, they must be batched, and there is then the danger that individual sprockets within a batch may be over- or undercooked. An undertreated sprocket will be too soft and will wear quickly. An overtreated sprocket will be too hard and may shatter in use.

This gives rise to an odd paradox: expensive cogs subjected to serious heat treatment wear better than cheapies, but at an increased risk of sudden failure. Brittle failures are rare, but they do happen, and here is a photo to prove it.

Hey everyone,

you hit the nail on the head, or the cog on the tooth. I took the cog to CityBikes today, and they called EAI. They immediately jumped to the flawed manufacturing process conclusion and asked for the destroyed cog, saying that they would send me a new one.

So for now the bike is out of commission while I ride the beater...

thanks

Has anyone here experienced a DA cog failing?

hmm... has anyone considered using an exploding cog as a weapon? someone should try to throw an exploding cog at some suvs, then run and take cover. i'm surprised you can buy them without a permit....

i kid, i kid.

It would be cool to sharpen the teeth and throw them like ninja stars

nope. I love 'em.