I have a Caad9 5 with 105. This chain is brand new. I went for my first post snow ride of the year on Saturday and snapped the chain twice. Once climbing and once on the flat.

It was lubed, in both cases I was not changing gear. I weigh around 200 right now and my legs while not savage, are mostly tree trunks. This is not power related in my books.

Either I have a derailleur issue or the seemingly straight cassette has an issue. I own a quality chain tool that I never leave home with. Chain breakage never strands me.

I bought the bike used and I put a new derailleur/chain on it. This is not the first time the chain has blown up. It took the derailleur with it last time. Should I be looking at the cassette? Looks straight, but we know looks are deceiving.

I blame all those damned hills in New West.

You can hardly go two blocks without facing a 15% wall of a road!

True, I live down on the river on Columbia Street. To go anywhere UP it's 15% grade with cold legs.

I'm thinking switching to a KMC DX10SC might help once I determine the root cause. Just for peace of mind.

So you keep buying cheap 105 chains, and you could have bought one good KMC for the same price off eBay? What is the definition of insanity?

Are you using the special connecting pin to reconnect the chain, or are you just pushing in the old rivet? If the latter, this would be your problem. Shimano chains do not use straight rivets, rather the rivets expand at the ends; whenever you push the pin out, it enlarges the rivet hole. The special connecting pin is oversized to fill the oversized rivet hole.

Operator error with the chain tool ?
Use a quicklink thingy.

+1

No operator error. I mark the chain link with a scribe to see where I broke it. Totally not there. When it broke on this ride, it was in two separate places.

I've been breaking chains with a tool for 30 years. I have it down pat. Hence my confusion. There was no skipping prior to it breaking. If your pinching the plates it will chug through the derailleur cage. It was smooth, the pins were spaced right.

Recap, it is not breaking where my tool has been. If it was, there would be no thread.

Zimm has an article on 10 speed chain breakage. Basically he says you can't tweek them sideways at all (like we used to do in the old days) when you install them.

I looked it up in the 105 Manual and this is exactly what they called it, page 10.

https://techdocs.shimano.com/techdocs...&bmUID=jh32IOP

What gear does it happen in? My wife snaps chains too, usually because she is pushing way to big a gear trying to go uphill.

First time it snapped on the 25 tooth. I was cresting my climb and had been in the gear for 30m. Chainline was nice and straight, no indication from the drivetrain that I was straining it. Low torque. I put some new, clean link in.

Second time it snapped on a different link on the flats by the river. Think I had it in 17 and wasn't hammering at all. Almost a cool down since I had a ugly turn coming up that I had to slow for.

Is there any merit to the idea that all the little marble rocks from the salt/sand trucks is hopping into my cassette? The roads I was on are filthy. Doesn't seem valid when I factor in how slow my climb was.

When it breaks, are the side plates cracked?
There was a rash of defective Ultegra chains cracking a year or two ago.

Well ok, this might have something to do with it.

https://techdocs.shimano.com/media/te...9830682230.pdf

I always have done chains up as A. They recommend B. Might be the lefty in me. I'll chuck this chain and start from scratch.

Yeah, the plates were cracked, but this isn't an Ultegra chain. I got my chain out of the UK on sale...

do you soak your chains too long in mineral spirits when you go to clean them?

If your plates are cracking, and not where you pushed in a pin, then it sounds like the same defect the Ultegra chains had.
Use a KMC or SRAM chain and fuggedaboutit.

I would call that a defective chain. Did both chains come from the same vendor? around the same time?
Its that, or you just have massive guads producing too much power.

First chain I broke myself. I shifted while climbing (powerstroke) and it took a couple spokes with it. It was the stock KMC chain.

This chain has broken 4 times and I've used up all my spare links. So it was only one chain from one vendor. I'm going to a SRAM 10 speed chain from MEC for now. I have no confidence in the chain and when I'm 40km from home that's not fun.

Are 28 inch quads "massive" to you?

I had some concerns since my bike came with ultegra so I just upgraded to a dura ace chain before I even picked up the bike, 1500 miles down no problems

my guess is you are underestimating your strength and the amount of force generated when ascending hills, and overestimating the strength of chains, together with a possibly defective chain.

sooooo... i recommend reducing your strength, ride only on flat roads, and use heavier chains.

I'd go with this one
https://www.amazon.com/Wipperman-Conn.../dp/B0028WQZMK

It's likely that it's just a defective chain. Same thing happened to me once, though it only took two failures for me to change the chain.

Does your generation of chain have the break-off pins to attach it? The 7800/6600/5600 era (10-speed) chains all (I think, not sure about cn-5500) have specific instructions about where you put the snap-off pin. There's a diagram in the instructions. And I usually have to look at the drawing to refresh my memory. But if you don't do it right, you'll make a weaker chain.

I'd take a look and see if your chain broke you put the snap-off attachment pin (assuming you used one). Also, though I have not used one, the newest generation of Shimano (7900/6700/5700) chains are directional. I don't know if mounting one backward would affect their strength.

is that one of those old-fashioned chains that permit you to push pins in and out with a clear conscience?