TallRider

I'm wondering about whether I should replace my 36t middle chainring on a 46/36/26 triple. Standard 5-bolt 110 BCD, but that doesn't matter here. What does is that I use the middle ring for the vast majority of my riding, and I'm worried that it's worn out and needs replacing for the second time. I replaced it two years ago.
When I line up the teeth with teeth from a brand-new 36t Sugino chainring, the gap is a bit wider, and I can tell that it's because the chain has worn down the teeth a bit. (And yes, I understand that these chainrings have different tooth profiles to aid in shifting, and I'm not mistaking that for wear.) What I'm wondering is, how much are the teeth worn, and how should I decide on whether to replace the chainring?
It's clear why and how a worn chain is a danger to wear out a good chainring. It's less clear to me how much a slightly worn chainring is a danger to wear out a new chain. (Basically, I'm thinking that if all teeth are equally worn, then a non-stretched chain will still be equally pulled by each tooth with which it has contact, so a new chain won't be worn out by an older chainring, at least not much.) So that's a general/theoretical question.

For specifics, here are four pictures of the chainring's teeth. The first two shots are from the inner side of the chainring, so the left side of each tooth is what pulls on the chain and is worn.



The third and four shots are of the outer side of the chainring, and in this view the right side of each tooth is what pulls the chain and thus is worn.



Again, this chainring's teeth are slightly worn, when lining them up with teeth of a non-worn chainring. The part that doesn't line up is the part where the tooth contacts and pulls on the chain link.

For comparison, here are a couple pictures of a very, very badly worn Campy 30t chainring. My brother rode 7000 miles, possibly a fair bit more, without changing his chain...

HillRider

Worn chainrings shouldn't hurt a new chain since they are Al and much softer than the steel chain. Replacement is necessary when the chain begins to skip on the chainring because the pitch mismatch is too great. Chainrings are pretty wear tolerant because the chain is applied to them under pressure as you pedal.

My son-in-law finally replaced his 42T ring when it began to skip. At that point it looked like a circular saw blade, very much like the 30T Campy ring in your pictures. Your ring actually looks pretty good and if it isn't skipping replacement isn't needed.

capwater

Worn rings are evidenced by a shark fin appearance, ie not symmetrical. If the teeth are just shorter, yet still symettrical you're good to go.

TallRider

Well, sort of. Teeth may be intentionally asymmetrical (to aid shifting) as well as being shorter, but not in the worn sort of asymmetry.
The question is [how] much shark-fin-ish-ness is a problem. Writing this post helped me realize that worn chainrings aren't necessarily a problem to wear out a new chain (whereas, a worn chain definitely is a problem to wear out new chainrings). So, basically, you can ride a chainring until the chain starts to skip on it.

Two things for the record
* the teeth on the 36t shimano chainring appear pretty close to symmetrical, but they are worn on the one side, a little bit. 0.1mm or so.
* Hillrider, I think the issue isn't the material of the chainring (you mention that aluminum is softer than steel), but rather the fact that even with worn chainring teeth, a new, non-stretched chain will still equally contact each tooth so the force of pedaling is distributed out. Whereas, with a stretched chain, the force of pedaling is borne by the first few teeth to pick up the chain, as opposed to all of them.

HillRider

Yes, that's right, but in your original post you specifically asked:


So I mentioned that the soft Al chainring wouldn't hurt the steel chain

TallRider

@HillRider: gotcha. I do think that the reason the chainring isn't much of a danger to the chain has to do with the mechanics of the interface between chain and chainring, not the fact that aluminum is softer. That is, I'm not sure a worn-out steel chainring (and I've seen them, too, looking like shark fins) is much of a danger to the chain either. But it sounds like you agree on that.

Thanks again for the input here. I finally put my old 5-bolt square-taper XTR crank back on my road bike. 180mm cranks (I'm 6'5"), run with an old (but good-shape) Dura-Ace 102mm square taper BB. Crank was designed for 107, but with 135mm-spaced mtn rear hub. So 102 BB spindle gets proper chainline for road bike. I'm running 50/36/24 chainrings, and it shifts flawlessly, gives me gears of a compact double with total bailout granny if I ever take this thing to the mountains.

HillRider

What an eclectic setup. Actually, I have found that the closer I can get the crank to the frame, the better for the chainline and the less angularity I have with the more extreme cogs. As long as the granny ring doesn't hit the chainstay, no problem with the shorter spindle.

TallRider

I'm with you here. I'm as narrow as possible right now - the DA (and Phil Wood also makes one) 102mm spindle is as narrow as they come in square-taper.

I overhauled my parents' bikes over Christmas, and my mom's touring bike has a Sugino AT triple crank (with the old 52/48/30 setup) that went with a 127.5mm bottom bracket, with adjustable chainline. Unfortunately, whoever had adjusted it back in the day had put the crank too close to the frame, so that the granny ring was worn at a nice angle. Still works, and since it's only a bailout gear anyway, not much of a problem that it was worn such.

Belugadave

Does anyone have any pictures of a worn out, needs replacing, shark fin looking chainring? I don't have any problems with skipping, but I have over 23,000 miles on my Ultegra 53/39 setup and it still seems like it is fine, but I'm wondering if I'm missing something here. It definitely doesn't look like the worn out one shown above and it really doesn't look like the other pictures either.

HillRider

Unless you are REAL strong or do almost no chain maintainance, chainrings can easily last 25,000 miles or more. I've gotten over 28,000 miles on two sets of 105 8-speed chainrings and they looked fairly good after all that time. If they still shift ok and don't skip, they are fine.

TallRider

See the last two pictures in my original post. The Campy chainring that my brother rode for thousands of miles without replacing or servicing his chain. He was experiencing crazy skippage with the worn chain (which was stretched worse than any I've ever seen), and it would have been even worse once I installed the new chain. I had to replace the crank (cheaper than buying 3 new chainrings) and the cassette as well as the chain.