With the park tool chain checker, do you replace at .75 or 1. When does the wear start damaging cogs?

At .75, you should be changing the chain soon. "Recommended"
At 1, you should change the chain, period. "Highly Recommended"

The chain will start damaging cogs around .7, actually. If you're at 1, take a look at the cogs + chainrings. You may need to change those very soon as well.

With a KMC chain it does not take very long to get to .75. IT always wears around that replacable link.

At .75, you should be changing the chain soon. "Recommended"
At 1, you should change the chain, period. "Highly Recommended"

Correct.


The chain will start damaging cogs around .7, actually.

Incorrect.

My experience is that small cogs will start wearing out fast when chain stretches beyond .75

Last year, I let the chain on my girlfriend's stretch a bit beyond 1 and ended up having to replace the two smaller cogs.

Best,
Ron

You're saying that the the cogs won't even be affected AT ALL until it's .75 ? As in, the very moment the chain would indicate a reading of .75, the cogs show the effects of damage, where as they wouldn't at .74... or .73.... ??

On mine it's printed right on the tool. ">1 Replace."

It indicates that .5-1 is okay, but I forget the wording.

Honestly, the reading comprehension on this forum is atrocious.

You're the one who's suggesting something magical about the .7 reading, suggesting that a reading around .7 is where the cogs will start being "damaged." I'm simply stating that you are wrong about this.

Parts will wear as you ride them. Sure, the further your chain is worn, the more it may wear your cogs. But there is nothing special about passing the .7 mark, contrary to your claims. When you see the .75 reading, it's a good time to buy a new chain, but the world will not end (and your cassette will not self-destruct) if you ride another few hundred miles.

I agree - you're not saint, either.

Well, duh - no ****, Einstein. But, really, everything begins to get "damaged" the second it's used... hence the point of .7 ...or .6 or anything for that matter. I didn't say there's something "magical" about .7, but more referred to the idea that the cog is already being "damaged" before .75. Go back to the OP's question - he asked when the cogs get damaged. I stated it happens before .75, even though that's the lowest a measuring device will offer as a point of consideration. What number would you have picked? None at all? If I write "it gets damaged the instant you use it" that may have come off as sarcasm, even though it's true.

Thanks for all the replies. After reading over everything, I have decided to change my chain every 150-250 miles to avoid unecessary wear to the cogs and rings.

Wouldn't it be cheaper to chanege cogs in this case?

I didn't see a wink...so I hope you are kidding. Any good 10s chain including KMC will provide 4-5000 miles of riding with only periodic cleaning and lubrication.
I typically change mine above .75% elongation.
PS: Cogs wear with mileage independent of chain elongation. Typically chainrings and many cassettes will go 10-15K miles without replacement.

That would be 52 chains a year for me.

Keep in mine that the type of riding you do has a lot to do with chain wear. Mashing up hills all day in the big ring puts a lot more stretch on a chain than high RPM riding on flats.

I use the .75 mark as a reference myself.