Replace chain or not?
 

My chain is "stretched" and the cassette shows signs of wear. If I just replace the chain, I think I will have shifting issues so then I will need to replace the cassette as well. On the other hand, the bike is shifting fine for now. Should I wait until the shifting goes bad then replace both? Any harm in doing that?

Possible damage to chainring too.

There is stretched and then there is s t r e c h e d, as said above you can be penny wise and pound foolish.

Not replacing on time will cause increased cassette, and even (front) chainring wear. For cheaper drivetrains, it is often the cheapest route (in the long run) to run it all to the ground - until the old chain starts finally skipping on the old cassette. If the chainrings are steel (not aluminium), they might even be good for another chain or two with the same "replacement strategy".

The price you pay is slower shifting, more chance of the chain to break, and, like mentioned, increased wear of the chainrings and sprockets.

When to replace the chain on a bicycle? - Bike Gremlin - Bicycles

Very true. The teeth on a chain ring only wear more slowly than those on a cassette/freewheel because there are often more teeth, and more teeth engaged to share the load. An elongated chain will only accelerate it's wear rate.

Shifting with a worn chain and worn teeth is often good.

Brad

How are you determining wear on your cassette?

I've been fooling around with bikes for decades and I can't determine cassette wear by looking at it. My test is to replace the chain and take the bike for a test ride. If I don't get skipping in my most commonly used cogs, I assume the cassette is still OK.

While we are on the topic of determining wear, what makes you think that your chain is stretched? If you're using one of those little chain checker thingies I'd calibrate it against a brand new chain before I took it's reading for gospel.

I am using a chain gauge.

Try measuring with a good steel rule; many chain checkers have been reported to show pessimistic readings, e.g. show 0.5% wear on a brand new chain. I suspect that they are designed to help bike shops sell chains.

[QUOTE=practical;20240834]I am using a chain gauge.[/QUOT
Which one?

Try measuring with a good steel rule; many chain checkers have been reported to show pessimistic readings, e.g. show 0.5% wear on a brand new chain. I suspect that they are designed to help bike shops sell chains.

A 'stretched' chain will ruin the cassette. It's not a shifting issue. If you wait too long before changing your chain the cassette will be worn and a new chain will skip with any decent pressure on the pedals.

As others have mentioned measure 11 or 12" of the chain with a steel rule. A new chain will measure 12". Once it's stretched to about 12 3/32" it's time to replace.

where is the data? how much.

In my opinion chain checkers are helpful in a shop situation, where one does not have the luxury of getting down and dirty with every customer's chain and a ruler, or perhaps for those with a huge stable of bikes, all of which are ridden thousands of miles per year. Other than that they're just another pretty tool for the board.

I recently replaced a chain on my old Uniglide cassette (I think it's Uniglide: 6-speed, first cog acts as lock ring, bike from circa 1982). The cassette looked absolutely fine upon close inspection, and the cogs look thick an sturdy.

Sure enough the new chain jumped like crazy (old chain ran fine, but was rusting). So, as per Sheldon's advice, I simply swapped the direction of the cogs, aside from the lockring cog. Chain and drivetrain are perfect now, aside from the tallest gear which still skips.

Point is, I stared and stared at these cogs for a long time, and I couldn't for the life of me discern any difference between the worn side and the 'good' side.

...over time.

The cassette wears as the chain lengthens. Once the chain is beyond 1% stretch, and maybe before, the cassette is ruined.