How to Buy New Cogs (And chainrings?)
 

I've been a very bad mechanic: I let my chain stretch a lot -- I'm not sure I how I let this happen. So now, of course I have to change the rear cogs.

I've always had freewheels in the past, but my 1997 M2 Pro has an 8-speed cassette.

I want exactly the same arrangement of cogs (or should I not worry about that?). It looks like I'll have to buy a complete cassette, yes?

Can I assume that any "8-speed cassette" will fit my wheel?

For this one, the sizes available are designated as, for example, 11-32, 11X32, {11-28T**, but they don't specify the exact cog sizes.

How do I evaluate whether I need to replace the chainrings?

Thanks!

Al

If the original cassette is a Shimano then any of their 8-speed cassettes will fit your freehub body and, yes, you should buy the complete cassette as individual cogs are both hard to find and too expensive to justify.

As to chainrings, unless they are so badly worn they look like shark's teeth or a circular saw blade, they are probably alright.

1. You'll need a cassette lockring tool and a chain whip to remove your old cassette. When you install the new one, don't be afraid to clamp down on the torque. A tight lockring squeezes the cogs together and minimizes damage to the freehub body.
2. While it is possible to source some individual cassette cogs, it's generally more cost efficient to replace the entire cassette.
3. Campy and Shimano cassettes use different splines so they can't be interchanged. SRAM cassettes are splined the same as Shimano. I'm sure some folks will argue the merits of SRAM vs. Shimano cassettes but I won't.
4. I seldom worry about the intermediate cogs sizes. Most cassettes are designed by the manufacturer to keep the steps between gears as equal as possible. Shimano mega range cassettes are an interesting exception to this rule.
5. New chainrings have teeth that are rounded on top. Worn chainrings are noticeably pointy.

Thanks, that's the info I needed. One more thing:

Is there any difference between 11-28T and 11X28?

http://bikeforums.net/attachment.php...hmentid=311342

I'm guessing that's just the way that the different sellers designated the cassettes.

Oops, one more question:

My current cluster is 11-28 (11,12,14,16,18,21,24,28).

Do you think I could go to a 11-32, or might that be too much for my derailleur? I wouldn't mind a little more help in the lowest gear (I have two chainrings).

Thanks.

Correct, they are the same

If you look at the dropdown selection box, the 11x28 is sold by Bike Nashbar, when you change to the 11-28T it's Amazon.

The PG850 is probably a lot lower spec than was on you M2 originally, but getting high end 8 speed has been hard for the last 15 years.

You haven't mentioned changing the chain, hoping you are doing this at the same time as the cassette, as a worn one won't mesh well with the new cassette

Knowing the current rear dérailleur you have would help, being a 1997, this should be Shimano, the model number should be stamped on the back of the parallelogram.

Hi,

Some derailleurs can't handle more than 28,
especially ones that don't have a B screw.

rgds, sreten.

It's a Shimano 600. On the back: RD-6401 (Plus the text "VIA[?] Shimano USA").

Yes, I have the new chain ready. Before my ride yesterday, I discovered the stretch, knew what would happen with a new chain, but tried it anyway, got skipping, and put the old one back on.

Not been able to find any info as to if this came in anything other than a SS (short cage) version, others may know; but if it is a short cage only, you are limited to the 28 max you already have (was this the original or was it a closer ratio?).

For the VIA, this means taken from Wikipedia - "VIA" ("Vehicle Inspection Authority") is stamped on all Shimano parts. It is an official approval stamp used to certify parts of Japanese vehicles – including bicycles. This mark signifies compliance with certain quality standards and is similar to the "UL" (Underwriters Laboratories) mark.

OK, thanks for taking a look at it. I'll just go with the 11-28 and play it safe.

This is the one I'm getting. I'm amazed it's so cheap -- it would almost make sense to just let the chain and cogs wear out together, and replace them both when they start to skip.

Hi,

Beyond a certain point it does, as long as you don't trash the front
chainrings as well in the process. Better to change the chain at lowish
wear but keep it. Then run the new chain the same sort of distance
and then keep swapping the chains until both and the rear are toast.
Some do the same with 3 chains per rear cassette.

AFAIK you go for a new (set of) chain(s) and a cassette based
on chain stretch well before skipping occurs to prevent trashing
the front chain rings. Running a chain and rear to skipping usually
means the chain is more trashed than the rear cogs, but you've
left it too late for the rear to work well with a new chain.

rgds, sreten.

Sheldon Brown's site has excellent tables showing the specific cog sizes for cassettes from the big players -- e.g., the XT & XTR 11-28 8s cassettes have different cog sizes than the cheaper HG50 cassette. Not just that, but also information about tooth transitions, spider assemblies, etc., that can help you customize a rear cluster from standard cassettes.

Here's the 8-speed cassette page.

It never skipped on the rear cog. This is how I discovered it: I'd come down a hill in the big chainring, then when I'd start up the next hill, I'd stand for a few strokes (still on the big chainring). This is when it would skip, and it was skipping on the chainring.

I got home and took a look, and this is what I saw:

http://bikeforums.net/attachment.php...hmentid=311734

The chain was running on the top of the teeth ("skating"). That's when I measured the chain.

The chainring teeth are pretty pointy.

http://bikeforums.net/attachment.php...hmentid=311735

Hi,

Looks trashed to me. Which is what happens if you run a worn
chain far too long, it trashes all the other drive components.

Out of interest would you mind measuring the chain tight with a 12"
rule. 24 link pins will match up when the chain is new and slowly
increase over 12" as it wears. How much over 12" is telling.

rgds, sreten.

OK, I will do that when I remove it. You're going to be shocked.

Hi,

You don't need to remove it and its easier to tension on the
bike. Derailleur tension should be enough with the ruler
supporting the chain dropping under its own weight.

rgds, sreten.

OK, over 12 links, the chain has stretched 3/8 of an inch.

It was a Nashbar NS-CN78 chain which I purchased on 2/4/2011 for $14.99. It has been ridden 4,238 miles on 146 rides in the 2 years and 2 months that I've had it. I lube the chain every 60 miles or so with 1 part chainsaw bar oil, and three parts mineral spirits.

Well, that would explain the wear to the chainrings....

Ergal, 7075 alloy, Being Harder, does wear longer than the stuff Sugino stamps out..
If the Teeth get die Cut, the alloy is chosen to facilitate that production technique.