How are 9-speed cassettes constructed? 8 Of 9 On 7
 

Towards the bottom of this page
http://www.sheldonbrown.com/k7.html
in the paragraph "8 Of 9 On 7" Sheldon mentions that you can put 8 cogs on a 7-speed freehub by taking one cog off a 9-speed cassette and use 9-speed spacers and shifters. If I do this with an 11-32 cassette, I end up with 11-28 after pitching the 32. But I've also heard that the two largest cogs are actually one piece. Is this true in all cases?

If you need to toss the small cog, I could get a regular road cassette and pitch the 11, which I wouldn't have much use for anyway, but I don't see how that could work...

How is this supposed to work?

I don't think you can pitch the smallest cog. You need it for the lockring interface. Eliminate one of the middle cogs instead.

Typically, the largest cogs are riveted or screwed together.
About 2 minutes per rivet head with a decent flat file and you can drive the pins out.

I did this recently on a 7 speed to make a custom cassette. IIRC, the 5 largest cogs were pinned together.

Depends on the cassette.

The lower end cassettes (LX and below) are composed of 9 splined cogs that are riveted or screwed together. If you have one of those, simply removing the rivets will allow you to throw away the largest cog.

On the more expensive cassettes the largest cogs are riveted onto an aluminum carrier to reduce weight and increase rigidity. If you have one of those, it's going to be a much more difficult project.

As another poster mentioned, the smallest cog is different from the others so eliminating it probably isn't an option.

The smallest cog has both a built-in spacer and serrations that mesh with the lockring to keep it tight. However, I've heard of it being removed and the lockring just tightened against the second cog but you want to keep track of it to make sure it doesn't loosen.

As RG also mentioned, lower line Shimano cassettes, LX/105 and below, have "full plate" cogs riveted or bolted together and you can remove the rivets or bolts, separate them and remove whatever cog you wish.

Higher line cassettes have the larger cogs fastened in groups of two or three on a "spider" and these cannot be separated as the individual cogs don't have centers to keep them on the freehub body. You can only remove the cogs that are individual.

Ah, that makes sense. I've taken the bolts out of 7-speed cassettes to put them on an 8/9 hub. I'll just take a close look at the 9-speed before I buy it. Thanks for the info.

I kind of hate to admit it, but I'm actually running EIGHT 7 speed cogs on an 8/9 FH.
I sanded the spacers down.:)
The lock ring is about 1 thread short of "properly" seating, but I don't see any problems with that.
I do have friction shifters though, so I didn't have to get the spacers perfect. Yhey are probably within .002" of each other though.

Originally, I wanted to make a "custom" 7 speed.
I had a-
12-14-16-18-21-24-28.
I bought a-
13-15-17-19-21-23-26
and made a-
13-14-15-16-18-21-24.
I then pick up a pair of wheels for $5 at a garage sale. The rear was trashed, but had an 8/9 FH on it, and the mind started turning.
I picked up a 2nd position 13T from LooseScrews.com and went to work!
(it turns out I can use the original 13T top cog as a 2nd position cog anyway)
I can now make up combinations using 12 to 19 in 1T increments, + 21, 23, 24, 26 and/or 28.
My combination "this week" is-
12-13-14-15-16-17-19-23. It works great for my flat "city style" riding. I stay on the middle (38T) ring, since I can rarely go more than 6-8 blocks without a stop. If I need higher gears, I have 4 more nicely spaced gears on the big ring. Actually, one is almost the same as M-8, but by the time I do a double shift to B-5 with my friction shifters, it works out about right.

As long as we're reporting on do-it-yourself cassettes, I've made 13x27 Shimano 9-speed cassettes by discarding the 12 and 13T first two cogs, substituting a 13T first position cog and adding a 16T loose cog to the middle of the stack using the spacer from behind the stock 13T cog. The only difficulty is finding loose 16T cogs as they are rare items. The result is 13,14,15,16,17,19,21,24,27. I never needed the 12 and the 16 is super useful.

Another approach is to buy a 105 level 13x23 9-speed cassette (they are a standard configuration), take the bolts out and remove the 18T cog. Then add a 9-speed spacer and what ever larger cog you wish behind the 23. I've used a 26 from an 8-speed but a 27 or 28 would work too.