custom cassette?
 

I have a 12-27 9-speed ultegra cassette that generally works well for my application, although sometimes I wish I could switch the 15t for a 16t. Is this possible?

It's physically possible but not easy because individual 16T cogs are all but impossible to get. Several years ago I made a couple of 13x27 9-speed cassettes by substituting a 13T first position cog for both the 12 and 13T stock cogs and adding a loose 16T cog and spacer.

However, I had a lot of trouble locating the 16T single cogs at the time and, for all practical purposes, they aren't available in either 8 or 9-speed form (both would work but you can't find either) by now.

Sure it's possible, but it's not a good idea, unless you want to find out the whole point of Hyperglide.

HG cogs are designed to sit next to the cogs they come with; your shifting will suck. Also, swapping the 15 for a 16 rather than inserting a 16 between the 15 and 17 will just result in an even bigger jump to the 14, which will majorly suck cause you're going faster, facing exponentially more wind resistance, and the last thing you want is a bigger gap (which you know the feeling of, since you're missing a 16).

What you're proposing is 12, 13, 14, 16, 17, 19, 21, 24, 27, right? This is never seen for a reason. That reason is that ideally, gaps between ratios should get smaller as they get higher, due to the exponential nature of wind resistance.

I'd suggest you buy a 12-23 and a 13t top gear and make up a 13-27, or skip the 13t and do 12-24.

Sheldon reckons just jamming a cog that doesn't belong in there is no big deal, but he seems pretty slap-dash on this subject, inviting folks to mix and match cogs of different thicknesses.

http://www.sheldonbrown.com/k7.html#custom

http://www.sheldonbrown.com/harris/k7.html#9cassettes

IME it works like crap if you don't preserve the proper relationships between HG cogs.

I'd like to point out why the mixing of cogs can become more of an issue. Since the advent of shift-while-standing setups, there was an immediate need to reduce the slippage during gear changes. This is because an un-seated rider needs continuous drive force on the pedals or there will be an unnerving swerving when a shift is done "under power".
The slippage I mentioned is hugely magnified whenever an HG-type cogset's staging is interrupted by well-intentioned reorganizing of the cogs, thus I can't recommend it.
For touring and commuting, where aggressive attacks on rolling hills (or sprinting) are de-emphasized, a re-staged sprocket stack works well enough, though will still be more sensitive to cable contamination and/or misadjustment. I'm quite sure this kind of riding is what the late Sheldon Brown had in mind.
I have tinkered a bit with cassettes, one of my 12-25t cassettes gets a 28t added on for hilly centuries. I lose the 13t rather than the 12t though, and don't make the 12-14 shift under standing efforts anyway, so there's a net advantage on those grueling rides. That, plus I just happen to have a titanium 28t cog laying around.

I've made several custom 9-speed Shimano cassettes successfully. The 13-27 that HillRider suggested works perfectly and I prefer it to the original 12-27. A single 16 is not easy to find but it can be done. Trading a 15 for a 16 is not a good idea, as said above, the 15 leaves a bigger ratio gap than the 16. Certain Shimano cassettes, road and mountain have single 16's. On some cassettes the 16 is joined with another cog but can be separated and used individually. Best thing to do is find cassettes that have what you need and salvage them.
My favorite 9-speed Shimano cassette is a 13 - 26 that I put together from D-A, Ultegra, and XTR cogs. It's close ratio and very light. It shifts as smoothly as any new cassette.

I always build up my own cassettes to get the jumps where I want them. They may not shift as perfectly as a matched Hyperglide cassette, but I never shift while standing and haven't run into a problem so far.

My most custom cassette (8-speed) is made from four different cassettes and works great: 13 15 17 18 19 21 24 30

I always found myself switching between the 17 and 19, so I added the 18 in between. I never used the 11 so I cut it. The gap between the 24 and 30 is big but this idea came from my Megarange freewheel. I have a close ratio cassette in the middle and a bailout gear when I need it.

To the OP, I would not suggest replacing your 15 with a 16. Maybe add the 16 (if you can find one) and lose a sprocket elsewhere. Choosing ratios on a cassette is always a give and take. Until we adopt Nanodrive we'll never be able to have every gear we want with a perfect gap in between each one.

If you are willing to go to seven speed you can have a drive train with 10% spacing between each step. An 11-34 cassette spaced at 20% and a front crank with the rings spaced 10% gives you the OLD half-step. I have it on my tourer and it works fine.

If you don't mind giving up your 12 on the cassette, you could buy a 13-25 and combine the loose cogs on the 13-25 with the carrier cogs on the 12-27 for one custom cassette, and create another cassette out of the other two halves like this:

13-25: 13,14,15,16,17,19,21,23,25
12-27: 12,13,14,15,17,19,21,24,27
12-25: 12,13,14,15,17,19,21,23,25




Custom 1 : 13,14,15,16,17,19,21,24,27

Custom 2: 12,13,14,15,17,19,21,23,25

So Custom 1 has your 16t and 27t, and Custom 2 is exactly like a stock 12-25 you could use on another wheel.

I miss a lot the 14-15-16-17-18-19-20 I had back in the day :D Enough gears to go everywhere, well i was in racing condition also :)

I considered that approach when I built my 13x27 by modifying a 12x27 but it required buying two complete cassettes and I had no use for the 12x25 that survived the mix-and-match. Buying just a first position 13 and a loose 16 was cheaper at the time but not practical today.

Correction:

Custom 1 : 13,14,15,16,17,19,21,24,27

Custom 2: 12,13,14,15,17,19,21,23,25

See it?

I was going by the description on a store's website - it said the 5 largest sprockets on a 9-speed were connected to the carrier... I thought I remembered four but I went with what they said.

On most Ultegra 9-speed cassettes the larger 3 cogs are joined on a common carrier. On a 13 - 25 the 25 is a single.

On a Shimano 105 9 speed the gears are held together by 3 removable pins. This makes for a very easy cassette to make custom. On your Ultegra the last 5 cogs are joined together by 2 seperate carriers 21 - 24 - 27 together and the 17 - 19 together. Go to a 105 cassette and ask your LBS to keep a eye out for a retired cassette that will have the extra cogs that you want. Or go to Craigs List/ Ebay/want ads and get the desired cogs.

This is correct. The 105 9-speed cassettes have all full-plate cogs held with removable bolts and can be used individually. Ultegra 9-speed cassette have the #5 and #6 cogs on a spider and the three largest cogs on a single spider and they can't be used individually.

Typical Ultegra 9-speed cassettes have the 5th and 6th cog pinned together but the splined centers are part of the cogs and can be used as individuals if you can separate the cogs from the pins. The larger cogs that are pinned to a common carrier cannot be used as individuals. The 9-speed Dura-Ace cassettes that I've seen have the 5th and 6th cogs pinned to a common carrier, like the larger cogs, and cannot be separated and used as individuals.

Correction:

Nope.

Most Ultegra 9-speed cassettes have the largest 3 cogs connected to a common carrier. The next two (17 & 19 in this case) are pinned together but separate from the larger 3.

All those pinned cogs can be separated with careful use of the right size drill-bit...

But the ones on common carriers cannot be used as individuals because the splines are on the carriers, not the cogs.

Only if they are "full plate" cogs and are riveted or bolted together. The cogs on a common spider are not full plate and have their centers cut out to save weight. The spider has the splines for all three cogs and none of them can be used individually.

Kimmo appears to be saying that the 17 and 19 can be split (perhaps for 105 versions), but that that isn't the correction he was making. I'm not sure what he's thinking about.

I don't know either. After double checking it seems that your original proposed cassettes can be done without drilling out any pins. Kimmo's would require drilling out and separating the 17 - 19 clusters.