Hey let's ride.
Tandem cassette question
I have a set of wheels that will be here soon. There are tandem wheels with Shimano hubs, 145mm rear, Shimano 9 speed compatible. I'm wondering, I have 3 9 speed Shimano cassettes in my parts drawer. One is a mountain setup with a 12-30 something, the other 2 or road cassettes are 11-21 and 11-23.
Can I combine parts of 2 different shimano cassettes to come up with a range I like?
I was thinking something like 12-28?
Are road and moutain cassettes swapable if your deraileurs are long enough?
In theory you can, but it obviously depends on how the cassette was manufactured. Most of the newer, wide-range cassettes use carriers that cluster the largest sprockets (e.g, 16-23t on Ultegra or 18, 21, 24, 28, 32 on XT) together, leaving only the smaller cogs loose. The reasons for this are actually quite practical, e.g., it spreads out the drive train loading across the carrier or clusters of sprockets and, in turn, the hub's cassette freehub. If you've ever seen a cassette freehub that had 9 gouges in each and every spline from a cassette that had 9 individual sprockets you'll quickly appreciate the minor inconvenience of the modern cassette designs. But I digress...
Bottom Line: Most tailoring is usually limited to those smaller cogs which isn't a bad thing... until you wear out the "custom" cassette and realize that you'll need to find other cassettes to cannibalize to replicate the franken-cob. If you have a long-cage rear derailleur you're covered for anything from a 24t to 34t... and maybe even 23t. You just need to use shorter chains on the smaller cassettes and, conversely longer chains on the taller cassettes.
Note: I have a 12x32t franken-cob and a useless Ultegra 11x23 cassette sitting around somewhere.
Modern cassettes have ramping built into them. This ramping is designed to match the adjacent gears and ease the shifting between them. I am pointing this out now because of the numerous times I have had to explain to people why their hacked together cassettes don't work as well as a perfectly matched cassette in spite of the fact that they have been told by other sources that it should work with absolutely no issues or even the slightest change in shifting performance.
That being said, if you are still determined to give it a try since you already own all the necessary parts, there is one way I know of that will at least reduce the chance for possible shifting issues.
If you look at the center of any cog, you will notice that the splines only allow each gear or set of gears to be installed one way onto the cassette body. If you examine the shapes in the ramping on a perfectly matched cassette, you will notice that the ramping is roughly shaped like a chain and that it drops down so as to align the chain precisely with a tooth on the adjacent cog. If you mix cogsets, then these ramps may not align the same way. By filing out the one larger tab on a cog, it can be installed now in more than one orientation and by aligning it so that the ramping most closely matches what you would find on a fully intact cogset. This works well for adding one more lower gear and possibly a higher gear but it might not be a good idea to do this with multiple gears that have been fastened together with an aluminum holder.
The effect on performance when mixing cogsets is only apparent when shifting and then only between the mismatched gears on the cassette. For someone looking for that 11t gear that is not available with a 27t low gear, or for someone wanting a bail out gear added, the decrease in shifting performance over that one gear may not be an issue, especially if you are anticipating it and can put a little effort into the shift and ease on your pedaling at the same time.
I suggest you try the combination first before filing anything to see how that works. If the shifting is not working quite up to par then you can consider a gear if needed.
Notice how all cogsets, regardless of manufacturer use a lockring? It would save a few grams to just thread the gear on but then the alignment of the last cog would not be consistent.
I was thinking something like 12-28?
I believe that cassette gearing is available in the SRAM cassettes and would work better than a hacked together cassette. Shimano isn't happy about that either, especially with as well as they do work, and there has been rumors about a fix for that, only I think it would only be available for 10 speed.