customized 10 speed mountain cassette

Now that SRAM has a 10 speed 11/34 cassette (VERY expensive) has anybody tried modifying a 9 speed cassette to convert to 10 speed?
i read somewhere you can take a SRAM 9 speed 11/34, add a 30 tooth cog, customize the spacer width and get a 10 speed mountain cassette for road bike.
Anybody tried?
BTW, I'm also aware of JTEK shiftmate to convert 10 speed shifters to work with 9 speed cassettes, but just curious to know if anybody has tried the 10 speed route.
I have 9 speed 11/34 SRAM with all cogs loose so it would be question of finding a 30 tooth cog (where to buy?) and somehow getting narrower spacers. ( I also already have a mountain shimano LX RD)

Thanks!

Nobody?

Okay ...

The few times I've restacked a cassette it was to add a bigger cog onto an existing cassette ...

For example, I added a 28t to someone's 8-speed 12-24 stack ...

I added a 32t to one of my 12-27, 9-speed Ultegra cassettes.

I added a 26t to a 12-23, 9-speed Chorus stack that I have. On this particular cassette, I also changed the smallest cog to an 11t.

In all cases, I found it was easier to simply add the "appropriate" cog onto a given stack and then subtract one of the intermediate cogs in the 14-to-17 range.

THAT's my way of saying that if I were looking for a DIY 10-speed MTB cassette (vs. using a 10-speed IRD cassette) that I would start with a 10-speed Shimano ROAD cassette & piggyback a 9-speed 32t-or-34t LX cog onto the back end rather than trying to take a 9-speed MTB cassette & shave the spacers to the smaller width.

N.B. I found that an 8-tooth difference in tooth count in adjacent cogs was easily managed with Shimano rear derailleurs ... and, apparently, based on SRAM's gappy MTB stack & Shimano's MEGA-stack cassettes, my observation is not unique ...

I've only piggyback ONE cog onto the end of a stack; but, I guess I wouldn't have too many qualms about stacking a 30t + 34t on a xx-24/25/26/whatever 10-speed cassette.

Either a 'lone' 32t or a 'lone' 34t stacked on a 11-27 cassette would definitely work if it is just a bailout cog.

Of course, you need to decide which of the intermediate cogs you want to eliminate ... the 'extra' 10-speed spacer will be sandwiched between the "regular" cassette & the piggybacked 9-speed cog.

Thanks!
I've read a lot about IRD cassettes; they don't work.
The cassettes I have (10 speed) seem to have the last three cogs (biggest ) all in one assembly... so just break them apart?

Let me try this again ...

Because I wanted lower gearing, I piggybacked a larger cog onto an existing stack ...

So, presuming you have one of the following 10-speed Shimano cassettes:

11-21
11-/12-23
12-25
12-27

Then I would add the following cogs from a 9-speed LX cassette & probably choose the following stacks:

11-21 .~~~~~~26 30 34 --> 11-13-15-17~~~~~~~~~.18-19-21.~~~~~~26 30 34
11-/12-23~~~~30 34.. .. --> 11-/12-13-14-15-17~~~.19-21-23~~~~~~.30 34
12-25~~~~~~~30 34 . ...--> 12-13-15-17-19~~~~~~~21-23-25~~~~~~.30 34
12-27~~~~~~~30 34 . ...--> 12-13-15-17-19~~~~~~~21-24-27~~~~~~.30 34

12-27~~~~~~~32 . ... ~~--> 12-13-14-16-17-19~~~~.21-24-27~~~~~~.32

etc.

You might prefer to omit different intermediate cogs.

You could probably re-stack an 11-21 with just a 30t bailout cog ...

The fewer the number of cogs you piggyback onto the back end, the better the indexing will be -- if you only add one cog, then the shifting should still be perfect ... two cogs, & you may-or-may-not want to reduce the thickness of the last spacer by an infinitesimal amount (draw a flat file lightly across it once-or-twice) OR both by an even lesser amount ... three cogs could become problematic unless the thickness of one-or-two of the spacers is reduced.

I don't know how good-or-bad IRD cassettes actually are since I don't have one; but, I would always presume that a Shimano cassette would shift smoother than any of the others since everyone else is playing catch-up with Shimano's R&D with regard to ramping (and/or pinning when including chainrings in the discussion).

Thank you! Very clear explanation
This is really easier than what I expected. I'll try it this weekend. I'll be modifying a 12/15... We'll see how it goes.

Running into a problem. I took a 10 speed 12/27 shimano cassette, removed the 13 tooth cog and added a 30 tooth cog next to the 27. I could not make it fit. (Mavik Aksium wheelset with factory spacer for shimano). The new 30 tooth cog is a flat profile whereas the original 27 tooth cog has a slight recess that lets it sit farther into the hub...
Am I doing it right? Do I remove the Mavik spacer? Do I need a longer lock ring?

Thanks!

Yes, I think you simply need to remove the MAVIC spacer from the freehub body to get the DIY cassette to fit properly.

The spacer probably/undoubtedly compensates for the slight recess in the cassette's spider ... and, removing it will allow the single cog to 'sit' in the same plane as the 27t cog formerly/(currently) sat/(sits) in.

You should not need a longer lock ring.

BTW. You may need to tweak the stops & indexing because you will have theoretically bumped the cassette outward by a fraction of a millimeter (~0.18mm?) ... it might be fractionally more-or-less because I don't know how thick that MAVIC spacer that you remove is.

The function of the spacer is to ensure that certain cassettes like your 12-27 don't sit too close to the spokes and thereby cause the chain to be too close to the spokes when the chain is on the largest cog.