My buddy just fit 6 outta 9 cassette cogs on his Hope SS hub. I like it! I don't think I ever use more than my 5 lower gears on my 1X8, anyway, except on the road to the trials.
The tips below refer to running 6 out of 9 speeds, but the same principle applies for any number of cogs that come from a 7,8,9, or 10 speed shifter/cassette mech.
For this setup to work you'll need a long ass rear der limit screw or two, more on that later.
You have two options when deciding which range of shifter clicks will be "blocked out" (which 3 clicks you don't need from the shifter)
OPTION 1 - I prefer to set it up the shifter/cassette mech "normally" with shifter's 9th gear on the smallest cog and let the rear derailleurs limit screw keep me from clicking the shifter from 4th to 3rd gear. (in this example 9,8,7,6,5,4 would be your six gears and 3 would take it over the edge into the spokes)
- Your limit screw for small cog may be long enough to work properly (If it's not you can get by without one on the smallest ring by just using the cable tension as a limit to stop it from falling off small cog--takes some tightening and loosening and moving the cable in the clamp a few times to find that sweet spot where the cable is just at the perfect spot on the smallest cog without even having to set a limit screw).
- To keep the chain from going into the spokes in the biggest cog you'll probably need a loooonnnng ass limit screw. When I made my Surly 1x1 into a 1X3 speed I pulled the stock screws out of an 1996 XTR der and replaced with screws from a cheeseball Shimano SIS walmart bike derailleur. They were long enough.
- With this setup the 3 shifter clicks that are "blocked out" can't be shifted into and there's no extra "dead clicks" in your shifter
OPTION 2 - You may also choose to set up the shifter 6,5,4,3,2,1 with gears 9,8,7 "blocked out", but I don't like this option for many reasons...
- You can still shift into 3 dead clicks because the smallest cog's limit screw doesn't keep the cable from going slack when you shift past
- You need a limit screw on the smallest cog...can't pull the cable tension trick
- It may seem like you could get away without a limit screw on the smallest, but I wouldn't trust a stretchy cable with no limit screw when a thumb is pressing hard on the other end of the mech.