With the introduction of Dura-ace 9000 and 11 cogs on the cassette (and SRAM 22), the 9000 Dura-ace wheelsets acquired a wider freehub body to match the wider cassette, making 7900 and 7850 wheels incompatible with 11 speed. It seemed like a waste of some very nice 7900 and 7850 tubular wheelsets I have so I set about modifying them to make them compatible.
The 11 speed Shimano cassette is 2.85mm wider than the 10 speed one. As the 10 speed cassettes have a 1mm spacer behind them, this makes the feeehub 1.85mm wider. Conveniently the 10 speed freehub has stops at the end of each spline which are nearly 1.85mm deep. If these are machined down, and then just a fraction of the lip at the back of the freehub, an 11 speed cassette will fit.
However, this then puts the lip which acts as a stop for the cassette 'inside' the hub. The cassette would bind with the hub when the lock ring is tightened. Some modification to the hub is also required. I figured I needed to take off about 1mm of the drive side hub flange. Unfortunately, my freehub was off at a friends workplace getting accurately machined in a lathe, and I was limited to a dremel and no real means of measuring how much I was taking off the hub. Very carefully with a cutting disc on the dremel I went about taking 1mm of the inside of the hub.
I did this only on the part of the hub flange nearest to the freehub. The 11 speed cassette has a 0.5mm lip on the biggest cog, which will push the cassette out by as much from the end of the freehub. From there the cog goes outwards away from the hub even further so only a small amount of the hub close to the center needed to be removed.
I think I ended up take off more material than I needed. Being impatient to finish the work, and not having the freehub to test things, meant I took a stab at how much to take off. I also wanted these wheels to be backwards compatible to 10 speed, so I needed to take off 1mm in a large enough diameter to take the 1.85mm spacer that is required to run 10 speed. The result isn't particularly neat, but OK for a cutting disc. I plan to pull it all apart again and neaten and then polish it all now that I know the whole thing works.
My freehub came back machined perfectly, exactly as I'd hoped. A quick test showed my 9000 cassette fit on.
I bolted the freehub onto the hub and could see that I'd taken enough material off to prevent the cassette from binding.
Next I tested the 9000 cassette on the wheel and it cleared all parts of the hub. Count them, 11 cogs on a 7900 freehub. Adding spacers and my 10 speed cassette I put the wheel in my bike with 7970 Di2 and no adjustment was required. Everything worked fine. The RD is no where near the spokes on this wheelset.
I'm happy with the result, it worked first time and means I can start using 11 speed and not have to worry about the additional expense of new wheels.http://benmanson.com/review/7900_to_11/7900_to_11.htm
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