Originally Posted by zacster

An 11/12sp freehub HG Spline L needs both a 1.85 and 1 spacer behind a 10sp cassette for DA, Ultegra and 105 according to the Shimano Compatibility chart here: https://productinfo.shimano.com/#/co...544&acid=C-731 But that is only if you specifically have that freehub. Other manufacturers with compatible freehubs may have different spacing, you just don't know.

The 8/9/10sp freehubs required a 1mm spacer for 10sp cassettes, either road or mtb. 11sp freehubs are 1.8mm wider, hence they need an additional 1.8mm spacer. 11sp HG700/HG800 cassettes were made to be compatible wherever 10sp cassettes would fit and need the 1.8 spacer but used the 1mm in the cassette body itself so it does NOT need a 1mm spacer. They are 11-34 teeth and the 34 overhangs the rear, taking advantage of the dish so it doesn't take up additional body space. Other 11sp cassettes will only work with 11sp freehubs, and without a spacer. These used up the 2.8mm of extra space for the extra cog, with narrower cog spacing throughout, over 10sp.

11sp wheels are 131 spaced to make it all work. They fit on a 130 frame because the tolerances aren't that tight and they figured nobody would notice. They needed the extra 1mm to account for the extra 1.8mm of the freehub. The .8mm was probably shaved off some other part of the assembly. With thru axle 142 hubs (something I don't use myself) that spacing wasn't an issue.