Shimano 11 speed MTB cassette on an 11 speed road hub, which spacer should I use?

Question

I have a Shimano 11 speed mountain bike cassette and a Shimano 11 speed road hub. Without a spacer there is considerable wobble of the cassette, and I read that a spacer is required.
My wheels came with two spacers, that I measured with calipers at 1mm thick and 1.85mm thick.
Which thickness of spacer should I use?

Klaster_1 · Accepted Answer

For 11s MTB cassette on road 11s HG hub, use 1.85mm spacer.
Bike-Components.de lists the following:

1 mm: Thickness: 1 mm Application:

using a 10-speed (Road) cassette with aluminium spider (e.g. CS-7800, CS-7900) on an 8-/9-/10-speed freehub
using a 10-speed (Road) cassette on an 11-speed (Road) freehub (1 mm + 1.85 mm) Manufacturer Part Number: Y-1Z807000

1.85 mm: Thickness: 1.85 mm Application:

using a 8-/9-speed cassette (Road) and an 8-/9-/10-/11-speed cassette (MTB) on an 11-speed (Road) freehub
using a 10-speed (Road) cassette on an 11-speed (Road) freehub (1.85 mm + 1 mm) Manufacturer Part Number: Y-4T724000

Criggie · Answer

More general addendum, a cassette is secured by a lock ring at about 40Nm.
If the cassette still has room to move laterally when the lockring is bottomed out, then its too wide so add a spacer or replace with thicker.  Otherwise your shifting will be compromised and the bike will change up by itself.
If the lockring can't thread in, or doesn't have a couple of rotations of thread engaged, then remove a spacer or thin them down.  A lockring is labelled as needing a certain torque, which is "reasonably tight"   If there's not enough thread engaged then it can rip out, requiring a new freehub body.

Shimano 11 speed MTB cassette on an 11 speed road hub, which spacer should I use?

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