dynawolf

I took an SLX 12-speed gravel bike to the LBS for shifting problems. They added a spacer between the biggest cog and the spokes. (Left side of cog looking from behind bike)

Is this correct/required?

My 11-speed bike does not require a spacer but my 12-speed bike does require one? The 12-speed has one more gear and it is on an 11-speed freehub. This does not make sense so I am checking with this forum.

Thanks in advance!
-Rob

MilhouseJ

It makes sense if the largest cog would be too close to the spokes due to backspacing. Does it shift as it should now?

FBinNY 

I don't have enough info to judge whether it makes sense or not.

However, it wasn't working before and is now, so that's a solid clue.

bboy314

The mechanic did something to fix an issue, and you’re looking for advice from strangers on the internet, who aren’t looking at your bike, to verify your work? Seems like a way to give yourself unnecessary anxiety.

Some cassette/freehub combos require a spacer, some do not. It depends on how the cogs are dishes relative to the carrier, and overall width. If your bike is now shifting properly, thank the shop and enjoy the ride!

alcjphil

On an 11 speed free hub body, an 11 speed cassette with a large cog of 34 teeth or more will also require a spacer behind the cassette

well biked

As others have said, more specifics are needed to know exactly what you have and why the bike shop installed a spacer behind the cassette.

Canker

alcjphil probably has it. You probably have a ROAD 11 speed freehub and a 12 speed mountain bike cassette. Road bikes need a longer freehub to shove all those gears on the freehub body and mountain bikes don't becase of the way the cassettes are made. 11/12 speed mountain bike cassettes are machined out on the inside so the huge cog sits closer to the spokes. 11/12 speed road cassettes don't have enough meat on them for the same machining with the exception of the shimano 11-34, 32 is too small. So the spacer was forgotten or not big enough and the shop fixed it.

Also could just be manufacturing slop. I have a mountain bike that needs a very slim spacer when running 11 speed shimano mountain bike cassettes but didn't with 10 speed sram. Shouldn't need it for either.

Duragrouch

Y'know, that's a really good point which I had not realized before. I had thought the large aluminum carriers to which the large cogs on big 1X cassettes, and especially with skinny 11/12/13 cogs are attached, was for better lateral stiffness. But yes, that carrier can overhang the large cogs over the drive side hub flange, and thus result in less wheel dish. While I don't like not being able to pull the cogs apart for cleaning, I have to admit, that is an advantage.

bluehills3149

AFAIK all 12 spd cassettes require an 11 spd hub. Only 11spd mtb cassettes (defined as any cassette with more than 34 tooth large gear) requires a 1.85mm spacer to fit an 11 spd hub but no spacer on 8,9,10 spd hubs. But all 12spd cassettes require 11 spd hubs and hence the inclusion of a spacer is strange. Then again it could have been a very thin spacer to account for bad manufacturing tolerance so without seeing the bike it's impossible to know if this was a genius fix or a WTF.
Of course this assumes we're talking Shimano HG freehub and not Sram XD or Shimano's new Microdrive.

FBinNY 

Without knowing the particulars, it's impossible to know what the mechanic had in mind.

However, this may restore your confidence.

It's SOP on all derailleur bikes to try to position the freewheel or cassette as outboard as possible, of course allowing enough room so the chain doesn't rub on the frame.

We do this because there's no benefit from extra frame clearance, and a real benefit in keeping the RD and chain away from the spokes.

So, anytime I see an opportunity to move a cassette outward with spacer, I do so. It's one of those " can't hurt, may help" situations, and your mechanic might have had the same thought.

Duragrouch

Years ago I was going to space my 7 speed cassette out, to reduce the gap between the small cog and the frame just a bit, because the chain would drop in there and jam tight. But the spacer reduced the thread engagement of the lockring just enough to make me nervous, as well as the spline engagement of the small cog, so I didn't do it. So I would look for both on the mentioned modification. Both will assemble fine and "fit", that doesn't mean it's sound practice. I wish I knew the exact answer for your parts, but I'm not up on the arcanities of high-number cassettes and hubs. Fortunately for me, upgrading my rear derailleur from the poor design original (jockey pulley too far forward), eliminated dropping the chain off the small cog.

bluehills3149

The reason why road freehubs went wider with the introduction of 11spd was they needed more width to accommodate the extra gear and they had no choice but to foist a new hub standard on us. MTB freehubs could stay on the narrow 10spd standard and just machine the inside cog as the larger >34 tooth cog sits comfortably away from the spokes (the spokes angle in) and it was way cheaper to do this rather than adopt a new hub standard. Also since most used an aluminum carrier on the larger cogs, machining costs are minimal. And I believe the al carriers are to save weight as having multiple big steel cogs starts to get pretty heavy and the major difference between cheaper cassettes is a lack of al carriers. Note ultegra has 2 al carriers, 105 has 1 and the lower cassettes have none.

Duragrouch

+1. This is starting to make a whole lot more sense to me now, both carrier, and difference between mountain and road.