soyabean

I'm upgrading a 559 mtb rear wheel from 7s to 8s. This involves swapping for a taller freehub body.

Usually this is straightforward, but the wheel was really spoked for 7s only. When I put a 8s freehub+cassette on it, the #8 cog is so pushed out that the chain is almost touching the right dropout, with less than 0.5mm to spare. This appears to work but just bugs me because I have never seen such tight clearance.

If I remove the spacer for the freehub body, that will pull everything more towards the center, then the chain on the #8 will be a good ~2mm from the right dropout. Where the freehub body meets the wheel, it's flushed flat, so all the spacer really does is just jut out the freehub body.

Is that spacer required for a freehub body to work properly?

FBinNY 

Two things to consider, then use your judgement.

Is the spacer necessary for the freehub to clear and not rub on the shell?
Would moving the cassette more inboard bring the RD too close to the spokes in low gear?

FWIW- the chain is reliable and will not magically move over and rub if it clears, even if only by 0.5mm, so I wouldn't fret over it. To be sure, shift to high and use a pry to push the chain outward as far as it will go on the sprocket, to confirm that it cannot ever rub on the frame.

Or, after answering both questions, consider mounting the freehub without the spacer, then use a thinner spacer behind the cassette to improve spoke clearance, but stay clear for the dropout by 1mm or so.

grumpus

Moving the freehub effectively moves the dropout too, so you don't gain any clearance.

FBinNY 

Yes and no, but raises an interesting point.

The axle spacing is semi independent of the freehub, and managed with spacers to control OLD and offset of the freehub and shell. Your point is well taken, because removing the spacer behind the freehub will require an equivalent axle spacer to maintain OLD and dish.

What's interesting is that if there's a thin spacer on the left end, the OP might simply move it to the right side, solving his problem. Of course this will change the dish slightly, but not enough to fret over.

Reynolds 

Why not simply put a 0.5mm spacer between the drive side cone and locknut?

soyabean

I've already done this. I just don't know if it's legal.

When I got the freehub body towards the dish, I made up for it by adding axle spacer for the right dropout.

All other proposed workarounds do not work.

Simply removing an axle spacer from the left dropout (and stacking it on the right dropout) will also displace the dish. I'm very OCD about this and a 2mm off-dish is huge.

Simply adding axle spacers on the right dropout starts to widen out the stays, and that in turn also plays with the dish.

FBinNY 

Legal? The hub police won't be knocking your door anytime soon.

But it does depend on how OCD you are.

A 2mm change is much more than you need, so replacing it with two 1mm spacers on either side will move the shell 1mm to the left.

An added 0.5mm spacer on the right changes dish by only 0.25mm, and that's also too little to fret over the OLD difference.

So, you need to prioritize your goals; textbook perfect, spoke clearance, dish, OLD, etc, and proceed accordingly.

Me? I'd leave it as is and go riding.

KCT1986

FWIW, the attached shows the 'standard' spec for the measurement between the right side locknut and the 'backstop' of the splines (where the cassette stops). SRAM shows 40.6mm +/- (bottom left in diagram), Shimano's nominal measurement is 40.75mm, both for 8/9/10 speed freehub bodies. You can go a little shorter but run the issues of chain clearance with the dropout, or sometimes the seatstay (depending on frame & outermost sprocket teeth count.

Kimmo

If I was playing around with all this spacing, I'd be happy with the minimum reliable clearance between small cog and dropout, likewise between large cog and spokes. The standard arrangement has plenty of excess, which you can trim away to buy room to reduce dish.

There's so much excess in fact, that I have a 10s wheel on which I replaced the 10s freehub body with a 7s one, paired with a billet cassette, the back of which was machined to fit on the shorter 7s splines. While I was at it, I added 2mm to the OLD. All to minimise dish. Gap between RD cage and spokes in first gear is minimal but adequate; 3mm or so

Re-dishing a wheel isn't hard.

grumpus

Set up the hub to work with the gears, when you're done with that dish the wheel to suit - setting the hub spacing to suit the wheel dish seems like the tail wagging the dog. What are you even trying to do?