7-speed MTB cassette on 8/9-speed wheel hub, where does spacer go?

Question

Sheldon Brown says "Add a 4.5 mm spacer before installing a 7-speed cassette on an 8-, 9-, or 10-speed hub", but doesn't seem to show spacer placement in any picture. My direct experience is that the spacer below the cassette put the smallest gear so high that it didn't engage the hub splines. So which is right, spacer below or above the cassette?

I have a 20 y.o. MTB triple with 7-cog cassette, and I recently needed the rear wheel replaced (warped and cracked). Took it to LBS, they only had 8/9 speed wheels and so told me they had to add a spacer when mounting the cassette. No problem, I know 7-speeds are old; "you should adjust the derailleur so it doesn't drop the chain", cool I can handle that.

WRONG? This is where they installed the spacer, which I noted but had no idea at the time might be wrong. 1,2): LBS installed spacer below cassette, which led to 3) small cog NOT able to engage hub splines. (I verified spacer thickness was ~4.5mm.)

P1:
P2:
P3:

I assume the lockring friction was all that was keeping the small gear from spinning. Turns out I could spin on that cog just fine, but standing on a climb caused immediate movement and I guess lockring breakage. As soon as I stood up, I heard a bang and my feet fell out from under me; immediately I was falling forward over the handlebars and splashed on the ground with the bike on top of me (earning road-rashed arms and a fractured elbow, plus surgery for a plate and 6 screws). I thought I had broken the chain, but it turned out to be fine. Disassembling to put the bike in the car, I noticed the small cog was hanging loose and the lockring was gone.

RIGHT? Playing around with the assembly aferwards, it looks like on top is where the spacer should be placed, or at least fits best. 4) Smallest cog engages hub splines as it should; 5) the tabs in the spacer just barely contact the splines.

P4:
P5:

Sheldon Brown says "Add a 4.5 mm spacer before installing a 7-speed cassette on an 8-, 9-, or 10-speed hub", which does imply it should go under the cassette, but as my pictures show, ABOVE the cassette seems to make much more sense.

So am I right and LBS wrong? Or am I missing something?

FYI I haven't taken the wheel back to the LBS yet (still piecing together what the failure mode was) but will eventually. Still deciding how upset I am.

UPDATE (7/13)

I cleaned some of the grease off and took some better pics. First to clarify P3 above, there was a very thin spacer between two smallest cogs, I assume to set correct gear spacing.

P6:

P7 shows the buildup of parts on the hub, with the 4.5mm spacer underneath. First 6 cogs, add gear spacer (splines completely hidden), add 7th cog (no spline contact, it just spins). P7:

And the same buildup on the original 7-speed hub; P8:

What's more interesting is these pin heads protruding from the bottom of the cassette; side shot shows how much cassette is held off spacer by pins. P9:

Because I have these pins, do I need a different kind of spacer, one with recesses?

Answer 1

A spacer to convert an 8/9 speed freehub to 7 speed freehub goes behind the cassette, between the hub shell and the cassette. Most cassette lock rings will not bind on the spacer, they will go inside it if you place it on the outside.

In addition, the aluminum used in most cassette spacers is too soft, and would compress under direct compression from the lock ring. It would not stay tight.

Hope that helps.

Answer 2

I've just replaced a 10 speed cassette with 7 speed and had a similar problem with the studs protruding from the rear of the cassette. These studs can be ground flat using a grinder, as far as I can see they serve no purpose other than to hold the cassette assembly together when not on the bike.

Answer 3

Sheldon says the spacer should go on first, and that would make sense, to keep the sprockets the same distance from the end of the axle that they were on the old wheel. Then only fine tweaking of the derailer would be necessary.

Having just replaced the 6-speed cassette on my bike I can say that the small cog should only engage about half a cog's thickness of the splines -- you'll notice that the splines on the cog are only on the inner half and thus that's all that can engage anyway. Can't tell for sure with the grease, but it looks like that 3rd picture is showing about the right amount of spline.

My guess is that the LBS guy failed to get the small cog properly engaged on the splines when he put it together.

BTW: Why didn't you just have them replace the rim?

Answer 4

There are clearly some incompatibilities between your cassette body and and the new small cog that it's butting up against, as well as the additional protrusions on the back of the cassette.

You are correct in that the small cog must fully engage against the splines and it does not appear from your third photo is that it will be able to. Though the LBS put the spacer in the correct place, they apparently missed some other fundamental incompatibility. A new 7-speed cassette will work correctly with a 4.5mm spacer; but there were so many variants of older cassettes that it is difficult to be certain what will work.

Your best solution is probably to purchase a new cassette, as compatibility with an 8-speed cassette body using the 4.5mm spacer will then be correct. If price is particularly a sensitive matter, you could alternatively modify the spacer you have to fit appropriately. Off-the-shelf spacers for this situation don't exist, unfortunately. It looks like you'd need to notch it for the three prongs on the back as well as remove a small amount of material to account for the protrusions on the front of the hub.

Whichever solution you pursue, the mechanic that did this installation should have noticed the assembly failure and advised you to of your options rather than letting you ride off with an unsecured cassette.

Answer 5

You need to very careful mixing and matching freehubs and cassettes. If this had been me, I think I would have been pursuing a claim against the shop in question.

As suggested, the 3 bolts holding the cassette together could have been removed allowing the gears to slide further on. However, there are problems here also. If you run a cassette like this, the individual sprockets slowly begin to loosen. If you are a steady rider, not covering many miles you might get away with it. If you are a strong rider I would say never remove these bolts or 'rivets' !

This has happened to me twice, and it took a while to figure out what was going on. What happens is that when you are pedaling hard (in say the middle gear) it begins to move very slightly under the load. As this takes place it burnishes the sides of the sprocket and it's neighbours. This creates a small gap, and of course all the sprockets begin to move - and that creates more rubbing etc etc.

With those 3 pins installed, the load is shared across all the sprockets, and they effectively rotate as one locked assembly. Shimano know what they are doing with those pins... Sheldon's recommendation to remove these pins is misguided. Please don't remove them.

Answer 6

Re: Using a spacer and grinding down the 3 pins that hold the cassette together:

It would be simpler to grind 3 notches in the spacer itself, which preserves the original cassette strength. That can easily be done with a rat-tail file or power tool. I find it dubious that those pins simply exist for convenience, which seems like Internet bro-science that lacks engineering context. Some people even suggest removing the pins altogether. Why do a ghetto hack when you can file down a cheap spacer by about 2mm instead? There is enough metal left in the spacer to maintain its strength.

Answer 7

Interesting topic, thanks for opening this thread and I hope your health worked out. I bought a 7 speed shimano HG 50 for a 8 9 speed hub. I put a spacer ( 2.5 or 3 mm) between the casette and the wheel. Then tightened the lockring. there was still some play. I checked the technical drawings for this casette and noticed the sprocket spacer between the 2 smallest cogwheels was not there. This spacer is supposed to be 1 mm. So I get the impressions that with this 1 mm extra by adding the missing spacer between the two smallest cogwheels, the whole thing will be tight and solid. Still leaves the question, my original spacer , between wheel and casette, was less than 3mm, Sheldon Brown says 4.5, which is 1.5 mm more.

I must also add that my casette has 3 rivets, sticking out from the wheel side of the casette, ( some casettes have only 2 rivets on that side , causing the whole thing to wobble, and making it necessary to make at least 2 'openings ' in the spacer for the rivets. Anyway, there is a good youtube video by rjthebikeguy who also uses a 4.5 mm spacer.)

Because my casette has 3 rivets sticking out, there was no need to make those openings, causing the casette to be pushed away from the wheel an additional 1 mm. Bit confusing, when it is written down. I guess there are different 8 9 speed hubs on the market, with different lengths, that might vary 1 2 or maybe even more mm. Anyone knows if the missing sprocket spacer ( between the 2 smallets cogwheels ) is necessary ? I bought the casette new, it was just not there.

Thanks, Update! Just to show how easy it is to make mistakes: I checked the description, and it turns out that my casette does Not have this 1 mm spacer between the two smallest cogwheels. Because my casette is 12 28 , and appearantly with that number of teeth, no need for the spacer, All other combinations ( 13 to 34 have the 1 MM spacer, in a different place. )