Take the 2-minute tour ×
Bicycles Stack Exchange is a question and answer site for people who build and repair bicycles, people who train cycling, or commute on bicycles. It's 100% free, no registration required.

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: enter image description here
P2: enter image description here
P3: enter image description here

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: enter image description here
P5: enter image description here

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: enter image description here

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: enter image description here

And the same buildup on the original 7-speed hub; P8: enter image description here

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: enter image description here

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

share|improve this question
    
I accepted zenbike's answer as most correct for my actual question. As to the implied question "why did the proper placement result in incorrect engagement", I think lantius was right: LBS missed some basic incompatibility between old cassette, new wheel, and proper spacer. I took the parts back to the LBS, and they ground down the spacer to make it thinner. –  Joe Bronikowski Jul 24 '11 at 13:21
    
Sorry, I know that this thread is old. But for others who might read this. Sheldons Harriscyclery states that: "If your cassette has bolts holding the sprockets together, you will need to remove the bolts. They are not necesssary, only intended as a convenience for rapid cassette swaps." If you do this you wont have a problem. –  Paulg Apr 18 at 15:31
add comment

5 Answers

up vote 6 down vote accepted

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.

share|improve this answer
    
Thanks, 2 reasons it does belong underneath –  Joe Bronikowski Jul 11 '11 at 23:14
add comment

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?

share|improve this answer
    
I'll get some better, less greasy pics :-) I honestly didn't even think to replace just the rim. –  Joe Bronikowski Jul 11 '11 at 23:18
add comment

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.

share|improve this answer
    
I think you're right about seating & incompatibilliy. I will get some better pics. –  Joe Bronikowski Jul 11 '11 at 23:16
    
I have a similar spacer that came with three notches as supplied. –  armb May 8 '13 at 14:03
add comment

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.

share|improve this answer
add comment

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.

share|improve this answer
    
Welcome to Bicycles SE. This site, like other SE sites, does not function like a typical forum. Users on this site ask questions and the community attempts to answer them. This question was about the placement of spacers. Additional advice such as this would be better placed as a comment rather than an answer in its own right. –  jimirings Mar 5 '13 at 22:40
add comment

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.