Rag joint blues

Centering the sprocket on the hub isn't really necessary. What you want to see is near perfect concentricity with the outside diameter, the teeth of the sprocket with the hub.

Make a pointer using something like a clothes hanger wire and attach it securely to the bike frame. Set the end of your pointer at the edge of the sprocket teeth then rotate the rear wheel watching to see if the teeth stay at the same place compared to the pointer as the wheel turns. Leave the sprocket bolts snug but not tight and use a plastic head hammer or a block of wood and tap the sprocket in the direction it needs to go to get it to spin concentric with the hub. Keep doing this process until the pointer and the outer edges of the sprocket teeth are always aligned through a complete revolution of the wheel.

Centering the sprocket hole to the hub is a good way, BUT, it depends on the sprocket hole being prefectly centered to the teeth. I've seen a few that weren't and even if centered on the hub the sprocket would not spin true. This is why I abandoned the hub/sprocket hole centering concept.

While you're doing the centering you'll also want to make sure the sprocket runs true in the lateral plane. It can't wobble side to side. It must spin true in both axis as viewed from the side and from the front/back .

If you opt for the sprocket adapter approach you must have an accurate measurement of the hub diameter. It can't be 'close' or 'about right' it must be a precise measurement down to the thousands of an inch. Compare your measurements with those offered by adapter makers for an adapter that will fit your hub. Some of them will even make a custom adapter but they aren't cheap. Good luck.

Tom

Toothy,
Another thing to look for is the thickness of the rubber parts. It isn't uncommon to find one, or both, that were cut on an angle. In other words one side can be thicker/thinner than the other. This will assure you a wobble.
You have two options if that is the case. You can order a new rag joint or you can grind the rubber parts so they are flat and consistant thickness across their face.

It's easy to see that if the rubber parts aren't flat that when the sprocket is tightened against them that the sprocket will have a side to side wobble. Roller chain doesn't like that and can derail.

Tom

Toothy said:

OK....I'll look at the rubber also. When you say tap the sprocket, is it ok to hit the teeth or on the inner rim. I would rather not take the coaster brake apart again.

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You want to tap on the sprocket teeth. That's why I suggested using a plastic hammer or a block of wood. You don't want hammer on the teeth with a steel hammer.

Tom

maniac57 said:

I disagree. Takes me about 10-15 minutes to install most rag-joints. I use a screw gun with a socket and tighten it with the screw gun clutch to keep things even. Works like a charm.
15 minutes beats $90 in my book.....

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Yep. Set the screw gun clutch to a desired torque and start tightening in a cross pattern, increasing the torque until all nine fasteners are to the desired value. That should be about 60 inch pounds. While you're doing it keep spinning the wheel to make sure you're not tightening a wobble in.

The only problem I've ever had was with sprockets that were warped or rubber pieces that weren't cut correctly. Both can be fixed with a little effort.
Once installed correctly the much maligned 'rag joint' will perform perfectly.

Tom