Gates Belt Alignment Problem
 

Had rear section of Calfee modded for Gates drive (ring clearance), now trying to get that set up and having some problems.

First problem that frame mod was not adequate for stoker timing ring clearance. But can fix that with about 3mm of spacers.

Second problem is that I can't get belt to stay on the stoker ring, it wants to walk off towards the frame. Have tried to fix by shifting front timing ring in and out (figuring out would be the way to go, but tried both directions in case, have gone +2 and -3mm relative to stoker ring), but this does not seem to help.

Any ideas?

Side note, from Gates site see that rings set up such that shoulder is on opposite sides for front and rear rings, for the tandem rings shoulder is on the same side. Wonder why the difference?

Having worked with a few different types of belts over the years, the first thing that stands out is that they must be running in exactly the same line, i.e., dead parallel. That means if you add 3mm of spacing to one BB, you MUST add 3 to the other, or the belt WILL walk.

This is so critical that, IIRC, at least one of the major manufacturers won't "guarantee" that your tandem will accept a Gates belt "upgrade". What they're really saying is that although your earlier model bottom brackets are certainly within tolerance for a chain drive, they are not "dead parallel".

^ Co-Motion specifically will not warant a Gates retrofit. According to them the BB's have to be exactly parallel. Before the Belt drive, they didn't allign the frames to the same tolerance that they do on the new ones in that regard.

If you get the chain line exactly straight, (which you can fine tune by sliding the eccentric side to side) and it still walks, it may be that the BB spindles are not precisely parallel.

Figured it out. As close as I can tell, for my set-up the front ring needs to be 3-4mm outboard of the rear ring for the belt to track properly. Needed 3mm of spacers to get the rear ring off the chainstay, and 11mm of spacers up front. BBs are both DA 118.5s, so I guess the captain timing crank/spider is sitting a few mm inboard of the stoker timing crank/spider. Took awhile fiddling with spacers/bolts/nuts to come up with the magic combination.

11 mm of spacers seems like a lot. Could you slide the eccentric a few mm's left, and not use so many spacers?

Agreed, 11mm of spacers would worry me.

Calfee eccentric doesn't slide.

In fact it is "only" about 7.5mm of spacers. Thought I was working with 1, 2, and 5mm spacers, measuring the assembled bits realizing they are maybe 0.5, 1, and 3mm spacers. Can't get a direct shot at gaps with vernier, but it is close to 2mm of spacers in the rear, and 7.5mm in the front. The front does feel pretty solid, I have shortish (maybe 5mm) ti bolts into long stainless nuts, and have some longer steel bolts I could also use. Pics attached, FYI.

But 7.5mm is still a lot, so I will see if Craig has any thoughts on a better approach. In the meantime, if others have ideas I am all ears.

Attachment 142739Attachment 142740Attachment 142741

Craig thinks spacers are OK, and that the eccentric may be allowing the front BB to get a little bit off-axis.

Is anyone running this system with the flanges on opposite sides (like they do for singles)?