Tandem BB question
 

(I've posted this query in Tandem Cycling and Bicycle Mechanics as well.)

I'm reviving an early 80's Peugeot TH8 with peculiar history and found something curious. Both BBs were assembled by someone with the fixed cups on the left instead of the right. Since French threading used right-hand thread on both sides it is possible to interchange L and R bearing races.

From other evidence it looks like someone had been attempting to upgrade from a Peugeot-branded Stronglight double to a TA triple but never finished the job. One problem that I have now is that the chainnring bolts of the triple hit the chainstay. (This might explain why the attempt was abandoned.) Both spindles are Stronglight 120mm, shown on the Harris website as correct for a TA road-triple, but the proper length for a TA tandem is 127mm.

So I have two questions.

Is it possible that the fixed cup holds the bearings further to the outside? If so assembling them on the wrong side would have moved the chain line to the left.

Would anyone have a 127mm ISO spindle? I believe another 7mm would be sufficient.

Thanks!

Hi Jim. I saw this over in the Mech forum and I am trying to pic the whole thing in my mind. the 127mm spindle certainly sounds right for a Tandem where the triple will be.

I don't really understand why the PO put the fixed cups on the left but I would certainly pull them off and start from scratch. is the triple going in the Stoker position?

Thanks for the reply. Yes, the triple goes in the stoker's BB. Yes, the spindles are asymmetric and installed with the long end on the right. One more spec, for what it's worth, both BB shells are 68mm wide.

I was just talking with southpawboston (picking up some parts) and he proposed an interesting idea: The two cups are the same thickness. The fixed cup always sits flush with the BB shell, and the adjustable cup usually sits outboard a little more to allow space for the lockring. So perhaps the previous mechanic swapped them to move the spindle to the right in order to get more clearance. Except it wasn't enough.

Another idea is that by using a thicker British spindle (but with ISO taper) it would force the bearing races farther apart, thus moving the adjustable cup on the right further outboard, and the spindle itself half of that distance. I don't know if that's the right thing to do. A longer spindle is the real solution.

Thanks again.

Simply set the bike up with front drive. You will also get the added benefit that you can more readily shift and trim the front derailleur and can use every combination of chainring and freewheel cog.

Not sure if that would work. I seem to recall not having enough adjustable cup thread left in the BB shell after trying this once (don't recall what I was using though). I could be wrong however.

Interesting idea. Does the chain weight over such a long run of chain make it sag too much?

I have a tandem with a front drive, and it has guide for the bottom section, but the top section doesnt sag that much.

Nice looking bike! Yours too, Citoyen du Monde, but the pic doesn't come through enough for me to see it clearly. I will certainly consider the front-drive arrangement.

In the meantime I went over to Harris and rummaged through a few boxes of spindles, found a bunch of longer ones but with slightly different bearing spacing. And a few shorter ones too. For $5 it was a safe bet, and seems to have worked out okay. The chainrings moved a bit further out than optimal but certainly workable. This initial build-up is a feasibility experiment anyway before I deal with some aesthetics questions (like decent paint). So I can re-evaluate the drivetrain then.

Thanks for the suggestion!

Or a cross over drive? I've just converted mine and it works a lot better now!

http://s623.photobucket.com/albums/t...t=DSCN0440.jpg

http://s623.photobucket.com/albums/t...t=DSCN0443.jpg

http://s623.photobucket.com/albums/t...t=DSCN0445.jpg

http://s623.photobucket.com/albums/t...t=DSCN0446.jpg

http://s623.photobucket.com/albums/t...t=DSCN0436.jpg

One question. How does the bottom guide/pulley mount and where would one get such a device? I guess that's two questions, isn't is? Oops, I mean three, don't I?

I called TandemsEast today to ask about them and the conversation didn't go, ah, smoothly. He insisted that a front-drive arrangement wouldn't work, that it would eat the chain, that it would be an expensive mistake. I'm willing to risk it for now. I like the looks of the two bikes without an lower support, but I should consider the options.

Thanks,

Jim,

I switch my Schwinn from same side timing and drive chains to opposite sides. Since mine did not come with an eccentric BB for chain adjustment, it had this ugly bracket and chain pulley system. Kind of crude but it got the job done. This is how it originally looked.

http://i125.photobucket.com/albums/p...t/P1010014.jpg

When I moved the timing chain to the left side I still needed to be able to take up the chain slack. I tried several different approaches, but ended up using this:

http://i125.photobucket.com/albums/p...nTensioner.jpg

It is for converting a multi speed to a single speed that has vertical dropouts. It is intended to be mounted in the RD mount on the rear drop. I used spacers from my local ACE along with some quality stainless nuts and bolts to snug everything up. It works well and much better than the original set up.

Sorry about the slightly out of focus picture.

http://i125.photobucket.com/albums/p...gChainView.jpg