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Freewheel/Chainring Alignment - jumping gears
#1
I just bought an old Schwinn Tandem that I am restoring to riding condition. This is a 5-speed bike. (I don't think anything here is specific to tandem bikes, but that's what it is). When I bought it, the (rear) chain was stuck between the two chainwheels on the rear (stoker) crankset. Owner said it was like that when he bought it, and he never had time to fix it. Now that I am into putting some things back together, the (rear) chain always comes off the chainring, when shifting to anything larger than the middle gear. This appears to be because the chainring is roughly in line with the smallest rear sprocket. So by the time the chain is shifted to the middle sprocket, the chain is deflected enough that it wants to come off the front chainring, after several revolutions of the crank.

Nothing appears to be bent too badly out of whack, and there seems to be no adjustments left anywhere to correct this alignment problem. The chainring is not perfect (it is bent a little bit -- don't know why), and the chain is old.

I don't want to replace these old Schwinn parts with new parts if I can avoid it. Does this sound like some kind of alignment problem, or is it really just a combination of less than perfect chainring, and maybe a stiff chain? Should the chain have zero bend when in the middle gear on the freewheel, or is not really true?

Thanks for any advice. -Jim
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#2
Ideally, the chain should run straight when in the middle rear cog, but that's not universally true. Is the front chain running straight or does it go out as it goes back to the stoker crank? If flaring out, it could be that the bottom bracket for the stoker is a bit too long and you could replace that without changing the crank (depending on type of BB).

First thing I'd do is straighten the bent chainring, that can drop a chain and you can usually get them straight without any major issues. May also be worth replacing the chains (or at least measure them for stretch). Chains are a wearing part that needs to be replaced occasionally anyway.

If it seems like the placement of the stoker crank is right, you may be able to put a spacer behind the freewheel and move that out slightly (if you have enough room between the dropouts. Last resort could be to get a "chain keeper" that will help prevent it falling off. But that's curing the symptom, not the disease.
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#3
Thanks for the reply.

I suppose I should take care of the chainring. This is a stamped steel affair on the Schwinn - it's not just a ring on a spider. I think my only choices are to attempt to bend it back true, or find another old Schwinn to cannibalize. It's a one-piece crank. I tried removing a spacer between the chainrings to move the outer ring inward a bit, but I don't think that made much difference, as the chainrings naturally sit a little distance apart. Front chain seems to be straight and true.

As I bought it, the bike came with a chain "keeper". There was a front derailleur mounted for some reason at the stoker crank position (on a 5-speed bike). The inside of the part that would move the chain was scarred up pretty good, indicating that the chain had been rubbing on this quite a bit. I took it off, thinking this must have been some mistake at the factory. Now maybe it was either a factory or bike shop fix for a problem that they could not fix. This is one of the reasons I suspect that this bike always had this problem.

I checked the frame alignment with the Sheldon Brown string trick, and it seems to check out true. There is no free space between the dropouts when I install the rear wheel. The only way I can think of to change the alignment is to either bend the rear (out of alignment), or to add spacers on the rear axle to move the entire rear wheel out on the chain side. This would at least mean bending the rear stays out to make that fit. The rear stays are rather stout, and there is a third stay in between the seat stay and the chain stay, so I am leary about trying to bend any of that around.


(09-05-2010, 11:19 AM)DaveM Wrote:  Ideally, the chain should run straight when in the middle rear cog, but that's not universally true. Is the front chain running straight or does it go out as it goes back to the stoker crank? If flaring out, it could be that the bottom bracket for the stoker is a bit too long and you could replace that without changing the crank (depending on type of BB).

First thing I'd do is straighten the bent chainring, that can drop a chain and you can usually get them straight without any major issues. May also be worth replacing the chains (or at least measure them for stretch). Chains are a wearing part that needs to be replaced occasionally anyway.

If it seems like the placement of the stoker crank is right, you may be able to put a spacer behind the freewheel and move that out slightly (if you have enough room between the dropouts. Last resort could be to get a "chain keeper" that will help prevent it falling off. But that's curing the symptom, not the disease.
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#4
If there is space between the outer edge of the freewheel and the inside of the dropout/stays, you may be able to add a spacer to move the freewheel over. (Make sure you will have enough room for the chain when it is on the outermost cog.) Most cassette spacers will fit behind a freewheel or you can use the lockring from an old 3 piece bottom bracket.

Definitely sounds like the front derailleur that was on there was being used as a chain keeper. I doubt that was a factory solution. Maybe the crank was swapped at some point and it is slightly off from the original one's dimensions. Do the chainrings have a lip where the edge with the teeth is slightly offset from the main body of the chainring? I've seen these put on backwards which mess up the chainline. Just a thought.

Yeah, I'd be hesitant to start bending a tandem frame up. I'd rather try bending the chainrings some to see if you can get alignment better that way. But bending is always a last resort.
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#5
is your rear wheel original? sounds like the dish could be wrong.
Get on your bad pedalscooter and ride!
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