Just picked up a Super Course for a good price (seems to be a rash of these lately).

I decided just for the heck of it to install a Shimano Light Action rear derailleur (the previous owner had 'updated' the simplex drivetrain to a suntour 'u' rd and a low-level shimano fd) and accompanying shifters.

My problem is that when in the big ring up front (on a double) and two largest cogs in the rear (on a 5 sp freewheel) I get some pretty serious cross chaining that throws the chain off the ring. I wouldn't really mind except that this eliminates 2 of my 10 speeds and seems fixable.

So here are my questions:
1) the Light Action rd is long cage where the Suntour 'U' was short--this couldn't be part of the problem, could it?

2) Does it have to do with chain length? The long cage effectively shortens the chain, but it was somewhat too long to begin with. Now, when in the big ring up front and smallest cog, the jockey wheels are perfectly aligned vertically.

3) Is this just a standard 10sp dilemma, something to put up with and forget about?

Many riders avoid cross chaining because it adds unnecessary wear to the drivetrain: it does cause the rider to lose a couple of possible gears. That said, a properly setup bike should operate in all the gears unless the range spread is quite extreme.

I'm not familiar with that particular Suntour, or any of them really.

When you say 'throws the chain off the ring' do you mean it just drops the chain to a higher gear or throws it completely off freewheel and onto the hub? Could it just be a low limit adjustment problem on the RD? When you're set on big/big how do the pulley wheels line up in relation to the sprocket? They should be just slightly inboard of being centered and if they're any further you can use the low limit adjusting screw to return them to the proper alignment.

When I said the big-big combo threw the chain off the ring, I meant that the chain was being forced (inboard) off the large front chainring. The RD limits are properly set.

My other suspicion is that the bb spindle is too long--but, man, I really don't feel like messing around trying to find a shorter spindle for these cottered cranks. I honestly don't have much experience with cottered cranks and would rather just leave well enough alone.

I'll try and take some pictures this evening.

The bottom bracket is the culprit, probably. A straightedge parallel to the toptube should split the front rings, and center on the middle ring in the back. It sounds difficult to line up, but in practice it is pretty easy to get close enough to see if there's a chainline problem. Calculate the gear inches in each combination...at least one of those two ratios is a near duplicate, more than likely.

Kbjack,

Chances are the chainline (plane which bisects the two chainwheels) should be 43.5 mm offset from the center of the downtube. If this is the case and the cogset doesn't line up, it's the rear triangle needing alignment, not a different bb.

I can help you check this out, and I have a lot of free time these days. Gimme a call or pm.

I had that problem once, the crank wasn't on the spindle properly. Take out the cotter, and move the crank arm in closer.

Problem solved. Matt has a cotter press, BTW.

Have your derailleur-hanger's alignment checked at your LBS - unless you own the tool. It's a simple operation take a few minutes. And, if a problem is found, it is easily fixed.

Won't hurt.

PM(s) sent.