jadlc

Hey guys I'm new to the forum, infact this is my 1st post on the forum. I don't know what I'm doing wrong but every time I go for a ride (still in the process of restoring this bike), the chain keeps coming off the back derailleur. I also don't know what exact model my bicycle is...someone please help me figure this problem out.

Bill Kapaun

https://www.parktool.com/repair/byregion.asp?catid=53

Scooper

I think the bike is a late seventies/early eighties Collegiate Sport 10. If so, it probably has the Shimano front freewheel system (FFS) where the freewheel is in the front crank instead of the rear freewheel.

Look for a serial number on the front of the head tube below the head badge and just above the bottom cup. You'll be able to tell what year it is from the serial number. Also, there might be four small, very lightly stamped digits on the head badge itself. Those digits represent the build date. The first three digits are the day of the year (001-366) and the last digit is the last digit of the year.

https://www.geocities.com/sldbconsume.../79ccpg22b.jpg

https://www.geocities.com/sldbconsume.../79ccpg23b.jpg

Retro Grouch 

Try this:

1. But your bike in a gear combination that makes the derailleur arm hang straight down.
2. Prop your bike up vertically and look at it from the back.
3. The derailleur arm should point straight down. If your's points in toward the back wheel (my bet) grab it with your hand and bend it back straight.

I doubt you have a front freewheel bike because that's a one piece crank.

Scooper

Looking at the 1979 Collegiate Sport 10 catalog pages I linked to above, in the lower right corner of the second page:

"Schwinn-approved forged steel one-piece crank with Shimano FF front freewheeling double plateau 39-52 tooth chainwheels."

Retro Grouch 

Yeah, I actually thought about that afterward I wrote it and even thought that I had deleated that part.

I stand by my advice for fixing the OP's original problem. If that doesn't work, however, I've got another, low cost, idea for fixing it that's related to the front freewheel thing.

jadlc

Hmm, so what do you suggest? I really don't understand the whole front wheel rear wheel thing...someone please explain

Retro Grouch 

First, make sure that your derailleur is hanging straight. Most of the time that's the problem. If that doesn't fix it, try me again but you have to work through these things one step at a time.

flian

Your chain is not in your pulley cage. Is the chain coming out of the cage or was it not completely in there in the first place? The chain should run over both pulleys in an "S" pattern (A reverse S looking at the right side).

Retro Grouch 

Yeah, that's a Suntour derailleur. Those were designed so that the derailleur can be replaced without breaking the chain. The fact that the chain was off the pulleys is part of what makes me think the hanger is bent.

jadlc

It's just like that in the picture. When I do ride the bike the derailleur is pointing straight down and is in both of the pulleys correctly (in S pattern). For some reason the rear derailleur swings around even after I tighten it. Do you think I need a new freewheel cog/rear derailleur?

CharlesC

Does the chain come off when you are peddling or when you are coasting? If the latter the front freewheel probably isn't freewheeling and may need to be taken apart and cleaned.

BCRider

Well if the teeth on the derrailleur pulleys are worn or the pulleys are running stiff then that would sure spit the chain off. The two pulleys on the derrailleur should be able to spin very freely. No drag at all that you can feel.

If the pulleys are running at an angle to the chain path due to worn pivots or the possible bent hanger as already mentioned that would not help either. The lack of a closed cage to trap the chain makes all this stuff super important for you compared to "just" important for a closed cage derrailleur where the chain is more or less forced to stay in place.

Check the chain for stiff links. The links all must pivot very freely to your sense of touch. Even a slightly stiff link will kick over the pulleys and then with no closed cage it'll skip off the pulley likely as not.

Hopefully you see that there's a whole bunch of things that it could be and likely it's a combination of more than one. Try to check all the parts in the system for drag and also check everything for alignment. Derrailleur pulleys must be parallel to the wheel in both vertical and fore/aft Any angle in either plane and you'll run into problems. If you find any you'll have to determine if it's the hanger that's bent, the bushings and pins that are worn in the link arms or the support plate for the pulleys themselves that's bent.

Taking it to a bike shop that has the tool to check the hanger is a very good suggestion and should be high on the list of steps to do.

In the meantime flex and force the parallelogram of the derrailleur and see if it moves much which indicates worn pins and pivots. If there's excess movement where there shouldn't be you're looking at needing a new derrailleur. There's no practical way or even only slightly impractical way to repair it.

You can also go along all the links of the chain to check for any signs of stiffness due to rust or other wear. At the same time check the pulleys for free rotation.

If all that checks out OK and the hanger is straight then we really do have a mystery. But right now it's more likely just a problem that you haven't found yet.

Scooper

The reason Shimano developed the front freewheel system was so as long as the bike was moving forward, the rear wheel cogs were moving and the rear derailleur could be used to change from one cog to another without the rider pedalling. The FFS was almost always used with the Positron rear derailleur shifter, which was a very early indexed shifting system.

Here are contemporary Shimano advertisements explaining the FFS and Positron.

jadlc

looks like the problem is the derailleur...the local bike shop doesn't carry any for this model of bike, does anyone know where I can find one?

Stacey

What's wrong with the derailer, that it has been deemed "the problem"?

Sangetsu

How about a pic of the bike with the chain properly routed around the cog and through the derailleur?

flian

Pictures from the back, in line with the chain, would be good. Focus on the derailleur.