Simple Derailer question.?

Question

So on my wife's Trek 820 Antelope (Circa 1990) the chain won't stay on the top cog - the one by the pedals - it's the largest one.

It does go up to the top cog but then derails back to the middle cog once you let the button go.

Normally to get it to the top cog, you have to push your left thumb and hold it while you pedal. It doesn't "click" up like with my new Trek.

Any ideas as to why this is happening and the possible cure?

Answer 1

Yes, the cable has stretched one entire increment, so use the adjusting collar (on the rear derailleur) to bring it up a step.

It is possible that the limiter screw is preventing the derailleur from shifting far enough. That would be very unusual. Use caution when adjusting the limiter screw in order to avoid jumping that big gear and chewing up the spokes.

See harriscyclery.com for an article on derailleur adjustment.

They are also a Quality Bike Parts vendor and will have 6 and 7 speed shifters available in case yours is worn out. That's rare, but it does happen.

On a bike that old, it is possible that it is suffering from chain stretch, causing it to slip. Do measure a foot of chain, pin to pin. If this is 1/8th" or less longer than a foot, replace the chain. Past the 1/8th" mark, it is time to replace the crankset and rear cogs.
For upgrades, FYI:
*7 speed freewheels often fit into places where there were 6 speed freewheels before. *Sometimes these can't use the smallest cog that way, but you can get an 11-34 to do 12-34 quite easily just by setting the limiter screw onto the 12t cog (7 speed shifter required, Shimano HG50 or HG70 chain required, SGS size derailleur required, like the excellent Deore Top Normal SGS that you may already have).
*Sugino XD600 crankset is a brilliant, featherweight XT/Ultegra grade 26-36-46 square taper model that fits older high-end bikes just dandy.

See the articles at harris cyclery on derailleur adjustment and chains

Answer 2

All these people have it right- your derraileur needs adjustment and it's not an overly hard thing to do (just make small adjustments until you've got it right). Check out the park tools website, they have a good maintenence section which will show you how to "dial in" both derraileurs. If you're handy and the bike is otherwise in good shape this won't be hard.

Here's a question- when is the last time you took it in to the shop to have a tuneup and overhaul? My guess is you could stand to have new cables and housing put on, and if they haven't been overhauled recently the hubs, headset, etc could use a good cleaning. Take it into the shop, have them do a complete maintenance job and it will keep going for another 16 years.

Answer 3

It's called a friction shifter and it sounds like it needs some adjustment. You didn't mention the make so I can't tell you exactly where but there should be two screws (together) on the top of the derailleur, they will move the derailleur from side to side to adjust the distance you chain moves on the chain rings. there is also a possibility you may need to replace the shifting cables, over time they will stretch. Hope this helps.

Answer 4

The deraileur issues that the others here mentioned certainly sound like the most likely cause. However, if that doesn't cure it, there is a small chance that you may have a chainring tooth that is either bent, broken, or has a burr on it. I've had this happen a few times over the years that I have ridden.

Answer 5

Sounds like an adjustment issue. Look at the cable adjustment first if it worked right at one time and has not been dropped hard. Next look and the adjustment screws on the derailleur body and reset the stops.

Answer 6

What u need to do is turn derailer cable adjuster counter clockwise 1/4 to 1/2 turn at a time till chain stays in large clog.The problem is probably due to cable stretch ,