sonatageek

I am piecing together a road bike and installed new cables yesterday. It has friction stem shifters and a Suntour Alpha 3000 rear derailleur (RD). It will shift fine, up and down from the higher 4 gears, but if I shift down to the lowest one, it seems to get stuck.

The cable is not binding -- I can shift down and it does slack, but the derailleur does not move.

I have lubed everything and am wondering if I should just swap out in another RD from the parts box, or if anyone has run into this and has an easy fix.

jeanluc

Some ideas:

• the inner wire is too tensioned and it doesn't allow the derailleur's inner spring to move out.
• something's weird with the inner spring
• try oiling the parts that move relative to one another in the derailleur, perhaps friction prevents the move (dirt? rust? small pebble between the coils?). You do say you had lubed everything so I'm listing more to doublecheck.

DannoXYZ 

By "lowest" do you mean the cog with the smallest number of teeth on the outside? Or the inner-most one with the largest number of teeth?

Might be as simple as adjusting the limit-screw.

cny-bikeman

Yes, please explain more clearly. It's best to use largest/smallest cog, rather than gear. Do you mean that if you shift to the largest cog it does not want to shift back to the smaller ones? If so it is neither springs nor limit screws nor lubrication. One has to use logic when diagnosing. You can shift into the gear, so not the limit screw. The shifting is proper in every other gear and the spring is at the greatest tension on 1st, so it's not the spring or lubrication or an obstacle.

Either your chain is the wrong length, the large cog is beyond your freewheel's capacity or the "B tension adjuster (https://www.sheldonbrown.com/derailer-adjustment.html) needs to be screwed in to rotate the derailleur backwards further. As always check Park/Sheldon first to make sure you've done everything correctly, and the cog limitation of most derailleurs can be found with a little searching.

guidosan

It will shift fine, up and down from the higher 4 gears, but if I shift down to the lowest one, it seems to get stuck.

It sounds like a limit screw issue. Best thing is to just start from the beginning. Not sure how much experience you have, but just in case it is limited, go t the Park Tool website and start from the beginning here is the link:

https://www.parktool.com/repair/readhowto.asp?id=64

sonatageek

I am talking about the largest cog on the rear cluster. I have adjusted the limit screw and it will shift onto that cog, but the issue arises when I try go from the largest cog to the next smaller (or any smaller) - the inner cable with go slack, but the RD does not move. I am wondering if the cable housing that is the loop that feeds into the RD might be the issue. That would make sense, as the cable is slack, but I cannot tell if the slackness is making its way through that piece of housing.

I will take a look at that tonight after work.

AndrewP

It is called the lowest gear because there isnt a lower one to shift down to. You can only shift up from the lowest gear.

cny-bikeman

Please see my post above. If the cable were the issue most likely you would have MORE of a problem on other gears, as the spring tension is less as you progress to smaller cogs.

sonatageek

Point taken. I replaced the chain with one from the parts bin (the chain that came off was a rusted mess) and the wheels and thus rear freewheel cluster have not been used with this RD or chain. This is the first time I have really done a quick and dirty 'frankenbike' build --

sonatageek

So after fiddling with this this issue for more time than I should, I took a cheapy Shimano SIS rear derailiur off of a junk bike I picked up today from behind a bike shop and installed it with all of the other existing parts. Problem is gone.

There must have been a weak spring or some other issue with the existing RD. None of the cable, limit screw or anything else seemed to help.