karldub

When I adjust my rear derailleurs I generally unfasten the shifter cable and set the limit screws. Then I go back to the smallest cog, I fasten the shifter cable and start increasing cable tension with the barrel adjuster in small increments from there.

However, something's up with one of my rear derailleurs (Shimano RD-7800) right now. After setting limit screws right and starting with the smallest cog, when I start to fasten the shifter cable even with a pretty "slack" shifter cable, the rear derailleur just goes into shifting mode and changes to the cog next up, as if the cable had a lot of tension (is there some kind of inherent tension in the last part of the cable housing?). This derailleur didn't use to do this with my previous shifter cable (stock Jagwire with matching Jagwire Road Pro cable housing), but after I changed to an unbranded/generic shifter cable this happened (or it might be due to the derailleur getting bent somehow).

• The problem is: The only way to get the derailleur to go my smallest cog is too remove ALL the cable tension, and then I can't shift away from the smallest cog.

• Can this be due to the odd combination of cable housing and generic shifter cable creating a lot of automatic cable tension, or is it more likely caused by some kind of derailleur failure?

JohnDThompson 

How did you determine that the limit bolt was properly set? If it is properly set, attaching the cable without tension should not cause the derailleur to move at all. All I can think of is check the hanger alignment and "B-screw" adjustment.

karldub

To check, I unfasten the cable, and help the derailleur to reach its limits by going to the outer gears by hand. Has always worked on other derailleurs, and this one as well.

Yep, my thoughts exactly, that's why I don't understand it. Yes, I try to examine the hanger alignment.

mitchmellow62

Could the return spring in the parallelogram be losing tension? Possibly the slight increase in friction in the new cable/housing can't be overcome at the end range of the spring? I have a 9 spd XTR much used since the early 200's and can experience a similar problem. It's been my assumption that the spring is losing tension.

karldub

Thanks.

I found the issue. The new cable was slightly thicker than the old one, which when going through the tight jagwire dirt/deflector adaptor on the cable end, increased the friction such as it didn't push cable back through when shifting back to lower gears.

cyccommute 

Fire suit on! Here goes!

Arrrgggghhhhh! Limit screws will be the death of the world! If you are installing a derailer, have at the limit screws with wild abandon! If you are adjusting the derailer and the limit screws have already been set, leave them alone! They are not involved in any tuning of the derailer for shifting. Their only job is to “limit” the travel of the derailer inboard and outboard so that the chain doesn’t jump off the cogs on either side of the gear cluster.

Additionally, if you are setting limit screws for any reason, it should be done with the cable attached. Not having the cable attached won’t make as much difference on the high gear but it will make a huge difference on the low gear. Without the cable, the derailer wants to return to the highest gear/smallest cog (unless you are using a low normal rear derailer). The location of the derailer needs to be precise and if the spring it pushing the derailer towards right on the bike (from the back), it won’t be in the location it needs to be in.

You could have some drag between the cable and housing but that seems unlikely new cable. Is the cable hanging up on the frame somewhere? Check to see if you’ve hooked it over a water bottle cage or you’ve routed it through the wrong hole on the bottom bracket guide. Another possibility would be that your cable housing from the rear stop to the derailer may be too short. Check to see that the angle of the housing going into the derailer body is straight and doesn’t have some kind of kink to it.

Another possibility is a kink in the cable. This happens sometimes on installation of the inner cable, especially if you had some resistance to putting the cable into the housing.

Yet another possibility is routing of the cable around the anchor bolt. Do you have it on the right side of the bolt? There is usually a channel carved into the body of the derailer to help show you where the cable should go.

If the derailer was working well with the old cable, changing cable and having problems would point to the cable rather than the derailer as a problem. Pictures would be helpful.

EDIT: I didn’t see that you had found the problem but my advice on limit screws still stands.

cxwrench

Funny, whenever I set up a new derailleur I adjust the limit screws before attaching the cable. This has never caused any issues for me. Doing it this way keeps me from shifting over the large cog because the low limit isn't set. Trying to set limit screws w/ the cable attached can result in 'false' setting.
ETA: Now waiting for someone to throw a fit and tell me how wrong I am.

Litespud

that’s what I do - to my mind, limit screw setting and cable adjustment are largely separate issues - deal with the first before you start messing with the other

LAJ

No reason for anyone to throw a fit over correct and logical help.

squirtdad

that is how i have historically done that also..... but your are wrong and wear funny shoes

Iride01 

After you attached the cable to the pinch bolt, did you pull on the bare cable going along the downtube and then release it then see if there is any more slack to be taken up?

Doing that helps pull the cable housing ends into the stops and get them seated better. But not a guarantee, I still sometimes have to pull a little harder when I put the cable in the pinch bolt.

Just for the record, On a new installation prior to putting the chain on, I set my limits prior to attaching the cable too. I just push the DR body to the opposite side to get the low stop adjust. And actually after everything is all set up, when the bike is in the low or high gear, the DR isn't even against the limits.

However if something changed, like a different wheel and cassette is put in the frame, then doing the other way being argued for setting the limits works well too.

cxwrench

:d

70sSanO

I used to do it that way, but made a slight tweak to the process.

For index shifters I set the H (top normal) RD limit screw without the cable attached; make sure the “L” screw is backed enough for full range.

Connect the cable and work through the gears, make adjustments and then bring the “L” limit screw to where it is just touching, when in the lowest cog, and then very slightly back it out.

With old downtube/bar end friction, and index, you needed both limit screws to stop the shift lever, by way of the derailleur, from over shifting the chain.

Ratchet & pawl shifters can only shift the derailleurs so far so the “L” limit screw is not really necessary to stop the derailleur/shifter. If it is set with not quite enough travel the shifter ratchet will wedge against the pawl and will end up making it difficult to release the pawl. I still set the screw close as a precaution to keep the RD from going onto the spokes if something bad happens.

John