3940dxer

I have a Specialized Rubaix road bike (love that bike) with stock Shimano derailleurs. I’ve been riding for about 10 years and despite being fairly mechanical (I’ve serviced a lot of intricate tape recorder mechanisms over the years and have a pretty elaborate shop) I've always been a little intimidated by derailleur adjustment. A couple of weeks ago my dealer did a drivetrain cleaning and servicing but after I got it back I had a lot of chain rub on the front derailleur. The guy’s on vacation now but I decided ok, this is it, I’m going to buy a good service stand, park myself in the shop, and study and practice the adjustments until I’m comfortable with them and have a smooth running bike. With the ongoing covid lockdown, it seems like the right time.

So I started today but ran into a possible issue and would like some advice. The Youtube videos and how to articles I’ve seen say that after adjusting the L limit screw for the right gap with the small chainring engaged, I should turn the barrel adjuster to slacken the cable, and then repeat the L adjustment.

In doing this I noticed that when I turn the barrel adjuster, the cable doesn’t gradually move in and out at the derailleur end. It’s stationary for one or two turns, then jumps to a new position. This happens whether I turn the barrel adjuster CW or CCW. Similarly, I notice that if I push or pull the derailleur with my hand, when I let go it doesn’t always return to the same position. So I’m wondering if the cable or something else is binding, maybe causing inconsistent shifting, and maybe making it harder to adjust things properly.

Does that theory make sense? If so, what might the fix be? (This is a carbon frame and the cable passes through the down tube so it’s hard to see what’s actually going on there.)

I notice the same thing on my wire's Specialized Ruby, which is similar to mine and the same age.

Thanks in advance for any help.

Andrew R Stewart 

It sounds like cable friction. A single notch of an adjuster (assuming an M5 thread and 4 positions per rotation) is about .008" of cable length change. Not too much and well within the flex of a cable/casing/end cap system. When a shock or significant movement happens this amount will be "applied", so to speak.

Internal routed cables are well known for potential increase of cable friction (compared to an externally routed one) and the lesser ability to service the cable. Andy

dsbrantjr

Perhaps new cables are in order due to a kink or fraying somewhere along the run. However, do not remove the old ones until you have figured a way to replace them without needing to fish the new cables through the frame. Running some tubing over the old cables to act as guides for the new is one approach.