Shift Problem Diagnostics Needed
 

Solved

Here's the situation. I've got a low-end bike with balky rear shifting. The rear derailer is the cheapest of the cheap, a Shimano TZ-30 on a 14-28 7 cog freewheel; and it got banged and the pulleys were angled in towards the centerline. I gently straightened them as best I could and they look very close to proper alignment. I also replaced the god-awful cheap grip-shifts with old Suntour 7-speed indexed barcons and replaced the cable and housing at the same time.

The problem is that I cannot get the low gears (big cogs 7-6-5 / 28-24-21) to engage without autoshifting, skipping, etc. I've tried in both index mode and friction mode with the barcons and the problem is quite severe even with careful and slow shifts in friction mode. Since the freewheel and chain only have a couple of hundred miles and show no signs of wear related stretching, I doubt that is the issue.

Because this is effectively my first bike both as a rider and as a mechanic, and because I'm rather budget constrained, I'm trying to determine whether the problem lies with me, the shifter/derailer setup or the derailer itself. I've gone through various and sundry articles on sheldonbrown.com and other similar places and am stumped. I'd but a new derailer -- they are not expensive -- but if the rider or mechanic are at fault, replacing one working part with another seems silly.

Indexing is unlikely to work well mixing Suntour index with a Shimano derailleur, but it should be easy to make friction mode work very well.

Usually this is simply a matter of dialing in the adjustments, including the often misunderstood "B-screw" which sets the height of the RD. Find a tutorial and review it thoroughly disregarding ONLY those relating to trimming the index mode.

However, you're starting out with a handicap because you had a bent derailleur, likely installed on a bent hanger. So you'll need to make sure these are resolved before wasting time trying to adjust. Start by carefully eyeballing the idler cage, and checking with a straight edge, to make sure it's straight and the pulleys are on the same plane with no twist or misalignment between them. Next shift to a gear combination where the idler cage is close to vertical. Place a straight edge (yardstick or broom handle) vertical against the rear wheel, then stand behind the bike and eyeball to see if the cage is parallel to the wheel. You can repeat the same test in the horizontal plane (likely requires a friend to hold things for you), but it's harder to get a well alignd view, so do the best you can.

With the RD reasonably aligned, you should be able to get some semblance of reasonable shifting in friction mode if you follow the tutorial.

After you get the derailleur and hanger straight, and correcting any deficiencies like cable installation problems, follow this procedure from the beginning without skipping any steps. Park Tool Co. » ParkTool Blog » Rear Derailler Adjustments (derailleur)

Make small adjustments and smaller yet as you get close. 1/4 turn at a time may be too much; sometimes less than 1/8 turn will make all of the difference between perfect and so-so shifting.

1. 7-speed isn't as picky about perfect derailleur alignment as 9+ speed systems. You're probably close enough.

2. Friction shifters will overcome a lot of other problems. If it was my bike, I'd switch those barcons to friction and love it for what it is.

I've read differently, specifically for 6 or 7 speed Shimano, not modern/quality stuff. But, as you say, if I get friction to work smoothly, I'll deal with it.

I'll be a monkey's uncle if the p.o.s. RD-TZ30 has a "B" screw. I was looking for it since Sheldon Brown and others reference it; and since the problem seems to be tension/height related. The $5.00 OEM part does not have a ready way to remove the plastic housing and seems likely to break if forced. Will try to find a part diagram for the thing.

As far as the hanger goes, it seems as square as I can tell with a small steel square. The bend was in the pulley hanger. I've got it so that there is about 1mm or less of misalignment between the jockey and idler. Using my friend parallax, I can ensure that they are both straight in the front-back/up-down plane. Obviously I can ensure with the cable adjuster that one or the other is exactly aligned with the relevant cog, just not quite both.

When the bike is hanging, I can friction shift through all the gears, though the difference between the lower three cogs is very small -- requires finesse on the shift lever. When actually riding, even if not really pedalling enough to maintain speed, the shifts are useless.

However, you're starting out with a handicap because you had a bent derailleur, likely installed on a bent hanger. So you'll need to make sure these are resolved before wasting time trying to adjust. Start by carefully eyeballing the idler cage, and checking with a straight edge, to make sure it's straight and the pulleys are on the same plane with no twist or misalignment between them. Next shift to a gear combination where the idler cage is close to vertical. Place a straight edge (yardstick or broom handle) vertical against the rear wheel, then stand behind the bike and eyeball to see if the cage is parallel to the wheel. You can repeat the same test in the horizontal plane (likely requires a friend to hold things for you), but it's harder to get a well alignd view, so do the best you can.

With the RD reasonably aligned, you should be able to get some semblance of reasonable shifting in friction mode if you follow the tutorial.[/QUOTE]

I've gone through and gotten it to work well while hanging on a stand (step ladder, weights for balance and carefully placed bungees) for repair. Just does not work well on the road.

Just found this RD-TZ30 PDF: http://si.shimano.com/php/download.p...-TZ30-2225.pdf . Seems like there should be one by the lower limit screw. Off to look...

Thanks to all who helped. Turns out that the positional washer that attaches the cable to the derailer was not installed right. Doh! Putting the washer on properly and therefore fixing the cable pull made things work as intended. Spiffy indexing with a shifter that is almost as old as I am! :)