I've obtained a new bike for my brother, and in tuning it after a rebuild. Sora shifters, Sora rear derailleur and RX100 front. The RX100 front derailleur doesnt seem to move sufficiently with the Sora lever.

With the front derailleur on the small ring, adjust right out to the tiniest gap between chain and inside plate on first gear, the throw on the derailleur doesnt push the chain onto the big ring. You have to hold the shifter RIGHT inside and give it 3 or 4 revolutions for it to catch and change. Then it rubs, with no adjustment.

Is it a mismatch between components??

Ideas?

Jim

Is your high limit screw set too low? Do you have enough tension on the cable? Those are my first guesses.

Disconnect the cable and loosen up the high limit screw. Reconnect the cable, with proper tension, and try again.

RX100 may be the wrong ratio for modern road shifter.

If you look at the actuator arm on the old derailleur, you will see that the arm is approximately the same length on each side of the pivot screw. The new Shimano road ratio results in an actuator arm that is approximately 1/2 as long on the cable side as on the cage side.

It would appear (from many recient postings here about this problem) that Shimano was using something more like the current MTB ratio back then.

Try adjusting the high limit screw first but if that changes nothing, you will need a new derailleur.

Madmax, you gave this advice when I posted about changing to 105 shifter and it wouldnt shift the chain enough and you were absolutely right about older shimano FD being the wrong ratio. I got a new derailleur and it fixed the problem. In fact my old derailleur was also an RX100 7 used with downtube 7 speed drivetrain.

The only correction to your explanation of why the ratio is wrong is that it was nothing to do with the arm lengths which was similar to the new 105 FD I bought. Comparing the old and new FD, I noticed that the new one has the arm at right angle with the cage (looking from above) but the old one has a bigger angle. The effect was that on new FD when the arm is pulled a certain distance, the cage was moved directly outwards but on the old RX100 some of the movement was forward therefore the lateral movement was less than the new one. A full push of new brifters must pull the cable enough to move the new FD the adequate lateral distance but the old RX100 must need a greater pull provided by downtube shifters.

Whatever the reason, the modern brifters do not provide enough pull for adequate lateral movement to shift the chain. You will definitely need a new one. I just did this upgrade to RX100 last weekend.

By the way, RX100 rear derailleur is also not compatible with 10 speed but is compatible with 9 speed. It simply does not have the adequate lateral movement to reach the tenth cog. I tried it with the shifter and also by hand pulling to maximum stretch.

10-speed cassettes and 9-speed cassettes are the same overall width. Actually, 10-speed is very slightly narrower, which is why it comes with a spacer for 8/9-speed hubs.

Well in that case RX100 rear derailleur isn't nine speed compatible either. I assumed it was becasue I could reach 9 of the 10 speed cassette.

OK, some answers...

The high limit screw was so far out I actually removed it from the derailleur, so its not that.

I'm betting that the preset click positions on the shifter arent far enough apart. There just seems to be not enough throw between clicks for it to properly shift and stay far enough outboard. Sure, it will travel far enough, but it wont lock into the outer ring position. On my 105 9-speed setup the front throw is the same, but the FD arm travels about twice as far.

I'll get a Sora or Tiagra FD and bolt it on and try, but I'd like to bet that fixes my problem.

Jim

Be sure the shifter is in its lowest position before attaching the cable to the derailleur. If part of the cable is reeled into the shifter before attaching the cable there may not be enough cable left to pull the derailleur far enough.

Al

Al, I made sure of that, even with the feather and second-stop on the Sora shifter, and its still not pulling enough.

New Sora derailleur coming from LBS tomorrow so I should know soon enough.

Jim