coffeecherrypie

I converted an early-90s bike from 7-speed to 8-speed with new wheels (going from 126mm to 130mm), new 8-speed cassette, and new Claris brifters (which look sharp now that they've gone to under the bar tape routing). The bike was originally all RX100 except that a long time ago someone swapped in a wide-range 7-speed cassette and an 8-speed STX RC rear derailleur to go along with it. As I expected, rear shifting is working just fine. However, the front derailleur (RX100) will rub the chain in either big chainring/smallest cog or little chainring/largest cog, depending how I adjust it. I have backed out both limit screws as far as they want to go.

I'm not sure if this is happening because the range of angles the chain has to sweep is larger with 8-speed than with 7-speed or because the range of cable pull of the indexed brifters is smaller than it was with the friction downtube (I think it was friction up front and indexed in the back). In hindsight I should have made sure it would work with 8-speed with the downtube shifter still there but oh well.

Anyway, about to start a session of adjusting FD height, angle, etc in the hope that I can get it to work. I have two specific questions:

1. Is there any reason to *expect* this setup not to work up front and
2. Assuming I can't get it to work, should I next try a different FD or should I try going from a 7/8 speed chain to 9-speed or thinner chain.

Andrew R Stewart 

1- The system will work as it will. Given the tolerance range some chain rub might or might not remain in some crossing combos. Refresh our memories and tell us if the Claris shifters have trim features. If not then the chance of rub is far greater then if the shifters can trim the cage position.

2- A 9 speed chain will only lessen the amount of possible rub by half the width difference (between it and an 8 speed chain, that's about .2mm at most of clearance gain). If the rub is greater that this width half difference then the 9 speed chain will likely rub some too. Note that as the chain get's narrower the tendency to ride on top of the middle/small ring's teeth when down shifting (and not settle into the teeth's valleys fully) grows.

One aspect to understand is the ring to ring distance is reduced for 9, 10 and more speed cranksets. So frt ders designed for 9 speeds are only intended to have to move enough to match the ring to ring dimension. A 9 speed ft der with a 7/8 speed crank means that the der is likely to not move enough to prevent rub. Andy

coffeecherrypie

Thanks Andy. It's not crossing combos though, just hardest ratio (big/small) and easiest ratio (small/big) that rub. Thanks for the info about width, it looks to me like I would need at least an extra mm, so going to a 9-speed chain wouldn't help.

coffeecherrypie

Huh this suggests I do need a new FD: https://www.bikeforums.net/6352110-post5.html

Maybe I'll just run with one downtube shifter for now. At least if that works I know it is a cable pull issue.

dsbrantjr

Observing where it rubs on the cage is key to diagnosing this. I would first make certain that the FD limit screws are not too tight. The first step is setting up the low limit to make sure that the small front/big rear does not rub, but only a millimeter or so of clearance is needed. If you leave too much clearance you may run out of pull at the other end of its travel. Leave the low limit alone from now on. Then slack off the high limit screw and shift to big front/small rear, it is easier to shift by pulling the exposed cable rather than using the shifter, to get the shifter adjustment out of the picture. If it rubs on the outside of the cage you do not have enough derailleur throw or the indexing is too loose if using the shifter, if it rubs on the inside you need to slacken off the indexing adjustment, it has shifted too far. If you can get satisfactory shifts (or you can over-shift past the big ring) then tighten the high limit until you cannot then back it off until it shifts OK. I cannot say whether your Claris shifter is compatible with your older derailleur. A narrower chain might be helpful, as might a judicious widening (by bending) of the derailleur cage; as I said before it depends on where it is rubbing.
I would make certain that the FD is dead parallel to the chainrings, because any angle will make it rub on one ring or the other, and the height should have little effect.

dsbrantjr

That worked for at least one Tour de France winner.

Iride01 

When you backed the limit screws all the way, did you ensure that there wasn't any cable tension that may have kept the front DR from going all the way to the low limit or that there was enough pull on the cable to pull it to the high stop?

If you have a mix match of shifter and DR, then someone else will have to say if that is okay.

2x or 3x front? Anything special about the crankset that it might have wider ring spacing than cranks made for that group at that time period?

coffeecherrypie

Both limits are backed out. Basically I think the derailleur is not sweeping through enough range but it sure seems like it could because I can adjust the cable tension so that it won't rub on the small ring, or won't rub on the big ring, but not both--so it's not a limit issue. (2X setup, seems to be a normal RX100 crankset which I think was 105 with worse finishing). I have been playing with the angle, maybe got it to work a little better. I just realized I have a 9-speed Sora derailleur in a box somewhere so I may try that next, although the box is in storage blech. For the meantime it's now really only rubbing on little ring-biggest cog so rideable, just noisy on steepest ascents. Thx for all help.

79pmooney

Try disconnecting the cable and simply observing where the FR sits (inner ring) and push it fully out until it hits the stop (outer ring). Does that do the trick? If yes, it's a cable pull/FD-shifteer mis-match issue. The friction shifter will solve that nicely, If no, the set screws are not fully out, the FD isn't up to he job or it isn't aligned/setup to best match crankset and chain. (Have you checked chainline to make sure the crankset is sitting where it is supposed to be inboard/outboard?)

I'm not up on the standards but I played with a bunch of FDs to set up a triple using a 5-speed standard triple crankset, 9-spped rings and a 9-speed chain, using a Phil BB to bring the cranks in as far as possible. (The old Suginos had far less Q-factor than most modern cranks. A narrow Q-factor allows me to keep running my OEM knees. $$-wise saving me enough to buy FDs to play with by the case.)

Ben

coffeecherrypie

I haven't done exactly that but depending how I adjust the cable tension, it can swing inboard enough or outboard enough with room to spare, just not both, so, right, it must be a cable pull issue. I can't find much documentation, except for the link I previously posted, for the idea that the cable pull ratio changed at some point, but for now that's my working hypothesis. I'm going to try a 9-speed derailleur next once I dig it up from storage. Or just bite the bullet and spend 30 bucks on a new R2000 front derailleur.

icemilkcoffee 

Which RX100 front derailleur do you have? The FD-A550 was designed for downtube shifting, and the pull ratio is not right for STI brifters. The FD-A551 is the right one to use.
The FD6401 was designed to be used with either downtube shifting or STI shifting. It has two cable clamp positions:

The 'north' position further from the pivot, is meant for DT shifters. The south position closer to the pivot is meant for STI. So clearly the STI uses a different pull ratio than the DT shifters. The FD-A550 doesn't have this dual position clamp. If you are want to experiment, you could file a groove south of the clamp screw and try to duplicate this. But the path of least resistance would be to just buy a FD-A551