Jordanmilo

I just put brifters (sorry, but it's a perfect word) on my vintage 7x2 Italian frame that previously had friction shifters.

When I stopped working on it last night I had the rear derailleur shifting perfectly.

Today I hooked up the front derailleur, and got that shifting dialed in. Now the rear derailleur works perfectly when the chain is on the small (36) chainring. But the rear is very reluctant to shift when the chain is on the large (50) chainring.

The primary issue is that the chain doesn't want to shift down from the largest (24) sprocket; it takes to clicks to get it off that sprocket, and then it shifts down two cogs.

Could the chain be too short? It's not according to Lennard Zinn's formula: chain on small rear, large front, the pulley cage is vertical.

I've read that people have had this problem when the derailleur cables are crossed; that's not the case here.

I don't know if I'll ever use (or should use) the large/large combination, but it seems like I should be able to if I want or need to.

I've used the barrel adjusters to try to cure the problem, but that just seems to create other problems.

Thanks for reading; thanks for your help.

rccardr 

Are you using indexing derailleurs? Most friction derailleurs will not work properly or consistently with indexed shifters.

Jordanmilo

Yes. Shimano 600 "tricolor" (RD-6400). Shifters are brand-new MicroShift 7-speed, which I've read are Shimano spec as far as pull ratio goes.

rccardr 

Did you align the derailleur hanger? That could be the culprit.
Generally speaking, ‘failure to release’ when the shift cable is in highest tension (largest rear cog) is caused by friction in the cable/housing or the RD itself.
If the RD is new to you, it might require cleaning or lubrication to ensure free movement throughout the full range.
The cable adjuster on the RD should be used to ensure that the chain is running properly between the cogs- not too close to either the next or former cog.

Jordanmilo

I did not align the hanger. I'll look into that, and I'll give the derailleur and clean and lube. Cables and housings are all brand new.

dedhed

How's the rear loop look? Nice smooth bend?

Jordanmilo

Looks ok to me. I'd post a pic but I took the derailleur off to clean it.

Jordanmilo

I don't have a gauge so I screwed an old hub into the hanger. The end of the hub's axle seems parallel to and lines up flush with the small cog of the freewheel, so I think we can say that the hanger is straight. (I also put a level across both flanges and they are off by the same amount as the top of both "cheeks" of the saddle--of course, the bike on the stand isn't level.)

masi61

What 7 speed cog set are you using? Is it a cassette or a freewheel?

Could you take a photo of the RD-6400 while it is in the “big-big” combo? We might be able to evaluate if there are chain length issues or whether the upper pulley is positioned well (or not so well).

masi61

That is a very original method of “eyeballing” it. Maybe show a photo of your improvised alignment device?

Jordanmilo

Not a very clear photo I'm afraid. I saw RJ The Bike Guy use a whole wheel. I've had this Campy high flange hanging around since about 1978!

Jordanmilo

It's a freewheel: Shimano Dura-Ace 7400; 13, 14, 15, 17, 19, 21, 24

Jordanmilo

And if anyone can use the small/small photo here it is.

masi61

Chain wrap looks pretty good in the big-big. Dura Ace uniglide freewheels have a silky ratchet action of the freewheel body, but cog precision might be not quite as good for the Uniglide cogs compared to modern Hyperglide. I run 7400 uniglide seven speed cassette on my commuter bike and find that the indexing can be a challenge to have perfect click shifting on all gears until I get the derailleur hanger almost perfect in the vertical and horizontal plane. I had the Park Tools DAG-1 and found it helpful but flawed. When Park Tools came out with their new and improved DAG-3 I had to have one. Having now used it on 3 different bikes I can report that the DAG-3 is really great and inspires more confidence that you have dialed in the accuracy of the derailleur hanger in a way that inspires more confidence.

Jordanmilo

Hmm...Hanger alignment is interesting to me: Why would the bike shift up and down through the freewheel flawlessly on the small chainring but not on the large chainring? That makes me think that the rear derailleur is not capable of dealing with the "new-style" crankset, with its 36/50 chainrings. Shimano's catalog says "Maximum chainwheel difference: under 13t," which I have exceeded. https://www.disraeligears.co.uk/site...re_scan_3.html

Suddenly, I am leaning toward a new rear derailleur; they seem shockingly inexpensive, compared to what we payed back in the day.

rccardr 

Chain length looks fine.
What is chainline?
Assuming no weird chainline, no major friction in the cable/housing and no binding in the RD itself, this is likely an issue with hanger alignment.

andrewclaus

Could be a B screw adjustment issue--the top RD pulley rotates away from the rear cog under tension. You can also play around a little with axle position in the horizontal dropout, but not much with that tire size.

70sSanO

I would back out the low limit screw slightly. The non-index side (RD-L) limit screws are not needed with STI shifters. Index side screws (RD-H) start the whole index sequence.

It makes no sense that the shifting works on the inner and not the outer ring, but if the shifter pawl is slightly hanging up due to the stop, I’d at least try it.

John

KCT1986

Little hard to see in your little pics but your axle may be a little further back than Shimano's recommendation. See bottom of the attached.

Not sure how much it matters, haven't been able to test since don't have horizontal dropouts on any of my frames. Worth a try??...

Edit: forgot to mention, the X spec. is what's relevant. Also, note measurement is right angle to chainstay.