Hi Everybody,

As stated, I have a question about older Shimano 8sp. chainrings. I'm rebuilding an older bike with a straight-up Shimano 6400 (STI) 2 x 8 drivetrain, and I'm hearing a little knock, a little ping coming from the top ring with each revolution of the cranks at speed (this is only in the Sm. R./ Sm F. position, and only when it's really spinning; otherwise it's quiet & runs smoothly).

It's clearly a chainline problem; I can see that the chain (an NOS CN-HG90-8) is definitely rubbing the back of the top ring when the ping occurs, but I can't figure why since all the parts- save the chainrings themselves- are stock for 6400 and are either NOS or in VGC). The bottom ring is an SG A-42 and seems to be a tad thinner than the top ring; the top ring seems to be pinned but not ramped, and has no other markings besides "Shimano <52>." So- I have no idea whether it's 7sp or 8sp. In fact, I can't remember just offhand where I got these rings (they might be leftovers from a previous build or I might have picked them up at a swap-meet or something; who knows... ). At any rate, I can't seem to dig up any pertinent information about them on the web.

Is the inner ring possibly 9sp-compatible? Are these rings mismatched? Would I need a different top ring, bottom ring, or both?

Search "cross chaining."

Thanks for the response, but it doesn't actually answer my question; I already know what it's doing.

I'm looking for information about the chainrings themselves, since this is clearly a compatibility issue, not an issue related to shifting per se.


Edit:

I'm glad I can read & write German; German forums & databases are awesome when it comes to technical details like this.

It turned out that my outer chainring, the one stamped "Shimano <52>" was 7sp, and standard issue with a 1050-series crankset.

I replaced it with an actual SG A-52 (a surprise find at the bottom of my chainring pile), and voila: problem solved.

A question to you: Is the chain you are using a used one? If it's what has been running with the same cassette & chain-rings, I'm stumped. But if it's a used chain from another bike, then you need a new chain.

Despite the curt response by AnkleWork, he is correct. You are (were?) getting chainring rubbing only in the cross chained gear and that is common no matter what chainrings you use. It's also both hard on the chain and unnecessary from a gear selection standpoint. I'm glad you solved your perceived problem but cross chaining is a bad idea never the less.

As stated in my first post: the chain is NOS: New Old Stock; still sealed in package, never used, brand-spanking-new. As are the cassette, RD (& its pulleys), BB, & RH. The FD is VGC, little wear; STIs are newly rebuilt (& worked like a charm on my other bike).
The only weak point was that outer chainring, which- from its markings- the Germans quickly identified as being from an earlier 1050 (UG) crankset. Despite having pins, the UG ring is still mostly flat, whereas the SG outer chainrings are also slightly dished to accommodate the shift ramps on the inner side. This accounted for the millimeter or two difference in the chainline that caused the chain to rub against the back of the top ring.

The thing that threw me about the 1050 ring was its similarity to the inner SG A-42, which has a similar finish and is also mostly flat (at first they look like they should be a matched set); then it dawned on me: it's an inner ring for a double; it doesn't have to have ramps on the back.
Another puzzle was the bolt-tabs on the 6400 crank: the thicker 1050 ring fit snugly, whereas the SG inner ring looked too thin (ergo, why I thought it might be the problem instead of the outer ring); then I read that for the first year or two, the 6400 cranks were issued with thicker 1/8" UG rings, and only later with the 3 3/2" SG rings; the crank will actually accommodate both ring types.

As for cross-chaining in general, yeah- it's not the best thing. I've been telling my best friend that for 14 years and he's still too much of a retrogrouch to bother upgrading the 1055 chainrings on his RB-1 to match his HG cassette & chain. Instead, he just lubes the crap out of it & hopes for the best.

So, to recap, I knew what was going on; I just didn't know why. Then I did; now it's fixed: Das Ende.