Why is my chainline off?
 

I recently put dura ace 7402 8spd cranks on my merckx, along with the matching 7400 bottom bracket (113mm, italian). Shimano specs say the chainline should be 43.5mm but I get closer to 48mm. I measured the spindle (113) and the bottom bracket shell (70), spindle is in correct orientation, and cranks are fully torqued. The shifting is good apart from a tiny bit of chainrub against the big ring when in the small/small combo. So nothing is really wrong but I thought it odd the chainline would be that far off. Last pic shows 4mm offset from center.
https://cimg1.ibsrv.net/gimg/bikefor...95df622170.jpg
https://cimg5.ibsrv.net/gimg/bikefor...b7b191244f.jpg
https://cimg8.ibsrv.net/gimg/bikefor...128e9ba429.jpg
https://cimg0.ibsrv.net/gimg/bikefor...c3e37ea394.jpg

Just wondering if the cranks are compatible with a JIS square taper, but your BB spindle is ISO?

It sounds like you have already tried and turned the limit screws to no avail.

Crank and BB are matched as per shimano tech bulletin I posted. Both JIS I think.

That’s a head scratcher. The pic kind of looks like you’re measuring from the center area of the spider. If that’s the case, I’d say move the measurement to the top of the inside chainring.

I did another measurement from the top center of each chainring to seat tube (28.6), and split the difference, and got 47.8mm chainline.

What is the distance between the inner ring and the chainstay?

maybe 6 -7mm but the chainstay is indented. I tried fitting a 110mm phil wood symmetrical bottom bracket prior to this and clearance there was only 1 or 2mm.

Spindle in backwards? Is the distance between the crankarm and the chainstay the same on both side or does one come closer? I would have thought that a DA crankset would use a 108 or 110mm spindle.

According to the Shimano specs posted by the OP, the spindle is indeed 113mm and longer on the drive side, so it's assembled correctly.

Typical expected chain line from the bottom bracket centerline was 43.5 mm to the center of the gap between the chainrings to the center of the main 28.6mm tubes.
‘that centerline is a monster to measure correctly, BUT measuring to the tangent of the seat tube for argument’s sake would yield, 43.5 - 14.3 = 29.2 mm to the midpoint between the rings
I will let you measure that. Account for paint thickness, and 29.0 mm would be pretty good.

from the provided image and the high fidelity scale used… I cannot tell where the centerline is.

What's the cassette chainline? If it's the same as the crankset chainline (within 1.5mm), I'd leave it as it is. You said that the chain already rubs on small/small combo, so if you move the chainrings inboard it will rub still more.

I don’t have a solution to your chainline, but I have a thought on the above situation. It is not that common for a chain to rub against the large chainring when in the small chainring and the small cog. It can happen with a very short chainstay, or an undersized inner ring (probably less likely with a 130mm bcd). I had one bike with a smaller inner ring that exhibited that issue.

The problem would be less prevalent with a 48mm chainline since the small-small angle is not as severe. (Edit: With a 43.5mm, it would be worse.)

Just wondering, how does the chain line up with the center of the freewheel/cassette?

John

seat tube to center of small ring is 30mm.

Hard for me to get exactly but using a long steel rule the current chainline is about the 4rth cog up. Should be at 4.5. But it shifts fine through all the cogs.

Right or wrong I’ve never bothered measuring the chainline. As long as the inner chainring doesn’t rub the chainstay and I get good shifting through all the cogs, I’m happy.

Op only specifies it's oriented correctly which often means setting the lettering so if they were visible to the rider they would be readable while sitting in the saddle. However, I have come across spindles that were printed upside-down and only figured it out when measuring the space from the crankarms to the stays. Although that typically means the crank sits too far inboard, it doesn't mean you shouldn't rule out all possibilities. And regardless of what the paper says, I still don't remember them using that long a spindle, I may be remembering wrong, but that still seems wide for a DA spindle.

On the last pic the DT centerline should be at the "sharp edge" of the BB shell lug. It's hard to measure accurately even if he's on the correct points, because its hard to see how the correct points on the bicycle line up on the scale or hard to get a caliper positioned correctly, due to optical parallax and just the lack of stable fixturing. I don't have an alignment table and I don't have perfectly steady hands.

There's also the question of whether the frame is aligned correctly in the first place.

Is there a riding or chain tracking problem? It's not clear if your measurement is accurate enough. Maybe your chainline should not get fixed, just for the sake of "blueprinting."

I am not seeking to fix the chainline, I am just pointing out an observation I made. Also the driveside crankarm clears the chainstay by 10mm, non-driveside by 9mm.

out too far then.

Until I saw that Shimano spec I too thought that the spindle should be 110.

Since you are within 2.5mm and you are happy with how it shifts, I agree that there is no reason to "fix" the chainline. If you were so inclined, one day, you could probably dial it in slightly closer.

The "why" is always an odd phenomenon. I built some bikes where things should, or shouldn't, fit exactly how they ended up, but function takes precedent.

I have read that early DA used ISO, Campy, but I have always thought that was 7200 and 7400 was JIS. I ran 7400 cranks for a long time with, oddly enough, a Performance individual cartridge bearing BB and had no issues. All I remember is that I ordered out of a catalog and in my ignorance it all worked.

But if you really want to, do a search for Shimano FC-7400 jis iso and you will find enough conflicting info to make you head hurt.

John

I think I read somewhere that 7400 was JIS "low profile" which is similar to ISO enough to substitute one for the other. Anyway that is why I chose the o/g 7400 bottom bracket, to avoid non- compatibility issues!

Is the frame straight? Rear wheel centered?