I've Googled the cause of this, but all I find are threads about front derailleur adjustment and not cross-chaining. Duh. I have a slightly different problem. I have an old cheapish square taper crankset that I've been using on my vintage road bike. Every time I put down the power (probably about 400 watts max), the big ring rubs BOTH sides of the front derailleur. This also happens sometimes when I am just cruising along. Yes, I have trimmed the derailleur, it's right smack in the middle. I'm not cross chaining. This happens even in 52x14. My front derailleur is aligned with the CHAIN, so it's perfectly straight. If I raise my derailleur, the shifting becomes unpredictable. Is it a square taper problem, or cheap crank problem? I have a two-piece shimano crankset that I can use, but I would prefer to keep the vintage styling.

What BB are you using? It could possibly be a flimsy BB but not likely as even the cheapest versions are usually rigid enough. Have you check if the chainrings are straight, bolts tight, crankarm bolts tight on the BB axle, any play in the crankarms? Are you running the correct front derailleur for the groupset speeds i.e. not running an 11sp derailleur with a 7sp chain which may be a little narrow for the wider chain. Frame cracked? Some frames are just flimsy and will rub no matter what but not that common.

What's the run out on the chainring look like? Many (perhaps most) chainrings aren't straight.

Another possibility is that the derailer has been bent either by accident or design. People used to bend the derailers to get better response.

Finally, you may just have a frame with a weak bottom bracket. Hard effort is bending the frame out of line and the crank is staying stationary.

I'm running a 7 speed rig. The derailleur is the right one for the speeds. I'm using an old Suntour Cyclone. I'm using a SRAM PC-830 chain. The derailleur doesn't rub on the ends (where slightly bent inwards to prevent overshifting), but rather in the centre of the cage. This happened with the stock Huret derailleur too. As for a weak frame, probably not. I tried the two piece crankset a couple weeks ago and there was absolutely no rub. I could align the derailleur and just barely put in a penny with no rub. I'm pretty confident that the chainrings are true.

If the two piece does not rub then logically the crank has to be part of the problem. Rubbing on both sides obviously can't be an adjustment issue, and I don't think square taper spindles bend/flex. If the BB is correctly adjusted then your choices are to use a different BB or to up your cadence, so that you aren't mashing as much on the pedals.

I was afraid I would get that answer.

If one crank produced a better result, then I would suspect that the chainlines for them are different.

Or possibly a difference in the number of teeth between the two might make a difference, but I'm less certain that your cranksets would have that much difference.

With Friction Shifting you can simply re center the FD cage to not be rubbing , after you shift on the front or the back.

The chainring is rubbing on BOTH sides of the derailleur. Are you suggesting that I ride with one hand on the bars and one hand constantly trimming the derailleur? That would be a bit tiresome with downtube shifters.

Again, the chain is rubbing on both sides of the derailleur cage. That is due to flex, not chainline.

Is your bike the '82 Austro Daimler Alpina of your signature? Your problem sounds a lot like that of a strong, not so light rider on a flexy steel frame (or one of the old, skinny tubed aluminum or titanium) bikes. If this is the case, there isn't a lot you can do. You might try a newer derailleur with a wider cage but this might now get hit by your probably old school straight cranks in high gear. You could have a framebuilder stiffen the BB area with gussets or other support but this would be: ugly, expensive and require a paint job. If this is the case, a stiffer BB and ckankset will help, but it will be like building your house of cards with stiffer, stronger cards.

BITD (that includes 1982) we all new of bikes and riders where your issue happened. It didn't to riders like me (6'+, 145# and a slow twitch, long limbed climber, but guys 5'8 and 175# sprinters could do that to my Fuji Pro, I am sure.

Edit: I saw your post above that you are using a Cyclone. Narrow cage. Quite susceptible to what I described above. There are lots of wider caged derailleurs. (I love the Cyclones and did all my racing on them but they do have limitations. Not having much room for flex, either frame or crankset being one.) Or you can just replace this frame with a Klein of the same era. Problem solved. For you and the next guy twice your weight and strength. Or you might be able to find a chain that is quieter so you don't notice the rub. This suggestion is actually serious.

Ben

Have you checked the BB for side to side play (I.e., out of adjustment)?

You either know, or you don't. Find out!

Try a narrower chain. Sure, shifting performance is supposed to be worse on the rear, but how much worse you won't know till you try.

When you say it rubs both side, you do mean that it rubs the inside when in the large rear and the outside when in the small rear, correct? Or is it rubbing alternately either side when in the same rear gear?

That's my bet too. Back in the olden days we used to get that a lot.

Yes, the bike in question is in my signature. The bottom bracket is not the problem. I swapped out the old crankset for the Shimano 105 5800 cranks. It solved the problem. Completely. There was no rub or flex whatsoever. Unfortunately, it doesn't really fit the style of the bike. I'm not getting rid of it though. I also checked to see if my chainrings were true, and they are. I took apart the old cranks completely, cleaned them up as best as I could, wiped down the tapers, and WITH A TORQUE WRENCH, I tightened them on to 30 ft lbs. This improved the rubbing problem a bit, but they still rub. I also made sure that the cranks were on the least wobbly taper. It looks as if I should be looking for a wider derailleur. Are there any Suntour derailleurs with wider cages?