Front dérailleur is not working properly. New crankset (x2, tried a second one) and both new and old chain (kmc and Shimano Ultegra). Freshly waxed on one and used on the other.

It seems to be random, sometimes shifting up to the big ring properly, other time it just doesn't quite make it.

Alignment is *perfect*. Distance between teeth and derailleur is 1.5mm (tried 2mm as well). Aligned 1.5mm in on the tail, then made straight and perfectly in line with the teeth via the 2mm angle pushing out screw, whatever the heck the official name is. You know what I mean

Broke out the shims to measure gaps and do the rest of the alignment via the app. First setting in the app for the front deraileur is "by spec" at 0. Next is between 7 and 11. At 7 it just barely touches when the chain is under tension and 11 it definitely touchesd. Second setting is at 12, again barely touching.

With the above, it barely works. Rarely switches when I tell it to change gears.

Changing that to +2 giving me a 1.5 or so mm gap for the first setting between the outer wall of the derailleur and the chain, then absolutely maxing the other two items gets me "shifting" but its a bit random.

Also randomly get it throwing the chain off to the outside.

I am guessing the unit has worn out the bushing inside? It DOES move, but the fact it sometimes works and sometimes doesn't makes me think the bushing is getting out of round and I am getting random behavior. Is the above a symptom of that failure mode? With about 8k miles on the derailleur that is supremely disappointing. Enough to drop Shimano and switch to SRAM. (but the 1499 price for a groups with only what I need is slowing that decision down.)

Anyone have any ideas? I cant find a single shop in northern va with an r8150 in stock to try to swap it out, plus for the cost... ugh I dont want to spend another 300 dollars to "find out" that it is just going to happen again.

Is the derailleur new or old? You didn't say. Freshly mounted?


The most likely source of your problems is whatever you changed recently. Sounds like the crank. Is it actually mounted correctly so it can't drift laterally through the BB? Are the rings straight? Is it set to the proper chainline for a road front derailleur, or is it a wide crank like GRX? Did you put spacers under the BB cups or on the spindle?

Crank is 100% perfect, using a pes515. Bottom bracket is bb infinite bearing setup with Shimano bearing in T47i (proper trek frame sized version). Has the delrin topcaps installed. There is no play, no noise, and it is dead straight. No other spacers etc. Torque wrench set everything and there is no excessive play in anything. Spins smooth even under extreme load.

Derailleur has 8k miles on it. (stated above, but possibly not clear).

It started throwing the chain off the big ring randomly, which is why I changed the crank rings (Tried both a 50/34 and an unused (came in a package deal) 52/36 chainring.

Battery is fully charged btw. Tried unplugging and plugging the FD back in and similar troubleshooting.

What chainrings? Are they actually spaced like Shimano 8000 rings?

Throwing the chain off randomly sounds like there is some other issue. And possibly that issue has damaged your FD.

Magene 50/34 (same brand as the crank) AND Shimano 52/36 chain rings. Same issue with issues getting to the big ring on both and both are brand new sets.

The derailleur DOES move, and when it moves "enough" the shift is crisp and fast. Other times it shifts and, from above, doesn't look like it is pushing hard enough to get it over the hump and switch up to the big ring. Like it is stopping a few MM short of the shift point.

Works every time on the stand of course.

Either your FD is dying or your crank is moving laterally.

Tried with a Shimano 8100 crank just to compare?
when you put the 52 on, did you also use the 36?

Found a local bike shop with an fd-r7150 in stock. Slapped it on and every shift is butter.

loud because the 7150 is a louder unit, but butter.

i Just couldn’t bear spending another 300 bucks on the ultegra if it’s the bushing that is failed.