Thoughts on trimming the tailpiece vs leaving the loop in a flexible drain?

I had the same issue when I put in a new Kraus sink last year. That flexible accordion pipe is a magnet for gunk, expecially if it has that upward loop you're talking about. I left mine with a slight belly for a few months and the smell got pretty bad becuase water just sits in those ridges. I ended up pulling it out and it was filled with slimy grey sludge. You definately want to trim that center tailpiece. I used a simple hacksaw to take about two inches off mine so the drain path stays on a consistent downward slope. Keeping the water moving fast is the only way to prevent those ridges from clogging up with coffee grounds or grease. If you can, I'd actualy suggest swapping the flex hose for a solid PVC P-trap kit from Oatey or similar. It takes a bit more measuring, but it saves you from having to take the whole thing apart again in six months when the sink starts draining slow. If you stick with the flex, definitely shorten the pipe so it doesn't loop.

I actualy disagree with cutting that tailpiece down right away. I made that mistake on my old Kohler setup and ended up having to buy a whole new assembly beacuse I cut it too short to ever fit a standard PVC P-trap later on. Those flexible hoses are often against code in many areas because the ridges catch hair and grease regardless of how straight you get them. Instead of focusing on the length of the center piece, I’d reccomend ditching the accordion hose entirely. I swapped mine out for a telescoping plastic kit from the local hardware store. It allowed me to bridge the gap between the sink and the wall drain without needing to hack away at the tailpiece. If you cut it too short now, you're stuck if you ever want to do a proper hard-piped job. Just get a slip-joint extension and a real P-trap; it’s much more reliable than trying to make a slinky pipe work.

Those accordion pipes are a modern curse. Back when I was an apprentice, we'd use a miter box to get a clean square cut on a brass tailpiece so the slip-nut washer would seat perfectly. If you're trimming that plastic, watch out that you don't cut it so short that the nut bottoms out on the threads before it's actualy tight on the pipe. Use a file to smooth the edge of the cut—any rough bits will snag debris. A real plumber wouldn't leave that slinky under a sink; it’s just asking for a Saturday night emergency call.

By allowing that upward loop to manifest, you’re effectively creating a secondary trap wich, combined with the extreme turbulence caused by those internal corrugations, significantly diminishes the scouring velocity required to move particulate matter out of the assembly. If any portion of that flexible tube rises above the actual wall-drain inlet, you’ve basicly engineered a double-trap situation that will inevitably cause air-binding and sluggish drainage. Before you take the hacksaw to it, verify your vertical drop with a tape measure to ensure you can maintain a consistent quarter-inch-per-foot pitch, otherwise, you're just trading one hydraulic failure for another.

Honestly, those accordion things are a nightmare. Beyond the gunk, the way they pull on the slip nuts is a recipe for a leak. If it’s looping up, there’s too much tension on the seals and you’ll find a puddle under your sink soon enought. I’d just ditch the flex pipe entirely and get a proper slip-joint kit. It’s way easier than you think. But yeah, if you're stuck with it for now, you gotta trim it. Just don’t go too far or you’re back at the hardware store for the third time today.