Laser alignment of derailleur

[handlebar]

I've ridden my Radrunner daily for over a year. Since it was new, I have occasionally experienced a jump that seemed to be from a side plate riding up on a sprocket tooth. It seemed as if the derailleur was not aligned with the rear sprocket, but it looked okay to me.

Recently, I bought a laser level to check the slopes of streets I ride. It occurred to me that if the bike were parked on level concrete, the laser plumb line of my level would show if the derailleur and the sprocket were aligned. I removed the chain from the sprockets. I set up the level behind the bike moved the back of the bike a little until the green light hit the sprocket teeth. If the derailleur had been aligned, the light would also have hit its teeth.

The light missed the derailleur teeth. It took force to spring the arm into line.

Backpedaling produces clicking, like the ratchet in a fishing reel. The next day, I happened to backpedal and noticed that it was much quieter. I guess the clicking comes from the side plates of the links, hitting the teeth of the derailleur. I guess the side plates are barely touching the teeth now.

[handlebar]

Since it was new, I have occasionally experienced a jump that seemed to be from a side plate riding up on a sprocket tooth. It seemed as if the derailleur was not aligned with the rear sprocket, but it looked okay to me.
.....
Backpedaling produces clicking, like the ratchet in a fishing reel. The next day, I happened to backpedal and noticed that it was much quieter. I guess the clicking comes from the side plates of the links, hitting the teeth of the derailleur. I guess the side plates are barely touching the teeth now.

Chain tensioner! "Derailleur" was the closest term I could think of.

Well, it "jumped" again. Other than that, it was quieter, but the laser showed that some of the change from pulling the arm had been temporary. The tensioner arm is mounted with two spacer washers. I ground down a flat washer to make a third. Now the laser said the alignment was perfect, and when I turned the pedals backward, I could see that the links were perfectly centered on the tensioner teeth.

It could still "jump" when I pedaled hard, so the problem is at the front sprocket. Unlike old-school chain for one-speed bikes, this chain can flex from side to side. The run from the back sprocket to the front is long. I believe a bike with a derailleur system normally has a chain guide clamped to the seat tube, to keep the chain centered where it's pulled onto the drive sprocket. One can't be mounted on the seat tube of a Radrunner, but maybe a chain guide could be installed. It would be simpler to put up with having a link side plate occasionally climb a tooth. It could be worse. Apparently, some mountain bikes lack chain guides, and the chain can jump off the sprocket.