i regularly use various bikes, a single speed, a mountain bike, and a couple of ebikes. i've been thinking of starting a new project converting a normal single-wheel trailer into a motorized self-propelled trailer. I need some input.

primary consideration is that i want it to be self contained, mostly meaning it carries its own battery and electronics, and preferably i do not want any cabling from the trailer to the bike.

i want the trailer's motor to engage when the bike is in motion, and provide torque to offset trailer weight, etc.

the idea i have in mind is using an accelerometer tied to the throttle input of the motor controller to detect when the trailer is in motion and engage the motor. this way i could just start pedalling and then the trailer's motor kicks in, and disengages when the bike's speed drops below a threshold (or when i hit the brakes). i'd also have a switch of some sort to prevent the motor from engaging when the tow bar is not connected.

is this stupid? is there a better way? am i missing something important?

As a novice, I come seeking wisdom.

I have an old Rockhopper that I'm looking to strip down to the frame and rebuild into a commuter. I've been learning a lot about gearing and all that, and have been researching parts. I see that there are a lot of 1x10 groupsets and a very small number of 2x9, but nothing really with a lower number of gears on the cassette.

To my untrained mind, a 1x10 would introduce more stress and flex on the chain than a 2x5 would, especially while riding on the extremes. And, with a 2x5 setup, you could (probably?) have better ratios - I'm thinking it should be possible to set it up in a way where one chainring is better suited for climbing, and the other for higher speeds.

Yet, I don't see any 2x5 or even 2x7 sets out there. What am I missing?

Another question: would it even be feasible to custom-build my own drivetrain with a 5-gear cassette, to use with 2 rings on the front, and if so, how complicated would it be?

My 8-year-old son asked this question and i couldn't give him a definite answer. So he's wondering if it would do the same thing as a balloon pushed underwater in the bathtub (which kind of makes sense to me, due to the density differences, not just gravity alone).

But I told him I'd ask those more knowledgeable than me.

Context: my ebike has 19mm nuts on the rear axle (and 15mm on the front). I have an old Proto wrench that's a 15mm/18mm, so i'm wondering if i could replace the rear nuts with 18mm nuts instead, so i can carry around one less wrench.

The bike is an Eahora Cupid with a 1000w motor if that makes any difference.