Really Compact Gear-Down Thingie

Depends what you mean by easy - planetary gears are a classic compact method, since you didn't mention cost as an object. But it probably is. If using chains, timing belts, or belts, a jackshaft is the classic serious gear-down method.

Something that can easily be stuck onto an existing gizmo, preferably while leaving the linked-to motor intact.

In a world of unlimited lead time and with no object to cost, "best" would be to get a motor custom-made for the purpose. That's kind of the opposite situation from what we have.

Not cheap, but have you seen the NuVinci hub:

They're using it.

I use to keep up with the bicycling stuff, many years ago... Someone started making a gearbox back then that mounted in the frame, above the crank. Never heard anymore about it or saw one in the wild. This might give you some more ideas:

I see you can still get the old 3-speed hubs for ~$50 too. You would have to figure out a way to mount it and fab a special sprocket to go in place of the spokes...

Is the purpose to make an electric drive vehicle or a bicycle with electric power available?

If the first then possible a dual front crank, i,e, two sprockets, might work - from motor to crank sprocket, from second crank sprocket to rear wheel. Note that dual or triple cranks are pretty much the standard on sports bicycles these days.

If the latter then I would think that there would have to be some method of disconnecting the motor from the pedals in some manner.

Whizzer, which was a belt driven internal combustion motor design used a separate belt drive from the motor to the wheel. See

EE, huh?

Did he use the mid-drive style because the bike doesn't use standard wheels/tires? I paid $209 for the 800w front hub motor style, complete with brake levers, thumb throttle, controller, pedal assist ring/sensor, bag, and rear package tray. They run as low as $159 now.

Here's a 1kw rear for $185.

I believe that the lower wattage motors are designed for assist, not to provide full power/torque, especially for long term use like hill climbing.

It already has a reduction unit built in, Tim. There's not much you can do with a small motor like that.

I hope he isn't driving the pedals. Sprag clutches (cassette or freewheel, in bike terms) are cheap.

There are lots of details that are perfectly sensible in his context, but which I don't feel that I have permission to discuss publicly. Alas.

Just these: (A) it's a weird application, and (B) in its context it makes perfect sense.

I think I'm missing the problem - looking at your question, you refer to dr iving the pedals. Looking at the picture on Amazon, they are driving a sepa rate sprocket on the rear wheel. What's the problem with changing that driv en sprocket (on the wheel) to a larger size?

But do like the idea of the Sturmey-Archer 3-speed. Just get that mounted i n the middle of the drive train, and you're good to go.

Customer can't easily get bigger sprockets for his chosen rear hub.

That is on the table, but I'm looking at alternatives.

I use "thingie" in the title to not give anyone a false impression of what I do or don't know.

I'm working with a client who's using one of these in his specialized bike:

He's currently using the 250W version, but we're having problems of insufficient torque -- this is partially due to re-purposing a board, and we're on top of that part, but I'm thinking ahead (well, borrowing trouble, in proper engineering fashion). So we thought -- hey! We'll use the 350W version (that's the one in the link).

The problem is that the 350W version actually has a lower current-to- torque constant -- it gets its higher power through pulling more current for the torque, and delivering more speed.

So -- any thoughts on an easy way to gear the thing down? I just recommended to the customer that they put two chain rings on the crank, so that they can drive the pedals at one speed and the bike at another.

Oh, I dunno, take a look (for example) here:

They have a pretty big selection. You c an, with some machining & welding, get these to fit pretty much any hub. Yo u could even cnc mill an opening in the larger sprocket to be a snug fit ov er the existing sprocket and either weld or mechanically fasten them togeth er.

Greetings Tim, Last summer I was working on a project that I thought would need a larger sprocket than I had. I looked into making my own first. It would have been pretty easy for me to mill one 'cause I have a machine shop and a couple CNC mills that would do the work, I just needed the tooth dimensions, which I found easily enough on the web. But I also found more than one site that sells little gas engines for bicycles that sold several sizes of large sprockets and a couple sites that would make custom sizes. As I recall the prices were pretty cheap. I ended up avoiding the need for a custom sprocket and don't remember which bike motor sites were selling them but it didn't take me long to find them when I looked last year. Cheers, Eric