RC motor 1 stage reduction to largest cassette ring on 22" bike?

[neptronix]

Need a sanity check.
Today i noticed that bike casettes can go all the way up to a 52T tallest gear.



+

10 speed cassette with 51T largest gear



Doing the reduction math..

52v x 50kv = 2600rpm * 0.825 = 2145RPM estimated loaded

530 RPM on 22" wheel = 35mph / 56km/h

We only need a 4.04:1 reduction ratio, so a 12T or 13T drive sprocket would work

You just need to make a mounting plate ( easy ) and some way to interface a 13T bmx freewheel to the motor shaft, yeah?

What could go wrong? is it a bad idea to use the 51T chainring as a drive mechanism?

 

[fechter]

It will be pretty noisy and the 13t drive sprocket won't last very long, but I think it would be strong enough.

 

[glennb]

- Harsh on the freehub
- Lose access to top of cassette
- Relatively expensive chain replacement
- 2000rpm restricts power potential

... amongst other things.

Left side drive is more practical. I.e. Use V-brake frame and six bolt disc hub with corresponding six bolt sprocket.

I'd use an Alfine hub with Gates CDX. No need to wax it

(I know you appreciate good technology. CDX is special. Don't let Chalo dissuade you with his saying belts need to be so taut that bearings are damaged. It's nonsense. I just ticked over 10,000km on one. It's floppy, no tension to speak of, has never fallen off, and shows zero signs of wear. Only maintenance is a quick blast with the garden hose after every ride, which isn't even necessary and takes all of fifteen seconds).

 

[ferret]

Overheating would be my concern, especially if planning to run anywhere close to 4500W.

Heat generation is inversely proportional to total reduction ratio for a given system. That why most high power systems use high kv motors with a high reduction ratio despite being more complex than low kv low reduction systems.

The overheating problem would we worse with outrunner motors which are slow to shed heat from the winding compared to inrunners.

Avner.

 

[qwerkus]

I think he only means to use the largest 51T as motor drive, and keep the lower sprockets for a (reduced) derailleur.
There are a couple of issues though, that springs to mind:
1- Chainline. The Largest sprocket is the one closest to the mid line, meaning that tire width will limit your setup. That same chainline also determines your motor placement, since the sprocket have to align, so finding a spot to mount it without interfering with pedals, or the second chain, will be tricky.
2- Strength. Those super wide range cassettes are designed for weak thin chains, and are not very though. At the power levels you mention, you can easily snap any 11/128" chain.

You may want to assemble your own cassette, using a very large granny gear and only 7 lower ones for shifting. By carefully managing spacers on a 40+mm freehub, you might be able to push the chainline further away and still use a 1/8" motor chain.

Nevertheless, if going down the RC motor route, I'd advise a left side drive too. You'll have to settle for rim brikes, but it's way less overall hassle, because you have plenty of space. Trickiest part is designing a solid adjustable motor mount, and lasering a spider to mount the rear sprocket.

 

[scianiac]

I mean I think it would work but with some downsides.

With the tiny rear wheel and that Kv it's not the ideal case for the motor but not terrible either, added fan cooling may be required if you want to push that much power for more than the occasional quick acceleration.

Lower gear ratio higher Kv better, maybe stack the cassette with an adapter to use a large 58T or 60T chainring as the rear. I really like this setup but I also had the whole driver body to use for the adapter. I think you'll loose at least the the top lowest 2 gears either when you account for chain interference.

I think the chain will be plenty strong and durable enough if you have good chain line which may be tricky with mounting and tires and the other chain. Mounting the motor to the seatstay with the motor inside the circumference of the wheel would be good but 20in does not provide a lot of space. I think all this thing that chains are too weak for X power is very flawed. It depends totally how you use them, if you use them with high reduction, high chainspeed and with good chainline even thin chains can take a ton of power. Width wise I think most chains actually have about the same tensile strength of 8000-10000N, most MFG just aim for the 8000N industry standard and don't both making them much stronger, wider chains normally means cheaper materials, part of the reason why they are cheaper.

Left side single stage belt reduction I think may be almost as easy or easier if you have a 3D printer and has a lot of advantages and even allows using a larger wheel since you can make the pulley even larger if needed.

 

[neptronix]

It will be pretty noisy and the 13t drive sprocket won't last very long, but I think it would be strong enough.

Is this because bike chain is inherently noisy at high speeds and there's no way around that?

Overheating would be my concern, especially if planning to run anywhere close to 4500W.

Yeah, i would expect 2000w peak, 1000w continuous out of this setup since i'm running it at low RPM relative to what they rated the motor at.

Lower gear ratio higher Kv better, maybe stack the cassette with an adapter to use a large 58T or 60T chainring as the rear. I really like this setup but I also had the whole driver body to use for the adapter. I think you'll loose at least the the top lowest 2 gears either when you account for chain interference.

Hm, these exist? if so, the gear with the biggest tooth count could be of an 8 speed type for added strength.
I'm okay with losing some gears.

No 3d printer here and limited fabrication abilities. Building the mount is the easiest part, though.

 

[scianiac]

Hm, these exist? if so, the gear with the biggest tooth count could be of an 8 speed type for added strength.
I'm okay with losing some gears.

I know I've seen one somewhere, I think on the high voltage EV youtube, I just 3D printed mine but either way they use the whole driver body. The trick would be to make one that doesn't, something like a laser cut piece of aluminum with chainring holes and that replaces to two largest gears on a cassette with an aluminum driver body that all the gears mount to, just spitballing here. I just used cheap aluminum folding bike chainrings, first a 60 and then a might lighter weight 58, what's nice is they are narrow wide so keep the chain on well.

Still at that point buying or making an adapter for a left hand drive sprocket is easier, you just have to use rim brakes or play spacer games to fit a disc brake. The reason I like a left hand belt drive is you can use a smaller pulley for more reduction, those pullies are made to fit on those motors, it's quieter, if you wanted to run a disc brake you don't have to worry about contaminating it. Technically the 3D printer isn't the hard part, you can just have somebody else 3D print it, it's the design that requires some skill but not much more than mounting the motor. I've seen this done enough times though you could probably find somebody's 3D models for the big pulley and brake disc mount adapter and just have those made.

 

[ferret]

Is this because bike chain is inherently noisy at high speeds and there's no way around that?

The smaller the sprocket the more noise is generated .

Yeah, i would expect 2000w peak, 1000w continuous out of this setup since i'm running it at low RPM relative to what they rated the motor at.

Running at lower RPM usually generates more heat than high RPM.

Hm, these exist? if so, the gear with the biggest tooth count could be of an 8 speed type for added strength.
I'm okay with losing some gears.

https://www.aliexpress.com/w/wholesale-60t-chainring.html

No 3d printer here and limited fabrication abilities. Building the mount is the easiest part, though.

 

[qwerkus]

Just saw this today: aliexpress.com/item/1005007950505897.html?
Might be interesting for your project.

 

[neptronix]

https://www.aliexpress.com/w/wholesale-60t-chainring.html

Yea i'm looking for an adapter to a chainring.

Just saw this today: aliexpress.com/item/1005007950505897.html?
Might be interesting for your project.

View attachment 375631

Interesting, but much too small tooth count and looks like it would eat quite a bit of space for gears. Not sure how i could utilize it.

 

[ferret]

Back in the day, before wide range cassettes and extender cogs were a thing, I wanted to convert a 11-36 cassette to a 11-42 cassette so I made a custom adapter for a 42t cog from an old cassette cog.

Avner.