When is a transmission /reduction gearing appropriate?

Boy Scout

1 mW
Joined
Feb 20, 2022
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17
I am considering an EV conversion for an old minibike. These frames are heavy, the bikes were built for rough off road, mud, hills. Never very high speed.

I am tempted to get the most powerful mid drive motor I can afford, but most motors advertise a maximum top speed. How do I match an appropriate motor for this application ? Do I need some sort of reduction gearing ? (Pictures are just an example)
 

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You could do a lot with that, either hub motor, mid drive motor or a hub motor as a mid drive.

How fast do you want to go?
What is the terrain you will be riding?
You could have low speed 20-30ph 36v but 80a which means 3000w, or you can super fast 40-60mph with 72v at 40a for the same 3kw.
Since there is no suspension, Your best bet would be 48v or 52v if you find a deal, match the controller probably 40 or 50a in those small wheels should be lots of fun.
 
#219 go kart chains.

They are very strong and you can buy the Sprockets as large as 96 teeth very easily for $20.

The front sprockets are more difficult to find and for me I took a #35 sprocket and simply ground it down until it fit the #219 because they are pretty close.

So maybe a 14 tooth front and 96 tooth rear gives you almost 7 to 1 gear reduction.

Then find a decent motor (5 kW) that spins to about 4000 rpm and you are set.

It's what I use. I have another ebike with 10,000 miles using a #219 chain. (realistic estimate too)

file.php
 
I remember those! Yep, small torquey low-geared mud monsters. Didn't like to turn so well. Marketed to farmers, hunters, woodsmen.

That yellow frame one looks like it already has a large rear sprocket (red). Is that like the one you are considering OP?
 
Frankly, posting this from Alaska. I have lines on several frames, but all under snow this time of year !!! (Posted those pics just as a reference). Several old manufacturers; Tote Gote, BearKat, Packmule, but dunno what I’ll find

Plan to map out a route this spring, and hit several farms, gold claims, hunting camps, and see what they have laying around !!! I figure they have been pushed into the weeds, one the engines are no longer repairable. But they are simple machines. Maybe EV conversion will give them new life !!
 
know the following:

- how fast you need to go
- what rpm your wheel needs to turn for that speed
- how much power you need to go that speed

get a motor that makes the required power, and learn what rpm the motor spins when it's doing so

find a gear ratio that matches that motor rpm to wheel rpm.
 
I currently have a 72v battery for a 3000w motor. There is a various prices for motors with these basics specs. Given 72v & 3000w, what other considerations would make one motor better suited than another for this application ?
 
Boy Scout said:
I currently have a 72v battery for a 3000w motor. There is a various prices for motors with these basics specs. Given 72v & 3000w, what other considerations would make one motor better suited than another for this application ?

You doubled the difficulty of your project by choosing a high voltage battery when you want relatively low RPM.

Top speed is a function of both power and wheel torque you have available.

Do you know the RPM per volt for your motor? If so, we can work from there.
 
calab said:
There are 2wd versions of those, quite cool and neat tbh.

The 2WD bike I'm aware of is Rokon. They're expensive and sought after.

1970-rokon-trail-breaker.webp
 
Ural was the one I was thinking of just couldnt think of the name. I thought they might have started earlier, I couldnt find nothing specific in my 2 second search other then "Modern day" headline here - https://en.wikipedia.org/wiki/IMZ-Ural
It is to bad the yearly motorcycle show and the yearly international car show have been cancelled, I always love going to those and sitting on and in the vehicles. Hopefully next year as I am sure they cancelled very early on to save money. Funny how the home and garden show went ahead, those are more local with higher profits for the companies that sign up in the boom years of reno during covid.

Chalo said:
calab said:
There are 2wd versions of those, quite cool and neat tbh.

The 2WD bike I'm aware of is Rokon. They're expensive and sought after.

1970-rokon-trail-breaker.webp
 
Chalo said:
You doubled the difficulty of your project by choosing a high voltage battery when you want relatively low RPM.

Top speed is a function of both power and wheel torque you have available.

Do you know the RPM per volt for your motor? If so, we can work from there.

I haven’t yet selected a motor. That is at the heart of my question. If I am going to build around this battery, if I had several 72v3000w motors to select from, what other spec’s would indicate one motor over another would be better suited for this project ?
 
You need the lowest RPM per volt that you can get, to simplify the job of getting sufficient gear reduction. I think you should check out the Cyclone 2-4kW gear motor. It's rated 900 RPM at 72V. With a 13T freewheel on the output shaft, it's fairly simple to get the wheel RPM down into a useful range.

https://www.cyclone-tw.com/product/1/data/12
 
Thanks Chalo, that is the thinking I am looking for.

New to the forum, researching and trying to educate myself. Thanks to all that tossed out an opinion. All info is helpful !!
 
Another option that might be reliable and economical is to use a 3kW class hub motor mounted inside the frame and driving the rear wheel through a chain. Hub motors tend to run even slower than gear reduced shafted motors, and they usually run quieter too. Potentially they could be cheaper than shafted motors, and the mountings might be easier to make.
 
I am looking to build something similar to get to my mining claim and explore our property in the mountains, as age and health has limited my hiking ability. My thought is a mini bike frame, with atv low pressure tires, motor above the front wheel and if needed a mid-drive one also, with battery-controller in the center. Even something like a Razor Rambler 16 with fatter tires and a front drive might work, smaller and lighter is gooder...The red circle in top pic is looking straight ahead from about 8'-10', eye level 5', so it's pretty steep. Any thoughts or suggestions...? Thanks!
 

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