Possible question for an engineer? sprocket ratio?


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Old 06-30-19, 10:26 AM
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Possible question for an engineer? sprocket ratio?

First off I wasnt really sure where to post this, if a mod can think of a better place to move this to please do.

Working on an electric bicycle project for a good friend/adopted grandfather of mine and ive come across some low end power issues. He gave me an adult tricycle and asked if I could put a motor on it so I put a 2000 watt electric motor on it and wanted to keep the pedals operational so I kept it in the drive line. The bike runs smooth until you hit a slope and the motor gets pretty warm and slows quite a bit so I want to put a bigger sprocket on the axle and since ill be doing so I want to also rethink my approach on the drive line on including the pedals or not, ive posted a small diagram of my set up. "figure A" is my current set up and "figure B" is what im wondering if it will be more efficient? If I put a separate sprocket on the axle and a direct and separate chain to the motor instead of including the pedal in the same assembly will that make a difference? Yes the pedals have an "idle" mode so they spin free and not with the motor when you are cruising. This is an adult trike so it has (3) (27'') bicycle tires on it.

added a quick image just so you get an idea of what it looks like...
 
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Old 06-30-19, 12:11 PM
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Secondary sprocket.......that'll be a tough find.

The smaller the drive pulley/gear the lower the load on the motor. However the motor needs to spin faster to overcome the small pulley/gear size. Not sure if you're going to be able to attach the motor into the pedal drive chain.

Typically the front tire is driven if the pedals are to remain.
 
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Old 06-30-19, 12:30 PM
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I assume the 2'000 watt rating of your motor is it's locked rotor or burst current rating. Basically the amount of current it takes to release the magic smoke.

Figure A is not less efficient than B. The big drawback is the gear ratio for the pedals and motor are pretty much tied together. I'm assuming the bike has a good ratio now for pedaling so your only option to lower the ratio when using the motor is to put a smaller sprocket on the motor but there are limits on how small you can go. You could look into putting a gearbox on the motor you currently have. Or, You can have a jack shaft with another set of sprockets between the motor and axle to gain gearing freedom.

Figure B is the better way to go because it allows you to keep the same gear ratio for the pedals and allows you to change the ratio for the motor drive. Unfortunately getting two sprockets on the back axle is going to be a real challenge.

Option C might be to search Chinese websites like Alibaba and DH Gate to look for a electric bicycle wheel. Then you could just swap out the front wheel. Many come with the battery packs and motor controllers and feature dynamic or regenerative braking.
 
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Old 06-30-19, 02:09 PM
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"Figure A" is already in play and is working just not as well as id like. The current set up it goes about 40 mph on flat pavement then hesitates once it hits a slight hill. Id rather it go 20 mph all the time hill or no hill. I wish there was room for a torque converter because the motor I got spins at 4k rpm.

My plan to attach a separate drive sprocket is to use spacers. The current sprocket is mounted to a differential with 4 bolts, I was going to remove the bolts put 4 spacers between the original sprocket and the new sprocket and re-insert new longer bolts.
 
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Old 07-03-19, 07:49 PM
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Did you look into a variable speed pulley/belt for the motor? I'm guess that might drive the weight considerations over board.
 
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Old 07-03-19, 07:51 PM
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Yes I did, many are called torque converters of such and they are actually pretty large, too large for this set up. I wish they made a miniature one for my set up.
 
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Old 07-03-19, 08:00 PM
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Yeah, I think that would really fit nice. I wonder if there may not be something for your type of application. I take it what you're seen is one on the pedals? Maybe there is a small one to fit on the electric motor?
 
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Old 07-04-19, 04:11 AM
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If you need a source for the sprocket look online at racing go karts. Many of them use split sprockets so it will be easier to install and they are available in a multitude of sizes.

 
 

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