Sprocket problem. Help!

Sam

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Aug 11, 2019
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Hello everyone, I hope one of you can help me. I try to make an ebike but I dont know how to install the rear sprocket. I want to put a # 25h 74t or 80t ... I need to use an adapter? which one?

Thank you really much for your help.
 

Sam

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Aug 11, 2019
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8
I try to put a picture of the bike but I dont know how. If you type "jesse james bicycle" on google you will see the small chopper bike I am talking about....

I buy the "for the evil genius" book but it dont talk about "change a sprocket for a bigger one" lol
 
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Radical Brad

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Welcome, and thanks for your support!

It seems like everyone (and their dogs) is calling their motorized bikes "Jessie James", so I found one of thousands to discuss...

3694

There are many options for this kind of sprocket mount, but here are the ones I know of and have tried with opinions...

1) Bolted to the spokes.
This is by far the easiest option, but also the worst. All I can say is... don't bother.

2) Bolted to a disc brake hub.
This works well, and can take perhaps 2 or more horsepower. I did this by hand filing out a sprocket from a 1 piece crank...



3) Custom machined hub.
Best option for sure. With all the gas bike kits around, you can surely get one for under $50 from the many online sellers.
A proper sprocket hub with 36 spoke holes could easily take 4 or more horsepower on a decent 20 inch BMX rim.

Brad
 

Sam

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Aug 11, 2019
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Yes this is exactly the bike I got.

I dont know if I will take the pedal out.. I think yes.

The method with the disc brake adapter seems nice but I wonder if there is a bearing attach to the sprocket? When the engine goes ahead its fine but if the engine does not turn and the wheel yes, without bearing.. what gonna happen? :p

You say it will be better on a decent 20in bmx rim but does the large tire will fit?

Sorry for my english I usually speak french and this is my first diy bike so maybe I ask stupid question but my sprocket make me crazy.. lol

I buy this 👉 https://www.amazon.com/JRL-TEETH-SPROCKET-POCKET-ROCKET/dp/B01N53F9RE/ref=mp_s_a_1_1?keywords=sprocket+25h+74t&qid=1567210891&s=gateway&sr=8-1

I hope I will find a way

Thank you!
Sam
 
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Radical Brad

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I wish I could help, but I think that your questions would be properly answered on the forum for the supplier of the gas engine bike.
You will see that this corner of the web isn't much into "gassers", mainly because I am all about electric!
I cannot really advise based on what you have shown, as I do not know these components.

Brad
 

Sam

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Aug 11, 2019
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I dont want to put a gaz motor :p but an electric 48v 20a 1000w motor with lithium battery but I can take my chance to another forum too.

I am better in electric than mecanic..

Your disc brake adaptor idea is great but I need to mount it to a freewheel? How?

Thank you!
 

Radical Brad

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Ok, now you are in my wheel-house!

The freewheel part will be the difficult part for sure.

The main problem being that the pawls creating the ratcheting drive are not really designed to handle the extreme torque of a motor like that. 1000 watts continuous may be able to deliver 3 times that during acceleration, so that is many more horsepower than would normally be seen under human power.

I have seen mid drives attempt to use standard BMX freewheels in a jackshaft arrangement, but these simply wont last on a motor of your size.

So your goal is to be able to use human only power at times?

Brad
 

Sam

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Human power should be usefull if I run on low battery but I dont know how to put both.
 

Radical Brad

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That is certainly the difficult part.
Look up "sprague clutch". I used one on a motor / human project once.
These are not exactly budget items, but the can handle the torque.

The only other options I can think of...

1) Just let the motor freewheel. If the controller is "open" when idle, the friction won't be too bad. Good enough at least for "legal" reasons that might force you to have human power.

2) Multiple BMX freewheels on a jackshaft. This one is tricky though, because if you don't align all pawls to click at once, 100 freewheels will be no better than just one - they will just break one after the other. Kind of like when a Karate master in a demo smashes 20 board in a row with small gaps between them. Not impressive at all, because it's no different than smashing one board!

Brad
 

Sam

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I think I will change the hub for one like this


And use this to mount it on.




What do you think about this ?
 

Radical Brad

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All of those components look like a great match together.
Might I ask the following?...

  1. Total RPM of your motor output shaft.
  2. Number of teeth on motor sprocket.
  3. Total diameter (with tire) of your rear wheel.
Brad
 

Sam

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Aug 11, 2019
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Rpm 3000, 11t on motor sprocket and total diameter with tire is 21 3/8 inch. Pretty large tire.
 
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