Aligning the back sprocket to the engine sprocket?

[bfgmovies]

So ive been motorizing a second bike recently, it's a girls cruiser bike and I'm using a 38T sprocket in the back, however to get it aligned right I will need to put both the rubber things on the outside of the wheel. Should I get a third rubber circle thing to put on the inside?

 

[Bikeguy Joe]

Yes..
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[bfgmovies]

Ok well.. I don't know how I mixed This up but it was misaligned so that I had to make it closer to the wheels not farther out, so I managed to fix that lol