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Old 03-28-2013, 03:37 PM
ProDigit ProDigit is offline
Motorized Bicycle Elite Member
Join Date: Mar 2013
Location: Miami, FL
Posts: 137
Default Re: direct drive an 18T fixie sprocket?

Thanks for the answer! I was desperately looking for the smallest rear sprocket.
I probably go with a 32; here's why:

I did some non-engineering, quick calculations and found the following:
(this is all pretty theoretical, only to have an average 'feel' of what to expect when equipping a bike with the kit)

The max speed I am willing to go on a bike would be 40-45MPH.
Since the 49cc engine, even on scooters, tops ~40MPH, I think I'll be taking the 66c engine.

The average cruising speed I would want to have would be between 25 and 32MPH
In this 25-32 MPH range, I want the engine not to rev too high, so round about 2500 RPM.

Most people have said that their 44T sprocket on a 26" tire gives them 32MPH.
This concludes that a 32T sprocket gives 44MPH; as according to the calculation there are only constants (Pi, Radius, front and rear sprocket teeth, RPMs).
So a 32 sprocket should give ~44MPH.
Since the bike is bigger, and at higher speeds there's more wind resistance, I am expecting to get ~37-38MPH out of the 49cc engine (since the engine just simply can't keep up).

To top 40MPH, I will equip it with a 66cc kit, and I'm still contemplating on the bike (which I will do in another topic).
I presume the 66cc kit should top ~40-45 MPH.

Presuming that the engine does 32MPH @5500RPM with a 44T sprocket, and that the minimum RPM not to bog out would be ~1000RPM; At that speed the bike will go 32/5.5*1 = 5.8MPH.
Converting that to a 32T sprocket, I would be going 5.8*44/32 = 8MPH.

at ~2000RPM the bike should be doing ~16MPH.
~2500RPM it should be doing ~20MPH, which is not the best, but still a great cruising speed.

If I would equip it with any smaller rear sprocket, I might lose top end speed, by over-gearing; so some compromise has to be made.

This ~20MPH speed is about the max speed you're comfortable with pedaling on a beach cruiser.
Beach cruisers have low gearing, good for fast acceleration, low speeds; thus it has the pedal power to go to ~10MPH, after which the engine easily takes over.

In the beginning, the engine does not have sufficient torque to accelerate fast, at which it is possible to further aid the engine with the pedals until ~20MPH (at which the engine RPM is doubled).
After this, every RPM that gets added to the engine, and every moment of time, where the bike is accelerating, less and less the pedals can keep up, until the engine fully takes over.
It is that situation I would want to see in a cruiser bike.

I think the math is pretty solid (with some variables taken from people's experiences (like top speed) etc...

So based upon this calculation a 32T sprocket is about the best you can get for a 66cc engine on a 26 in wheeled bike.

Last edited by ProDigit; 03-28-2013 at 03:40 PM.
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