Sprocket ratio - should I change drive or driven sprocket?

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Got a 10T drive sprocket and a 44T driven sprocket (4.4:1) ratio. Want to change ratio to increase top speed. Would I be better off changing the drive or driven sprocket? Got a 100cc engine with 26" tires (but im sure the engine is more like 80cc.)

If I was to get a 40T driven sprocket would I expect a ~10% increase in max speed? And ~15% with 38T?
 

Nickt919

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I'm yet to try a bigger drive gear but I've tried a few big sprockets. I tried from a 36 to a 44T sprocket.
I have not tried a bigger drive sprocket because everything I've read about it says space is very tight. The bigger gear up front may take some modification to accommodate it. There are enough rear size sprockets to get what I wanted with-out changing the front gear but the idea of a tooth bigger up front would give a 415 chain a little bigger radius to wrap around.

I've found..for me at least...the engines seem to tolerate a 10/38 the best.
This holds true for me on total stock to moderately modified engines.
I ride on all flat land....this is a big consideration. The biggest hill I go up is a curb.
This ratio on flat land gives me the best combination minimum pedal assist taking off and top speed.
Any ratio higher than a 10/38 my stock engine really couldn't un-wind and lower ratio left untapped speed to be had.

While on the topic of sprockets...the quality of the cheap CG Chinese sprockets is crap. I went through many cheap sprockets...both counter and rear sprockets. Most come with quality like Fred Flinstone chiseled them out with a hammer and chisel. Out of round, non-uniform teeth. If you really want a smooth drive chain it helps to buy CNC cut gears. They not widely available but they can be bought.
 

ImpulseRocket89

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Got a 10T drive sprocket and a 44T driven sprocket (4.4:1) ratio. Want to change ratio to increase top speed. Would I be better off changing the drive or driven sprocket? Got a 100cc engine with 26" tires (but im sure the engine is more like 80cc.)

If I was to get a 40T driven sprocket would I expect a ~10% increase in max speed? And ~15% with 38T?
Since I just happen to have my calculator handy for another thread we can look at a few combinations to give you an idea of what kind of speed increase you would be looking at.

First we need to establish your maximum RPM. I am just going to use 6000rpm as a baseline since it is likely the most realistic, unless you have done some port work.

With your current setup, your top speed at 6000rpm is 25.7mph, give or take a bit.
With a 40 tooth sprocket that top speed at 6000rpm is now 28.3mph,
With a 38 tooth your top speed at 6000rpm is now 29.8mph
And with a 36 tooth sprocket, your top speed at 6000rpm is 31.4

Basically, in really rough numbers, every 1 tooth of back sprocket gives you roughly .75-.8mph at 6000rpm.

Here is the real kicker. A little port work to the cylinder to increase the powerband even 1000rpm will make a greater difference in top speed potential. If your engine could now top out at 7000rpm the top speed of the bike would be 30mph with the 44 tooth sprocket which matches the 38 tooth at 6000rpm, but would have better torque.
 
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Very good info. I ended up ordering the 38T. I am currently porting the engine, theres quite a lot of junk that I am filling down. That on top of the leaner jet as well should all make for an interesting change in riding performance.
 

ImpulseRocket89

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Very good info. I ended up ordering the 38T. I am currently porting the engine, theres quite a lot of junk that I am filling down. That on top of the leaner jet as well should all make for an interesting change in riding performance.
If you are porting it, then you may want to stay with the rich jet. Porting will change the efficiency of the engine and thus change the fueling requirements.
 

Sidewinder Jerry

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One of the first things you need to determine when doing speed calculations is what the internal engine reduction is. You can do this by removing the cover plate over the drive gears and counting the number of teeth on each gear. The internal reduction will be either 3.1:1 or 4.1:1. You can then use this to help calculate Total Reduction.

Here's my speed formula:

(RPM × Wheel Diameter × π)÷(1056 × Total Reduction)=MPH
 
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