Watercooled motobicycle help

[5-7HEAVEN]

You will need to spin this engine to 9,000rpm for max power.

You might need 40:1 gearing for pulling away from a stop at the bottom of a very steep hill.

For highway speed, gearing of 12:1 would work.

An 8-speed cassette or an 8-speed internal gear hub(IGH) and a shift kit would work very well.

I ran my 5hpCY460 engine and my Tanaka 47R engines thru this gearing.

Both engines absolutely loved it.

My First gear was 46.36:1 and 8th gear was 15:1.

Estimated 46.4mph @ 9000rpm.

 

[5-7HEAVEN]

If I'm correct, this engine has a different clutch bolt pattern, a larger clutch and a larger bell.

It is is different from the standard 76mm clutch bellhousing.

So the 5:1 pocket bike(pb) transmissions or any common tranny will not bolt on.

A high-winding engine falls flat on its face without low (numerically high) gearing.

 

[5-7HEAVEN]

That 39cc without a tuned expansion chamber will only put out up to maybe 2 - 3 hp at 6000-7000 rpm. I could see 4.95 hp at 9000 out of that with an expansion chamber.

This engine reportedly produces 3.5KW.

1 KW = 1.341 hp.

3.5 x 1.341 = 4.69hp @ 9,000.

 

[5-7HEAVEN]

For the chinagirl 49cc and 66cc 2 stroke motorized bike engines, a 1:4 gear ratio (engine:wheel) generally works best if you only have one sprocket.

For yours I'd say between 1:4 and 1:5 if you only use one sprocket. That will get you a general idea. I'll let someone else answer further that may know of this specific engine.

China girl engines have a 4.2:1 internal gear ratio.
A 4:1 ratio results in 16.8:1 ratio, a 5:1 ratio yields 21:1 ratio.

This engine does not have an internal gear ratio..

Using 1:4 and 1:5(sic) ratios on this engine only makes 4:1 and 5:1.

It will burn its clutch, without the bike moving from a stop on flat ground.

 

[SouthSideKruizuh]

If I'm correct, this engine has a different clutch bolt pattern, a larger clutch and a larger bell.

It is is different from the standard 76mm clutch bellhousing.

So the 5:1 pocket bike(pb) transmissions or any common tranny will not bolt on.

Without the 5:1 pb tranny, you will not be able to get anywhere near a 40:1 gear ratio.

A high-winding engine falls flat on its face without low (numerically high) gearing.

I dont have the engine yet (in the mail o,n the china express) so I can't verify bolt pattern, but from what I have been able to find it is 71mmx71mm bolt pattern and a 77.5mm id clutch bell. I got the engine from aliexpress and I am assuming it seems like a clone of the 39cc mta4 engine or more similarly a clone of the polini water cooled pocket bike engine, it uses a very similar looking clutch housing. Since I can't find any direct info on this exact engine, I guess I'll have to wait to get it to confirm clutch size

 

[SouthSideKruizuh]

How many teeth on the engine's clutch?
If it's 10t, you need a very large 25h sprocket on the jackshaft.

Resourcing 25h sprockets to slip onto a 5/8" or 3/4" jackshaft is futile.

Luckily, there are adaptors that'll fit large 25h sprockets to these jackshafts.

Staton Inc

3 Prong, 5/8” ID x .9" wide Adaptor with 2 set screws for freewheel sprockets. 1.37 x 24 tpi CW - Right Hand Threads. Includes the two M6-1.0 set screws.This Freewheel sprocket adapter will fit the following single and multi speed freewheels with 1.37 x 24 right hand threads.They also can be...

www.staton-inc.com

A 25h 55-tooth pocket bike rear sprocket could fit onto your jackshaft.

25H 55T 54mm Rear Chain Sprocket For Electric Scooter Razor E300 Compatible #25 | eBay

â Quantity: 1pc Rear Sprocket. not from your country. If you would prefer.

www.ebay.com

It is 4.53" diameter, same as a 28t bicycle sprocket.

It looks like 8 teeth from the from the picture. Again I'm assuming it's the same clutch sprocket as the 25h pocket bike one so I could change it to a 6t-10t

 

[YetiWoodz]

China girl engines have a 4.2:1 internal gear ratio.
A 4:1 ratio results in 16.8:1 ratio, a 5:1 ratio yields 21:1 ratio.

This engine does not have an internal gear ratio..

Using 1:4 and 1:5(sic) ratios on this engine only makes 4:1 and 5:1.

It will burn its clutch, without the bike moving from a stop on flat ground.

I don't understand. A ratio is just a fraction. A 10 tooth drive to a 40 tooth wheel sprocket reduces to 1:4.

 

[SouthSideKruizuh]

How many teeth on the engine's clutch?
If it's 10t, you need a very large 25h sprocket on the jackshaft.

Resourcing 25h sprockets to slip onto a 5/8" or 3/4" jackshaft is futile.

Luckily, there are adaptors that'll fit large 25h sprockets to these jackshafts.

Staton Inc

3 Prong, 5/8” ID x .9" wide Adaptor with 2 set screws for freewheel sprockets. 1.37 x 24 tpi CW - Right Hand Threads. Includes the two M6-1.0 set screws.This Freewheel sprocket adapter will fit the following single and multi speed freewheels with 1.37 x 24 right hand threads.They also can be...

www.staton-inc.com

Using this:

47 Tooth Rear Sprocket for #25 Chain

47 Tooth Rear Sprocket for #25 Chain

cart.electricscooterparts.com

This 47t is 3.87", same as a 24t bicycle sprocket.

A 7t clutch and 47t jackshaft yields 6.71:1.

A 12t jackshaft(#35 chain) connected to a 36t rear sprocket is 3:1.

6.71 x 3 = 20.14:1.

Possible 34.6mph @ 9000rpm.

Now you're in the ballpark with a single sprocket combination.

JMO, a shift kit with multi-speeds would really wake up this engine at every shift.

Lol that's where my confusion is I have the sick bike parts shift kit and all the numbers got me spinning. So the engine hooks to the 17t jackshaft sprocket, then transfers to the 10t pinion hooked to the 44t chainring sprocket at the pedals. That is connected to a 36t sprocket that is hooked to my derailleur and 14-28t 7 speed cassette. Phew....

 

[5-7HEAVEN]

Lol that's where my confusion is I have the sick bike parts shift kit and all the numbers got me spinning. So the engine hooks to the 17t jackshaft sprocket, then transfers to the 10t pinion hooked to the 44t chainring sprocket at the pedals. That is connected to a 36t sprocket that is hooked to my derailleur and 14-28t 7 speed cassette. Phew....

Yay! Shift kit!

Your engine will LOVE you for it.

Use a 6t clutch sprocket.
Chain it to a 47t(#25h)jackshaft sprocket.
The 47t is 3.87" diameter, about the same as a 24t(#35) sprocket.

On the right side jackshaft, use a 9t(#35) Sick Bike Parts(SBP) sprocket.
Connect to a 36t outer chainring sprocket, freewheel and crank arm.
Use a 24t inner chainring SBP sprocket.

Your first gear(28t) ratio will be 36.56:1.

Your final gear will be 18.28:1.

38.1mph @ 9000rpm in 7th gear.

For more low gearing, swap in a 14-32 for 41.78:1 ratio.

You could literally climb trees with this stump pulling 1st gear.

Perfection.

 

[5-7HEAVEN]

If you want to use the 10t/44t sprockets you already have,
your gearing increases to 40.21:1 in First, and
20.11 in 7th gear.

34.6mph @ 9000rpm.