Gear Ratio with Predator 212cc and jackshaft

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Predator303

New Member
Apr 26, 2012
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San Juan, Puerto Rico
Not quite sure where to post this but maybe someone with experience with this engine can give me a tip before I order the sprockets.

I bought the engine from somebody in this forum and he ran the following setup

13t clutch
22t JS in
9t SJ out
48t rear sprocket

I get a ratio of 9.026



He said he had around 50mph at 6700rpm

Now my goal is a good cruising speed of 50mph at a lower rpm

The website where I can order the jackshaft only offers the JS sprockets from sizes 10 to 18 and it would be great to order everything from one place.

So if I order the clutch with 12t then 18t in, 10t out and to a rear sprocket of 44t (which i still have around here somewhere)

I get a ratio of 6.600



If I calculate right, this would mean I can go same speed (50mph) with about 4904rpm. Which sounds perfect but then again I think it might be a little too long the ratio because of the start from dead stop.

Now from what I heard the engine got lots and lots of torque so here is the question:

Do you think this is a too long ratio for launching from dead stop?

Compared to my current bike with a slightly tuned (higher compression) 80cc running a 34t sprocket in the rear I can still perfectly launch from dead stop without paddling so im pretty sure with a monster like the Predator it should be no problem with this ratio setup. Or not? ;)

The clutch I want to get engages at 2.000rpm
 
Last edited:

dmb

Active Member
Dec 4, 2010
1,354
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36
lakewood ca
carefull if thats the one i think it is. it can spin the hub clamp right through the spokes. looks like you got your wish. very good deal. now buy a big shoehorn. dennis
 

Predator303

New Member
Apr 26, 2012
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San Juan, Puerto Rico
Can somebody explain me the different options they sell here and why you would either recommend a jackshaft or torque converter? And also - what rear sprockets are being used in the case of the converter??

 

potbunny

New Member
Nov 26, 2012
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Bloomington, IN
what did u order and where from? is your engine stock, or is the governor removed? i am also making a bike and want a good speed, did yours turn out ok?
 

potbunny

New Member
Nov 26, 2012
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Bloomington, IN
i dont think i want to remove the governor as ive been told that it reaches 5500 rpm max with it...i was looking at 50 or so its on a chopper with pretty thick tires. i am completly ignorant to gear ratios, clutches, rear sprockets and all of that jazz, i honestly have no idea what im doing at this point. i really just want to run a chain from a 12 tooth clutch that engages at 2800 to a 55 rear sprocket and see what i get...do you think that will work? what are your suggestions?
 

JonnyR

New Member
May 13, 2012
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ronkonkoma, new york
all governors are set to 3600

and you need a 10:1 reduction from the motor since its a chopper you dont need the offset like the rest of us do but it may need to go the other way so you may still need a jack shaft
to work right it would be a 120T rear sprocket a 55t if you could make the power to pull that gear is something like 80-90 MPH the gear reduction is the other reason we use a jack shaft so we can use a 40-55T rear sprocket its taken up at the jack shaft
 

potbunny

New Member
Nov 26, 2012
26
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Bloomington, IN
so i need a jackshaft? what sizes do i need on the jack shaft and rear sprocket? the same as predator did above? i would hate to take out the governor and i dont want to unless i really have too. what size sprockets should i use to get maximum speed?
 
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JonnyR

New Member
May 13, 2012
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ronkonkoma, new york
it idels at 2200 if your clutch in gauges at 2800 and the govenor is at 3600 that gives you 800RPM to use

the sprockets arnt hard to work out its division mostly if your clutch is 12T and your rear is 55T thats 4.58:1
with the suggested AGK jack shaft ratio 12T clutch 22T in 9T out and a 56T rear is 12.44:1
if you switch to a 44T rear or a 11T on the output of the jackshaft it brings it close to 10:1
 

potbunny

New Member
Nov 26, 2012
26
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Bloomington, IN
and what is my top speed? is that the fastest i can possibly make it go to start out from a dead stop? i found a clutch that engages at 1800 to 2000, would taht help?
 

JonnyR

New Member
May 13, 2012
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ronkonkoma, new york
no because it engauges below idle so you cant start the engine without it engauging top speed with the governor is not going to be to high 30ish without it 50 ish
 

JonnyR

New Member
May 13, 2012
1,203
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ronkonkoma, new york
thats about the highest geat ratio it should pull stock with the governor you will never see 50MPH without it you should be able to do it but the rest of the bike is going to need extensive modification wheels bearings tires forks the stronest frame you can find its not a cheap build alot of the guys have $1500-2000 in there 212cc bikes