Having trouble figuring jackshaft ratio

Mrgoodcat

New Member
Jul 25, 2021
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I'm putting on A42CC poulan pro Chainsaw motor And I'm building a jackshaft to go to my 7 speed rear freewheel gears I used a gearing calculator with 6t on the clutch to 44t input - 32t out to a 14t as my highest gear goin 14 -16-18-20-22-24-32 with high gear ratio of 3.208.1 max speed 44.643 mph @ 5500rpm
With the saw rpm max 9000 will this work???
 

Sidewinder Jerry

Well-Known Member
Dec 19, 2011
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Rockwood, TN
I'm putting on A42CC poulan pro Chainsaw motor And I'm building a jackshaft to go to my 7 speed rear freewheel gears I used a gearing calculator with 6t on the clutch to 44t input - 32t out to a 14t as my highest gear goin 14 -16-18-20-22-24-32 with high gear ratio of 3.208.1 max speed 44.643 mph @ 5500rpm
With the saw rpm max 9000 will this work???
None of what you're wanting to do will work. Do a simple kit build. Non kit builders generally aren't concerned with a budget.

It's sorta like this; it'll cost you many times more to buy a frame and components to build a bicycle than just simply buying a whole bicycle.

Same is true when comparing a DIY build to a kit build.
 

JerryAssburger

Active Member
Mar 11, 2017
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Salem Oregon
The good part is, you'll always have enough "gear" for the motor.
Realistically, the motor puts out 2- 2.5hp tops, and that puts you in the neighborhood of 35 mph top speed.... so you gotta gear it for at least one of your gears to get you there at 9000 rpm. Naturally, no-one wants to scream around at top rpm, except to accelerate to your desired speed. I haven't the slightest clue what a decent "cruise rpm" would be for a chainsaw motor. My admittedly arbitrary goal would be top cruise rpm at the engine's torque peak, but again, who knows on a typical chainsaw motor what THAT is? On my STOCK 66cc China Girl bike, it liked what worked out to 5000 rpm- it didn't vibrate, it pulled decently, and it wasn't threatening to grenade on me.
So at present, your top gear is too tall, but that's okay- you have many lower gears to pick from. I had cobbled together a bike with a Briggs 3hp and using the 7 speed cluster on the rear wheel- the lowest gear topped out easily at just under 30, and had decent take-off. The next gear up was my fav, because the bike could hit 35 easy and still pull from a stop. The third up would pull 38, but the bike had to start out in one of the lower gears, first. The last 4 gears- I couldn't use- too high. (unless down the side of a loooong ski-slope, maybe?)
Sidewinder is RIGHT, as far as how much of a PITA it is to cobble a non-kit together and then end up not saving much money after-all. The newer "100cc" kits are approaching the $140 mark ($180, with the essential rag-joint eliminator) ...and that's a bargain. I SO want to pull the trigger on one just to have it at the ready.
I only built my first bike with the Briggs because I had no clue these kits existed.
 
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