Yerf dog spiderbox gear help

[John316]

I recently picked up a Yerf dog spiderbox from a friend. It is in bad shape. Carb was seized up, engine seized up. I drained about 1/2 gallon of water out when I drained the oil. I pulled apart the head to look at cylinder, but decided the engine wasn't worth salvaging. I looked at all the available options for a new engine and choose to go with a 13 hp predator with 40 series torque converter, jackshaft mounted on pillow block bearings. I already have the engine and I'm welding on the frame to mount it.
The only issue I still haven't figured out is what to do on the sprocket mounted on the rear axle?
It appears to be a 31 tooth for a #530 chain. I want to go with a #35 or #420 chain and sprocket set up because of availability and variety. (I was thinking of going with a 12 tooth drive sprocket and a 60 tooth driven). I wanted to keep max speed around 35-40 mph.
I am having a hard time finding a 60T sprocket with 3-1/8 bolt hole cir (2-1/4 hole to hole) bolt pattern.

Anybody know where I can get a 60 T replacement sprocket in a #35 or #420 ?
Or any other Ideas that might help. Is my calculations on the gears seem close to the MPH I am looking for?

[ckau]

This is part of an article I had posted some many moons ago. It may help or totally confuse you.
Working out gear ratios are a tough problem because of so many variables. This is the math but keep in mind a jackshaft will change things up unless you keep all the gears on the jackshaft the same as the drive sprocket. That axle sprocket is tough, There are numerous gear
& sprocket company's out there that can make a sprocket to your specifications, google it. or you can save some bucks by buying a blank 60T and drilling out your bolt pattern.
Working out the ratios is the easy part . It’s just some simple math formulas you can do on any calculator. The hard part is deciding what you need . Unfortunately the only true way to know is try them. There’s probably a dozen different trans gear ratios available but only a few axle ratios available without having some custom stuff made. When more than one pair of gears are used together, that is called a Compound Gear Train. The gear ratios for each individual gear pair are multiplied together to compute the overall compound gear ratio
Using a basic stock Yerf with no reverse as example the basic math formula for getting overall ratio:
Trans gears- 13/40 13(A) , 40(B)
Axle gears 13/31 13(C), 31(D)
Divide A into B (40/13) = 3.08 [trans gear ratio] (E)
Divide C into D (31/13) = 2.38 [axle gears ratio] (F)
Multiply E and F (3.08x 2.38) = 7.33 ( overall drive ratio) (G)
This means for every complete rotation of the axle sprocket the motor turns 7.33 times.
Another way to see it: @7,330 rpm the axle sprocket make approximately 1,000 rotations ( 7.33 to 1)
The higher the final ratio number (G) the more torque and acceleration.
Lower the number (G) = more top speed.
Keep in mind this number (G) is to the axle sprocket. Tire diameter plays a part in this too. Taller tires lower the number. There’s another formula to work out the ratio between tire diameter and sprocket rotation
To figure the true final ratio you will want to use IPR (Inches Per Revolution) which means the number of inches the buggy will travel per each revolution of the motor
To Figure IPR:
Rear tire Circumference (X) 22in. is a popular tire size.
Multiply by ( C) 13, outer trans gear. If with reverse this number would be 16
Then divide by ( D) 31, axle gear. (X*C) / D = IPR
( 22 x 13 ) / 31 = 9.22 . This means the buggy would travel 9.22 inches for each revolution of the motor.
The higher the IPR number the faster you go !
With reverse: (22 x 16) / 31 = 11.31 Which shows with a reverse unit your covering more ground. with the same motor rpm.
Are you confused yet? I am! At this point I usually get a brain meltdown. You can play with these formulas to see what different gears will do. It can help to make some sort of educated guess before spending time and money .

[John316]

Thanks for the input. I will post some picture after I get the engine and jackshaft mounted

[John316]

I have been modifying the rear frame to accommodate the 420 engine while I am waiting on the rest my parts to come in. I did decide to go with a #40 chain, 9 tooth drive sprocket and 60 tooth driven. The 60 tooth sprocket is 9-3/4" in diameter so I had to do some chopping and re welding to get things where they will work. I was trying to get 7:1 gear ratio since I am going with 22" tires, but 6.6 is close enough. Most of my driving will be trails with some steep hills so I wanted to start out low. I figured I could always get a 10, 12 or a 14 tooth drive sprocket for the Jackshaft and increase the speed if I need to.