1970 Junky Rupp gear ratio

[ugmold]

Lately I have been doing some work on my 1970 Junky Rupp Enduro/Scrambler. I replaced the wheels (non-rupp) and got them pretty straight. I purchased a 52 tooth #41 chain rear sprocket that will work with the rear wheel and now need to figure out what gear ratio to go with. Looking for a optimal setting for both power/torque and top end. Not interested on a Torque Converter at this time, mostly because the wheels have no brakes and I plan to use a band brake on the clutch.

I have seen the "ratio calculators" but not to sure even what I am looking for so here are my specs.

Tecumseh 3.5, rpm ? 3200-3600 ?
Clutch is 10 tooth (although I could go with buying another) #41 chain
Rear Sprocket: 52 tooth #41 chain
Jack Shaft currently has no sprockets.
Tires are 10 inch, height top to bottom
Tire circumference 51.5
Tire height 16.25 - 16.5

Here is some recent work, Part 7, if you are curious: [video=youtube_share;A_lbVlb3Sio]https://youtu.be/A_lbVlb3Sio[/video]

 

[Havasu Dave]

I'll try and help you out. I use a spreadsheet developed by Jon Pardue a few years ago. He was an active member on here, and the spreadsheet is somewhere on line probably as an .xls.

Anyway, if you retain a ten tooth clutch, as well as the rear axle sprocket of 52 teeth and using your tire diameter of 16.25, an input sprocket on the jackshaft of 14 teeth, with an output sprocket of 12 teeth will give you a final axle ratio of 6.1:1.

You're looking for adequate torque to propel the machine, not top speed, so 6:1 is considered the target ratio. You can go up or down, depending on how much torque you make, at what RPM it is made, and how much the machine with rider weighs.

Note with the above ratios, and the engine max'd out at 3600 RPM, your theoretical top speed is 28 MPH. Use a 13t input sprocket on your jackshaft, and your ratio goes down to 5.6:1 which might work if you are light enough, but probably mean a sluggish start. But your max. speed shoots all the way up to 30 MPH.

 

[ugmold]

I'm about 190, 6 feet. Do ratios and horsepower mix? What I mean is if opted for a 6hp would 6.1.1 still be desirable?
Is this the same Jon Pardue who's the electrical whiz? I have one of his Honda S90 wiring charts tacked to the wall right next to me.
This is much appreciated my friend!

 

[Havasu Dave]

I'm about 190, 6 feet. Do ratios and horsepower mix? What I mean is if opted for a 6hp would 6.1.1 still be desirable?
Is this the same Jon Pardue who's the electrical whiz? I have one of his Honda S90 wiring charts tacked to the wall right next to me.
This is much appreciated my friend!

In a word, "yes." If you have gobs of horsepower (what you are really talking about is torque) you can get away with higher geared acceleration. The Predator engine crowd (6.5 HP nominal) do this at lower ratios in the "fives."

There is a whole other subject with clutch engagement RPM and building engines with specific power bands in mind, and even drive types that can branch off of the simple question of "gear ratio."

The single most effective performance enhancement you can make to a mini bike is to shed weight. You ought to see my grandkids grab the throttle on a 2.5HP Briggs and Stratton Cat Mini Bike! When they take off, I look on with jealousy.

Yes, it is the same Pardue who's schematic you have hanging on the wall. :laugh: Both of them know their stuff, and Jon is easy going and will work with anyone on an electrical challenge.

Go with the lower gear ratio first. If you don't like it, install a 13t on the input side and try it. Those calculators are purely theoretical speed calculators, and will give you ratios. But how a machine performs- where the best torque is RPM-wise, how it's tuned, altitude, rider weight, advertising lies on the shroud, governor settings, all of it comes in to play. 6:1 really just gets you in to the ball park.

 

[ugmold]

Thanks again. I remember my 1st minibike in about 1969, Bird Wren I bought from a rich kid for $15, blown engine, painted fluorescent green. Brought it home cleaned it up, blue paint, got a 2hp Briggs at the dump. The Rich got a new 5 hp Mini bike, we raced I blew his doors off, I must have weighed about 98 lbs...lol. That 2hp took my every where, plenty of power.