Another gear ratio with jackshaft question

[freakincage]

Hello Gurus,

It's been a while since I've had to post a question because I can usually find it by searching...but this has me kinda stumped.

Some of you may know that in the fall I picked up a beat up Sears Roper to revive. It was missing many parts, including the factory jackshaft, so I have to fab one up. I'm now at the point of putting the drivetrain into it and I have a couple of jackshaft questions.

The previous owner had the chain going directly from the clutch to the rear wheel and as a result the rear wheel had to be off center to get everything to line up. I want the rear wheel centered in the frame, so I need the jackshaft.

The power plant is an ungoverned Briggs 5hp. No other mods, just no governor.

I have a 12T clutch for the engine and a 60T sprocket on the rear wheel (5:1 ratio).

Locally, the smallest sprockets I can get to for the jackshaft are 16T.

I was thinking of this set up:

12" diameter wheels

12T clutch to 16T input on jackshaft (1:1.333 ratio)
16T output on jackshaft to 60T rear sprocket (1:3.75 ratio)

Based on what I've read to calculate this, the final ratios works out to be 4.999:1 (1.333*3.75), resulting in a top speed of around 25MPH and decent enough torque to get going.

I've also read that it is the final gear ratio is all that matters, but that doesn't really make sense to me because of the variables added with the jackshaft.

What are some thoughts on my proposed gearing (aside from "try it and see")?

Thanks in advance,
G

 

[landuse]

On 12" tyres you should be OK I think.

Try it and see

 

[freakincage]

"Try it and see" LOL.

Do you (or anyone) know what the original gear ratio for the Sears Roper was? I can't seem to find that info. Is it really just the end ratio I should be shooting for? I don't need to pop wheelies, but I would like to have decent acceleration, so I am willing to trade a bit of top speed for torque. Is there a gearing that you feel would be better than what I'm proposing?

 

[firemanjim]

You should pm rmm727 for a factory reproduction jackshaft bracket. He does awesome work.... I got one for my roper.... Fits like a glove. I'm using nos 2 speed set up....

 

[freakincage]

This thing had been basterdized before I even got it and there isn't even the factory mounts or anything. I think a simple custom set up is easiest for me as fabbing stuff isn't a problem. Thanks though.

 

[JoeKR]

I think you'll find you'll be going over 25mph with that ratio and no governor. I also see your area is board flat, so you don't need stump pulling power.
With both jackshaft sprockets 16t you're just transferring power from one side to the other, that just seems like a waste.
I'd go 12t to 20t then 16t to 60t. That gives you a 6.22 to 1 final ratio.

 

[chancer]

You should pm rmm727 for a factory reproduction jackshaft bracket. He does awesome work.... I got one for my roper.... Fits like a glove. I'm using nos 2 speed set up....

He is just sayin, nothing really about your questions (I think) but the bracket he had made was AWSOME. If you want an AWSOME bracket pm rrm727http://www.diygokarts.com/vb/showthread.php?t=28680&highlight=bracket+made
Check out this bracket:

 

[freakincage]

I think you'll find you'll be going over 25mph with that ratio and no governor. I also see your area is board flat, so you don't need stump pulling power.
With both jackshaft sprockets 16t you're just transferring power from one side to the other, that just seems like a waste.
I'd go 12t to 20t then 16t to 60t. That gives you a 6.22 to 1 final ratio.

Although where I live is flat as a pancake, the area where we play isn't, so I do need some pulling power. Plus some of the people that will be playing on this are around 200 lbs. I don't need crazy top speed. How would a 6.22:1 ratio work for what I'm describing?

My understanding is that lower ratio = more torque and higher ratio = higher top speed.

Also, does anyone know what the factory ratio for the Roper was and with that ratio, how did the bikes perform?

Thanks.

 

[landuse]

My understanding is that lower ratio = more torque and higher ratio = higher top speed.
Thanks.

You are correct

 

[JoeKR]

Although where I live is flat as a pancake, the area where we play isn't, so I do need some pulling power. Plus some of the people that will be playing on this are around 200 lbs. I don't need crazy top speed. How would a 6.22:1 ratio work for what I'm describing?

My understanding is that lower ratio = more torque and higher ratio = higher top speed.

Also, does anyone know what the factory ratio for the Roper was and with that ratio, how did the bikes perform?

Thanks.

Ok, knowing that bit of info, I'd go w/a 7.5r by using a 12t into 24t and keep the 16t to 60t. When you only have one gear ratio you had better get it as right as possible. And that goes for just about anything, I always set my single speed bicycles for my particular build and fitness.

I'm battling this issue on my drift trike, cause I'm needin' some short gear, and I'm just now installing a jackshaft of dubious design.
I can't help you with the original gearing, but I don't think it would apply here. That thing was designed when we were all a little lighter.
Good news is with all this extra work, I'm losing weight!