Final drive ratio

[mrmagoolew]

I have a mini bike with a jackshaft. I have a 11 tooth clutch, then back to a 30 tooth jackshaft gear then to the other side to a 12 tooth. Then I go to a 48 tooth final drive. The only problem is that I just don't get any speed out of it. Should I have a larger or smaller on the back. Even with the engine wound out, I only get about maybe 15 MPH. Am I doing something wrong? Do I go ahead with a bigger back sprocket? I was thinking of 88 teeth or so. Any Ideas. Lew

 

[bajagokart]

so for you engine to jackshaft its a 30/11 which is 2.73:1 and from the jackshaft to axel its 48/12 or 4:1 so your combined total is 10.92:1 combined ratio (2.73 * 4)
that is really really really low for your minibike to say the least lol
i would probably get a 36 tooth on your axel and a 22 to 24 on your jackshaft which would give you a 2:1 and a 3:1 so your final ratio would be 6:1 ish which is much better

 

[r_chez_08]

What engine have you got powering the bike?

 

[mrmagoolew]

Believe it or not, the gearing is exactly the same as when I got it. That's what was on it when I replaced all the broken up stuff. I just couldn't understand why it would only go so fast with a 5 HP motor on it. I'm going to have to see if I can go with those gear recomendations or not. I don't know how much smaller I can go in the rear. I'll also have to check out that jackshaft. Thanks, Lew

 

[r_chez_08]

Yeah 6:1 like blaz suggested, or maybe ever 5:1 would work, depends on tyre size.

 

[solomon]

you could just change the 12T sprocket from the jack shaft going to the rear sprocket to a larger one.

 

[mrmagoolew]

Hi, Solomon, Would going up to 18 teeth help much? I don't have a clue! Lew

 

[Doc Sprocket]

Honestly, if you're going to go 5 or 6:1, measure your rear axle-to-ground-clearcance and see if a 55-66T rear sprocket will fit. If so, just get rid of the jackshaft, and it's additional weight and moving parts.

 

[mrmagoolew]

Hi, Toystory, I have a 48 tooth. I was thinking along the same line as you about getting rid of the jackshaft, but I have a swingarm to contend with. I'm not sure that getting rid of the jackshaft is an option. I'll look into it again. As far as a sprocket size goes, I can probably fit an 80 tooth sprocket and still have room to spare. Getting rid of the weight sounds great too. All the extra parts not being used or causing trouble sounds good too. I guess I'll try this 48 tooth sprocket. Now I'll have to make another brake cable bracket. Lew

 

[Doc Sprocket]

With the swingarm problem, you may be able to solve that with a spring-loaded chain tensioner.

 

[bajagokart]

Or to make it simple, go 10T on the motor and 60T or 50T! Remember the smaller the sprocket the less torque you get but the more speed you gain!

 

[solomon]

Swap the 12T to a 26T and u have made the gear ratio 5:1, Can't get any more simple than that. How tall is your rear tire? And what engine are you running?

 

[Doc Sprocket]

Or to make it simple, go 10T on the motor and 60T or 50T!

That is essentially what I told him. But since his clutch is 11T, no sense buying a new one. The same ratios can be accomplished with 55-66T on the axle.

 

[mrmagoolew]

Hi, It turns out that my clutch is 12 teeth. I was able to get it out into the light and actually count the gears. The engine I'm running is a Briggs and Stratton. It's 5 HP, and has a manual type governor. I was told it was from around 1974. It's painted a coppery color. Getting back to the clutch. It's 12 teeth, and I have a 48 tooth sprocket. So that should be a 4:1 ratio, right? Would that be OK. I weigh about 240#, and that might be going too much the other way. I'm mostly going to be riding in the dirt, on rolling terrain. What do ya think? Lew

 

[bajagokart]

4:1 might be kinda high for 5hp im thinking . . . minimum i would do is 5:1 so get a 60T on the rear if at all possible

 

[mrmagoolew]

Hi, Guys, It turns out that the jackshaft will have to stay. It and the swingarm crossmember is in the way if I try to go to one single chain. So, now it's back to using the jackshaft. Now, if I would like to get the ratio down to 5:1, what would I have to do? As is, the clutch and jackshaft ratio appear to be 2.5. So the other one would have to be the same ratio? If that's correct then if I kept the 48 tooth rear sprocket, I'd have to have around a 20 tooth sprocket, right? Well, I found an 18, so maybe that'll be close enough? Lew

 

[devino246]

Hi, Guys, It turns out that the jackshaft will have to stay. It and the swingarm crossmember is in the way if I try to go to one single chain. So, now it's back to using the jackshaft. Now, if I would like to get the ratio down to 5:1, what would I have to do? As is, the clutch and jackshaft ratio appear to be 2.5. So the other one would have to be the same ratio? If that's correct then if I kept the 48 tooth rear sprocket, I'd have to have around a 20 tooth sprocket, right? Well, I found an 18, so maybe that'll be close enough? Lew

ratios are multiplied. you would need a ratio of 2:1 going from the jackshaft to the wheel.

 

[bajagokart]

so switch it to like a 10T on the jackshaft to a 20T on the axel or a 12 to 24 etc. etc.

 

[solomon]

a 26T sprocket going from the jackshaft to the rear sprocket will get you right around 5:1 with your current setup.

 

[mrmagoolew]

I was able to find a 24 tooth, and I don't think it will be so large as to interfere with the gas tank on the engine. I'll have to give that a try for right now. Funds are always limited. So many projects, not enought money!! I'll let you know how things turn out. The sprocket and hub should be here any day now. I still have to build that brake cable bracket though. I'll get to that tomorrow. Lew