Hi everyone so a few years ago i started restoring what i believe to be a sears mini bike but some personal issues have gotten in my way. I am finally now working on the bike again i have almost all my egg in a basket but i can't seem to figure out this jackshaft gearing as it was missing as long as i've had the mini bike. Right now i have a 12 tooth clutch and a 60 tooth drive sprocket i know this to be 5:1 i would like to improve low end and im not sure what size sprockets to choose? Thanks for any help.
jackshaft formula
John, you mentioned "jackshaft." You might know you need four sprockets for that, but not sure from your description. If all you have is two (clutch sprocket and rear wheel sprocket), then just do what gumpit suggests to increase that 5 in the 5:1 ratio that you already know about.
If you have a jackshaft as well (two more small sprockets on a separate shaft--you would have two chains, too), then you can just play with those two little sprockets.
There are online calculators for this, and plenty of info from running a search on the forum. Or, you can use this formula to calculate the resulting ratio:
C = # teeth on the clutch sprocket
S = # teeth on the rear wheel sprocket
Jc = # teeth on the jackshaft sprocket connecting to the clutch sprocket
Js = # teeth on the jackshaft sprocket connecting to the rear wheel sprocket
Ratio = (S / C) x (Jc / Js)
So, with a 12-tooth clutch, a 60-tooth rear wheel sprocket, and a jackshaft with 18 teeth on the sprocket connecting to the clutch, and 13 teeth on the sprocket connecting to the rear wheel sprocket, you have this ratio:
Ratio = (60/12) x (18/13) = 6.92:1
Then, if you want to figure your top speed, do this:
R = the ratio you calculated above (with or without the jackshaft)
D = outside diameter of the real tire
P = RPM of the engine
K = 60 / 12 / 5280 = 0.000947 (this is a conversion factor)
MPH = (P / R) x 3.14 x D x K
In the jackshaft example above, suppose you have a rear wheel with 13.5" diameter, and your engine is running at 3600 RPM:
MPH = (3600 / 6.92) x 3.14 x 13.5 x 0.000947 = 21 MPH :thumbsup:
John, you mentioned "jackshaft." You might know you need four sprockets for that, but not sure from your description. If all you have is two (clutch sprocket and rear wheel sprocket), then just do what gumpit suggests to increase that 5 in the 5:1 ratio that you already know about.
If you have a jackshaft as well (two more small sprockets on a separate shaft--you would have two chains, too), then you can just play with those two little sprockets.
There are online calculators for this, and plenty of info from running a search on the forum. Or, you can use this formula to calculate the resulting ratio:
C = # teeth on the clutch sprocket
S = # teeth on the rear wheel sprocket
Jc = # teeth on the jackshaft sprocket connecting to the clutch sprocket
Js = # teeth on the jackshaft sprocket connecting to the rear wheel sprocket
Ratio = (S / C) x (Jc / Js)
So, with a 12-tooth clutch, a 60-tooth rear wheel sprocket, and a jackshaft with 18 teeth on the sprocket connecting to the clutch, and 13 teeth on the sprocket connecting to the rear wheel sprocket, you have this ratio:
Ratio = (60/12) x (18/13) = 6.92:1
Then, if you want to figure your top speed, do this:
R = the ratio you calculated above (with or without the jackshaft)
D = outside diameter of the real tire
P = RPM of the engine
K = 60 / 12 / 5280 = 0.000947 (this is a conversion factor)
MPH = (P / R) x 3.14 x D x K
In the jackshaft example above, suppose you have a rear wheel with 13.5" diameter, and your engine is running at 3600 RPM:
MPH = (3600 / 6.92) x 3.14 x 13.5 x 0.000947 = 21 MPH :thumbsup:
I have a bolt on rear sprocket when me and my dad re did this bike when i was 12 that was an upgrade we made. The welded bracket for the jackshaft had always been there and i want to use them. Also the distance from the motor to the rear wheel is a little long and without an idler on bumpy terrain it used to toss the chain the jackshaft would eliminate this issue.
Sounds like you are all set for a jackshaft, as you mentioned up front. You can use the formulas I posted, or any of several decent online calculators (just Google "jackshaft calculator.") :thumbsup:
You will lose top speed, but gain low-end torque, by getting that 5:1 ratio upwards of 6:1 or more, depending on the size of the engine.
You will lose top speed, but gain low-end torque, by getting that 5:1 ratio upwards of 6:1 or more, depending on the size of the engine.
The formula for gearing with a jack shaft looks like this.
If you buy a 14 and 12 jack shaft gear you'll get this
(Jack shaft gear ÷ clutch gear ) × ( rear sproket ÷ jack shaft gear)
(14÷12) × ( 60÷12)
(1.14) × (5)
= 5.8 to 1 ratio
All you then do on a piece of paper is chage the jack shaft sproket sizes. After a couple of sums you'll find what you need.
If you're a big guy anything beteeen 6 and 8 to one will work:thumbsup:
Hope it helps
If you buy a 14 and 12 jack shaft gear you'll get this
(Jack shaft gear ÷ clutch gear ) × ( rear sproket ÷ jack shaft gear)
(14÷12) × ( 60÷12)
(1.14) × (5)
= 5.8 to 1 ratio
All you then do on a piece of paper is chage the jack shaft sproket sizes. After a couple of sums you'll find what you need.
If you're a big guy anything beteeen 6 and 8 to one will work:thumbsup:
Hope it helps
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they will becoming tomorrow ill be laying my white base coat that ill soon shoot metallic blue over.
But back to my math at hand and mind you im very piss pore at this.
60/12 x 15/13 which gives me a 5.7 does this seem ok for a decent cruiser mini mind you ill be running a predator clone with a OldMiniBikes performance package.
But back to my math at hand and mind you im very piss pore at this.
60/12 x 15/13 which gives me a 5.7 does this seem ok for a decent cruiser mini mind you ill be running a predator clone with a OldMiniBikes performance package.
been playing around a bit before the break down it did about 26 MPH man that seems faster when your 12 lol.
so with a 5.7 ratio 3600 rpm motor and a 12.5 tire that gets me to 23.47 mph im just concerned this won't be fast enough for me but i now live in a hilly area so i need low end torque and want speed as well.
For size i'm fairly light at 160.
so with a 5.7 ratio 3600 rpm motor and a 12.5 tire that gets me to 23.47 mph im just concerned this won't be fast enough for me but i now live in a hilly area so i need low end torque and want speed as well.
For size i'm fairly light at 160.
Very good, but do not change the clutch leave it a 12 like you have.
I would try
(14/12) x (60/12) = 5.83 - so you'll only need to buy a 12 and 14 sprocket
or
(15/12) x (60/12) = 6.25 to 1
Keep the clutch and rear sprocket as is and just keep playing with the jacks shaft sprockets till your comfortable
nice one:thumbsup:
I would try
(14/12) x (60/12) = 5.83 - so you'll only need to buy a 12 and 14 sprocket
or
(15/12) x (60/12) = 6.25 to 1
Keep the clutch and rear sprocket as is and just keep playing with the jacks shaft sprockets till your comfortable
nice one:thumbsup:
What you need is going to depend a lot on the engine itself.
A little 2.8hp on my Doodlebug does well with a jackshaft for an 8.8 ratio and me at 150lb. But I took it on a trail with some bigger hills and wished I had more power. Top speed 20 or so, but I turned up the RPM to 4000 or so because the jackshaft is easier on the engine.
6.5hp Predator on my Sensation is fine with 5:1 ratio. The thing is a beast, and I lean forward when taking off. Does not need a jackshaft except maybe in reverse, to take the torque -down- some. But I can't take it on a trail without jarring my teeth out, and I don't trust the kids on it in the woods. I know what it feels like to hit a tree.
I actually like my lower-speed Doodlebug with the 2.8 engine and jackshaft, better than the 6.5 Predator with a Sensation frame bolted onto it :laugh:
A little 2.8hp on my Doodlebug does well with a jackshaft for an 8.8 ratio and me at 150lb. But I took it on a trail with some bigger hills and wished I had more power. Top speed 20 or so, but I turned up the RPM to 4000 or so because the jackshaft is easier on the engine.
6.5hp Predator on my Sensation is fine with 5:1 ratio. The thing is a beast, and I lean forward when taking off. Does not need a jackshaft except maybe in reverse, to take the torque -down- some. But I can't take it on a trail without jarring my teeth out, and I don't trust the kids on it in the woods. I know what it feels like to hit a tree.
I actually like my lower-speed Doodlebug with the 2.8 engine and jackshaft, better than the 6.5 Predator with a Sensation frame bolted onto it :laugh:
Thanks i have decided im going to keep my 12t clutch and run an 18t incoming jack shaft gear and a 16t out going gear and my 60t rear sprocket and that will get me at 5.6 that should be a decent compromise.
But in other good news i actually found out what mini bike i have i now know it's a bird and should of had a small luggage rack kinda cool now if only i could find the right foot peggs.
But in other good news i actually found out what mini bike i have i now know it's a bird and should of had a small luggage rack kinda cool now if only i could find the right foot peggs.