Fat Tire minibike

[karl]

I was thinking of making a fat tire minibike. There is plans for it on this website. I have a couple of 3hp briggs and I am thinking of using on. I am going to gear it down to 12-15mph. I got 18in-9.5in tires from a lawn mower that could work. I am looking for a 100 tooth sprocket and I cant find on. Could you make one? IS there any templates online? Also, when using the speed calculator on this site, how do you factor in the jackshaft? Do you just add them together? Any help would be great.

 

[Doc Sprocket]

How big would a 100T sprocket be? Jackshaft!!! Anyway, multiply the ratios to get the final reduction. For example, 10T engine 20T j/s in would be 2:1, then 10T j/s out to 30T axle would be 3:1, so 2:1 x 3:1= 6:1. For the calc, you could just enter this as 10T clutch, and 60T axle- same thing.

 

[karl]

Ok I want this geared down for my woods. I got tons of mud (stuff you can barely walk through) and my 1980 Suzuki rm 100 is hard to handle in the mud. I want to put a rack on it and it will be the "tractor" of mini-bikes

 

[jrmints1415]

try 8:1 or 9:1 with a jackshaft

 

[r97]

sounds good, i have been thinking of building a similar thing to ride on trails and stuff.

the one suggestion i have is that i would move up to an 8hp or so engine if you can, i'm not convinced a 3hp engine will do much good spinning an 18" tire geared for 20 through any mud at all. of course ii have been proven wrong before so its worth a try, but again, i would go for 8hp if you can.

 

[karl]

Im not gearing it for 20mph, Im gearing it for 13mph. I dont have a 8hp engine and right now. Im broke so I gotta make do with what I have. Also, a 3hp is way more fuel efficient.
My gearing will probably be 10 tooth to 72 tooth, then a 10 tooth to a 72 tooth. So It will be about 14:1 gear ratio. You haft to remember 18in tires. Does that sound good? That should gear it down to about 12mph and the 3hp will be able to push it.

 

[r97]

like i said i got nothing against you trying. maybe it will work great, i cant be sure.

also a 10:70 or 7:1 ratio on one side of a jack shaft paired with another 7:1 ratio on the other side would not equal a 14:1 ratio, it would actually equal a 49:1 ratio because the ratios are multiplied, not added.

 

[Doc Sprocket]

My gearing will probably be 10 tooth to 72 tooth, then a 10 tooth to a 72 tooth. So It will be about 14:1 gear ratio. You haft to remember 18in tires. Does that sound good? That should gear it down to about 12mph and the 3hp will be able to push it.

Your math is out. Those gears will equate to about 50:1. You multiply for compound reduction, not add. 10 to 20 and 10:72 will be close to 14:1.
Edit- r97: Your turn to beat me! LOL

 

[anderkart]

Also, when using the speed calculator on this site, how do you factor in the jackshaft? Do you just add them together? Any help would be great.

You could experiment with some different gearing combinations using this jackshaft calculator: http://www.compgoparts.com/TechnicalResources/JackshaftRatioCalculator.asp

It can be a little tricky the first time, let me know if you need any further guidance.

Don't forget to enter rear tire circumference instead of diameter.
Just enter a 2000 clutch lock-up rpm.
Enter 3600 engine rpm unless your governor has been removed.
Don't worry about entering your chain pitch. (that's just for an accurate chain length readout)

They've just got their clutch mounted on the jackshaft output instead of the engine, so dont let that confuse you while entering your gear sizing info. Your clutch located on the engine wont make any difference to your your gear ratio/top speed readouts on this calculator.

 

[karl]

So more like a 10 tooth to a 25, then a 10 tooth to a 72 That will probably be the best gear ratio. I cant work on it now because im grounded, hopefully this weekend I can get a start on it

 

[Doc Sprocket]

Ouch!

 

[robbie]

So more like a 10 tooth to a 25, then a 10 tooth to a 72 That will probably be the best gear ratio. I cant work on it now because im grounded, hopefully this weekend I can get a start on it

That would give you an 18 to 1 ratio.

Assuming that you don't use a governor on your engine, and that it will probably get up to 4500 RPM, that would give you 13 mph. Your engine may be able to go faster, but I think 4500 is a useful number.

You already have the tires, and the numbers you quoted for your gears are easy to find.

I think the 3 hp engine will do the job nicely. We have a mini bike with a Harbor Freight 2.5 HP engine. It goes 20 mph on a gravel driveway and only slows down a little bit on the hill. If I geared it for 13 mph like the bike you want to build, it would never slow down.

 

[karl]

As you can see im bad with gear ratios, but I forgot my clutch is a 11 tooth, and I use the link that andercart had for the jack shaft calculator, and I got a 16.36:1 gear ratio.11 tooth to a 25, then a 10 tooth to a 72. And with the governed rpms , it said 11.67 mph. The plans that are on this site says it will go 10mph, so this should be close

 

[robbie]

Sounds like you're on the right track, but you may want to eliminate the governor. If you leave the governor on, it will feel like driving a bulldozer, or a tractor in low gear.

 

[karl]

I want it to go slow, and I ride lawn tractors all the time. It is quite relaxing driving slow. I cant start it now because the barn is locked up.

 

[karl]

I got a 3hp briggs running, I got plenty of tires, lots of 3/4 conduit, I haft to borrow a pipe bender, will start making the frame soon. When I start the frame I will make a new thread of the build.