Proper gear ratio?

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hefferzon

New Member
#1
#1
Hello.

I recently bought an old (1968) 2 hp flat head Briggs on a local swap meet. It seems to be in good shape except for the gas tank, which has seen better days.
Now I'd like to build something fun with it and I thought of a minibike. I don't know if the 2hp is strong enough though, what would be a good size rear sprocket if I use 6" wheels and normal tires? I was planning on using an 11T (#35 chain) Max Torque clutch as it seems to be the most common with 5/8" bore.

//Jesper
 
D

dwadholm

Member
#2
#2
Without a jackshaft you'd want at least a 72 tooth which would put you at 6.5 to 1. You aren't going to climb any hills and it'd be slow off the line with an adult. You could also find an 80 tooth which would help at 7.1 to 1. Smaller tires like 410 X 350 would help with low end grunt.

With a jackshaft you could gear up to 8.0+ but your top speed will only be around 15 mph...depends what you want to do.
 
H

hefferzon

New Member
#3
#3
Ok, thanks. I found 4" whells that are approx 10" diameter in total(including tires). Will these work with a 60T sprocket and no jackshaft? According to my calculations I will get a speed of around 16 mph at 3000 rpm. By the way how much will these small briggs rev?
 
D

dwadholm

Member
#4
#4
You might be OK with a 60 tooth setup since you are going with a smaller wheel (really it depends on the diameter of the tire).

Most engines are governed to 3600 RPM. Tough to say if that's true for an old 2 hp Briggs. Maybe someone else will chime in.
 
H

hefferzon

New Member
#5
#5
dwadholm said:
You might be OK with a 60 tooth setup since you are going with a smaller wheel (really it depends on the diameter of the tire).

Most engines are governed to 3600 RPM. Tough to say if that's true for an old 2 hp Briggs. Maybe someone else will chime in.
Click to expand...
The tires are 10" diameter, so they are just 2.5" bigger than a 60T sprocket, whitch leavs me with just above 1" ground clearence from the sprocket. That may be a problem, I don't know..
 
D

dwadholm

Member
#6
#6
hefferzon said:
The tires are 10" diameter, so they are just 2.5" bigger than a 60T sprocket, whitch leavs me with just above 1" ground clearence from the sprocket. That may be a problem, I don't know..
Click to expand...
Unless you are going to be dragging a knee around corners it should be OK. Are you going to use square shoulder tires or rounded? Square might "protect" a little better while turning.
 
D

dwadholm

Member
#8
#8
Those Taiwanese hubs and bearings will not stand up to the speed and abuse of a mini bike for very long. Lots of side play in those bearings... You will want to find a used set of actual mini bike wheels and some squared off tires if you intend to go with that setup.

So, right now you just have an engine? Are you going to bend up a frame yourself? You might be able to find a roller frame you can fix up for $50 around Craigslist.
 
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hefferzon

New Member
#9
#9
Problem is I live in Sweden and here minibikes are more or less non existing. Shipping + tax/customs for an old frame to Sweden will be expensive. I know those cheap bearings are crap, but they are easily swapped out for quality ones as they are standard metric size. The hubs are another story though. I was thinking of making the frame from 7/8" hydraulic pipes, should be strong enough I think.
 
D

dwadholm

Member
#10
#10
I see what you are saying. Well, you can certainly give them a shot. It doesn't need a super robust setup with a top speed of 15 mph! Just make sure you can mount a sprocket to it as well...might take some shade tree engineering to figure that out. The nice thing about mini bike wheels is they usually have the bolt holes set up for a spacer and sprocket.

Half the fun of building a mini bike is just figuring stuff out. Make a plan, buy the parts, test and see what works and what needs reworking.

It bears mentioning that if you can find a 5hp on an old tiller/snowblower it might be a better starting point but I understand you gotta work with what you have on hand!
 
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H

hefferzon

New Member
#12
#12
Yes, a 5hp is certainly a better choise. I got one of those 5hp I/C from a leaf blower a some time ago, but it's dedicated for a Cushman airborne "look alike" kind of thing that I'm building from scratch.

The 2hp was just something that I got real cheap and now trying to find a use for, except just starting it and look at it:smile:
 

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