looking for a little gear ratio help from the pros

V

Vandoog

Member
#1
#1
Hey y'all, I'm working on planing out a diy build from an Old Briggs 80202 3hp I got running thanks to this forum. I have a pretty odd italjet bambino mini dirtbike frame I plan on cutting up to build a more traditional looking minibike. Probably just going to reuse the swing arm and fork. I also have a good deal of 420 chain from the king o lawn mower the engine came off of.

I've seen some ratio calculators but frankly I am a little confused about calculating. My rear tire measures roughly 15 inches in diameter and about 3 wide. I'd like to ride on dirt if possible, weigh about 145 lb, and prefer a bike that can climb a lil hill to one with an extra couple mph top speed. I'd like to avoid running a jackshaft if possible and plan on building the frame around that once I get a clutch and rear sprocket.

over here on this go kart ratio speed calculator i'm seeing that a 12/60 would give me a top speed of 32 mph which seems like a lot but I imagine the low end would be terrible?

I'm wondering if anyone has a similar wheel size and engine and could drop some knowledge my way?

0.jpg 109118805_759848178161044_5652517405235229536_n.jpg
ps. frame was free but the fella kept the original engine hence modding it to run my 3hp briggs
 
Last edited:
V

Vandoog

Member
#2
#2
I'm leaning towards a 10t max torque clutch in the front and a 60t sprocket in the back. Anyone know a cheap source for a sprocket? They are surprisingly expensive online
 
jeep2003

jeep2003

#3
#3
that should work but its going to struggle with that big of a wheel. It would be a challenge but you could use the manual clutch and sprockets from the reel mower on the jackshaft. Id shoot for more like 10 or 12 to one with that wheel and 3hp. On flat pavement you could get away with 6 no problem
 
V

Vandoog

Member
#4
#4
jeep2003 said:
that should work but its going to struggle with that big of a wheel. It would be a challenge but you could use the manual clutch and sprockets from the reel mower on the jackshaft. Id shoot for more like 10 or 12 to one with that wheel and 3hp. On flat pavement you could get away with 6 no problem
Click to expand...
hmmm good to know. I had a feeling the ratio calculators I've been looking at might be a little bit ambitious. The calculators are helpful but it is definitely tricky for someone building their first bike to speculate about what the low end might be like.
 
SAS289

SAS289

Well-Known Member
#5
#5
Those speed calculators are nice reference but take a couple things for granted. First is the load. A 130 pound guy will get a higher top speed than a 220 pound guy. Second is the power available at the given RPM. The power available at 4K RPM is very different depending on the size of the engine being used.

We ask a lot from these small engines. Think of this.. Moving a 300 lb load with a 3HP engine is comparable to moving a 3000 lb car with a 30 HP engine. This wouldn't work on a car because the gearing required would produce just above farm tractor top speeds. But a mini bike could still be fun even with lower top speeds.

I would say 7/1 would be the minimum. But try for 9 or 10/1. The higher the numeric ratio the easier life is for the engine and clutch. There are no solutions for 1 speed gearing. There are only trade offs.
 
V

Vandoog

Member
#6
#6
SAS289 said:
Those speed calculators are nice reference but take a couple things for granted. First is the load. A 130 pound guy will get a higher top speed than a 220 pound guy. Second is the power available at the given RPM. The power available at 4K RPM is very different depending on the size of the engine being used.

We ask a lot from these small engines. Think of this.. Moving a 300 lb load with a 3HP engine is comparable to moving a 3000 lb car with a 30 HP engine. This wouldn't work on a car because the gearing required would produce just above farm tractor top speeds. But a mini bike could still be fun even with lower top speeds.

I would say 7/1 would be the minimum. But try for 9 or 10/1. The higher the numeric ratio the easier life is for the engine and clutch. There are no solutions for 1 speed gearing. There are only trade offs.
Click to expand...
Oooops amidst my confusion I ended up ordering an 11 t clutch and a 72 rear sprocket last night. Wasn't really sure what to do an wanted to get the project started. Gonna see if I can change that 11 to a ten, thanks for the help!
 
minibikin'

minibikin'

Sho-nuff
#7
#7
First thing, you're going to be unhappy with the 2hp. Its going to need gearing of 7:1 or taller to move that around. Not trying to be pessimistic, but it's reality. Second, if you're insisting on using that engine, put a jack shaft on the bike( I would regardless). It will help with gearing options and chain length. I would normally recommend a TAV for dirt but the 2hp doesn't have enough power for that.
 
V

Vandoog

Member
#8
#8
minibikin' said:
First thing, you're going to be unhappy with the 2hp. Its going to need gearing of 7:1 or taller to move that around. Not trying to be pessimistic, but it's reality. Second, if you're insisting on using that engine, put a jack shaft on the bike( I would regardless). It will help with gearing options and chain length. I would normally recommend a TAV for dirt but the 2hp doesn't have enough power for that.
Click to expand...
Not sure where the 2hp is coming from? Its a 3hp briggs but thanks for the tips, sounds like jackshaft is definitely going to be the way to go. Also I know from the pic it looks like a big ol dirtbike, here is a shot from online of one or scale:
21803851995_214239de94_c.jpg
 
minibikin'

minibikin'

Sho-nuff
#9
#9
Vandoog said:
Not sure where the 2hp is coming from? Its a 3hp briggs but thanks for the tips, sounds like jackshaft is definitely going to be the way to go. Also I know from the pic it looks like a big ol dirtbike, here is a shot from online of one or scale:
View attachment 266032
Click to expand...
Ugh... my eyes lol. Tire circumference is a key factor in static gearing. Your 11/72 gearing with that diameter tire should net you a 6.54 static gear ratio and 24.53MPH @ 3600rpms. Throw a jackshaft in there and go 11clutch, 12js 11js 72 rear and you have a 7.14 static gear ratio. Jackshaft sprockets are cheaper and easier to change to fine tune the gear ration than changing clutches and rear sprockets.
 
V

Vandoog

Member
#10
#10
minibikin' said:
Ugh... my eyes lol. Tire circumference is a key factor in static gearing. Your 11/72 gearing with that diameter tire should net you a 6.54 static gear ratio and 24.53MPH @ 3600rpms. Throw a jackshaft in there and go 11clutch, 12js 11js 72 rear and you have a 7.14 static gear ratio. Jackshaft sprockets are cheaper and easier to change to fine tune the gear ration than changing clutches and rear sprockets.
Click to expand...
thanks for the tips its a pretty silly looking thing, I plan on cutting the frame stretching it and welding something that hopefully looks a lil more like a tradition mini bike. def gonna ditch the plastic and lil tank and all that. I'm mostly just stoked about the fork, swingarm and wheels
 
You must log in or register to reply here.
Top