Sprocket size/Gear ratio questions

[brendonv]

Hey guys i have a 2.5HP Briggs and Stratton engine and want to build a mini bike. Im stuck on which gear ratio is the best to get the most speed i can out of the engine. Ive heard 5:1 or 6:1 ratio is good. Im thinking of getting a 12T to a 72T or a 12T to a 60T. Which ratio would be the best? The clutch im thinking of getting has a 12T sprocket already on it which is why i use the 12T in both.

Thanks,
From Brendonv

 

[Russ2251]

You will get more speed with smaller driven sprocket, but there is a price to be paid. It will take longer to get to top speed.
In a short race, say 100 ft, the guy with smaller driven sprocket will beat you.
Perhaps at 300 ft, you will pass him. Just an example.

 

[brendonv]

ok so its like this?

Smaller sprocket(60T): More acceleration, less top speed
Bigger Sprocket(72T): Less acceleration but more top speed

how much difference in speed are we talking here?

 

[Russ2251]

ok so its like this?

Smaller sprocket(60T): More acceleration, less top speed
Bigger Sprocket(72T): Less acceleration but more top speed

how much difference in speed are we talking here?

Just the opposite.
Punch in numbers here:
Speed Calculator

 

[brendonv]

alright so:

2.5HP Briggs with 12T clutch on it going to a 60T sprocket on the wheel will give me speeds of about 25-30mph, correct?

Oh one other thing, ive heard a lower gear ratio is more prone to burn out your clutch. Will i have any problems with the clutch burning out if i use a 60T sprocket?

 

[crazycart]

In a short race, say 100 ft, the guy with smaller driven sprocket will beat you.
Perhaps at 300 ft, you will pass him. Just an example.

The guy with the smaller driven will lose in the 100 ft, but win in the 300 ft.

With a 2.5hp, i would go with a 10-11 on the clutch and 72 on the axle.

 

[Russ2251]

No problems unless used off road.

 

[Russ2251]

The guy with the smaller driven will lose in the 100 ft, but win in the 300 ft.

With a 2.5hp, i would go with a 10-11 on the clutch and 72 on the axle.

My bad ...I miss read/posted.

 

[brendonv]

the thing is my local store only has a 12T clutch, the 10T are real pricey. Could i instead go from a 12 to a 70T? I want good top speed though, like 30mph. What sprocket will achieve this? I still cant believe i dont need a jackshaft for this.

 

[Russ2251]

the thing is my local store only has a 12T clutch, the 10T are real pricey.

Do an on-line search for centrifugal clutches. Can be really inexpensive compared to the local guys but must take into account shipping costs.

Could i instead go from a 12 to a 70T? I want good top speed though, like 30mph.

Sure. Check speed calculator as referred to in earlier post.

I still cant believe i dont need a jackshaft for this.

Why would you need one? I don't like j-shafts and don't use them. Just complicates things and adds weight. Sometimes they are necessary because original design may have demanded it.
Get us some good clear pics of your setup.

 

[brendonv]

Get us some good clear pics of your setup.

I have no setup atm, im going to buy my sprocket and clutch first then going to buy 2 wheel borrow tires, looked at some today and there awesome! About 10cm thick and 30cm or so high. Then build my frame around that.. I think a 6:1 ratio will be good. Will give me real good acceleration and i think 45km/h will be heaps being so low to the ground. I have 2 motorized bikes and i get up to 50km on them and thats pretty darn fast.

The higher the ratio the less strain on the clutch so the clutch will last longer than having a 5:1 ratio. One question, how big in diameter ruffly are the 72T sprockets?

 

[jr dragster T]

PS wheel barrow tires will wobble like crazy and are NOT safe! Usualy the hub with the cheap bearings is pressed into the rim and will come apart when cornering.

 

[Russ2251]

how big in diameter ruffly are the 72T sprockets?

Just under 9 inches, roughly.

 

[brendonv]

PS wheel barrow tires will wobble like crazy and are NOT safe! Usualy the hub with the cheap bearings is pressed into the rim and will come apart when cornering.

Another guy on this forum has used the same tires as what im getting on his mini and seems to be running ok. There either trolley tires or wheelbarrow. I cant remember but now i know the size of the sprocket i think the 10" wheels will do fine. I was going to get 14" or 16", ill get the sprocket and see what fits best, what tires do most people use? Size and type.

 

[EagleTalons]

Did you ever get your mini bike done?

 

[brendonv]

Did you ever get your mini bike done?

Nah i built the go kart instead. I might build a minibike later on though. Once i get a bit of cash. XD