Go kart gear ratio with briggs 5hp with 9.5in tall tires

[RustyRedneck]

I hate this. It takes all the fun out of it and it becomes another abstract formula. Here it is, BUT the learning comes from understanding it the way RRedneck describes it.


Cheater Formula (using only the most basic required information).
Multiply 4 things together, then divide by the 5th. Easy like Sunday Morning.

MPH =
(0.002975)*(Engine RPM)*(# of Teeth on Clutch)*(Diameter of Tire in Inches) / (# of Teeth Rear Sprocket)

For example...

((0.002975)*(3600)*(10)*(9.5)) / (54) =
18.84 MPH

Because I hate not knowing where strange numbers come from...
The constant 0.002975 comes from
((3.14159) * (60 min per hour)) / ((12 in per foot) * (5280 ft per mile))
And for completeness, 3.14159 is pi. May god have mercy on your soul if you didn't know.

I'm not really sure what a kilometer is and my watch doesn't read metric time, so sorry rest of the world. You can make your own cheater formula. LOL


Sine, Cosine, Cosine, Sine, 3.14159!!!!!
https://www.youtube.com/watch?v=5NgjkwQ-7lY

Thanks Bob. I hadn't spent enough time to simplify the formula like I've been meaning to. I'll probably be using your simpler layout from here on.

 

[bob58o]

Cool! Now...can someone throw a jackshaft into the equation? Say increase from 18.8 mph to 37.4 mph with it?

Generally Jackshafts make karts slower and increase torque.
If you want to double your speed at the same RPM. Then you need a rear sprocket with half as many teeth.

3600 RPM
9.5" Tires
10T Clutch
27T Rear Sprocket

((0.002975)*(3600)*(10)*(9.5)) / (27) =

37.6 MPH, which is what I assume you meant. 37.4 is kinda out of left field.

I wouldn't gear it for 37.4 or 37.6 MPH at 3600 RPM.
How about 40 MPH at 5000 RPM?
40 = ((0.002975)*(5000)*(10)*(9.5)) / (x)
40(x) = (0.002975)*(5000)*(10)*(9.5)
40(x) = 1,413
x = 1,413/40
x = 35

35T Rear Sprocket.

So with 9.5" Tires, 10T Clutch, 35T Rear Sprocket...
You will go 40MPH at 5000 RPM. That is a 3.5:1 Gear Ratio

3.5:1 may be a stretch (even with tiny tires), but definitely better than 2.7:1 which is what you asked for (38 mph at 3600 RPM).

I think 4:1 would be golden with your set up.

9.5" Tires
10 T Clutch
40T Rear Sprocket

25 MPH at 3600 RPM
35 MPH at 5000 RPM.

---------- Post added at 05:45 PM ---------- Previous post was at 05:34 PM ----------

And now I realize that the OP is not the same person who posed the JS question. So there's that.

 

[fasterSr]

Yeah well I'm sorry I hijacked the thread too. I am really struggling with this power to ground stuff...Chrysler built my trans/rear ends for me so yeah, now it's a whole different kind of animal. Never built EVERYTHING from scratch before and with so many choices. Just as I think I got 6 to 1, a little voice says."yeah...no...start over"...Ugh.

So sorry for that. It's that need for speed thing...hope you all understand.

And Thank You bob58o very much...for the rest of the story.

 

[bob58o]

I will work through an example of how a kart will put torque to the ground.

Lets assume...
You have a clutch that engages at 2000 RPM.
The engine makes 10 lb-ft of torque at 2000 RPM.
You have a 10 Tooth Clutch.
You have a 50 Tooth Rear Sprocket.
You have 12" Tires

The 50T:10T gear ratio means that the engine's 10 lb-ft or torque gets multiplied by five (from 50T/10T). So now the 10 lb-ft crankshaft becomes 50 lb-ft at the rear axle. The 5:1 gear ratio multiplies the torque by 5. But this also means that the axle is spinning 5X slower than the crankshaft. The crank is spinning 2000 RPM, but the axle is spinning 400 RPM (2000/5).

So at the axle we, have 50 lb-ft.
And we have 12" tires.
Torque is Force x Radius, not Force x Diameter, so we use 6" Radius, not 12" Diameter

Torque = Force x Radius
Force = Torque / Radius

r = 6"
r = 0.5 ft
Torque = 50 ft-lb

Force = Torque / Radius
Force = 50 ft-lb / 0.5 ft
Force = 100 lbs.

Now if you have a jackshaft.
Say 10T clutch to 15T sprocket
Then 10T sprocket to 50T sprocket.

You have two reductions.
15T:10T, 1.5:1
and
50T:10T, 5:1

Multiply the two together.
5 x 1.5 = 7.5:1

So with that set up, Instead of multiplying the torque at crank by 5, you multiply it by 7.5. Ten lb-ft at the crank multiplied by 7.5 is 75 lb-ft at the axle.
Also means that the axle is spinning 7.5X slower than the crank. If the crankshaft is spinning 2000 RPM, the axle is spinning 267 RPM. 75 lb-ft at the rear axle divided by 0.5 ft radius provides 150 lbs of Force. More Reduction = More Torque = More Force = More Grunt

Now if you have 75 lb-ft at the rear axle and had 24" Tires, then radius is 1 ft.
75 lb-ft / 1 ft = 75 lbs force. Bigger tires = less grunt.

If you have 75 lb-ft at the axle and 6" Tires, then radius is 0.25 ft.
75 lb-ft / 0.25ft = 300 lbs force. Smaller Tires = more grunt.

2000 RPM, 7.5:1 gear ratio, 24" Tires = 19 MPH, Less Grunt - More Speed
2000 RPM, 7.5:1 gear ratio, 12" Tires = 9.5 MPH, More grunt - Less Speed
2000 RPM, 7.5:1 gear ratio, 6" Tires = 4.75 MPH, Even More grunt - Even Less Speed


For 5-7HP Engines Using C Clutches...
GR = Gear Ratio
D = Tire Diameter in Inches
T = Peak Engine Torque in Lb-Ft
W = Total Weight (Vehicle plus Passengers) in Lbs

GR = 0.01179 * W * D / T

18" Tires
9 Lb-Ft Peak Torque
500 lbs total weight
GR = 0.01179 * 500 * 18 / 9
GR = 11.8:1

13" Tires
7.3 lb-ft Peak Torque
250 lbs total weight
GR = 0.01179 * 250 * 13 / 7.3
GR = 5.25:1

It seems to be ballpark close for the ranges I've tested.

 

[fasterSr]

I want first gear muscle car torque. Not first gear Ford truck stump pulling torque and without sacrificing top end speed. So you see one of my many problems that are on the board.

I'll start my own thread after I learn enough to ask the right questions. Thanks to all for the very informative thread!

 

[Denny]

I want first gear muscle car torque. Not first gear Ford truck stump pulling torque and without sacrificing top end speed. So you see one of my many problems that are on the board.

I'll start my own thread after I learn enough to ask the right questions. Thanks to all for the very informative thread!

I see one very disappointed little boy in your future in (after you look in the mirror you'll see him too).

Denny

 

[bob58o]

I see one very disappointed little boy in your future in (after you look in the mirror you'll see him too).

Denny

You don't tell them Santa isn't real, you just put one of these under the tree.

https://www.ebay.com/itm/COMET-TORQ...685830&hash=item5423027d08:g:RbMAAMXQTT9Ru8dN

Hmmm? Black Spring in the Pic? Cheaper to paint black, then green, or red?

$56? Holy cheap now. Nobody should be using 3/4" CC's on yard karts when TC's are $56. Unless you have to. How low will they go? It seems to be cheaper every time I look. Is this a knock off of a knock off?
https://www.youtube.com/watch?v=pRtVMLwh6mY

I want torque ... without sacrificing top end speed.

2 Letters.
TC
Best of Both Worlds!!! Just like HOT ICE.
https://www.youtube.com/watch?v=gF8rlghyxJU

 

[Tpdingo]

I just remembered, my kart has a 12 tooth clutch and a 36 tooth sproket with 9m5" tires and it works fine so 4:1 should be excellent.

 

[fasterSr]

I see one very disappointed little boy in your future in (after you look in the mirror you'll see him too).

Denny

Hmmm. Can't be done OR not done by me yet?

https://www.youtube.com/watch?v=ZXMnHXnOz3Q

If these things had a streering wheel......ah well...we'll see.

 

[bob58o]

Hmmm. Can't be done OR not done by me yet?

Generally we deal with gearless transmissions. Or single gear transmissions. Meaning you start in the same gear you finish. If you want bottom end, you give up top end. If you want top end, you give up bottom end.

Or you use a Torque Converter, which gives you the bottom end, without sacrificing the top end.

For example, If 6:1 is a good mix of bottom and top end, 8:1 may be great bottom end, but suffer on the top end. 4:1 may be faster on the top end, but suffer on the bottom end.

Say you like 4:1 for top speed. I'll show you how to get there, without giving up the bottom.


Using a Torque Converter is like having an automatic transmission. Say you have 54:12 with a TC. That is 4.5:1.
4.5:1 becomes 12:1 in low gear. 4.5:1 becomes 4:1 in high gear.

You still keep the top end, plus you gain so much on the bottom.

AND Who doesn't like a bunch on the bottom.
https://www.youtube.com/watch?v=-D99n9f3vU4