OzFab
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The first thing you need to know is
Low ratio number (eg: 2:1) = high gear = high speed
High ratio number (eg: 12:1) = low gear = low speed
Lets say you have a 12 tooth clutch on your engine & a 72 tooth sprocket on the axle, that gives you a drive ratio of 6:1.
How did I get that? by dividing the number of teeth on the axle sprocket by the number of teeth on the clutch sprocket: 72/12=6
The drive ratio determines your top speed, as well as the amount of torque available for acceleration & there must be a compromised balance between the two...
For example, if you use a 4:1 ratio (regardless of other factors) it will have a huge top speed but, you will destroy any clutch attempting to reach it; likewise, if you have a 12:1 ratio, acceleration power will be excellent but, top speed will be lousy...
There are many of variables to consider when deciding on a drive ratio:
-kart size & weight
-engine capacity/output
-tyre size
-transmission type
All these variables must be considered before making a final decision...
A 6:1 ratio is perfect for a small kart with 13" tyres, a 6.5hp engine & a centrifugal clutch but, would be useless on a buggy with 22" tyres; for that you would need a much lower gear ratio…
So, why can’t I have a 6:1 ratio on my buggy with 22” tyres?
You can, if you want to burn out a clutch every 10 minutes…
As mentioned,
So, now look at the size difference in the tyres; 22” is almost double 13” so, you must use a lower ratio to compensate…
Quickly, just go & punch the following numbers into our speed calculator
Diameter in Inches: 13
Engine RPM: 3600
Teeth on Axle: 6
Teeth on Clutch: 1
Then hit the Convert button…
The number that appears in the MPH box should be 23; that is the estimated top speed of a kart with the above specs…
Now, go back & change just the Diameter in Inches to 22 & hit the Convert button; the number in the MPH box should now be 39.
You see, by changing the height of the tyres, you change the estimated top speed. Why? Because you changed the overall drive ratio; now go back to the calculator again & change the Teeth on Axle number until the MPH gets close to 23; what did you end up with?
So, isn’t a higher top speed better?
No! Whatever you add to top speed, you take away from acceleration; here’s the compromised balance I mentioned earlier…
By having a gear ratio that achieves a high top speed, you add unnecessary pressure to the clutch. Torque is what’s needed to get the kart moving; if the torque output of the engine is lower than the torque needed to move the kart, the clutch will suffer. The easiest way to overcome this problem is to adjust the drive ratio, usually by increasing the size of the axle sprocket…
I have a 3hp engine & 20” wheels on my kart; will a 6:1 ratio work?
Without considering any other aspects, the only thing guaranteed to happen with such a setup is the destruction of the clutch; let me explain why:
Torque! Torque = turning effort
A 3hp engine with a governed max RPM of 3600 will produce a maximum of 4.38 ft/lbs of torque
A 6.5hp engine with a governed max RPM of 3600 will produce a maximum of 9.48 ft/lbs of torque
The axle sprocket:engine sprocket ratio then acts as a torque multiplier, the tyres then act as a torque reducer…
What does that mean? Let me explain:
Let’s say you have 6:1 gearing (12 tooth clutch & a 72 tooth axle sprocket), a 6.5hp engine & 13” rear tyres:
To find the available torque at the wheels, first you need to multiply the engine torque by the drive ratio:
9.48x6=56.88 ft/lbs
Now we need to find the ratio between the axle sprocket & the tyre; a 72 tooth #35 sprocket has an outside diameter of 8;814” so
13” tyre divided by 8;814” sprocket = 1.475
Now we divide the axle sprocket torque by the axle sprocket/tyre ratio:
56.88/1.475 which gives us 38.56ft/lbs of torque at the rear wheels.
So, what we you change only the tyre size to 20”; what happens then?
20” tyre divided by 8;814” sprocket = 2.27
56.88/2.27 gives us 25.05ft/lbs of torque at the rear wheels.
So, what do you think will happen if, between the weight of the kart & the rider, a minimum of 30ft/lbs is required just to move[/] it? The short answer is you’ll destroy the clutch…
Back to the original question, if you use a 3hp engine in place of a 6.5hp, 6:1 gearing & 20” tyres, you’ll end up with 11.58ft/lbs at the rear wheels
I have a 12 tooth clutch & I need a drive ratio of 9:1 but, I can't find a 108 tooth axle sprocket…
This is a common problem; the only way to remedy it is to incorporate a jackshaft
What is a jackshaft?
It’s literally a shaft that hold 2 bearings & 2 sprockets that allow you to add compound gearing to the drive line, providing a much larger range of ratios. The two bearings are anchored to the kart frame so that the two sprockets are aligned with the clutch & axle sprockets...
Let’s look at the example above; you have a 12 tooth clutch on the engine & a 72 tooth sprocket on the axle but, you’ve decided to change the 13” tyres for much larger 22” tyres, now you need a 9:1 drive ratio to compensate for the larger tyres…
So, the current drive ratio is 6:1; what size sprockets will you need to achieve 9:1? Well, there’s no “magic number,” you just have to play with numbers until you find the correct ones…
There are many online calculators such as AGKs, Comp Go Parts & Burris Racing that will simply spit numbers out but, I’m going to show you how they work.
Ok so, let’s start at the axle & work back: So, the axle sprocket (A/S) has 72 teeth; to make life simple, let’s put a 12t(ooth) sprocket on the output end of the jackshaft (J/S out); now, just for examples sake, for now, we’ll put another 12t sprocket on the input end of the jackshaft (J/S in). With that configuration, I can tell you the drive ratio is still 6:1.
How do I know that? Well, for starters, the two sprockets on the J/S cancel each other out because they’re the same size but, also because I used a formula to figure it out:
First, divide A/S / J/S out (72/12=6)
Then divide J/S in / C/S (clutch sprocket) (12/12=1)
Now multiply the two numbers together (6*1=6)
Well, that didn’t change anything so, lets increase the size of the J/S in sprocket to 24t & see what happens:
A/S / J/S out (72/12=6)
J/S in / C/S (24/12=2)
6*2=12
Ok, now that’s too low so, let’s split the difference & use an 18t:
A/S / J/S out (72/12=6)
J/S in / C/S (18/12=1.5)
6*1.5=9
So, you see how we have to play with the numbers to find the desired result? And that was a simple one…
With a jackshaft, you can use smaller sprockets to achieve the same ratio as what you would use if you just have a clutch and axle sprocket. A jackshaft is often used because there is either no ground clearance, a large ratio is needed or if you are using a Torque Converter
More to come...
Low ratio number (eg: 2:1) = high gear = high speed
High ratio number (eg: 12:1) = low gear = low speed
Lets say you have a 12 tooth clutch on your engine & a 72 tooth sprocket on the axle, that gives you a drive ratio of 6:1.
How did I get that? by dividing the number of teeth on the axle sprocket by the number of teeth on the clutch sprocket: 72/12=6
The drive ratio determines your top speed, as well as the amount of torque available for acceleration & there must be a compromised balance between the two...
For example, if you use a 4:1 ratio (regardless of other factors) it will have a huge top speed but, you will destroy any clutch attempting to reach it; likewise, if you have a 12:1 ratio, acceleration power will be excellent but, top speed will be lousy...
There are many of variables to consider when deciding on a drive ratio:
-kart size & weight
-engine capacity/output
-tyre size
-transmission type
All these variables must be considered before making a final decision...
A 6:1 ratio is perfect for a small kart with 13" tyres, a 6.5hp engine & a centrifugal clutch but, would be useless on a buggy with 22" tyres; for that you would need a much lower gear ratio…
So, why can’t I have a 6:1 ratio on my buggy with 22” tyres?
You can, if you want to burn out a clutch every 10 minutes…
As mentioned,
So, now look at the size difference in the tyres; 22” is almost double 13” so, you must use a lower ratio to compensate…
Quickly, just go & punch the following numbers into our speed calculator
Diameter in Inches: 13
Engine RPM: 3600
Teeth on Axle: 6
Teeth on Clutch: 1
Then hit the Convert button…
The number that appears in the MPH box should be 23; that is the estimated top speed of a kart with the above specs…
Now, go back & change just the Diameter in Inches to 22 & hit the Convert button; the number in the MPH box should now be 39.
You see, by changing the height of the tyres, you change the estimated top speed. Why? Because you changed the overall drive ratio; now go back to the calculator again & change the Teeth on Axle number until the MPH gets close to 23; what did you end up with?
So, isn’t a higher top speed better?
No! Whatever you add to top speed, you take away from acceleration; here’s the compromised balance I mentioned earlier…
By having a gear ratio that achieves a high top speed, you add unnecessary pressure to the clutch. Torque is what’s needed to get the kart moving; if the torque output of the engine is lower than the torque needed to move the kart, the clutch will suffer. The easiest way to overcome this problem is to adjust the drive ratio, usually by increasing the size of the axle sprocket…
I have a 3hp engine & 20” wheels on my kart; will a 6:1 ratio work?
Without considering any other aspects, the only thing guaranteed to happen with such a setup is the destruction of the clutch; let me explain why:
Torque! Torque = turning effort
A 3hp engine with a governed max RPM of 3600 will produce a maximum of 4.38 ft/lbs of torque
A 6.5hp engine with a governed max RPM of 3600 will produce a maximum of 9.48 ft/lbs of torque
The axle sprocket:engine sprocket ratio then acts as a torque multiplier, the tyres then act as a torque reducer…
What does that mean? Let me explain:
Let’s say you have 6:1 gearing (12 tooth clutch & a 72 tooth axle sprocket), a 6.5hp engine & 13” rear tyres:
To find the available torque at the wheels, first you need to multiply the engine torque by the drive ratio:
9.48x6=56.88 ft/lbs
Now we need to find the ratio between the axle sprocket & the tyre; a 72 tooth #35 sprocket has an outside diameter of 8;814” so
13” tyre divided by 8;814” sprocket = 1.475
Now we divide the axle sprocket torque by the axle sprocket/tyre ratio:
56.88/1.475 which gives us 38.56ft/lbs of torque at the rear wheels.
So, what we you change only the tyre size to 20”; what happens then?
20” tyre divided by 8;814” sprocket = 2.27
56.88/2.27 gives us 25.05ft/lbs of torque at the rear wheels.
So, what do you think will happen if, between the weight of the kart & the rider, a minimum of 30ft/lbs is required just to move[/] it? The short answer is you’ll destroy the clutch…
Back to the original question, if you use a 3hp engine in place of a 6.5hp, 6:1 gearing & 20” tyres, you’ll end up with 11.58ft/lbs at the rear wheels
I have a 12 tooth clutch & I need a drive ratio of 9:1 but, I can't find a 108 tooth axle sprocket…
This is a common problem; the only way to remedy it is to incorporate a jackshaft
What is a jackshaft?
It’s literally a shaft that hold 2 bearings & 2 sprockets that allow you to add compound gearing to the drive line, providing a much larger range of ratios. The two bearings are anchored to the kart frame so that the two sprockets are aligned with the clutch & axle sprockets...
Let’s look at the example above; you have a 12 tooth clutch on the engine & a 72 tooth sprocket on the axle but, you’ve decided to change the 13” tyres for much larger 22” tyres, now you need a 9:1 drive ratio to compensate for the larger tyres…
So, the current drive ratio is 6:1; what size sprockets will you need to achieve 9:1? Well, there’s no “magic number,” you just have to play with numbers until you find the correct ones…
There are many online calculators such as AGKs, Comp Go Parts & Burris Racing that will simply spit numbers out but, I’m going to show you how they work.
Ok so, let’s start at the axle & work back: So, the axle sprocket (A/S) has 72 teeth; to make life simple, let’s put a 12t(ooth) sprocket on the output end of the jackshaft (J/S out); now, just for examples sake, for now, we’ll put another 12t sprocket on the input end of the jackshaft (J/S in). With that configuration, I can tell you the drive ratio is still 6:1.
How do I know that? Well, for starters, the two sprockets on the J/S cancel each other out because they’re the same size but, also because I used a formula to figure it out:
First, divide A/S / J/S out (72/12=6)
Then divide J/S in / C/S (clutch sprocket) (12/12=1)
Now multiply the two numbers together (6*1=6)
Well, that didn’t change anything so, lets increase the size of the J/S in sprocket to 24t & see what happens:
A/S / J/S out (72/12=6)
J/S in / C/S (24/12=2)
6*2=12
Ok, now that’s too low so, let’s split the difference & use an 18t:
A/S / J/S out (72/12=6)
J/S in / C/S (18/12=1.5)
6*1.5=9
So, you see how we have to play with the numbers to find the desired result? And that was a simple one…
With a jackshaft, you can use smaller sprockets to achieve the same ratio as what you would use if you just have a clutch and axle sprocket. A jackshaft is often used because there is either no ground clearance, a large ratio is needed or if you are using a Torque Converter
More to come...
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