August 18th, 2017

Drive Systems: The Black Box With Gears and Sprockets

Most of us when we look at a go kart for our first time we see all these whirring parts, whizzing chain and a loud engine. We see lots of smoke. We see the go kart ripping around the track we are impressed with what we see, but we are kind of put back with: “What is this? What is this thing composed of?”

If someone at that point in time were to ask you “ Could you make me one of these go karts?”

You would probably say “I don’t think so.” And the most daunting thing about it would probably be the drive systems. And second to that would be the steering system, which we will talk about later in a different site.

But we are talking about drive systems today and they really are not that difficult: basic gearing and torque has to do with sizing the motor up to the wheels so it will push the go kart forward.

The drive systems that are out there are basically clutch systems. The clutch systems exist in two different natures really and they are:
  • The direct drive system with a belt
  • The direct drive system with a clutch
The way the belt system works, is that the belt acts as a clutch. The engine is either moved to get the belt tighter, or a tensioner comes to the belt to make the it tighter. And this action of tightening the belt causes the engine to gradually engage drive.

A clutch is a mechanism that takes heat energy and gradually takes the power of the motor and engages it to the rear wheels.

For a clutch system to be understood it should be explained.

You basically have a spinning motor, not like an electric motor where you have instantaneous torque when you turn on the motor. A lot of us are familiar with the wheel chairs that you see that the handicapped move around on: there are no clutches in there. They have a direct drive system, the motor has enough torque to start from zero and go. But even then it take a lot of energy for that to happen.

But, we are talking about an engine. An engine by its nature is spinning, and it requires itself to be spinning to get any power. There are other engines that can start at zero, for instance the two cylinder steam engine, and the will have instantaneous torque, like the old Stanley Steamers.

But we are not talking about them, we are talking about the gas motor or a Diesel. And the engine by its nature is spinning. And it has a flywheel on it to keep it going. It has to over come compression, it has to overcome the four cycles in the system. And when the flywheel does that it has to keep the system going. There is spinning energy in the flywheel. And the gas, the fuel is burned, it contributes to he spinning nature of the flywheel.

The bottom line is that you cannot just drop a load on it. You can just expect it to start going right away with out steadily engaging the load to it.. So what has to happen is that you have to clutch, or something that dissipates heat energy and gradually engages the load.

One way to envision this is to have a one disc spinning. Envision in you mind a disc spinning on the floor. It is a pretty sizeable disc, like the size of a garbage can lid, and it is spinning freely. Then you take another disc like a frizbee and you put it on the garbage can lid. When that frizbee when it hit in the center will start to spin, but not instantaneously. If you watch it, it will start from zero and then gradually increase to the same speed of the garbage can lid

Actually what is happening there is that the frizbee is contacting the surface of the garbage can lid, dissipates frictional heat enegy until finally the garbage can lid and the frizbee are going the same speed. What is being mimicked there is a clutch. Heat energy is being dissipated by friction, and that is a key to understanding a clutch.

So a clutch is designed to gradually engage a load and do that in a relatively quick time

There are different kinds of clutches: there are centrifugal clutches, there are liquid clutches, there are friction disc clutches, there are belt style clutches that we talked about before. All these have the same characteristic in that they gradually engage the load and cause heat energy to be dissipated during that loading process.

The purpose the clutch is that you do not have the engine engaged all the time. That is quite dangerous. The clutches first purpose is to gradually engage the load, the second purpose is to disengage the load when desired. A very dangerous thing to have is to have the load engaged all the time. Lets say you have the engine running and you want to stop, you would not be able to stop. You have to disengage the engine from the load.

Stopping is one of the most important things about a gokart (brakes is something we discuss in “Brakes, stopping the beast”). Part of braking involves disengaging the motor.

In other sections we discuss other style clutches. We are not going to do that here. What we are going to do is discuss the drive system in general. My basic point is that clutches are a vital component to the drive system. (See discussions on clutches “Clutches, engaging power.”)

Drive Systems

The drive system is basically trying to take the engine power, which is being dished out, and put it to the drive wheels with out stalling the engine. Stalling the engine really means that the engine quits, it does not have enough power. The ideal match is to have the engine have perhaps 10% -15 % more power than the gokart actually requires. The reason for this is that (and this is for your calculation) when you get your horsepower ratings they are telling you that the engine is rated at “X” amount of horsepower (i.e. 5 HP at 3600 rpms). If you use that torque-value-calculation in your calculations you will be pressing your motor to its limits, in fact you may be overheating it. So you want to give it a little safety factor. So if you give yourself a cushion of about 10% -15% above what is actually required then you will be alright.

When I do my calculations I base them on a zero to full speed acceleration. What I am looking for really is time, acceleration, and rpm, values to see what the calculations are dishing out. I have long belated formulas that you can use, or you can use the program, which is available on the Go Kart Guru Engineering Tools page.

Basically what we are looking at are two components (three at the most). The two components that we are worried about right now are the drive gear from the motor, and the drive gear on the wheels. We are assuming this is a very basic system. There basically is drive gear on the engine and a drive gear on the drive axel or wheel.

We are assuming in this calculation that our wheel is 12 inches in diameter. Nothing fancy about this system, just clutch, a drive gear and a driven gear on the axel.

Now typically what you will have is a very small gear, something like 12 teeth on the drive gear on the clutch (about 1.5 inches pitch diameter). Whereas on the driven gear on the axel you will have something like 72 tooth or larger perhaps 84 tooth. Which is close to 8,9 or even 10 inches in diameter.

What you are doing is you are taking the torque and you are transferring it through the chain into the drive gear on the axel. Now this may sound very basic, I am doing this on purpose to keep it simple.

For all those who do not understand what torque is: torque is really quite simple. It is a force being put on a radius. It is a spinning force it you will. It sounds oximoronish, but that is really what it is, a spinning type force. But we had to lable it so we call it torque.

Well the spinning force can be translated into another spinning force down below, into that driven gear. That driven gear transfers that spinning force into the tire. The tire then translates its spinning force, through the radius of the tire, as a force into the ground. That is how come the go kart goes forward.

So we have a couple of torques that we are dealing with. But what we want to do though is to balance out the torque from the engine to the torque going to the drive gear. It is really just a balance equation. It is like you are sitting on a teter totter and you are sitting on the teter totter, and you notice that you have a really big kid on one end and a really small kid on the other end. Well the small kid is not going to sit real close to the center, he is going to be sitting way out, and the really big kid is going to be sitting way in. That is really kind of what we are dealing with here. You need a big gear to get a large force.

So you the engine which is really putting out a relatively small amount of torque and we want a large amount of torque being put into the tire. So the best way to look at this is: big gears make a large amount of torque, small gears driving big gears make a large amout of torque. Small gears by their nature are putting out relatively small amounts of torque, big gears by their nature are putting out large amounts of torque.

So when you look at an engine. Say you wanted to make something to go. What you are going to be looking at is having for the most part a very small gear put on an engine or a motor or an electric motor for that matter, and a very large gear being put on the drive shaft. The purpose for this obviously is that we are reducing the amount of torque.

Lets jump to ratios. Basically, ratios tell you how many spins there are to the wheel in th rear, to the engine. So, let say we have a sprocket that has 12 teeth, and a we have a sprocket that has 72 teeth. You know that 72/12 = 6, or you have 6:1 ratio. All that means is that the rear wheel are spinning 1 time for 6 revelutions on the engine.

That is realatively descent. On a go kart you probably want to have around 6:1 to 8:1 is relatively descent. On some of these torque converter clutches they have 19:1 which is designed to really pull you out of a dead stop relatively easily with a relatively small engine. That would be a variable transmission, and that would increase you ratio with out doing anything to the chain. But what we are talking about here is a one speed ratio.

The idea here is a very simple system. We do not want to get very complex with transmission and so forth. All we want is a one speed drive. We just want to press on the gas and go.

5 hp 3600 rpms

12 th
72 th
12 in diam
300 lbs
8 second 200 foot span

Go to the program, press calculate and you can see the speed and so forth.

What we are trying to do here though, in the drive system, is take the torque from the motor and get it to the rear wheels and make sure the engine doesn’t stall or it just cant do it. So ideal thing is engine it self put out a certain amount of horsepower

HP = T RPM/5252 or HP = T RPM/63024

T= HP 5252/rpm

F = ma

a = F/m (Mass = slugs)

Basic physics….

The program I provide is designed to do a lot of the crunching.

Web is designed for pounds, not kilograms.

If you are interested in the equations, they are available in:
The How to Build a Go Kart pdf

 
 

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