Drive Calculations: 5 Steps to Get That Go Kart Running Properly

It is astounding how many go kart builders are emailing me and asking why their go kart clutch smokes.  It seems I have written several articles on the subject…but perhaps, just perhaps I am not getting you the information you need.  So here goes…why is my clutch smoking?  What can I do to make my go kart run better?

There are 5 steps toward designing a go kart drive system they are:

1. Figure out the Weight of the Go Kart
2. Figure Out Your Engine HP
3. Get All the Drive System information correct
4. Figure out what your go kart will do on a 10 degree hill
5. Make Drive System adjustments and recalculate
Those five steps may seem pretty inane or simple, but are pretty important.

First of all,  figure out the weight of the go kart accurately.  You may want to use a scale and go out and weigh the go kart.  The challenge is getting the weight.  To get the weight use the scale twice, first measure the front section of the go kart, then place the scale in the back and measure the back section of the go kart.  Add the two weights together and that will be the total weight for the go kart.

Now as you probably were guessing, the size of the drive is definitely dependant on how well the go kart will perform.  For example, my son and I just did an experiment our selves this weekend (in preparation for this article) and we did this exact process….weighing the go kart, then weighting ourselves.

The following data then was collected:

Father: 185 lbs
Son: 79 lbs
Go Kart: 228 lbs

We then used this data for later in our analysis.

So to recap, weigh the empty go kart, then weigh yourself.  You may want to double cross check your numbers by weighing yourself in the go kart.  This also is a great opportunity to figure out the Center of Gravity of the go kart.  Be sure when you weigh up the go kart for CG that you place the scale under the wheels.  This takes some thinking…hint we used a large 2×6 placed on its side on the scale on, then set the go kart on it.

Figuring out the HP

To get a proper read on what the HP is going to be when you are starting from a stop, or trying to climb hills, the horsepower needs to be calculated.   First the engine manufacturer tells you what the HP rating is on the go kart by placing a sticker that says “5.5 hp”  Trouble is that 5.5 hp is at a rating.  A hint is that most engines are rated at 3600 rpms.  So if your engine says 5.5 hp, it is 5.5 HP at 3600 rpms.

That seems simple enough, but the reality of  a go kart is that 3600 rpms is not going to be how fast your engine is turning over from a dead stop, or when you are climbing hills.  Typically clutches engage at around 1500 to 2200 rpms. So the torque value that you want to use for your calculation is going to be in that range.  The torque at 2200 is not going to be the same as the torque at 3600 rpms.

This is key when evaluating the go karts performance.  Torque is the turning force that an engine puts out, and the turning force is not constant but linear.  As the rpms increase on an engine, the torque out put increases.  That is why you get pushed back into your seat, because the engine is putting out more and more torque as the engine starts going faster.  (That is to a point, then the torque actually tapers off, that is when you can feel the push in your back taper off as the rpms cross a peak)

The question is, how do I calculate the torque at the engagement speed?

The assumption can be made that the torque is linear and is zero at zero (0) rpms.  Having that in mind and using simple algebraic line equation methods the calculation proceeds as follows:

The equation for a line equals:

Y= MX + B

Y in our equation is going to equal Torque
X is going to equal rpm

B = zero, because the intercept for the Torque to RPM is zero.  In other words, you do not have 1 inch pound of torque at zero rpm, you have zero torque at zero rpm

That being said, the equation changes to this:

Torque = M * RPM

To get the initial torque that the engine puts out at 3600 rpms we need to use the HP that the engine is rated at:

HP = Torque *RPM/63000

Solve for Torque

Torque = HP*63000/RPM

Torque = 96.25 in-lbs

We plug in our values to get the following equation and solve for M

96.25 in-lbs = M * 3600 rpms

M = 96.25/3600 or M = Torque/3600 or M = (Hprated *63000/3600)/3600 or M= .00486 * HPrated

So the final equation is:

Torque  = Hprated *.00486 *RPM engagement

Rpm engagement is 2200 rpms so the torque is:

Torque at 2200 rpms = 59 in-lbs

This final equation is vital for our go kart performance calculation.

Drive Train Layout

The drive train layout is what makes the gokart climb hills or stall and go backwards down the hill.  As we point out in the Go Kart Building 201 book the basic drive line is composed of the following:

– Clutch drive sprocket
– Axel driven sprocket
– Wheel size

These three components are what is needed to get the calculation started.

The best way to collect this data is to go out and measure each component in its diameter.  There are a bunch of short cuts, but the safest method is to measure the diameters of everything, clutch sprocket, main drive sprocket and the tire.

Once this information has been gathered, tabulate it and store it for the next step.

In our example the variables will be:

Clutch Sprocket =  1.12”
Drive Sprocket = 8.6”
Wheel Size = 13”

Climbing Hills Calculation

What you will discover along your way of making go karts is that driving on flat paved surfaces is a lot different than driving on rock driveways and climbing hills.  You will soon discover that climbing hills is a lot tougher on the go kart that just going on a flat straight away.

The reason for this is that the weight of the go kart now starts to pull the go kart down the hill, where as before it was just rolling along.

The following equation tells us how much force is required to get the go kart to go up a hill:

Force Hill = Weight Go Kart * SIN(Angle Hill)

The typical hill is 10 degrees and the sin for 10 degrees is .17

So the Balancing force to just hold the go kart in place on the hill is for example:

(228 (gokart) Lb + 185 (person)lbs )*.17 = 70.21 lbs

The next step is to calculate what the go kart will do with the horsepower it has.  So we go through the calculations covered in Go Kart Building 201 and come up with a force that is pushing the go kart:

Force Go Kart  = (Te* Rd/Re*Rw)

Te = Torque Engine = 59 in-lbs
Rd = Radius Drive Sprocket = 4.3 inches
Re = Radius Clutch Sprocket = .60 inches
Rw = Radius Wheel = 6.5 inches

Force Go Kart = 48.41 lbs

As you can see the force that the engine will push the go kart is 65 lbs, to climb a hill it must put out 70.21 lbs.  The go kart will actually go backwards down the hill with this set up, because the amount of downhill force is greater than the engine can deliver.

The go kart to be exact will go down the hill at – 5lbs/12 slugs = .41 ft/sec^2 which would appear like a slow crawl backwards. 

Upgrading the Drive Train

If you have ever ridden ten speed bike you will understand that using the largest sprockets in the rear by the wheels and the smallest sprockets in the front by the sprockets is the way to go.

The same applies to drive systems.  The steeper the ratio, the faster you feet pedal, the faster the engine spins.  Bottom line is that the rear drive sprocket is not large enough.

The question is, what will be large enough?

Well, iterative calculations will help solve that problem.  The simple solution is to start making the rear sprocket larger and larger until you have reached your optimum sprocket size.

So that is just what we will do.  The rear drive sprocket currently is 8.6 inches in diameter, we are going to crank it up to 12 inches in diameter to see what we get for force:
Force Go Kart  = (Te* Rd/Re*Rw)

Te = Torque Engine = 59 in-lbs
Rd = Radius Drive Sprocket = 6 inches
Re = Radius Clutch Sprocket = .60 inches
Rw = Radius Wheel = 6.5 inches

Force Go Kart = 90.76 lbs

The question now is, is that enough?  At the surface the force of 91 lbs is greater than the hill load of 70 lbs, so that is encouraging, but is that enough to really get the go kart moving up the hill?

The answer lies in the acceleration.  Acc  = (91-70)/Mass = 20.7lbs /12slugs = 1.73 ft/sec^2.  The go kart will go up the hill, but it will climb slowly.  A decent rate of climb is closer to 6 to 8 ft/s^2

So we will recalculate until we get an acceleration of at least 6 ft/sec

Acc = (Force GoKart- Force Hill Climb)/(Total Wieght/32.2)

Force Go Kart = Acc (Mass)+Force Hill Climb

Fgkt = 6 ft/s^2 (13 slugs) + 70 = 148 lbs

We need to get 148 lbs out of the drive system.  So we can calculate for what the optimum sprocket diameter is:

Force Go Kart  = (Te* Rd/Re*Rw) = Acc (Mass) + Fhill

Solve for Dd = Drive Sprocket Diameter

Dd = 2*[Acc (Mass) +Fhill]*[Re*Rw]/Te

Dd = 2*(6*13+70)*.6*6.5/59

Dd = 19.56 inches

Well as you can imagine a sprocket that is 19.56 inches in diameter is much larger than the wheel, which is 13 inches.  It would be physically impossible to use a sprocket that is larger than the wheel.   Ideally what should happen is that the sprocket some how gets divided in half.  That is where a jack shaft comes into play.

Bottom line for a jackshaft is that overall ratio should equal the ratio of the system that we just figured would be the best design.

A jackshaft system calculation is as follows:

(Re/Rjg1)*(Rjg2/Rd) = Re/Rd(ideal)

Re =.6
Rjg1 = unknown
Rjg2 = .75
Rd = 4.3
Rd (ideal) = 9.78

Ideal Ratio = 16.3:1

Solve for the Unknown sprocket diameter

Djg1 =  2*[(Rd(ideal)*Rjg2)/ Rd)]

Djg1 = 2* 9.78*.75/4.3

Djg1 = 3.41”

So the real question from this whole mess of equations was, “What would be the ideal sprocket size for my go kart?”  It is based on the degree of the hill, the amount of acceleration I expect to get, how much the go kart weighs, how much horsepower I have, what is my expected drive line system…

 Djg1 = 2*[Acc (Mass) +Fhill]*Re*Rw*Rjg2/ Te*Rd

Acc > 6
Mass = Weight /32.2
Fhill  = Weight *.17 or Weight *SIN(degree hill)
Te = Torque of Engine = Hprated *.00486 *RPM engagement
Re = Rengine Clutch
Rw = R wheel
Rjg2 = Rjackshaft small output sprocket
Rd = Rdrive sprocket

When you use this formula you will get a drive system that will climb hills guaranteed; unfortunately it might only go 12 mph, but at least it will climb hills.
Bottom line is if you want to calculate the drive line you will need to know the:

1. Weight
2. Drive System Specifics
3. Torque at the engagement RPM
4. Calculate the Drive system
5. Recalculate so that it will work

If you would like more information on how to calculate the proper drive system for your go kart consider the Go Kart Building Drive Systems Course which includes the basic 201 course and the Vertical engine drive course.

The best value, however, is the Bundle which includes all the Go Kart Books, plus the Go Kart Plans for free.