Thursday, March 26, 2009

Go Kart Drive Line Problems: How Do I Match Up A Clutch?

One of the biggest problems people run into when making a go kart is figuring out the drive line.

I get emails that are asking me how to match up the clutch to the go kart, because the clutch is smoking.

Invariably what the problem is, is not the clutch, but the gear ratio.

A classic example is a go kart with 12 inch wheels being driven by a 8.5 inch drive sprocket.

The problem that occurs is that the demands of the go kart drive line require a steeper drive to engine ratio. The smaller 8.5 inch drive sprocket is requiring a ton of force in chain tension to get the go kart moving.

To get around this the ratio must be made steeper, and the way to do that is to increase the main drive sprocket size.


  • 5 hp @3600 Briggs and Stratton 4 Cycle Engine (Torque Level is: 88 in-lbs)
  • 325 lb go kart
  • 12 inch diameter rear tire
  • Drive Sprocket Diameter = 1.5 inch
  • Driven Sprocket Diameter = 8.5 inches
  • Approximate Acceleration on level ground is 8.56 ft/s^2
  • Pushing force is 85 lbs
(These calculations came off of the gokartguru engineering tool kit calculations page)

Now compare the same go kart with a larger drive sprocket

  • Driven Sprocket Diameter = 11.89 inches
  • Approximate Acceleration on level ground is 11.89 ft/s^2
  • Pushing force is 120 lbs

Nothing has changed on the engine, just the ratio in the rear sprocket to the engine has changed causing the go kart to accelerate faster and have 35 lbs more pushing capacity.

As a rule of thumb for every angle of hill climb multiply .017 times the weight of your cart to determine the amount of force required to push your go kart up the hill. So if your go kart weighs 325 pounds the force required to get the gokart up a 20 degree incline is 110 lbs. That is just to get it to the balance point.

Your drive system must put out over 120+ lbs to get the gokart up the hill.

As you can see the standard drive line is not going to be able to get that go kart up the hill, it is only putting out 85 lbs.

The bigger sprocket, however, would get the gokart up the hill. There is a problem though: the main drive sprocket is the same size as the wheel. So an alternate sprocket system is going to be required to get the go kart to go up hills.

As a rule of thumb for larger wheeled, one ratio go karts, the sprocket needs to be almost as large as the rear wheels, other wise you will have clutch slippage problems.

(Again this is a rule of thumb, you may be able to get a clutch that works better under the loading conditions, but the corresponding engine output has to be matched as well.)

Typical centrifugal clutches that you buy at the hardware store are not designed for prolonged engagement, and have significant slippage when encountering torque limits.

For example the 5 horsepower engine above puts out 88 in-lbs of torque. When the level of horsepower output is maxed out on the engine it will lug below the engagement limit on the clutch and just sit there and slip.

To overcome this, the horsepower needs to be increased and correspondingly the rpm increased to cause the clutch engagement to be more aggressive, seeing that the clutch engagement curve for a clutch is related to the square of the rpm.

Next time we will discuss what to do when you can’t find a sprocket that is going to work, and you just have run out of options…or have you?

Visit the GoKartGuru driveline calculations course for a more in depth discussion about drive lines for go karts.

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