Torque-A-Verter Is Slipping: What Is Wrong With My Go Kart?

By Robert Gamble “The Go Kart Guru” (Copyright 2009)

This article is written in response to a great question about torque-a-verters. Here is the question:

I have a cart with a 8 hp motor, comet clutch and a jack shaft but it still doesn’t climb hills well…. The clutch is set @ 2200 rpm, I am thinking the clutch needs to engage sooner to get better belt grip, could that help? I recently replaced my worn clutch springs with new stiffer ones, and they killed my torque, way to much belt slip now.”

The torque-a-verter or the variable speed clutch is used widely on various go karts. The clutch is particularly useful for making a go kart more all-around. In other words it can climb hills and still zip along at a decent clip.

Trouble shooting the clutch can be frustrating and sometimes dead-endish if you don’t understand how the clutch system works.

First of all the clutch is composed of three parts:

- The driver
- The driven
- The belt

The driver and the driven are basically pulleys that adjust automatically according to rpm and torque. The driver, uses centrifugal weights to gradually squeeze the pulley together and therefore cause two things to happen: 1.) a tighter engagement to the belt and 2.) a gradual increase in the torque radius of the pulley.

The gradual increase in the torque radius is what starts the variable transmission shifting or the change in overall drive ratio.

The tighter engagement of the belt causes a tension to be exerted into the belt. This tension is then transferred to the driven pulley. The driven pulley reactes to the amount of tension the belt is putting on the pulley. The harder the belt pulls, the larger the diameter of the driven pulley.

When the tension relaxes, the pulley then opens up causing the over all ratio between the driver and the driven to decrease. This causes the go kart to go faster, but loose the ability to climb hills, because now the ratio has been lessened or made less steep. Instead of having a 19:1 ratio it may now have a 6:1 ratio.

You can pulls stumps with a 19:1 ratio, but you will stall an engine at start up with 6:1.

One of the frustrations with a Snow-Mobile like clutch (or the variable clutch) is that it usually is set for a very high engagement speed. This corresponds to prolonged engagement time with the belt and the driver and can cause the belt to burn up prematurely.

To make the clutch engage sooner two methods are employable: 1.) lessen the spring pressure on the centrifugal weights, or 2.) increase the centrifugal weight mass.

The easiest method is to change the springs to be less tension. Use only the manufacturers recommended spring adjustments, however, because the clutch is designed to engage at particular rpms and torque settings.

The driven section can be adjusted as well. The tension on the cam spring can be adjusted in or out to change the rate of engagement. The tighter the spring the longer it stays engaged. The looser the spring the sooner it shifts and drops to a lower ratio.

And finally, the belt size and the C-C (center to center distance) between the driver and the driven is critical. Any less of a space and the system wont engage. To far apart and the system will burn up belts because it won’t shift properly.

And a word of note, if the driven to the go kart rear axel has too low of a ratio, the over all system may not work properly and will contribute greatly to the burning up belt situation. Because the torque requirements for the driven pulley are more than what it was design for, so the belt will try to engage, but will slip.

In plain English: if the rear driven sprocket to main axel sprocket has a ratio of about 3:1 you have serious problems, but if the system is between 6:1 and 10:1 then the system is probably set up properly.

To recap:

Variable speed clutches (or comet clutches/snow mobile style clutches) have four main areas of concern:

- The driver
- The driven
- The belt
- And the over all chain drive

The driver is adjusted using springs and weights. The driven is adjusted using a main spring adjustment. The belt must be the correct style and size for your system. Additionally, the C-C between the driver and the driven must be dead nuts on. And finally the over all ratio must be reasonable, other wise the system will bog down and smoke.
The Go Kart Guru is a go kart design specialist with over 25 years of go kart building, designing and repairing go karts.

Be sure to visit the Go Kart Guru web page for specials on books and videos that will help you design and build your go kart to be a real hummer.

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5 Responses to “Torque-A-Verter Is Slipping: What Is Wrong With My Go Kart?”

  1. I am having trouble finding comet clutch springs… I have an old set that is so light that the clutch grabs at idle, but I have massive torque with them and no belt slip. I bought the only set I could find (2200RPM) it works great but torque is terrible now.

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  2. My gear ratio is actually 14:1.. This is a utility kart designed to haul and tow

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  3. The springs that you bought are too stiff, and the old ones are too light, you will need to find a spring that is just in the middle. Check out a hardware store and see if they have any springs. If you have a part number of your clutch you should be able to buy the correct springs. Go to this guy…he has all sorts of comet clutch parts. http://www.gokartsupply.com/comet.htm

    One thing of note, if you have the wrong type of belt on your clutch that can have an effect on how the clutch grabs too. Some are designed with a “v” groove on one side only. If you are trying to run a
    “v” on both sides of the belt, then the clutch will slip.

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  4. Do you have a performance calculator similar to the one in your engineering toolkit that works with a CVT system? Or would I just have to plug in each gear ratio and plot it on a graph?

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  5. I am currently working on another book that has graphs that will work with the CVT. Right now, you are correct, you have to do the ratio system on paper and then force the program (by manipulating gears sizes) to get the ratios you are looking at. CVT also are different, so you need to make sure you have the range from the manufacturer.

    If you don’t then you will have to do as I did in the article about “Why is my chain breaking” and calculate it by hand by measuring the belt engagement points and so forth.

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