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International Karting Industry Buyer's Guide

GO KARTING!
A Guide To The World's Most Popular Motorsport.

Kart Expo International

The most widely used engine in fun karting nowadays is the 5-HP Tecumseh engine. The clutch for fun karts have a spring (or springs) to retain the shoes in place while the engine is idling. As the engine speed increases, the spring (springs) allow the shoes to move outward and contact and turn the drum, which has a drive sprocket. This drive sprocket is connected to either a wheel or axle, normally by a chain, which propels the kart.

A general rule for all clutches is to keep them clean and well lubricated. Gearing ratios are also of primary importance and are very different depending upon the type of kart and its intended use. A good rule of thumb for fun karts however, is a 1:6 ratio. What this means is, if a clutch has ten teeth, then the rear sprocket should have 60 teeth. If a live axle is used or if tire size gets over 15” in diameter, then this ratio should be increased. There is a point where a clutch alone is not practical and the only alternative is a jack shaft or torque converter to further modify the final drive ratio.

An important consideration when working with a centrifugal clutch is the total overall weight of the clutch. The more a clutch weighs, the more horsepower it takes away from the engine to rotate the unit. There is a direct correlation between clutch weight and output horsepower: the more weight on the shaft, the longer it takes to transfer power to the rear axle. 

Another factor when using a centrifugal clutch is precise alignment of the clutch sprocket with the driven (axle) sprocket. Many clutches have two set screws in the hub to mount the clutch to the engine crankshaft. However, this manner of mounting can often cause problems. Driven sprockets can often become warped either from the stamping operation or the subsequent heat treatment. Should this occur, a rigidly mounted clutch will create binding of the chain particularly at higher speeds, which results in either increased sprocket wear or thrown chains. 

We recommend using a bolt and washer in the end of the crankshaft to hold the clutch on the shaft for two reasons. First, it is much more secure than set screws, therefore safer. Secondly, by using a crankshaft bolt, the clutch can “float” on the shaft by about .060” thereby self-aligning itself with the driven sprocket.

One of the first things you should examine when looking at a clutch should be how easily the drum spins on the sleeve and how much wobble there is between the drum/sprocket and the sleeve. If you can get a .010 shim (1/4” wide) between the sleeve and the bushing, the bushing should be replaced. The oil in the bushing accounts for 15% of the weight of the bushing when it comes from the factory. During transit and storage at various facilities before purchase or installation, much of this oil leaches out, especially if it isn’t packaged in a plastic bag when it is received. Ideally you should submerge the replacement bushing in a good oil (10W30) and heat the oil to 130 degrees F for about an hour. If you buy a replacement sprocket with a bushing already press-fitted into the sprocket, put the whole sprocket and bushing into the heated oil. What you are trying to do is to replace the air in the pores of the bushing with oil just like it was done at the factory.

Over time the heat generated from the clutch will dissipate all the oil. Frequent lubrication of the bushing is the best preventative maintenance you can do to a clutch. A squirt of oil at the snap ring while it is warm is the best time to lubricate the clutch. If the clutch is fitted with a roller bearing which needs to be replaced, pack the bearing with a good quality automotive grease before press fitting the bearing into the sprocket

There are two things to remember: Keep your clutch lubricated  and keep it aligned! These two tips should help your centrifugal clutch run better and last longer. 
 

About the author: Jim Donovan is vice president of Max-Torque Ltd., a leading clutch manufacturer based in Naperville, Illinois. Their telephone number is 630-971-1957 or on the web at www.maxtorque.com. Mr. Donovan will be chairing the upcoming “International Clutch Symposium” at the 2002 Kart Expo International, February 23-24, in St. Charles, Illinois. For more information, see “Kart Expo Update” on www.e-KMI.com.
 
 

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