Karting Education: When 3 Wheels are Better Than 4 - Part 1

Karting Education: When 3 Wheels are Better Than 4 - Part 1

When 3 Wheels are Better Than 4: An Introduction to Kart Chassis Tuning

By Chuck Goodson


Although racing karts may look simple to the untrained eye, one unique aspect of kart design makes tuning more complex than you might expect.  Upon closer inspection, you see that the chassis of the racing karts has swooping shapes in the middle and a solid rear axle.  Most racing chassis have straight lines on the chassis and use a differential on the rear axle. 


So how does this make tuning different?


When any vehicle turns a corner or curve, the inside tire rolls a shorter arc radius than the outside.  Because of the difference in arc radius, the inside tire travels less distance when in a curve or turn.


On the front of a car or kart, the inside and outside tires are not connected and spin independent of one another.  But on the rear, that is where things are different.


A car uses a differential on the axle to allow the inside and outside tire to spin at different rates and negotiate corners.  In racing car applications, the differential can often be tuned to allow differing amounts of rotation between each side to maximize the balance of power and turning.


Diferencial Torsen (Autoblocante) / Self-locking Torsen Differential
Source:  MakeaGif

In karting, there is no differential.  Karts use a solid rear axle, which means that the inside and outside tire always rotate the same speed.  With both rear tires connected by a solid axle and always rotating at the same speed, the grip of the rear tires will resist turning inputs and make the kart maintain a straight line, or understeer.  If you have ever tried to move your kart around on the ground, you have experienced this resistance!


What is the solution to the understeer created by a solid axle? Using 3 wheels when cornering!


Karter turning with 3 wheels

Source: Curt Goodson

In the photo above the inside rear tire in slightly off of the track.  This kart is turning on only 3 wheels!!!


A key element in kart tuning is unloading the inside rear tire during cornering to allow the vehicle to rotate through the corner.  The tire does not have to lift off of the track surface, although it often does, it only needs to unload enough to allow the inside tire to slip along the shorter inside rolling arc.  In fact, you typically want the absolute minimum amount of lift necessary to unload the tire enough to allow the kart to turn as desired.


Most adjustments made on the kart and in driving technique are about managing the unloading of the rear tire and how that affects rotation and overall grip of the kart.  Since the inside rear tire in karting is lifted in corners, karts achieve 100% rear weight transfer in corners.  This is unlike other motor racing, where the rear and front weight transfer can be balanced through tuning tools like ride heights, suspension rates, and roll center adjustments.  In karting you achieve full rear weight transfer and only have 3 wheels providing grip on a corner, so many of the thought process about front and rear balance from conventional motor racing can be tossed aside.


So exactly how does a kart lift the inside rear tire in a corner?


Tune in to part 2 of this series "When 3 Wheels are Better Than 4: The Front Tires Do the Heavy Lifting" , where we will cover how front end steering geometry is the primary influence on rear tire lift.  Afterwards we will discuss how chassis flex, and weight placement work together with the front end to influence rear tire lift.


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