Karting Education: When 3 Wheels are Better Than 4 - Part 2

Karting Education: When 3 Wheels are Better Than 4 - Part 2

When 3 Wheels are Better Than 4: The Front Tires Do the Heavy Lifting

By Chuck Goodson

In our last post "When 3 Wheels are Better Than 4: An Introduction to Kart Chassis Tuning", we discussed why it is important to lift the rear tire to help a kart corner. Today, we will discuss how to lift the inside rear tire, or “jacking” the rear.

  • The single most impactful tool for lifting the inside rear tire is actually on the opposite end of the kart!

Understanding Jacking: The Role of Front End Steering Geometry

What happens when you turn the wheel on the kart with no one sitting in it? The kart starts to get tippy and rocks on a diagonal. This is part of how the rear tire can lift in corners. Why does it do this? The front end steering geometry causes the inside front tire to drop and the outside one to rise as you move the steering wheel. This is commonly referred to as “jacking”. There are two primary parameters on the front end that can influence jacking: Caster and Scrub Radius.

Explaining Scrub Radius and its Influence on Jacking

Let's start with scrub radius. Scrub radius is simply explained as the distance between the centerline of the tire and the centerline of the kingpin at the track surface. The name is used to describe the motion of the tire when the wheel is turned.  At zero scrub radius, the tire turns on its center line and has zero scrub.  At positive or negative scrub radius, the tire has to sweep on an arc to rotate, hence scrubbing.

karting scrub ratios explanations

Karts are designed with a positive scrub radius to provide more input for jacking through caster.

Adjusting Scrub Radius for Optimal Performance

  • Adding scrub radius increases the amount of jacking from caster.
  • Reducing scrub radius reduces the jacking from caster.

Methods for Tuning Scrub Radius

Scrub radius is adjusted by 2 methods:  Either adding/removing spacers on the spindle or by changing the camber.

When adding spacers, the centerline of the tire moves out, increasing the scrub radius.  Removing spacers does the opposite.

When adding camber, the intersection of the kingpin centerline at the track surface moves closer to the tire centerline, reducing scrub radius. Removing camber moves the kingpin centerline away from the tire, increasing scrub radius.

The Role of Caster in Creating Jacking Force

So we have explained what scrub radius is, but how does that lift the rear tire? It works together with caster to create “jacking” or lifting force.

Caster is the front-to-rear angle of the kingpin from vertical, with positive caster used in automotive and karting applications. Positive caster makes the steering want to center more easily and increases tire lean when turned. Excess positive caster increases steering weight and effort.

Caster Explanation

Positive caster when combined with scrub radius causes the front tires to swing in an arc that is not flat with the track surface. This results in the tire on the outside of a turn (red dashes in illustration) lifting as the wheel is turned and the inside tire (green dashes) lowering as the wheel is turned.

Fine-Tuning Caster for Optimal Performance

  • Increasing caster increases the backward lean of the kingpin and results in a larger difference in tire heights when the wheel is turned, increasing the jacking amount.
  • Decreasing the caster angle reduces the difference in front tire heights when turned, decreasing the jacking amount.

Analogous to a Wobbly Table: Understanding the Mechanism

Wobbly table
Source: Image by lifeforstock on Freepik</a>

Have you ever sat at a table with one leg shorter than the other?

What does the table do? It rocks back and forth across the diagonal with the two long legs staying on the floor and the short leg and its opposite corner tipping back and forth. This is what is being done in the kart. We are making a wobbly table of sorts!

When the front wheel is turned and the outside front tire raises, it becomes the short leg of the table. Only in the kart, we also lengthen a leg by lowering the inside front tire, further increasing the wobbly table effect. When the raised outside front tire and lowered inside front tire are combined with the outward forces of weight transfer during a corner, the kart tips outward and lifts the inside rear tire.

Accounting for Kart Flexibility and Weight Distribution

However, karts aren’t exactly like the wobbly table example. The chassis has flexibility that a table does not have. The table also has an evenly spread and unmoving mass that the kart does not have.

In the next blog post of this series, we will discuss how the chassis frame and weight placement work with the front end to influence tire lift.

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