Karting Education: When 3 Wheels are Better Than 4 - Part 3
When 3 Wheels are Better Than 4: Karts Are Giant Springs
In our last post "When 3 Wheels are Better Than 4: The Front Tires Do the Heavy Lifting", we discussed how the front end can create jacking in the rear of the kart.
Today, we will discuss how chassis flex and weight transfer create tire lift.
If you ask for opinions about chassis flex, you may walk away thinking it is some type of voodoo magic. It is really all about managing the roll resistance of the kart and carrying the jacking force from the front end to the rear axle.
So why is the chassis like a spring?
As we discussed in the conclusion of our previous article, kart chassis are designed to want to tip diagonally when the front wheels are turned. However, you won’t see the chassis tip diagonally when sitting still with the driver in it because the chassis is designed to flex.
Narrow Waist on a FA ALONSO KART FRAME - OK
The shape of the chassis, with its narrow waist, allows the front end of the chassis to twist and rotate somewhat independently of the rear. This causes the frame to load like a spring when the front tires are turned but not lift the rear. When combined with the lateral forces in a turn, the energy stored in the frame then lifts the inside tire.
Several non-adjustable parts of the chassis design, such as the diameter and thickness of tubes, tube metallurgy, the width of the waist, etc., influence how each kart responds and flexes. However, the biggest impact on the timing and amount of tire lift is from the timing and amount of steering input from the driver.
Marco Romero - DPK Racing Americas - KA100 Senior
Another key area for adjusting tire lift is how the rolling resistance of the kart is adjusted and where the weight is located.
What is rolling resistance?
Rolling resistance is the resistance of the chassis to roll side to side when turning. A high rolling resistance transfers more load to the outside tire in the corner. In other motorsports, the vehicles have springs, dampers, and linkages that are all part of managing front and rear roll resistance to tune the front to rear balance in cornering. In karting, all you have are chassis reinforcing choices that can be added.
The base design in all karts results in a much higher roll resistance at the rear than at the front of the kart by allowing the front to flex and move more. To visualize how this works, let's imagine an extreme. Pretend that the front spindle is made out of flexible rubber instead of steel. This would lower the front end roll resistance to near zero. What would happen when you go through a corner? The outside front spindle would bend and allow the frame to sag down to the ground. What would that do to the inside rear tire? It would allow it to lift extremely high, likely several inches in the air.
If you look at the base chassis design, the open front end and reinforced rear are designed to create a much stiffer rear with greater rolling resistance. This is all part of the pursuit of rear tire lift!
Now, as you consider where different stiffening adjustments can be made, you can start to visualize how they can add roll resistance to each end of the kart.
Front Bar on a FA ALONSO KART - KZ CHASSIS (Gearbox)
A stiffer front bar adds roll resistance to the front.
NO Front Bar on a FA ALONSO KART - OK (Direct Drive)
No front bar or a softer bar reduces the front roll resistance.
- Adding a rear bar, seat struts, stiffer axles, or longer hubs all work to add rear roll resistance.
- Shorter hubs and softer axles reduce the rear roll resistance
- A stiffer seat increases the overall roll resistance of the chassis but affects the rear more than the front due to where it attaches.
- Softer seats reduce the overall roll resistance but affect the rear more.
Another variable in rolling resistance that is frequently overlooked is the driver!
The driver can intentionally or unknowingly impact the rolling resistance quite significantly. Pushing forward and outward on the wheel in a corner can increase roll resistance at the front and reduce how much jacking is transmitted through the frame to the rear. Pulling back and inward on the wheel in a corner does the opposite. Also, pushing outward on the heel stop in a corner can increase front roll resistance and reduce the jacking effect. These are not tuning tools to rely upon but are great in-race adjustments a driver can make to work around a handling issue.
Curt Goodson - LO206 Senior
So what is the difference between jacking from caster and scrub radius (steering) and jacking from roll resistance (chassis flex)?
- Jacking from chassis flex is driven by the amount of lateral load generated in a corner. As a result, jacking is consistent with grip, regardless of steering input. However, if grip changes, the jacking will change.
- Jacking from steering is proportional to how much the steering wheel is turned, meaning tight corners will create more jacking than fast sweepers.
- Additionally, changes made in chassis flex, especially the front bar, impact how much jacking force from the front steering is carried to the rear of the kart.
- Chassis flex also affects how quickly a kart responds and how a kart handles bumps.
- Stiffer kart will react more quickly to inputs.
- Softer kart will handle a rough track more predictably.
Remember, the goal is to unload the rear tire and keep it unloaded until the kart has finished rotating; it does not have to lift off the track. If the rear tire lifts too high, the rear of the kart may start to hop. This occurs because the outside tire contact patch is affected when the rear lifts too far, reducing the grip of the outside tire. The outside tire can no longer support the lateral load, slides out, and the back of the kart sets down, then reloads, and lifts again. Not only is this uncomfortable to drive, but it also slows the kart down every time the rear tire sets back down.
None of the changes outlined in this series will add or remove grip from the tires. All we are trying to achieve is unloading the rear tire enough to allow rotation of the kart in the corner. Once the inside rear becomes unloaded, the rear weight has all transferred to the outside rear tire, which defines the rear grip. Unless the driver moves the steering wheel around erratically while cornering, the grip is essentially fixed once the rear tire lifts.
Thank you for reading this blog post. We hope it provided valuable insights into the fascinating world of kart chassis mechanics and how they influence your kart's performance. For a more comprehensive understanding, we highly recommend reading our other related blog posts:
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An Introduction to Kart Chassis Tuning: Dive deeper into the basics of chassis dynamics and learn how to get started with tuning your kart for optimal performance.
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The Front Tires Do the Heavy Lifting: Understand the crucial role front tires play in kart handling and how they contribute to the overall performance of your kart.
Stay tuned for future posts where we will discuss other factors that can tune grip and front-to-rear balance. We will cover the location of the center of gravity, tire pressure, and camber angles.
These articles are designed to complement the knowledge, offering you a well-rounded perspective on kart chassis tuning. Happy racing!
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