Handling constraints&setup

Introduction

“Handling” is the term used to describe the fundamental behavior of a vehicle being driven. It is often described in terms of the response a car has to driver input. For example, the car pushes (or has understeer) in a corner or the car is loose (or has oversteer) in a corner.

What is being described is the response of the vehicle from a combination of factors including how the weight is distributed in the car, how the suspension reacts to the driving forces, and how the tires contact the road surface.

By understanding the physics of handling, we can visualize the behavior of the car we are designing to optimize its performance. In our guide below we touch on the various elements that make up race car handling.

The most basic rule of handling and speed for a race car lies in increasing the speed we can go through the middle of the turn. It has been said before, and rightly so, speed gained in the turns will be carried throughout the lap. This is true for both circle track racing and road racing. A car balanced for handling and dynamics is as fast as it can be.

There are other factors that will make your race car faster, but most of the gains are turn related. Given that we can all agree on the above basic principle, we further break the gain down into three turn segments-entry, mid-corner, and exit. The slowest portion of the lap is spent in the mid-turn segment, so that is where we are most interested in gaining speed.

Granted, increased average speeds in the transitions of entry and exit help reduce lap times, but the gains there pale in comparison to gains we can achieve at mid-turn. Speed gained in the middle of the turn will be carried all of the way around the track.

For a track that averages 100 mph per lap, a 2mph gain at mid-turn represents a 3/10 reduction in lap times. I've seen fast cars run up on the slower cars by 5 mph or more, and that is the half- to full-second difference between First Place and Fifteenth. So, we necessarily start out our handling tuning with the mid-turn balance, both handling and dynamic balance.

Handling Considerations.:- 

Weight Distribution

Track width

Wheelbase

Static Weight Distribution

CG Location

CG Height

Weight Transfer

Acceleration Weight Transfer

Deceleration Weight Transfer

Turning Weight Transfer

Weight Balancing

The positions of the components in a race vehicle determine how its weight is distributed while it is standing still. This Static Weight Distribution will also affect the way it handles on the track. The tires connecting the vehicle to the track provide friction with the road surface and impart turning, braking and acceleration forces into the suspension (if any) and chassis. The Weight Transfer due to these forces largely dictates whether the vehicle handles as expected.

Locating Track width

Track width, as shown in diagram TW1 below, is the width of the car, measured between the centers of the tire contact patches. The track width is important because it determines how much weight is transferred by the mass of the car in cornering.

Understanding Wheelbase

The wheelbase of a vehicle, as shown below, is the distance between the front and rear wheels, measured at the centers of the wheels. The wheelbase is important, because it determines the weight transferred by the mass of the car in acceleration and braking as well as the yaw characteristics in turning.

Balanced and Imbalanced Static Weight Distribution

Every part of a race vehicle has mass and depending on where that mass is located relative to the tires, it will affect the how much weight is on each tire.

Static weight distribution is defined by two ratios:

• A ratio of the total weight on the front or rear tires of the vehicle.
• A ratio of the total weight on the left and right tires of the vehicle.

In example below, the car’s center is marked with a red cross, which lay half way along the Wheelbase, and half way along the Track Width.

The weight is balanced evenly between the front and rear: 50% front/50% rear. The weight is also balanced evenly between the left and right sides: 50% left/50% right.

In reality, most vehicles never balance their static weight this well because large components and their necessary positions do not allow it. Given a front engine and driver, a car might look like the example below,, The driver’s weight shifts the Left/Right distribution to 55% left/45% right. The engine shifts the Front/Rear distribution to 60% front/40% rear. Notice the position of the red cross doesn’t change, but the weight percentage on each side does.

Maintaining C.G

Knowing the weight distribution, front to rear and left to right, we can pinpoint where the CG (Center of gravity is located along the length (Longitudinal) and width (Lateral) of the car. The CG indicates the point that you could balance the car on if you were to jack it up underneath that point.

Height effects on C.G

Static weight distribution is just a 2D (Two-dimensional) concept, until we take into account the height from the ground of the same components described above. The CG height is determined by where the mass of the vehicle components are located vertically.

In example below, the car on the right has been lowered, so the engine, body, driver, and all other components are lower to the ground (CG Height “Y”) than the those in the car on the left (CG Height “X”):

Any vehicle that wants to turn at high speeds is usually as close to the ground as its intended road surface will allow. We’ll see why next in “Weight transfer”.

Key points in Handling

For all vehicles

• Minimize CG height
• Keep static weight distribution as balanced as possible (front/rear, and left/right). Later tuning will be easier than working with a large imbalance.
• Unless the surface is very slippery, reducing weight transfer in cornering usually gives more traction
• In cases where the surface is slippery, a higher CG or narrower track width will aid in transferring weight to outside wheels to give more “bite”. Narrower tires may also aid in this situation.
• Optimize Polar moment of inertia by packaging components as near as possible to the CG.

Weight Balancing

All of the above characteristics come together when a vehicle starts accelerating, turning and decelerating. Each of these driving operations is asking the vehicle to change its momentum in one way or another.

By accelerating, the driver is asking the vehicle to increase its momentum in a forward direction. By decelerating, the driver is asking the vehicle to decrease its forward momentum. By turning, the driver is asking the car to change the direction of its momentum from that which it’s already headed in.

Each change is brought about by the tires in contact with the road. The tires make the change (Spinning faster, slowing down or turning) and the rest of the vehicle reacts to the change and (hopefully) assists the tires in maximizing their grip. This is where weight transfer comes in.

Considering Acceleration

To demonstrate acceleration weight transfer, let’s use the rear wheel drive car in below as an example. The Center of Gravity (CG, the point where the mass of the car is centered) is always going to be above the ground. It will also always be located between the front and rear tires.

When you hit the accelerator pedal, the rear tires apply a forward acceleration. The reaction to the forward acceleration is the rearward shifting of the car’s mass (centered at the CG point) to the only point in the rear where that mass can go—the rear tire contact patch. An analogy might be the CG point “Falling” toward the rear tire contact patch. In either perspective, the shifting of the mass adds weight and traction to the rear tires.

In the acceleration process, the rearward shifting of the car mass also “Lifts” weight off the front wheels an equal amount. What weight the front tires lose, the rear tires gain.

Considering Deceleration 

The opposite of the acceleration weight transfer takes place during deceleration. The CG point is in the same place, so nothing has changed except the fact that the car now has momentum and we wish to dissipate that momentum by braking.

When you hit the brake pedal, the four tires slow down creating a negative acceleration. The reaction to the negative acceleration is the forward shifting of the car’s mass (centered at the CG point) to the only point in the front where that mass can go—the front tire contact patch. The shifting of the mass adds weight and traction to the front tires while simultaneously “lifting” weight off the rear tires.

Considering Turning force 

When a car turns, the weight transfer that takes place happens both laterally (or across the car’s track width) and longitudinally (along the car’s wheelbase). For simplicity sake, we’ll focus here only on the lateral weight transfer.

Just like with acceleration and deceleration explained before, the Center of Gravity (CG, the point where the mass of the car is centered) is always going to be above the ground. It will also always be located between the left and right tires (Within the track width).

let’s look at how turning causes weight transfer. Let’s assume the car is moving and has forward momentum in a straight line (As shown by the black arrow in WT3). As you turn the steering wheel, the tires change angle from straight ahead to the steering angle (Shown by the yellow arrow). The car’s mass, due to the momentum will wish to keep going in a straight line, but the tires connected to it are forcing it to turn.

when the car turns, the forward momentum causes the mass of the car to shift to the only place it can go—the outside tires. The car “Rolls” onto the outside tires, adding weight and traction to those tires, while simultaneously “lifting” weight off the inside tires, reducing their traction.

Is Weight distribution Important?

The tires on a car generate grip because of the vertical loading on them. Generally speaking, the more weight loading we place on a tire, the greater the traction it will provide.

However, this is not entirely true because tires do not provide a linear amount of friction (traction force) compared to the vertical loading. The law of diminishing returns applies, and so as more and more weight is loaded onto a tire, the traction it provides increases at a slower rate. The imaginary tire example shows this diminishing increase in traction.

This becomes important because we have four tires (at least on most racing vehicles), and if possible, we want to have each tire produce the most grip possible. Therefore, we (usually) need to design our vehicle to transfer the minimum amount of weight from the “unloaded” tire to the “loaded” tire for maximum grip. Transfer too much weight and the useful traction provided by “unloaded” and “loaded” tires is diminished.

For example: Let’s say we’re turning a corner, and in the process the outside “loaded” tire has 400 lbs of vertical load. The inside “unloaded” tire has 100 lbs of vertical load. Using the graph above, the total traction available would be 300 lbs + 100 lbs = 400 lbs.

Now, let’s say we lowered the CG, and turn the same corner at the same speed. The outside “loaded” tire now has 300 lbs of vertical load. The inside “unloaded” tire has 200 lbs of vertical load. Using the graph, the total traction available would be 250 lbs + 180 lbs = 430 lbs.

The lowered vehicle would give better traction and turn the corner better.

Consideration of Polar Moment of Inertia

Although CG is the center of the mass of the entire race vehicle, each component of the vehicle has its own mass and location as well. This is important because the further from the CG the components are, the harder it is to rotate or turn the vehicle. Each component is said to have its own polar moment of inertia.

If we use the analogy of a baseball bat, the concept becomes clearer. The longer the bat and the heavier the tips of the bat, the harder it is to swing it. It has a high polar moment of inertia. With an especially short bat that is heavy, we can still swing it in a fairly short time. It has a low polar moment of inertia. However, make the bat longer, and it takes us longer to bring our swing up to speed.

Circle of Traction

The circle of traction is a handling concept that involves how a race car uses the total traction available to it. If a race car is braking using every bit of traction it has, it won’t have any left to turn a corner. Likewise, if a car is using every bit of traction it has to turn, it won’t be able to brake or accelerate without losing traction and sliding.

Handling guide

For circle track vehicles

• Biasing the weight to the inside will reduce weight transfer to outside, increasing overall traction
• Use stagger and offset to aid turn in and weight transfer

For road course vehicles

• Having a balanced front/rear and left/right weight distribution will provide an optimal starting point for tuning the handling of a road course vehicle. If the static weight distribution is too imbalanced, more effort will be required to make the vehicle behave in a predictable manner. 

Thank you