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Suspension Technical Article - Issue #1

Author: Ali Allage
Page: 1
Last Updated: 7/29/2008

By: Bryan Hise @ JRZ USA

 


Intro

The purpose of this guide is to enable you to make the best use out of your JRZ adjustable dampers. This guide will cover the basics of how the tire works, how the damper works, how the damper influences the handling of your car, how to change the settings on your damper, basic vehicle setup, and how to go about troubleshooting the handling of your vehicle. Finally, some standard practices will be presented in order to assist you with common issues. For more in depth information, stay tuned for more JRZ USA Tech Articles.

Basic Vehicle Dynamics

The Tire

The racing and/or high performance tire is a complex machine that can be easily understood with a few basic concepts. For our purposes, what we need to understand is why controlling the load on the tire is so important. Then we can discuss what we use to control the load on the tire, and how we can go about getting these tools to work together.

Tires make grip by both friction forces and by pushing into the small irregularities on the pavement, which is called hysteresis. These mechanisms of grip are activated in different magnitudes given a tire slip angle and the weight on the tire. Tire camber and tire pressure also have an effect on this behavior and will be covered in later tech articles.

The slip angle is the angle at which the tire contact patch is pointed relative to that of the wheel. Slip angle does not mean your car is sliding exactly, what is normally perceived as “sliding” or “slipping” is in fact a slip angle high enough to make the driver uncomfortable or down right scared. When a racing or high performance tire is doing what it does best at the racetrack, it operates at a slip angle of around 4-8 degrees (road racing).


Figure 1 - Diagram of tire slip angle

For a given load and a given slip angle, we can determine how much cornering force the tire will make. What makes this important is that the tire does not have exactly “linear” behavior. This means that there is not a direct relationship between vertical load, slip angle and cornering force. Once we figure in tire temperature, wear, etc. the problem gets really messy. Again, for our approach to the problem, understanding the basic trends of the tire will enable us to get the most out of the car without buying an F1 team. At peak grip levels the tire is very sensitive to loading and slip angle. It is our goal to manage this load sensitivity for maximum grip and balance.

Going further, when we analyze a vehicles performance (in general) we look at what the front and the rear of the car are doing. This means that we look at the combined effort of the left and right tires on each axle in moving the car around a corner. This combined effort is where the meat of TLLTD (discussed a bit later) is. Because of tire load sensitivity, weight transfer can actually reduce the amount of grip the front or rear of the car can make. So by controlling the amount of load transfer spilt between the front and the rear, we can bias the grip levels to balance the car.

Below is some example data from a tire which shows basic tire behavior. The first diagram shows tire cornering force versus slip angles at three different vertical loads. Note the line of peaks, as tire load increases so does the slip angle at which the tire makes maximum grip.


Figure 2 - Lateral force against tire slip angle for three loads.
Image credit: Race Car Vehicle Dynamics, Milliken & Milliken

So it may appear that we can keep adding load and the tire will make more and more grip. So why not transfer the entire load to the left or right? Well besides most drivers feeling more comfortable on four wheels rather than two, the pair (front or rear) will not make as much grip overall. The diagram below shows this characteristic. Lateral force coefficient is the ratio of cornering force to vertical load. The higher the lateral force coefficient, the more grip it makes per lb of vertical load.


Figure 3 - Lateral force coefficient versus slip angle for three loads
Image credit: Race car Vehicle Dynamics, Milliken & Milliken

So if there were 1800lb total on an axle (front or rear) and the weight was split evenly left and right, there would be about 1,980 lb of cornering force available. If all of that weight was transferred to the left or right tire the axle would only make about 1,710 lb of grip! This would lead us to the conclusion that we want to minimize weight transfer. This is not exactly possible given that we cant change the design of the car very much in most forms of racing (however lowering ride height will help) it is now our task to control how this weight transfer is split front and rear and how fast it comes in as the car makes its way around the track.

Handling Modes

Before you are presented with a discussion of using your damper to get your car handling the way you want it, we must first go over some of the basic terminology and concepts of vehicle handling. Vehicle handling modes, understeer/oversteer, and suspension will be reviewed.

Vehicle handling modes describe the different types of motion your vehicle undergoes as you drive it. There are four modes of vehicle motion; heave, pitch, roll, and warp (one wheel bump). During racing conditions, the vehicle will experience combinations of these motions simultaneously. For our treatment of suspension setup we will discuss these modes individually for ease of analysis.

The first mode is heave (also called bounce.) This is when all four wheels on the car compress or rebound at the same distance and rate. The heave condition is the most basic and is generally a starting point for many suspension designs and setups. Springs and shocks are specified from a starting point from analysis in this mode. Heave motions are normally experienced over long road humps and dips as well as on high speed banked corners.

Next, there is vehicle pitch. Pitch motions are experienced during acceleration and braking. This mode occurs when the vehicles front or rear wheels compress and/or rebound at the same rate. During pitch, weight is transferred between the front and rear wheels which change the cornering balance of the car. Weight transfer during pitch is generally equal left to right in most applications. Dampers effect pitch by changing the rate at which this weight is transferred. By changing the rate of weight transfer, you can adjust how the car turns in and exits corners.

Vehicle roll occurs when both the left or right side tires compress or rebound equally. In this mode, weight is transferred between the left and right side tires. What is different about roll as compared to pitch is that the front and rear of the car do not split the roll weight transfer equally. The ratio between the front and rear roll weight transfer (also known as Total Lateral Load Transfer Distribution, TLLTD) is very important in setting up your car. By changing your suspension you change the TLLTD of the vehicle. Similar to pitch, dampers affect the rate at which TLLTD comes in, and this changes turn in and exit behavior. It is important to note that anti roll bars or sway bars only function in roll and in the next mode, warp.

Warp mode is also known as single wheel bump. Road irregularities, road race track rumble strips, and other bumps cause vehicle warp. In order for the car to maintain maximum grip in this situation, it needs to be outfitted with a soft suspension setup. Unfortunately, you car will not perform as well in the other three modes. It is this tradeoff that you will seek to balance to get your car handling at its best.

Understeer/Oversteer

Understeer and oversteer are two handling conditions that define how much steering a driver needs to input the get the car to turn at a certain radius at a given speed. The Understeer condition (also known as “push”) happens when a driver must add more and more steering to get through a corner. Conversely, oversteer (also known as “loose”) is a condition where the driver must steer less in order to keep the car from turning in more or from spinning out. These conditions can be understood by this approach: when the car is understeering, the front of the car has less grip than the rear and cannot turn the car in any more. When a car is oversteering, the front has more grip than the rear and the rear cannot hold the car in the turn any longer causing is to slide out. Understeer/Oversteer is also called “balance” and will be used for ease of explanation. The balance of the car can be different at in the three phases of the corner; entry, middle, and exit.

There are several parts of your car that mainly influence your handling balance; springs, sway bars, dampers, and the tire setup. Here we will look at each phase of the corner and how suspension adjustments affect the balance of the car. Later on we will discuss how your suspension components work together to get the balance right. We will start with mid corner for simplicity, and then move on to entry and exit.

Mid Corner

During the mid corner stage, it is assumed that the car is not accelerating or braking, such that the car is traveling at a constant speed. It is also assumed that car is cornering on a smooth road such that the dampers are not moving and affecting the balance. Here the car is in pure roll and we can develop the basis of the vehicle balance. When the car is in pure roll, the major contributor to balance is TLLTD, tire camber and tire pressure. For now we will assume that tire camber and pressure effects are negligible. When you change springs and bars, you change TLLTD. The gist of TLLTD is that there is a certain percentage of TLLTD on the front wheels at a given loading that makes the car neutral or right on the line between push and loose.

To set up the car more towards push, you add more front % TLLTD. To add % front TLLTD you can add front sway bar stiffness or up the front spring stiffness. Reducing rear sway bar or spring will give the same effect. You can reverse these adjustments to bias TLLTD towards the rear if the car is pushing or if you want a looser setup. You may note that from the previous section “Intro to Dampers” canister pressure does have a “preload” effect. While it is not as significant as changing spring rates and spring preload, adjusting canister pressure can be used to fine to balance. Canister pressure adjustment and recommended only for experienced users. A final note on mid corner balance and setup, your basic TLLTD effects the car greatly in entry and exit as well and you can make changes here to satisfy entry and exit needs.

Entry

On corner entry, the car will first pitch forward as you slow to entry speed and begin to roll as you turn into the corner. This means that weight is being transferred to the front and outside wheels, with your outside front carrying a significant amount of load. Getting the car turning in slow or faster is a function of adjusting the rate of this weight transfer. Low speed rebound (or just rebound if you don’t have 4 ways) is the general tool here. To get weight to transfer forward faster, lower the rear low speed rebound setting. This will load up both front tires quicker giving them grip to turn the car in. Similarly, you can reduce front low speed rebound to get the front pair of tires to bite sooner giving a similar effect. This may sound contradictory so consider that lowering rear rebound speeds weight transfer forward in pitch/braking while lowering front rebound will give the front end more grip during a roll/turn in motion.

Exit

When you finally get past waiting to see the apex and lay down the power, the car enters the corner exit phase. Here weight is transferred from front to rear and back from the outside to the inside tires. Generally, the rate of side to side weight transfer is slower than during corner entry. Setup on exit is similar, where you can increase the rate of rear weight transfer to put the power down (rear drive cars) by lowering front rebound. If the car feels a little loose on exit, try tightening up front rebound to keep the car stable.

Springs, Bars, Shocks, and Their Interaction

The vehicle suspension has three (normally) devices that provide force between the road and the chassis; springs, bars, and shocks. These members operate in different ways and are dependant on each other in their operation. Here we will review these devices, characterize their effects on the car, gain an understanding of their interaction, and discuss methods to get them working together.

First, we will study the spring’s contribution. The spring acts on the car all the time, giving the car control over bumps, in braking and acceleration, and in cornering. Choosing a spring rate is the first part of getting a baseline setup together. In general, choose spring rates based on track bumpiness, or your level of comfort for a street and track car. For example, for a smooth course, run a higher spring rate. For a driver who doesn’t want to feel a lot of road disturbances, choose a lower spring rate. JRZ’s professional engineering staff can assist you in developing your baseline setup.

Next we look at sway bars. The sway bar adds stiffness in roll and warp and is a great tuning tool for balance. Here the first interaction is studied. If you are happy with the balance of the car, however it is rolling too much, you have two basic options. The first option is to add some front and rear spring to hold the car up, or add some front and rear overall. The spring change might help hold the car up, but you may hurt entry and exit because the car don’t drive and squat the way it needs to. Generally, for smoother, faster tracks run more spring than bar and vice versa with slower bumpy courses. Because you can get the same stiffness in roll with a lot of combinations of springs and bars, it’s important to note what each part does. More importantly, you must consider that the front/rear springs and front/rear bars coupled together create the weight transfer balance side to side. The front/rear springs affect the weight transfer front to rear.

Finally, the shocks control how we manage the rate of which this weight transfers and as a result, the “platform” feel of the car. The shocks also control the energy of the springs and bars as the car moves around. Physically, these two topics are one in the same, however for our purposes, it is best to break the two down. The break down is that low speed damper adjustment affects the handling weight transfer and platform, where high speed adjustment affects the overall movement of each wheel. For example, high speed damping is used to control wheel hop as well as control the bumps and kerbs on track.

Intro to Dampers

Basic Function
The damper’s most basic function is to work with the springs and sway bars in keeping the tire in contact with the ground. The damper operates only when the suspension is in motion, meaning that the damper does not supply any force when your car is sitting still. The damper achieves these characteristics through a system of valves and orifices that are designed to meter oil flow. This oil flow metering is the force that acts on the wheel.

How Adjustments Work
To modulate the damper force, external compression and rebound adjusters are used. These adjusters accomplish this by restricting or freeing up oil flow through the dampers. In 3 and 4 way dampers, low speed adjusters meter oil flow when the shock is moving in a slower motion (for example, car rolling into a turn) and effectively “close” in high speed suspension motion.

Adjustment Effects
High and low speed damper adjustments are not mutually exclusive. While each adjustment affects the damper’s operation in its respective range, there is a small cross effect from each adjuster. This means that if you adjust your low speed compression 5 clicks stiffer, you will need to adjust your high speed compression one click softer to maintain the same high speed characteristic as before. These cross effects are sensitive to valving and additional information is available upon request.

Gas Pressure Effects
Adjustable pressure gas chambers in a remote canister or inside the shock can also change the way your damper works. Gas pressure has three effects on damper behavior: it acts as a spring, it provides “nose force” and it affects overall damping force. Gas pressure acts like a spring because the damper has to compress the gas chamber when as the shaft goes into the shock. “Nose force” is a term that describes the amount of force required to move the damper from a static or zero velocity position, it is also called “lifting force”. “Nose force” generally affects support/platform of the vehicle. Finally, gas pressure has an effect on overall damping force meaning that the lower the gas pressure, the lower the overall damping force will be in both compression and rebound. This last effect can be considered negligible. JRZ dampers are designed such that the spring and overall damping effects of gas pressure are negligible. Therefore, adjusting gas pressure in a JRZ is very similar to adding spring preload.

 



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