Oversteer / Understeer, and How to "Slip" past the competition.

March 16, 2018

Last week we talked about the performance of the tire, how it transfers work done by the engine to the track. Some of the concepts used in that article will be referred to in this weeks blog post. If you are not familiar with the traction circle, and weight balance in general, please go back and read the post from last week.

 

Many Sim Racers are familiar with the Terms Oversteer (OS) and Understeer (US), some may even know the tuning parameters that influence these handling characteristics. In the next few paragraphs, we will be taking a detailed look at the Science behind OS/US,  how we can use Telemetry data to identify corners where the car is in OS/US, and how we can tune the car to our driving style preference.

 

Slip Angles, What Is It, and Why Do We Care?

 

When our car is traveling down a straight, our tire and tire tread is for the most part non distorted. However as we corner, a cornering force is applied to the tire's contact patch. This force deforms the tire, this deformation of the tire is what gives us our Slip Angle, a video is given below to illustrate this phenomena.

 

 

The Slip Angle is defined as the angle between the velocity vector (direction of travel) and where the tire is pointing. Most individuals would think that deforming the tire would be a bad thing and result in a decrease in grip, however the complete inverse is true. Tires increase in grip with increases in slip angle, up to a limit. The limit is a property of the tire, and is usually given by the tire manufacture. Once this limit is reached, increases in slip angle will result in decreasing levels of grip. For most racing cars this ideal slip angle is usually around 8 degrees.  A picture of a typical Slip Angle vs Traction graph is given below,

 

 

From the blog post last week, we know one of the main limiting factors is our maximum grip force. Slip is a way to increase that overall maximum grip force and squeeze  more performance out of the car and decrease our lap times.

 

Now that we know we need some levels of slip, lets determine how we can determine if the car is balanced.

 

 

Neutral Steer

Neutral Steer occurs when both front and rear tires are slipping the same amount. The result is the car is balanced, and the car will respond to steering inputs. That is to say, the car will not understeer or oversteer.

 

With Neutral Steer, the car can maintain greater levels of lateral acceleration (cornering force) while maintaining the same steering input. A visual for this statement is given below,

It is important to note that although increases in Ay (lateral acceleration) result in constant required steering input, our slip angle will increase with increasing lateral acceleration, once our car exceeds the tire's optimal slip angle, the car will no longer be able to maintain the cornering radius (reduction in grip).

 

Oversteer

Oversteer occurs when the rear tires have a greater slip angle than the front tires. The difference in levels of grip will cause a moment about the cars center of gravity, causing rotation. This rotation will allow the car to turn into the corner, if the car oversteer is not controlled, the car will result in a spin. When the Car is in oversteer, counter steering will be required, to counter the effect of the moment caused around the center of gravity of the car. As lateral acceleration is increased the counter steering required also increases. The diagram for this behavior is given below,

Typically on a car that has a bias towards oversteer, the system will become unstable. The car will oversteer, and the oversteer will not be controlled, the rotation of the car will continue requiring more counter steer. Increasing counter steer will also increase the slip angle of the front tires, once the front tires exceed their optimal slip angle the car will spin.

 

Understeer

Understeer is the exact opposite of oversteer, understeer occurs when the front tires have a larger slip angle than the rear tires, this causes the car to track out of the turn. To counter understeer, more steering input is required, however if the car has already exceeded the optimal slip angle, giving the car more steering input will not bring the car back into the corner, and in some (most) cases, it will worse the affect of understeer.

The diagram for understeer is given below,

Most road cars have a bias towards understeer this is because, having the car track out of a turn is more stable, and less dangerous than a car that likes to spin. In racing, most drivers prefer some oversteer as it allows the driver to turn into the corner.

 

How to Use Test Data (Telemetry) to identify Handling Characteristics

Handling characteristics for a car is subjective to the driver. Some drivers prefer more OS, some prefer US, when one driver complains about the car, how do we quantify how far away from neutral steer we are?  To answer this question we need to get out a few equations.

 

In a neutral steer car, both front and back tire slip angles are equal, if we derive the slip angle equations in terms of acceleration we arrive at the two equations below,

 

However we know that in a car that is neutral steer the rear and front axles are equal in terms of slip angle. In turn, this also means our acceleration for front and rear tires are equal. Setting the two equations equal to one another, we arrive at the equation below. We will refer to this equation as equation one,

 then if we write acceleration in terms of corner radius and forward velocity, we have the second equation,

 if we plug equation 2 into equation 1, we arrive at the final solution,

So you may be asking, okay Zach why the hell do we care about this? And what I am about to say is going to change the way you setup cars forever!

 

We can now visually graph our neutral steer channel purely as a function of forward velocity and lateral acceleration, both of which can be outputted by most simulator telemetry programs.

 

By graphing our neutral steering angle, and comparing that to the steering angle from our steering wheel, we can very easily tell if the car is understeering or oversteering, if the car steer graph is over the neutral steel channel, the car is in oversteer, and vice versa for understeer.

 

Program Setup

To graph our neutral steer channel you are going to need a way to create your own functions, while some telemetry programs have a way to crate custom traces. I have found the best way, is to do the calculations in excel. I use Pcars Telemetry to capture data, and then I use the CSV export feature to output steering, speed, and lateral acceleration.

 

1) In excel,import the CSV data from the telemetry.

2) Create a new column to put the time in (each row is 50ms increments by default). Then covert the speed from KMH to M/S.

3) In the steering column, divide the steering angle by your steering ratio, and make the number negative (multiply by negative 1)

4) Figure out the cars wheel base. Project cars doesn't have a way to determine wheel base, so what I normally do is go to google, and try to figure out a best guess for the wheel base of the car I'm doing my analysis on.

5) Create a new column, and use the neutral steer channel equation from the previous section.

6) Graph the results as your y axis, and graph the time as your x axis.

 

An example of how I setup my table is given below,

 

Neutral Steer Channel

Now that we have our results in excel, and graphed, lets look at the results. Remember the Neutral Steer Channel, is the line at which the car is perfectly balance, that means the car is not in understeer or oversteer. If our steering line is over the NS channel, our car is in oversteer, if the line is under the NS channel our car is in understeer.

 

Lets look at our first example, using a telemetry program we captured the following data,

 

 

Looking at the two signals (steering (pink), lateral accel. (purple)) between the blue and red vertical line, we can see that as the driver increased his steering input, the car did not increase in lateral acceleration. This is to say, the driver was trying to turn the car more into the corner but because the grip levels for the car had been exceeded, their wasn't a resultant increase in cornering force.

 

Looking at the graph above is tedious, and is hard to quantify how much US we have. Now looking at the graph below which contains our steering input vs neutral steering channel as mentioned in the section above. We can see that our steering (orange line) is under the neutral steer channel (blue line). This indicates US which verifies our speculation that the car was in US in this corner. We can get a sense of how much US the car is in by comparing the magnitude of deviation from the neutral steer channel (orange line - blue line)

 

 

Now lets look at OS. The test data from the telemetry is given below,

 

 

 With the graph above we can see that the steering input oscillates, as we get a general decrease in lateral acceleration, this indicates oversteer. Using our handy dandy neutral steer channel graph we can easily see this by comparing the steering vs neutral steer line, the results are shown below,

 

 

Again we can get the same information for either type of graph, however using the steering vs the neutral steer angle is a much quicker way to analyse data and determine exactly what the car is doing, as well as the magnitude of OS or US the car is in .

 

Tuning the Pain (OS/US) away!

We now understand what tire slip is, how to identify the handling from tire slip using our neutral steer channel, but how do we tune the adverse handling out of the car? The answer to this question is by figuring out where the car is unbalanced, and manipulating the tires, suspension, dampers, and aerodynamics to balance the car.  The car can be defined as balance when all four tires have the same grip levels. Lets look at some of the things we can alter below.

 

Tire Tune

When it comes to tires, there are a several things we can change, ultimately we want to maintain the largest contact patch possible,  we can accomplish this by using things like pressure, temperature, and camber.

 

Using the PCTuner tool on this website, you can easily see if your cars are set up correctly, you will just need to run a few laps then record your temperatures and plug them into the tool, the tool will tell you if your tires are overloaded, if they over/under inflated, or if you can use more camber. A picture of the tool is given below,

 

 

Suspension / Damper Tune

The suspension and damper is arguably one of the areas where you can affect the balance of the car the greatest.  We can use things like the spring rate to affect the responsiveness of the car, as well as change the dampers to control the rate at which weight transfer occurs. A Video discussing how to setup a car suspension / damper is given below,

 

 

This tool is available for $10 under the store section of this racingsimtools.com

 

Aerodynamic Tune

Aerodynamic Down force is another way to adjust the balance of our car, if we notice that the car is neutrally balanced in slow corners (where aerodynamics don't play a roll), however we are either in OS or US in fast corners. We can either adjust the down force bias towards the front (for US) or towards the back (for OS). 

 

One important thing to note, if the car is not balanced in slow or fast corners, we should be working on our suspension setup first, only after we are satisfied with low speed cornering should we analyse high speed corner handling characteristics. 

 

 

 

 

 

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