Understeer vs Oversteer vs Driver

Before anyone gets confused with this steering business, it is important to define understeer and oversteer. We could go into the technical definitions getting into slip angles of the tires or accelerations of the front end compared to the rear. That seems over the top. When cornering, every driver has pushed his car past the limits of adhesion. It is understeer if he turns his front wheels, nothing happens and he hits a wall. Oversteer happens when he turns his front wheels and the back end swaps ends with the front, and hits the wall going backwards. For all you NASCAR fans out there, when a driver says the car is "pushing" or "tight" then he is referring to understeer. On the opposite end, it may be "loose" and oversteering.

Enough of that. There are many things to influence a car pushing or being loose, including the driver. If you have any more questions or are still confused, I am going to refer you to my friend Google.

During data analysis last time, the car appeared to be understeering through the corner exit. There are several strategies to determine the cause. First, data reveals if it is the car's fault, or the driver's fault. A quick look at the steering plot, and throttle channel can show this. During a car's transient maneuver, it can only change directions in a finite amount of time. The driver may turn the wheel very fast, yet the car cannot respond that quickly. To find how fast a driver can turn the wheel, take a similar speed and radius corner where the car was neutral handling. Last, take the derivative of wheel position with respect to time, and out comes a speed. Comparing the two will reveal how well the driver is trying to make the car corner. So if the velocities are in the same ballpark, its not the driver...not quite! The last phenomenon is power on understeer, or the driver hitting the gas pedal too early and not spinning the rear tires, pushing the front beyond its limit of adhesion. Sometimes a simple driver technique correction will fix the problem. Other times the differential is at fault. Since differentials are notoriously hard to tune without separate wheel-speed sensors, I will leave it to someone else to explain how this is done.

If all of these parameters are ok, then it is most likely the car setup causing this condition. Going back to the data, there is a small amount of understeer in every corner so an adjustment should be made. Several systems on the car can be used to tune this depending on the conditions. Since this is the current example: I used dampers to correct the problem. It only occurs on corner exit, a transient condition, as opposed to steady state. Put a few clicks of stiffness into the rear low speed bump, and done! problem solved.

As a disclaimer for everyone out there who are shouting "Hey you are taking grip away from the rear, you should always add grip to the the offending side to find a balance."
This is not always the preferred method, and there were other factors that went into my decision.

This is the end of any posts on how to correct vehicle dynamics problems, since it could go on for days. But it is a sample of what data analysis needs to be done for every corner between sessions to decide on the next setup. Unfortunately cars don't just understeer and oversteer, due to the examples listed above and many factors must be considered during every data analysis session.

Data Analysis Intro

One of the responsibilities of the race engineer is finding the optimal setup for each session out on the track. I utilize data from engine RPM, speed, throttle position, lateral acceleration, and position of each damper to determine what changes need to be made. This task usually takes a few hours of studying the data, looking at past data, and playing with numbers. No one has a few hours to spare at the track, between fixing any broken parts and prepping the car for the next session, I am lucky to have half a hour.

To solve this problem, I am turning to my new found friend called Matlab!
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Tonight I thought I would focus on the first project I coded up almost a year ago. Steering vs. Lateral acceleration plots are extremely useful to quickly determine what the car is doing. Understeer, oversteer, poor damping, and poor driving technique can all be seen (of course I have an eye on my driver, I mean, my setup is always perfect...not)

Steering is on the bottom, the scale is irrelevant since it is just the voltage read from the sensor. Lateral acceleration is on the side, measured in G forces. Each color represents a different corner. This graph shows there is not necessarily a corrilation between more steering angle and more lateral acceleration. Slip angle tire and vehicle characteristics dictate this curve.

You many now be saying "Wow Matt, way to be a giant nerd and reinvent the wheel, most data analysis software has this built in, show us the goods!"

Well this was my first foray into the world of Matlab and without any instruction from school I was quite proud of myself. Going back to the whole "time is money" concept, I needed to automate everything and make it easy to read. In addition, as soon as I got to the track I found this graph view was useless since I had no theoretical reference to see what the car should be doing. So I quickly fit a line to the data as a reference and split the graph up corner by corner, ending up with this:

The blue x's are what the car would be doing in a perfect world, with the red being what the car is doing in that corner. Corner camber, and conditions needed to be taken into consideration when looking at the data. As an example take a look at corner 12. At High Plains Raceway, this is a constant radius, flat corner. Consequently the data should sit right on the theoretical. It is very close, but upon closer examination at the end of the corner the driver is throwing in more steering and the lateral acceleration is way down (the lowest red points of the curve for all of us color and resolution challenged, ie me). This indicates some good understeer, and the next step is to find out why, which is exactly where we are going to leave off for the night, and pick up later!

First Post

Well seeing as how this is my first post, I should probably explain what this blog will be about and a little of what I do. Predominately this blog will cover my journey through the HUGE subject of racecar vehicle dynamics. Also all of my crazy ideas and projects will be included. Last will be anything else I think is worthwhile for everyone else to see. Don't worry there will not really be anything about my life in here since its fairly boring consisting of work, school, and if at the time, a girlfriend. 

Im currently a Mechanical Engineering student at the University of Colorado. I also am race engineer for Waterman Racing, currently campaigning a Ford Mustang FR500S in the World Challenge Series. Look for us on Versus channel this year!