Dirt Late Model Vehicle Dynamics - Part 1
Welcome to the first installment in a series on dirt late model vehicle dynamics. The purpose of this series is to provide a mathematical explanation to some conceptual ideas well known throughout the dirt racing community. Many people successfully use chassis engineering concepts to tune their race car without understanding the underlying mathematics. This series aims to close that gap and help the reader obtain a better understanding of the science behind what their race car is doing. No need to worry if math was not your strongest subject in school, mathematical concepts will be explained as needed.
Having a good understanding of what is going on with the physics of a race car is a very important tool for any racer to have. Over the past decade, an increased focus on race car engineering has crept into the dirt late model world. The days of just running what the chassis builder provided are gone. Today, the racer must be creative and innovative to come up with ways to get an advantage, and that often means tweaking the OEM chassis to meet specific needs. One should remember that a chassis builder is building a chassis to perform on a wide array of track configurations. Simply running a baseline setup will usually not result in the most optimum performance on a given night of racing. Understanding race car engineering and having the ability to adjust the race car to specific conditions is vital to getting the most performance and best finish in a race.
There are many debates out there today about rather or not an increase of engineering resources in the sport is a good thing or a bad thing. Like it or not, engineering has become a part of the dirt late model world. It is here to stay, and it will only become more and more important as the sport grows. Racers will be wise to get ahead of the wave now and start understanding it rather than fighting it.
The first part of this series is a discussion of technical resources. By no means should the reader regard this series as an all-encompassing set of information. There are many resources available on the topic of race car engineering. One only needs to query Amazon.com for “race car engineering” to start building their library of knowledge. The problem with most race car engineering books, however, is that they focus on pavement racing applications, not dirt. Most books about dirt racing only talk about conceptual ideas for tuning with no analytical topics addressed. The two worlds of racing share many characteristics, but overall, they are very different. This should not deter someone from reading the books, there is much knowledge to be gained by studying them. A listing of suggested readings will be included at the end of this article.
As a dirt racer, you must be conscious of the fact that many of the analytical tools in pavement racing do not apply directly to the dirt racing world. One example of this is the use of tire data to help set up a race car. The engineer working on a pavement race car is very interested in the tire’s slip-angle and slip-ratio performance characteristics to help predict the balance of the race car. In the dirt world, this type of information simply isn’t readily available. Even if information was available, it probably wouldn’t be a very useful tool because of the ever-changing surface of the dirt race track. However, this statement is made a little tongue-in-cheek as it may eventually find its way into the dirt racing world. Who knows.
One should also understand that many conceptual ideas used to tune a race car discussed in various books and articles are not applicable when performing a mathematical analysis of a chassis. Again, this holds true when looking at how to analyze a pavement car versus analyzing a dirt track car. One example of this is the use of the roll-center to analyze a race car. The concept of the roll-center is often talked about in race car engineering books and can be a useful tool for conceptually understanding what a chassis is doing; however, the roll center is not used in any way when analyzing a dirt track racing chassis. This may be contradictory to what many of you reading this have read in articles and books in the past. To understand this, one needs to appreciate a few characteristics of the dirt late model chassis. First, as compared to a pavement race car that has relatively little suspension travel, the dirt late model has a large amount of suspension travel, especially on the right front and left rear of the car. This means that the motion ratios of a dirt late model will change much more as compared to a pavement race car. Often, when analyzing a pavement race car, the engineer may simply assume that the motion ratios are constant throughout the operating range of the suspension. This is not the case on a dirt late model race car, especially on the rear suspension of a live axle decoupled suspension. (i.e. the four-link suspension utilizing birdcages and a lift arm) When analyzing a dirt late model chassis, one needs to accommodate for the changing motion ratios. Secondly, many paved track race car engineers can effectively calculate roll moments by using the moment-arm-method to determine chassis balance. This can be done only if there is minimal suspension motion and if both the front and rear roll centers can be clearly defined. This is not the case on a dirt late model chassis. Many people will approximate the rear roll center of a dirt late model, and this is okay for conceptually trying to understand what is going on, but this is not adequate when mathematically analyzing a race car suspension. Furthermore, the moment-arm-method is a rather archaic engineering tool. Remember that this method was developed during a time when computers were not readily available forcing engineers to use pencil and paper graphical methods to develop suspension systems. Today, there is more computing power available in the cheapest of computers than could have ever been imagined fifty years ago. There are much better ways to utilize our computers to analyze a chassis.
The key point here is to understand the difference between conceptual tools used to help the human brain comprehend what is going on with a chassis, and analytical tools used by a computer to analyze a chassis. The two are very different, and the latter is what will be focused on in this series.
In part 2, we will start diving into the mathematical system of equations that govern the dynamics of our race cars.
If you find this series helpful, then you can help me by thinking of Bartlett Motorsport Engineering next time you need to by parts for your race car.
The following is a list of suggested readings on the topic of race car engineering.
• Race Car Vehicle Dynamics - by William and Douglas Milliken
• Race Car Design - by Derek Seward
• Race Car Engineering and Mechanics - by Paul Van Valkenburgh
• Tune to Win - by Carroll Smith
• Engineer to Win - by Carrol Smith
• Fundamentals of Vehicle Dynamics - by Thomas Gillespie
• Vehicle Dynamics and Damping - by Jan Zuijdijk
• The Multibody Systems Approach to Vehicle Dynamics - by Michael Blundell and Damian Harty
• Chassis Engineering - by Herb Adams
• Tire and Vehicle Dynamics - by Hans Pacejka
• An Introduction to Race Car Engineering - by Warren Rowley
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