We should never be afraid...
We should never be afraid to make changes to our setups in order to match the changing track conditions that are so common when racing on dirt. Today, the top teams are not only making changes, but have learned exactly which ones will make the car better as the track surface evolves.
Change, change, everywhere a change... , a message from social protests of the '60s, could have been written in reference to dirt track racing. Throughout an event, most dirt tracks change surface characteristics, some in a dramatic way. Granted, there are some so-called dirt tracks that are always oiled and maintained to the same grip. These are exceptions and won't be addressed here. They are more like asphalt anyway, and a team racing a dirt car on those tracks would do well to set up their cars more closely to an asphalt car setup.
The most difficult tracks are the ones in which the type of dirt or clay needs a lot of moisture in order for the track crew to be able to groom the track. A promoter will often plow up a track during the week preceding the event, water it throughout the morning of the event, and then roll it while still pouring water on it until practice time. From then on, unless it is watered again, the track will begin to dry out and go through several phases of moisture content.
The Hobby Stock setups (A)...
The Hobby Stock setups (A) and the typical Late Model setups (B). For wet and sloppy conditions, usually the first situation we encounter during practice and possibly qualifying...
What makes these various conditions so hard on our setups is that the g-forces are changing along with the declining availability of grip as the track begins to dry out and go slick. As the g-forces change, our setups must change too. The difference is in the grip factor and mostly deals with the level of moisture present in the first inch or two of the surface material. Here is a list of phases of grip that a dirt team might encounter on a typical race day, along with the associated estimated g-forces that the car might experience:
1. Wet, sloppy, and rough: 1.6 to 1.7 g's (peak g's but very difficult for the driver to maintain a good line)
2. Very moist but graded and compacted: 1.5 to 1.6 g's (the best the track will be all day and very fast through the turns).
3. Hardened and more packed as the surface begins to dry out: 1.3 to 1.5 g's (still good corner speed while losing a little grip off the corner).
...we need to set up the car...
...we need to set up the car stiffer to withstand the high g-forces as the car gets thrown around in the ruts.
4. Black slick, a condition in which the track still has enough moisture to keep the material packed, but has hardened and is taking rubber: 1.1 to 1.3 g's (becoming difficult entering and turning in the middle of the turns).
5. Dry, a condition in which the top layer is now drying out and losing material in the form of sand: 0.9 to 1.1 g's (loss of grip is now getting substantial, and the car will not turn well or get bite off the corners. Driver touch is most important in maintaining momentum).
6. Dry slick, a condition in which the track has lost considerable moisture and is now very dry and slick with little grip: 0.7 to 0.9 g's (The cars are now very slow in getting into and through the turns, and traction under power is very low. Small gains in mechanical grip can provide substantial gains in lap times).
To maintain zero rear steer...
To maintain zero rear steer through chassis motion, we need to arrange the rear links at certain angles. The swing arm Z-link (A)...
We can see how it is possible to experience a high 1.7 g's all the way down to a very low 0.7 g's in a single day. There is no way a team can run well in all of the conditions while maintaining the same setup. What we need to do is think about how these conditions, one by one, affect a car and then set it up accordingly. We will examine some physical rules that deal with chassis dynamics so we can arrange the setup to meet the conditions.
Let's use two of the most popular types of dirt car-a touring Late Model and a metric-style Hobby Stock car. For each type of car, we will set up for the six conditions listed above. The changes will be realistic and will not involve cutting or welding the chassis. Time is a factor and we want to make positive changes that can be accomplished in a short period of time.
This track condition means the tires are getting a lot of grip and the surface tends to have ruts that grab the car hard. To be able to control the car, we need much stiffer springs and a more balanced setup front to rear. The car's center of gravity (CG) needs to be lower when the grip is high. A low CG number for a dirt car is around 17-18 inches off the ground.