Dirt stock car wedge

We left off last time talking about balancing left side weight to wedge. I think there needs to be a little more clarity on what wedge can actually do to a cars handling and how large wedge can actually be used to give you an advantage. As we previously explained wedge is the difference in static weight between the left rear and right rear tires; and since weight transfer side to side and back to front when the car enters the corner, the left rear is typically the most statically loaded tire on the car. What we did not get into too much depth on is how to track tune wedge to use it to an advantage.

Tire slip angle

First we should see how a car turns. When a car goes down a straight, with all four tires pointed in the same direction, we’ll call this a neutrally balanced car. This is most times not the case, but for an easy explanation, we’ll accept this as a constant. As you start to turn into the corner, the front wheels are turned putting a “twist” in the contact patch. As the tire rotates, the leading edge enters this “twist” and the trailing edge is then untwisted as it leaves this contact patch area. The energy from the untwisting of the tire exerts a force on the hub and spindle in the direction they are turned which pushes the tire in the direction it is pointed; thus turning the car. Many people have heard the term slip angle. Slip angle is basically the angle of the “twist” in the tire. More on tires in future articles.

If the rear tires on a dirt stock car have too much traction, they will simply over power the front tires and the car will go into a push; or the car will be tight, whatever terminology you would like to use. Slip angle will technically ad traction to the front of the car. The more you turn the tires, the more traction you add to the front, the better chance you have for your car to turn. In a perfect world with a perfectly balanced car, there is only so much you should need to turn the front tires. Excessively turning the tires would lead to more scrub and indicate a tight car.

Now back to how wedge fits into this whole picture. For a car to turn, even one with significant amounts of rear steer, the outside tire must slip at least a little bit; an exception to this is a car with four wheel steering. The more wedge you add to the car the less load the right rear tire will have on it, allowing the car to turn easier. Many racers theorize this, on high wedge race cars, as allowing the car to pivot around the left rear tire. This can be well suited for a paper clip style track, a point and shoot style track, or a short corner high banked track where you might need to get the car deep into the corner, turn it quick, and accelerate hard off the corner. This isn’t always the best idea for large radius slick tracks, where you need plenty of side bite while rolling off the gas into the center of the corner. Although it is not the correct way to go if you are trying to balance your car, but I’ve seen plenty of cars in the three hundred to four hundred pound range win races.



Mark Donahue; “The Unfair Advantage”

If you think of what an actually good balance to a car really is, the high wedge stuff really doesn’t make sense. In pure tuning thought too high of wedge will never get the rear tire imbalance to equalize anywhere on the track to give the rear tires equal loading and maximum traction. There is actually a real good book that points out over and over again the importance of proper balance. The book is called “The Unfair Advantage”, and is written by the late Mark Donahue. Mark took a brief retirement from racing too write this book in the 70’s. Although it is slightly dated and about tuning road race cars, it is still one of the best racing books I’ve ever read. It is written by Mark Donahue’s total point of view about how he thought about racing and more importantly tuning all of the race cars throughout his career. Car balance is one of his highest priority in tuning his cars. He said he often spent hours on the skid pad testing and testing to try to solve balance issues with the cars. It is interesting to see how he went about solving problems and developing the cars. Some say he was probably one of the greatest driving engineers in racing.

 

If you are interested in purchasing this book you can click on the link above which will take you directly to Amazon.com. The book is actually pretty inexpensive at $14 or $15 for a paperback version. It’s well worth the money.  Next time we’ll dig into right front spring rate and how it can affect balance. Till next time, race hard.

Kevin