Source : https://www.hotrod.com/articles/ctrp-0407-ackermann-steering-system/
Tuning Your Steering System To Reduce Drag
One of the critical elements of proper chassis alignment is the steering system. The front wheels must work together, just like the two ends of the car, so that they can produce the greatest amount of traction to turn the car. This area of chassis setup falls under the umbrella of those systems on our cars that can ruin an otherwise great setup if not designed correctly.
We have published previous stories about Ackermann, its definition and technical details about why it’s important. However, we have learned more about this important subject. We need to continually consider Ackermann and how it affects our overall chassis setup package.
What is Ackermann Effect?
Ackermann effect is a phenomenon associated with an automobile’s steering system. A steering design that incorporates Ackermann causes the inside (closest to the radius of the turn) wheel to turn a greater amount than the outside wheel. We need this difference in steering angle because the inside wheel runs on a smaller circle or arc than the outside wheel.
Ackermann effect is named after the man, Rudolf Ackermann, who discovered and did research and development on the subject. Early in basic automotive design and development, engineers discovered the need to design an exact system for steering a production car so that each wheel tracked correctly when the car was negotiating a turn. The ideal system would compensate for large radius turns as well as for tight, “turn right at the stop sign,” type of smaller radius turns.
History offers that many of the very first owners of automobiles were concerned about tearing up their circular gravel driveways, and the Ackermann designed into the car helped keep the wheels tracking correctly and reduced the primary cause of rutting in the driveways.
Do We Need Ackermann in Our Race Cars?
There have been many opinions about the use of Ackermann in our race cars and whether it really helps. Numerous older books and articles on the subject extol the benefits of Ackermann to help the car to turn. Are these articles correct? The answer is yes and no. Here is why.
In our past, going back some 30 years, the suspension and steering systems in oval track stock cars were strictly stock units that exhibited characteristics of the intended use-to drive around the neighborhood and down the interstate. Converting the car to circle track racing was beyond its original intended use. It is easy to understand why some of the systems may not have worked well on the racetrack.
Early crewchiefs did not understand, nor did they have the technical knowledge to develop what we now know as a balanced setup. This is where the suspension systems are working together and doing the same thing when the car is in the turns. This balance makes a lot of good things happen, such as causing all four tires to work hard and providing consistency in the handling balance between tight and loose.
Because most cars were not properly balanced-although some did get there through trial-and-error guessing-the left-front tire usually did little work. This was evidenced by several indicators: 1. cool left-front (LF) tire temperatures compared to the left-rear (LR) tire; 2. a need for a stiff right-front (RF) spring, as the RF corner supported most of the front load off the car in the turns; and 3. excess RF tire wear and heat. If the LF tire had little weight on it in the turns, then teams learned that excess Ackermann actually helped the tire to generate more heat and turning effort.
A drag-link system is correctly designed with angled steering arms. The system inside the steering arms will produce unwanted Ackermann effect, which needs to be canceled. The result of turning the steering wheel is as follows: 1. The driver turns the steering wheel left; 2. The drag link moves to the left; 3. The drag link moves forward as the Pitman arm and the idler arm rotate around fixed points; 4.
So, when we hear older, more experienced crewchiefs tell us that using higher levels of Ackermann really helps their race cars to turn better, we can believe them. Now that we have learned how to properly balance the car, we do not need the excess Ackermann in our steering systems. In fact, if we do not remove the excess Ackermann effect, it will have a reverse effect on our handling and actually make the car develop a push as the two front tires fight each other to go in different directions.
When we have Ackermann effect present in our steering design, the toe is increased. Conversely, with Reverse Ackermann, toe is reduced when we turn the steering wheel. There are many different static settings for frontend toe that are dependent on the size of the racetrack, the banking angle, and the type of tire used. Most short-track stock car teams use toe-out to stabilize the frontend and keep it from wandering back and forth across the track. Conventional wisdom tells us the car will need more static toe-out for the smaller radius tracks. At racetracks of more than a half-mile, less toe-out is required. The amount of toe-out used typically ranges from 11/416 to 11/48 inch.
Rack-And-Pinion Steering
The rack-and-pinion steering system is designed to use straight-ahead steering arms and produces very little Ackermann effect if both arms are the same length. Since both wheels turn the same amount as the rack moves back and forth, the wheels steer an equal number of degrees.
Regardless of the amount of static toe-out you use, the toe numbers can be very different as we turn the steering wheel. If our car gains toe when it is steered, commonly referred to as toe-steer, then we have Ackermann effect. If our car loses toe when it is steered, we have Reverse Ackermann effect.
We need very little Ackermann effect in most situations when racing on an oval track. Even on very tight quarter-mile tracks, the LF wheel will only need an additional 11/416 inch of toe over the RF wheel to correctly follow its smaller radius arc. That is 0.112 degrees, or a little over one tenth of a degree. You can imagine my reaction when a racer tells me that he or she only has a couple of degrees of Ackermann in the car. A degree of Ackermann equals 11/42 inch of toe for an 85-inch circumference tire. So, if we have 2 degrees of Ackermann in our steering systems, that equates to an additional inch of toe when we turn the steering wheel. We would never think of setting an inch of static toe in our cars and then go racing.
While this points to the fact that we all need a correctly designed steering system, most racers and many car builders still do not fully understand the steering systems in their cars and how they work to produce or cancel Ackermann.
What causes Ackermann?
There are several ways that your car can produce Ackermann effect. The most common is by installing the wrong spindles or other steering system components on the car.
Over the last few years, teams and car builders have worked hard to reduce unsprung weight-the weight of the wheel/spindle assembly. One way to accomplish this was to install a lighter spindle.
At first, car builders began using smaller, compact car spindles on the stock frontends of full-sized cars that were designed with the drag link steering system. At the same time, custom spindles were being fabricated for the newer design and popular rack-and-pinion steering system. These later spindles were different in design from the stock spindles because they had steering arms that were pointed straight ahead from the ball joint instead of being angled in from a top view at the tie rod end like the ones used on the drag link steering systems.
In the mid-’90s, some car builders swapped the heavy cast-iron stock car spindles that had been used with their stock-based drag link systems for the lighter “rack” spindles intended to be used on the rack system. The result was a steering system that produced excess Ackermann effect. This hurt the turning performance on those cars.
A simple, yet accurate way to measure Ackermann is to use a laser or string to project the alignment of the wheel/tire out in front of the car. If you place targets in front of the tires at a distance of exactly 10 feet from the center of the hub, you can mark where the tires point at straight-ahead and then at wheels-turned positions.
A select few racers discovered the problem and corrected it with different length steering arms. They dominated racing during that period because their cars turned better than the competition. Today, with a better understanding of Ackermann and the amounts needed by the cars, we can measure our Ackermann and correctly adjust any excess toe steering quickly and easily. Remember, no amount of chassis setup adjustment will overcome excess Ackermann effect and the loss of front grip associated with it.
How to Test For Ackermann Effect
Here is a simple, easy test: First, scribe a line on each front tire around the entire circumference at about the middle of the tread. Use a toe bar with adjustable width pointers and measure the toe with the wheels pointed straight ahead by measuring the front and rear width on the scribe line and subtracting the widths.
Turn your steering wheel the same amount as the driver would steer at mid-turn, and then recheck your toe using the same method. You will be able to record the true amount of gain (Ackermann) or loss (Reverse Ackermann) in toe. Toe plates that read in degrees are not accurate enough for this purpose and may slip as the wheels are turned.
The use of a quality laser alignment system will do the same thing as using strings or toe bars, only more accurately. Given the importance of having correct toe gain from steering input, investing in a laser system would be considered a reasonable investment that would provide a sizable return if a problem were present and eliminated.
If your car gains or loses toe, there are a couple of ways to correct the situation. You can adjust the length of your steering arms to compensate for Ackermann effect. Lengthening the left steering arm will reduce the amount that the wheel turns, which reduces Ackermann effect. The opposite is true for the right steering arm-we would need to shorten it in order to reduce Ackermann. We can also change our drag link to move the inner ends of the tie rods forward to reduce Ackermann, or rearward to reduce Reverse Ackermann effect.
If your spindles were not designed for your steering system, change to the correct spindle design and possibly have some lightweight spindles fabricated to the exact specifications as the correct spindles.
Caution: Do not make spindle changes without knowing how the change will affect your moment-center location. You may be making a positive change in your steering system and a negative change in the moment-center design.
Make sure you know how much each of your tires is steering, and balance your setup so that both front tires are working together to steer your car. A good-turning race car is one that will have more turning power and is therefore more capable of running up front and winning races.