An excess of Ackermann steering...
An excess of Ackermann steering effect would not be noticeable on the closest car because only one front tire is touching the track. The back car would definitely be affected as both front tires are firmly on the ground.
The articles I most enjoy writing are the ones that make the most performance difference. I have included Ackermann information in my general alignment articles in the past and have dedicated whole ones to the Ackermann effect. You might also have noticed that I will repeat myself every two years or so, or sooner if the need arises. This is one of those times.
The reason I tend to harp on certain setup phenomenon is because of several things including the fact that we have new racers coming into the sport all the time, new readership regardless of the time spent in racing, and because some of us need reminding from time to time. You might have gotten the message, but you may move to another team and it might not have gotten the message. It's helpful in your argument for certain processes to refer to a recent article, any article, in Circle Track.
Ackermann is important because it can ruin an otherwise great setup. Knowledge of it is important for all types of auto racing, not just circle track. We have readers from all walks of racing life. We have confirmed reports of our magazine ending up in the bathrooms of Sprint Cup teams too numerous to mention in the limited space we have here. Sprint Cars, dirt cars, asphalt Late Models, SCCA cars, Formula cars, IndyCars, and Formula 1 cars all must be concerned with the effect of Ackermann, and they certainly are.
In this day and age, for both the dirt racers and the asphalt teams, modern setups dictate a closer look at many areas of chassis geometry and alignment. If we prioritize the various areas of concern, Ackermann would rank right up there near the top. More importantly now, modern setup trends in both dirt and asphalt racing dictate that we need to take a closer look at our Ackermann situation.
Years ago it was fairly common to see a dirt car with the left front tire up off the track in the turns or see tire temperatures on an asphalt car's left front tire that were the coolest of the four. These were the result of unbalanced setups where the rear suspension desired to roll much more than the front suspension. In those days, greater amounts of Ackermann could be desirable, or at least less harmful to our handling. If the LF tire is off the racing surface it can do no harm.
The History of Ackermann
Effect Ackermann effect is a mechanical phenomenon that is 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 do need a slight difference in steering angle between the front tires because the inside wheel runs on a smaller circle or arc than the outside wheel. The key word here is "slight."
Ackermann effect is named after the man who did extensive research and development on the subject. Early on in basic automotive design and development, engineers discovered the need to design a 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 radii turns.
An early story 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 steering helped keep the wheels tracking correctly and reduced the primary cause of rutting in the driveways. Modern racing stock cars have little in common with production cars, so we need to readdress the issue of Ackermann.
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. Were these articles correct about this subject? The answer is Yes and No. Here's 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 original intended use, primarily driving around the neighborhood. Converting the car to circle track racing was beyond its original intended use. So, it's easy to understand why some of the stock systems may not work very well on the racetrack.
Early crew chiefs didn't understand, nor had the technical knowledge to develop what we now know as a balanced setup. This is where, as we have explained many times in CT articles, both of 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 including helping all four tires to work harder, and providing consistency in the handling balance between tight and loose.
Since many cars in the past were not properly balanced, the left front tire usually carried less loading and did little work. This was evidenced by several indicators: 1) cool LF tire temperatures compared to the LR tire, 2) a need for a very stiff RF spring as the RF corner took most of the front load of the car in the turns, 3) Excess RF tire wear and heat, and for dirt, a LF tire that had no contact with the racing surface much of the time. If the LF tire had little or no load on it in the turns, then teams discovered that excess Ackermann actually helped the tire to generate more heat and turning effort when it was in contact with the track.
Modern Day Trends in Setup In today's racing world, the dirt cars are more balanced in their setups and the LF tire does much more work. This trend has made the dirt cars more consistent and faster under most conditions. With the asphalt teams, we see a move toward larger sway bars, softer springs, and stiffer LR springs. This arrangement causes the LF tire to be much more in contact with the racing surface, carry more loading, and to work harder than ever before. If the front tires don't track exactly where they should, there will be problems getting the car to turn.
When we have Ackermann effect present in our steering design, it means that the toe-out increases as the steering wheel is turned and with reverse Ackermann, toe is reduced. There are different static settings for front end 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 front end and keep it from darting back and forth across the track. Conventional wisdom tells us that the car will need more static toe-out for the smaller radius tracks. At big racetracks of more than a half-mile, less toe-out is required. The amount of toe-out used typically ranges from 1/16- to 1/4-inch.
The truth is, we need very little Ackermann effect in most situations when racing on an oval track, be it dirt or asphalt racing. Even on very tight quarter-mile tracks, the LF wheel will only need an additional 1/16-inch of toe over the RF wheel to correctly follow its smaller radius arc. That is 0.112 degrees or a little over 1/10 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 a 1/2 inch of toe for an 85-inch circumference tire. So, if we have 2 degrees of Ackermann in our steering systems that would equal 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 all of this points to the fact that we all need a correctly designed steering system, many racers and car builders may not fully understand the steering systems in their cars and how they work to produce or cancel Ackermann. Here is an explanation of how it works.
Ackermann is actually the...
Ackermann is actually the increase in toe-out of the front tires when the wheels are turned. To measure toe, we measure the difference in distance between the front of the tires and the rear of the tires and subtract. This car has toe-out, meaning the front width of the tires is wider than the rear. If we subtract the rear (80 1/8 inches) from the front (80 3/8 inches) we see that this car has 1/4-inch of toe-out.
You have seen this sketch...
You have seen this sketch before, but really study it. I did some very accurate calculations that were made using a commercial coordinate geometry software program to see just how much Ackermann is really needed. The results indicate that we do not need whole degrees of Ackermann, but decimals of degrees.
The 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 sterring 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. The stock-type spindles with top view angled steering arms were intended to be used on the stock based drag link steering systems. When racers started using the lightweight rack spindles with straight steering arms on the drag link system, it produced a considerable amount of Ackermann effect and caused a considerable gain in toe when the steering wheel was turned.
What causes Ackermann? There are several different ways that your car could be producing excessive Ackermann effect. The most common is when we install the wrong spindles or other steering system components on our car.
Over the last few years, teams and car builders have worked hard to reduce the unsprung weight, or the weight of the wheel/spindle assembly. I'm not sure why. One way to accomplish this was to install a lighter spindle.
At first car builders began using smaller, stock compact car spindles on the car's front end that were designed for the drag link steering system. At the same time, custom spindles were being fabricated for the newer design and very 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-1990s, 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 that were intended to be used on the rack system. The result was a steering system that produced excessive Ackermann effect. This hurt the turning performance on those cars.
A very few racers figured out what the problem was, corrected it with different length steering arms, and dominated racing during that period because their cars turned better than the competition. Today, with our better understanding of how Ackermann works and what amounts our cars really need, we can measure Ackermann and correctly adjust out any excess 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 Check For Ackermann Effect There are a few ways to check for excess Ackermann in our race cars. The best way is to use a laser alignment system to measure how much each front wheel turns and compare the two. The laser system can also be used for rear-end alignment, right-side tire alignment, and bumpsteer.
A less expensive, but adequately accurate method is to use strings to measure your Ackermann. I have used this method and, if done carefully, it will yield the results we are looking for. Almost everyone has used strings to align a race car. A string pulled tight is always straight, we can count on that. So, if we pull a tight string across the outside of each front tire sidewall and extend the string to the front 10 feet, we can take the measurements necessary to see how much Ackermann we have.
The procedure is as follows: 1. put the front wheels straight ahead. 2. pull a string across the outside of each front tire (avoid the lettering portions of the sidewall) and place a mark on the floor (on a piece of masking tape) where straight ahead is. 3. turn the steering wheel approximately the same amount the driver would in the turns where you race. 4. again, make a mark on the floor at 10 feet where the string extends from the outside of each front tire. 5. measure the distance between each set of marks for each wheel and compare the left wheel with the right wheel.
The Ackermann Toe Chart (previous page) shows how much toe gain relates to differences in the left and right wheels for different size tires. We can average the left and right tire sizes and look at that number when finding our Ackermann on the chart. Remember that if the left wheel moves farther than the right wheel, then we have Ackermann, or toe gain. If the right wheel moves more than the left wheel then you have reverse Ackermann or loss of toe.
A drag link system is correctly...
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 cancelled. 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. The inner tie-rod ends move forward with the drag link. 5. The outer tie-rod ends (at the forward ends of the steering arms) are forced wider apart as both tie rods become less angled from a top view creating additional toe-out. The angled steering arms produce a reverse-Ackermann effect which serves to cancel the system's attempt to spread out the wheels. So, the system shown can produce very little Ackermann effect.
Modern day setups for both...
Modern day setups for both dirt and asphalt result in the left front tire carrying more load whereby the negative affects of excess Ackermann are more pronounced. The more load a tire carries, the more work it does. If the two front tires are pointed in opposing directions, then they will fight each other and the result is less front grip and a push that can't be corrected with usual setup changes.
A very simple yet accurate...
A very 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 again at "wheels turned" positions. The difference in distances between the right and left tell us whether we have excess Ackermann or not.
Solving the Excess Ackermann Problem If your car gains or loses toe, there are a couple of ways to correct the situation. You can adjust the length of one or both of your steering arms to compensate for Ackermann effect. This works best for a car where the steering wheel is always turned to the left as opposed to a dirt car that sometimes has the wheels turned to the right. Lengthening the left steering arm will reduce the amount that wheel turns with a measured steering input, 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 tire rods forward to reduce Ackermann or rearward to increase it or 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 ones fabricated to the exact specifications as the correct ones. I did that way back in 1997 with a brand-new car.
Our old car, which won a Regional NASCAR championship, had the old cast spindles and the new one had the rack light spindles. The new car did not turn well. We replaced the light spindles with the old-style cast spindles and the problem went away. We then asked the car builder to fabricate new ones to the same dimensions as the old style cast ones.
He asked, "Why?" We said never mind, we were paying him, just do it. A few weeks later, a new car arrived at one of the same car builder's dealers in Nashville with spindles identical to the ones we had fabricated. Coincidentally, I was there to help with a seminar and recognized the copy.
For a rack-and-pinion steering system, moving the rack forward in relation to the outer tie-rod ends will reduce Ackermann. Most Dirt Late Model cars use the rack systems. So, we don't have the convenience of only having to improve our Ackermann effect in one direction, it must be correct for left or right turning of the wheels.
Asphalt Late Model cars are also designed with rack systems. Instead of changing the length of the steering arms, it might be best to move the rack and keep equal length steering arms when working to reduce excess Ackermann.
A 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. This problem relates to spindle height differences where the upper and/or lower ball joints will change height with a spindle change. This changes the upper and/or lower arm angles and along with that the moment center location.
Make sure you know how much each of your tires are steering and reduce the Ackermann effect if needed. Then, when you balance your setup, both front tires will be working in perfect alignment to steer your car. A good steering race car is one that will have more turning power and is therefore more capable of running up front and winning races.
The chart shows how much difference...
The chart shows how much difference in turning distance at 10 feet relates to toe. For an 86-inch circumference tire, a 1 1/2-inch difference between right and left wheel movement at 10 feet out results in 0.342 inches of additional toe. When doing this test, make sure which side turns more. If the left wheel turns more than the right wheel you have Ackermann. If the right turns more, you have reverse Ackermann and the car is then losing toe.
In a Dirt Late Model or an...
In a Dirt Late Model or an Asphalt Late Model with rack-and-pinion steering, moving the rack fore and aft changes the Ackermann effect. This method of eliminating excessive effect is much better than changing one or both steering arm lengths and ending up with unequal lengths. Most manufacturers mount a plate for the rack that is slotted to allow for fore and aft movement.
This spindle has a slotted...
This spindle has a slotted hole where the tie-rod end mounts to the steering arm. This allows the team to adjust the length of the steering arm to control the amount of Ackermann effect.