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.