If you can make the tires happy, you will automatically make the driver happy. Aiming all four tires in the right directions will optimize their performance and make them happy in the process. Cross weight, camber, toe and tire pressure are the primary alignment adjustments that can produce happy tires. That is what racing suspension alignment is all about. But how do you learn what the right adjustments are? If you look, listen, and measure what the tires on your car are trying to tell you, you can learn everything that you need to know about how to tune your alignment. That knowledge can guide the process of improving the suspension alignment of your racing machine, and every detail is specific to your circumstances.
The complexity of independent suspension gives you numerous adjustments, and that also means that you need a way to measure and adjust each one. When you can measure your alignment accurately, you can adjust it with confidence. To start with, make sure that the big-picture basics of your car are right. Don’t assume that anything is right on the car you just bought, or if you just had a hard thump against something. Make sure that the body is not twisted, the wheelbase is the same on both sides, and the subframes are not offset or skewed. Careful measuring of heights and plumb bob marks on the floor will reveal those things.
CAMBER AND PRESSURE
The goal of camber and tire-pressure adjustments is to get as close as possible to the same contact patch pressure against the pavement across the width of each contact patch. This maximizes the cornering grip that can be produced by that tire. It is practically impossible to directly measure contact patch pressure, but you can measure the temperature of the tread. Tread temperature varies with the amount of work that each portion of the tread is doing. This variation of temperature directly correlates with contact-patch pressure. The time-honored method of measuring the variation of temperature across the width of the contact patch is a needle probe pyrometer. There are three significant disadvantages to this method.
First, those measurements can only be made after the car stops in pit lane, so the tires have cooled somewhat. Second, the temperatures that are measured represent the combined effects of the most recent several corners, braking zones, and acceleration zones. Third, interpreting the temperature spread requires a guess or unverifiable guidance about the optimum temperature spread across the tire. Some racing folklore calls for a 40-degree spread as the optimum, with the inboard edge hotter than the outboard. Other bits of guidance call for other spreads or equal temperatures across the tread.
Those days have passed for one of the most significant effects on race car performance. For the cost of one or two sets of tires, you can outfit your car with a data logger and six infrared tire temperature sensors. Locate three sensors aimed at different locations across one front tread, and three across one rear tread. The data from those sensors will tell you how hard each portion of each tire is working while you are cornering, braking, and accelerating. That is the information that will guide the true optimum camber and pressure adjustments for your race car and tires. If the center of the tread is hotter than the sides, that tire has too much pressure, and vice versa. If the outboard part of the tread is hotter than the inboard part, the tire needs more static negative camber, and vice versa.
There are two alignment adjustments that are important for racing, but they are rarely found in a factory maintenance manual. The static roll angle of your car is called tilt, and it is one of the two uncommon racing adjustments. Tilt results from a difference in left- and right-side ride heights. It’s different from rake, which is the difference in ride height between the front and rear.
Zero tilt is the goal for cars that turn both left and right equally. It is easy for the tilt angle to drift away from zero while you are making ride height and cross weight adjustments. Find a level place on your frame, firewall, or floor pan that you can set a bubble level so that you can monitor and adjust the tilt angle. If you discover that the tilt angle is not zero, it will take a lot of work to reset all the other alignment adjustments back to where you want them after you zero the tilt. So, set that first, check it often, and reset it back to zero as soon as it moves. That will save you a lot of extra work later.
The other uncommon suspension adjustment that is important for racing is bump steer. “Communication Theory” in the October 2013 issue of Speed News covers this topic. That article describes how to measure bump steer, and how to make suspension adjustments to minimize it. While tilt is almost the first alignment adjustment that you should make, bump steer is almost the last one in the order of adjustments. That is not because it is the least important. Rather, it is because there is a strong interaction between caster adjustment and bump steer.
That is just one example of the interactions that are typical of suspension alignment adjustments. When you make some adjustments, they will affect other adjustments that you don’t want to change. This makes the process of fine tuning your alignment an iterative process. Here is the order of adjustments that will minimize the amount of repetition that you are in for:
- Geometry package, springs, and other configuration options
- Hot tire pressures
- Ride heights
- Ride heights
- Cross weight
- Bump steer
- Zero and attach anti-roll bars
LIVE AXLE ALIGNMENT
Normal live axles are carefully designed and manufactured to locate the differential axis, the axle shafts, and the wheel bearings all on the same centerline. Like many other racing-optimized assemblies, racing axles are different. They do not have to be straight, and there are advantages to a bent axle. It is possible to take advantage of the modest flexibility of the axle shafts, or barrel-shaped splines on their inboard ends, to build an axle with camber, toe, or a combination of both. Either of these angles can improve rear axle grip, but the angles are set when the axle is manufactured and they are very expensive to change. A car that competes on courses with tight corners will benefit more from rear toe in than negative camber. Conversely, a car that competes on tracks with large corners will benefit more from negative camber. The sidebar explains why these choices are preferable.
THE DEVELOPMENT PROCESS
There are two keys to getting the most from your alignment development process. The first is being systematic. Start by writing down every detail of your current setup. Only make one change at a time, and then write down the result of that change. Make your next change based on carefully reasoned logic and a review of your post-session notes for your last few outings. The other key to a successful development process is making one handling adjustment every time the car comes to a stop, without exception. That makes every outing a step in your car’s development process. Track time is expensive, so you might as well learn something from every opportunity you get. Optimizing a race car is a long process, and it is one of the most important competitive elements of racing. If you do a better job of developing your car than your competitors, they will end up behind you on the track.
All the back issues of Speed News are freely available from the NASA website, so detailed articles on many suspension related topics are just a click or two away. That is a great compilation of racing knowledge, and each article covers one element of the racing suspension development challenge. For example, “Damper Tuning” is in the September 2013 issue. “Weight Matters” in the August 2012 issue of Speed News shows you how to make a wheel scale and setup pad on a shoestring budget. It also shows a cheap and simple toe string system so that you can measure toe settings. That will let you make adjustments or repairs at the track with confidence. A much more thorough compilation of motorsports engineering and tuning advice is in my book, called Think Fast – The Racer’s Why-To Guide to Winning.
Developing the suspension alignment of a race car involves a lot of wrench time, precise measurements, sound logic, thorough and precise documentation of your adjustments, and a constant focus on learning from the messages your tires are sending to you. This focus enables you to implement a hugely important principle: Don’t guess when you can know. After you have homed in on the optimum alignment adjustments, your tires will be happier, and so will you.
TURN RADIUS EFFECTS ON TOE SETTINGS
Have you ever wondered why competitive racing suspension alignment has front toe out and rear toe in? This is a result of two effects. The first is a simple consequence of geometry. That each tire is in a different place relative to the turn center produces a slip angle on each inboard tire that is different from the outboard tire on that axle. The second effect is camber thrust. When a tire rolls with a camber angle and a load on it, the tire produces a lateral force toward the direction that the tire is cambered. A pair of tires on a front or rear axle with negative camber has effective toe in because of camber thrust. Static toe out reduces this effective toe in, while static toe in increases it.
Dynamic toe-in is defined as the inside tire slip angle being smaller than the outside tire slip angle. Conversely, dynamic toe-out is defined as the inside tire slip angle being larger than the outside tire slip angle. The usual static front toe-out and rear toe-in settings shift the dynamic slip angles closer to being equal, which produces more cornering grip.
The dynamic toe angles change with the turn radius, steering angle, Ackermann percentage, slip angles, camber thrust and cornering balance. This illustration shows the effect of varying the turn radius, with all other variables held constant.