Rear stagger is the difference...
Rear stagger is the difference in circumference of the two rear tires. The left-rear tire must be smaller than the right-rear tire on an oval track where we turn left. That is required so that the rear end, as well as the front end, will track correctly and follow the racetrack instead of steering left or right. Too little stagger and the car will drive to the outside wall. Too much stagger and the car will point to the inside of the track and usually be very loose.
Some cars running on smaller racetracks will need small amounts of Ackermann. Remember that one degree of Ackermann, meaning that the left wheel turns a degree more than the right wheel at mid-turn steering angles, represents a full 1/2-inch of toe. If we start with 1/8-inch of toe and add another 1/2-inch, we would have a staggering 5/8 inches of toe at mid-turn, which is too much.
On the smaller tracks, we could stand to gain 1/8- to 1/4-inch of toe to compensate for the small radius turns. We are causing the front tires to work against each other when we run more than that.
Insufficient rear tire stagger will cause the car to "point" towards the outside wall on exit. There is a correct amount of rear stagger for each track based on the overall tire diameters, track width of the rear tires, the radius of the track, and the track banking angle.
Too little stagger will cause the car to drive to the right as we get back into the throttle and the rear end moves on an arc that has a greater radius than the track at that point. The larger radius, if drawn onto the racetrack, would lead into the grandstands and that is definitely not where we want to go.
The rear end should be aligned...
The rear end should be aligned so that it is perpendicular to the centerline of the chassis and where the right-rear wheel centerline is inline with the right-front wheel centerline. The car can stand a small amount of rear steer to the left on asphalt and a greater amount of rear steer to the left and right for dirt applications where track conditions dictate those extreme designs.
Another setup crutch is the use of excess brake bias at either end of the car to help solve a handling problem. If a car is tight or loose on entry, many drivers have learned that if they make changes to the brake bias, they can improve the handling and performance at that one point on the racetrack. While that is possible, it can lead to problems at other sections of the turn. Using the old trial and error routine, the driver will say the car feels better on entry with the different brake bias, but now wants to work on the mid-turn handling balance, which has gone bad.
So, more changes will need to be made to attempt to solve other newly created problems where those problems did not exist before. It's not hard to see that using a brake bias crutch to solve one problem can lead to more and more problems. Soon, the team is overcome with a complicated series of handling problems while the car is getting slower and slower.
We know the rear end should be aligned perpendicular to the car's centerline and inline with the right-side tires. If the car is tight, some teams will move the right-rear wheel back to help "free up" the car. Steering the rear end either by static alignment or by invoking rear steer to the right will definitely help make the car less tight, but at the expense of exit performance.
If the car is loose, a team might move the right-rear wheel forward or design more rear steer into the left to help tighten the car, but more often than not, the alignment will make the car too tight especially on exit off the corners.
There is an optimum amount...
There is an optimum amount of stagger required for each car running at each racetrack that has more to do with the radius that the car will drive through on exit under acceleration than maybe the mid-turn radius. We should never crutch the car by using more or less stagger than would be required to roll the rear end through that portion of the track without it turning left or right.
The use of the excess steering input or abnormal brake bias or any other of these crutches are indications that other problems exist and need to be fixed. What the car needs is an arrangement of spring rate layout, front geometry design (including moment center placement), Panhard bar height (rear moment center height), proper steering characteristics, correct weight distribution, and tire stagger, that will all work in combination to provide a fast and balanced setup. The last things to work with are finding the correct shock rates for optimum entry and exit transitional handling, the correct front-tire cambers, and the correct tire pressures, the latter two being adjusted based on the tire temperature readings.
Once all of these areas of setup are correctly identified and made to work together, the car will give the team what it wants, a fast and consistent handling package that the driver can use to win races.
When you identify your driving and set-up crutches and find the best fix for your handling problems, you and your crew will be more successful and have a lot more fun at the racetrack.
Coming Next Month: Camber Crutches.