Selecting tire sizes and predicting changes in stagger is somewhat a black art. The top teams always have a designated person who selects, sizes, and manages the tires. It is a very important job. If you talk to a dozen of these tire gurus, you will probably get different opinions on the procedure. But, you will also find critical similarities in each ones process. The following are important points to consider when working with your race tires.
Over-inflation WarningStagger is measured with the tires inflated to operating pressures. Safety Note: Some teams will stretch the tires by inflating them to very high pressures. DO NOT do this, ever. It is unnecessary and potentially dangerous. Team members have been seriously injured doing this.
Match Stagger to the RacetrackOne of the most important considerations to know is that rear stagger should match the racetrack. Using more or less stagger than the track requires will ruin your handling. If it does help at all, it is usually a crutch and a more correct solution to your handling problems should be found. A racetrack with a certain turn radius (average radius of both turns) will require a certain rear stagger amount. That number represents the difference in circumference of the two tires, the left being the smaller one, that would result in both tires turning the same number of revolutions through the middle of turns. This number should not change. The chart at the top of page 40 shows typical stagger amounts for different types of racetracks.
Type of Rear DifferentialStagger influences a car using a spool (both axles locked together all of the time) rear differential on entry to the turns as well as through the middle and off the corners. With a Detroit Locker type of rear differential, the rear wheels are unlocked when entering the turns. The Detroit Locker will lock both rear wheels under acceleration and mostly affects handling off the corners.
Less StaggerToo little rear stagger will try to turn the car toward the outside wall and cause a push on corner-exit. That is because the front wants to go that way (toward the wall) anyhow due to centripetal forces, and a little help in that direction will quickly develop into a serious push.
More StaggerExcess stagger will usually show up as a loose condition because the rear tires are not able to turn the car to the inside of the track, and so one or both rear tires will slip, causing loss of traction.
Important ConsiderationsCorrect stagger is a must. To arrive at the best tire sizes, we need to consider: 1) The correct stagger amount for the track we are running, 2) How much each tire might grow and how soon, and 3) How to quickly solve a stagger problem using the four tires on the car.
Correct StaggerThis is the amount of stagger that will produce equal revolutions of the rear wheels when driving around the turns. This factor is not a consideration at the front because the tires are not connected by an axle like the rear tires are.
Front StaggerA definite consideration for front stagger is stability under braking. Because there is a lot of weight transfer to the front under braking, the front tires and brakes work harder than the rears. If both wheels are turning the same number of revolutions, then the car will brake mostly in line with the direction the car is traveling.
If the left-front wheel is turning faster (i.e. a smaller tire circumference and therefore more stagger) than required, the car may pull to the left on entry under hard braking. If the front stagger is too little, the car may not pivot as needed and develop a push on entry.
The general rule is to maintain a front stagger amount that is close to the correct stagger needed for equal revolutions of the wheels.
Anticipating Tire GrowthPart of the art of tire stagger maintenance is being able to predict how much each tire will grow due to increases in pressure and temperature. As a rule, one or both of the right-side tires will grow more than anticipated. But because tire growth is mostly influenced by air pressure increase and heat (abuse), it is possible that the left-rear could grow more than the right-rear tire.
Example: If a car has too little stagger in the rear, but turns well enough to overcome the tendency to push, then the left-rear tire will slip and heat up. That is because, in most cases, the right-rear tire is the most heavily loaded tire at mid-turn and will not slip. One of the rear tires must slip, so it ends up being the left-rear.
Handling Affects StaggerMost of the time, tight or loose handling will determine which of the right-side tires will grow the most. A severe push will heat up the right-front tire and cause it to grow beyond normal. The rear tires may maintain correct stagger because neither of the rear tires is being abused. A much more common problem is the tight/loose syndrome. That is when the car is somewhat tight, but with increased steering input, the car reverses handling balance at mid-turn and goes loose off the corner. The right-rear tire suffers abuse as it slips and gains heat and pressure. Stagger will grow as a result.
Predicting stagger growth is mostly dependent on predicting the ultimate handling balance of the car. The easiest way to prevent stagger changes is to balance the setup in the car in the first place. We can see from all of this that stagger changes can be an indication of basic setup problems. Small changes in stagger (a ¼-inch or so) can occur due to slight imbalances in the setup of the car. This is where front stagger selection can help us if we need to make stagger changes at the rear.
If we pick two right-side tires, air them up to 30 pounds each, and one of the tires grows more than the other, we would probably put that tire on the right front. That is because excess stagger at the front will cause less adverse effects on handling than having incorrect stagger at the rear.
It is not usual to have same-size tires grow differently just from airing up. Mostly, tires change size during hot laps. If we choose the tire sizes in a certain order, we can make adjustments to stagger by switching the same-side tires.
If we size the four tires like the diagram (at left), we can adjust for either a growing left-rear or right-rear tire. These changes are meant for unusual growth of the tires due to handling problems and not due to individual tire construction differences.
Stagger Method ConstantsBalance your setup to help maintain correct stagger. Check tire pressures and sizes often. Get to know how your tires might grow under racing conditions so you can choose the correct starting sizes for each tire. Be among the first teams to the tire truck so you will have a better chance of finding the tire sizes you will need. Designate one or two tire specialists for your team to work with tires every week. Never let an inexperienced person air up and size your tires. Someone could get hurt. CT