In the last article, I talked about the uses of toe and how it affects both drivability and front-end dynamics. However, that is only one of the three basic geometric ways to move a tire on a race vehicle. Of the other two, camber is well-recognized, but for all intents and purposes, the least-understood of the three, caster, is sometimes the most overlooked about setups.
Everyone who has pushed a shopping cart understands caster. Caster wheels, when placed on heavy objects that need rolling, allow the object to maintain momentum in a straight line. In other words, once it gets going, it likes to keep going. Think of it as a way to love Newton’s First Law of Motion: an object in motion tends to stay in motion. Newton would’ve loved caster wheels.
Caster helps the old lady push her three bags of cat litter through the grocery aisle without breaking a hip. It also allows me to move 55-gallon steel tanks of liquid nitrogen with two fingers. (Getting it started is a two-handed requirement.) However, what we know as caster wheels are just half the trees of the proverbial forest.
So what does caster do? Well, to understand why something works, it’s good to know how it works. Caster is a measurement of how far “pulled back” a wheel is from its center. It’s measured in the same degrees found on a circle, and the more the wheel is pulled back, the more degrees of caster that wheel has. Conversely, if you can pull a wheel back (+ caster), you can also push a wheel forward (- caster).
So how does that translate to what you feel? Think about what caster does to wheelbase. Positive caster shortens wheelbase, and negative caster lengthens wheelbase. Custom-chopped motorcycles have an extreme amount of negative caster because they lengthen the wheelbase from neutral. Ask anyone who drives a motorcycle, and they’ll tell you that the steering on these bikes is very, very sensitive and does not correct itself. However, on a shopping cart that has a lot of positive caster, if you turn the cart and let go suddenly, the wheels will correct the line of travel and allow the cart to more naturally travel in a straight line.
Shorter wheelbase cars can turn much easier. This is why if you look in the street stock pits at your local bullring, most of the cars are shorter wheelbase models, like vintage Camaros. The late ’70s model Camaro was (and still is) an extremely popular racing car simply because its wheelbase is the shortest allowed by many local rules. It’s so ubiquitious around the country that iRacing based its street stock on this car model!
So wouldn’t it serve best to pull both wheels back to maximum caster and shorten the wheelbase? Well, yes and no. Caster feels different to a driver than what a setup may show, and most of it has to do with the camber and toe settings. However, what most drivers feel when they say “caster” is actually a “caster split.” Think of it this way: maximum caster split would be the LF hub pulled back as far as possible, and the RF hub pushed forward as far as possible, causing the shortest wheelbase on the left side, and the longest wheelbase on the right side. So what does this do to the feel of a car in the corner? Well, try it out in a test session. You’re going to find the left side of the car is going to “pull” the chassis in the corner nicely, but it’s also going to turn a little too nicely off the corner as well as the front tires fight each other to gain grip!
Caster split is a mean devil. It can cause massive headaches with setups, but it can also be just the thing you need to make your setup even better.
Usually, drivers have a feel for a certain caster setting, and leave their setups alone in this regard. Dale Earnhardt, Sr. was known to prefer a higher caster split, and I’m sure there are top-tier drivers from all backgrounds who prefer different caster settings. Although caster does make an impact upon dynamic toe and camber, it is often adjusted according to driver feel.
So then why do nearly all circle-track setups have a LF wheel at negative caster and a RF wheel at positive caster? It has to do with the self-straightening tendencies of positive caster and self-turning tendencies of negative caster. As the car is cornering and the tires are under stress, the LF will want to turn itself MORE (because of the increased toe-out) and the RF will want to straighten itself (toe-in). If you can see this in your mind, the caster is actually working to toe the front-end around the corner by itself!
So, in short:
Positive caster in both front wheels will make the car slightly unresponsive in the steering, and will heed less to setup changes (less dynamic toe-out).
Negative caster in both front wheels will make the car feel responsive, more “touchy” and sensitive to track changes and tire temperatures (more dynamic toe-in).
Positive caster split (LF more + than RF) will make the car turn-in easily, and turn-out easily. Drivers complain of too much caster split as having no control with the steering wheel, and the car wants to fight itself into and out of each left-turn.
Negative caster split (LF more – than RF) will make the car turn less easily in the corners, but drivers will feel like they have significantly more control with the wheel; less “slop” in the steering. The dynamic toe will also be helping the chassis to turn throughout all parts of the corner.
Understanding caster is just one more trick to have in the bag when it comes to finding that extra tenth or that comfort level in the setup. After all, confidence makes you faster, right?