Track Alignment: How It Is Different and Why It Matters (Part 1)

Chapter 1. Caster Angle

Every car that’s born to be driven deserves a proper track alignment. Our technicians are seasoned drivers on the racetrack themselves. With engineering background and working experience in professional race teams, they understand how each parameter on your car corresponds to specific handling characteristics and also know that every driver has their own driving style. Track alignment is just the tip of the iceberg in our knowledge base.

For track driving, maximizing the tire contact patch means everything. Today, as the first chapter of a long series of blogs, let's delve into a detailed analysis of how caster angle affects your car’s handling and feel.

Caster Angle & Caster Trail, Picture courtesy of "Race Car Design" by Derek Seward

When looking from the side of a vehicle, caster angle, as depicted in this figure above, is the angle between the vertical line drawn through the wheel center and an imaginary line that represents the steering axis. On a typical family car with MacPherson strut front suspension, this line is defined by the strut top and the lower ball joint. On many sports and race cars with double-wishbone front suspension (such as every generation of the Mazda Miata), it is a line drawn through the upper and lower ball joints. If you extend the two lines all the way to road level and draw two points exactly where they intersect with the road, the distance between the two points forms what is called the caster trail. Since the center of the tire contact patch is behind the point where the imaginary steering axis contacts the ground, this moment arm causes the steering wheel to self-center after being turned. Many drivers understand that increasing the caster angle can lead to a heavier steering feel, which allows easier and more precise steering correction. But is that all there is to it?

One benefit of having a larger caster angle is that as the car turns in, the more heavily-loaded outer wheel gains negative camber, which generates more lateral force through camber thrust, allowing the car to corner at a higher speed. In the era when racing cars had no power steering, the saying of “add caster to go faster” often holds true, as the main drawback of adding more caster was driver fatigue.

On this basis, you might think increasing the caster angle is always a good thing. However, as with many things in motorsports, having more often doesn’t mean better, and balance is the key. 

One caveat of running significant caster angle (especially when combined with king pin inclination) is that it aggravates the effect of wheel lift. To put it simply, when cornering, the lightly-loaded front inner tire gets pushed downward by the steering geometry, and therefore, the lateral weight transfer across the front axle gets reduced while it increases on the rear axle. Due to the tire’s vertical load sensitivity, the front grip improves while the rear grip worsens. As a result, the balance of the vehicle shifts towards oversteer as the car turns in.

The other downside of having a very large caster angle on your car is that the caster trail (as discussed earlier) now forms a significant portion of the self-centering moment. Therefore, it becomes more difficult to read the effect of the tire’s pneumatic trail through the steering wheel. Pneumatic trail is a topic we will get into very soon, but for now all you need to know is that it communicates the understeering feel to the driver through the steering feedback, and running a large caster angle often overshadows it. 

On many cars that utilize eccentric bolts to provide camber & caster adjustment, there is a limit to how much overall camber & caster you can achieve. Many modern cars have electronic power steering systems that’re calibrated for daily driving. The side effect of that is the steering wheel now feels very muted and doesn’t take much effort to turn. If you prefer having a heavier steering feel, you may have to turn up the caster and sacrifice the front static camber a little. Combining that with the other issues we discussed earlier, you can see that balancing the two metrics and optimizing the on-track performance becomes a bit of art in itself.