I’ve heard that a few of our readers would like to know a little more about things like camber and toe, and the effects the basic suspension settings have on vehicle stability and control. Before reading this, keep in mind that the optimal setup for any combination of car and road can vary a lot. This is just a guide to help understand what three settings do:
- Camber
- Toe
- Caster
There are many more variables in the suspension setup, but these three seem to be the most easily changed, and have the largest effect when tuning the car.
Camber is the angle of the wheels from vertical when viewed from the front. Negative camber means the top of the wheels is closer to the center of the car than the bottom. Positive is the opposite, with the top of the wheel further away than the bottom. The measurement is degrees off from vertical.
Usually the suspension in a car is designed to decrease camber as the suspension compresses. This way, when the body rolls as it goes through a hard corner, the outside suspension compresses and pulls the top of the wheel in, the inside decompresses and pushes the top of the wheel out, counteracting the roll from the body, keeping the tire closer to perpendicular with the road.
The main idea behind changing the camber angle is to maximize the tire’s contact patch for when you need it most. Typically it is set slightly negative to maximize traction during hard cornering. The downside is less traction when traveling in a straight line.
Positive camber causes more wear on the outside edge of the tire, while negative camber causes more wear on the inside edge of the tire.
Toe is the angle between the wheels and the car’s centerline when viewed from above or below. Toe-in means the tires point inwards, ie front of the tires are closer to the car’s centerline than the rear of the tires. Toe-out is opposite, with the front of the tires out and the rear in. The measurement is degrees off from parallel with the car’s centerline.
Toe mostly affects straight line stability and turn-in response. Toe-in improves straight line stability, negating the effects of things like surface irregularity, bumps, crosswind, and generally makes the car want to travel in a straight line.
The downside of this is that the turn-in response is reduced. Consider that the inside tires must travel through a smaller radius when turning than the outer tires. When turning with toe-in, the inside front tire will have a smaller angle of turn than the outside tire, meaning that it wants to go through a larger radius, and is fighting against the outside tire during a turn. As the weight is transferred to the outside tire, the effects of the inside is reduced.
Conversely, with toe-out, the car will be unstable at high speeds, anything that transfers weight to one side of the car will make the car want to turn in that direction because the tire is pointed outward. Keeping this in mind, it seems a contradiction that toe-out improves steering response. Remember what I mentioned before about the inner and outer tire’s turning radii. With toe-out, the inside tire tries to turn a tighter turn than the outside tire, which is exactly what we want. This way, the tires aren’t fighting against each other until the weight transfers to one side.
However, just like camber, any toe away from 0º increases wear on the tires; Toe-in causes more wear on the outside edge of the tire and toe-out causes more wear on the inside edge of the tire.
Caster is slightly more difficult conceptually, and it only applies to the steering wheels. The angle between the axis upon which the wheel turns and vertical is caster. The best example I can think of is a bicycle. The front wheel rotates about an axis that is not vertical, but is angled so that the axis of rotation is in front of the contact patch. When viewed from the side, positive caster means this axis of rotation is tilted backwards, the top is towards the rear of the car and the bottom is forward. Negative camber is when this axis is tilted forward.
What does this do? Well, when the contact patch is behind the steering axis (Positive caster), the wheels want to travel in a straight line, and will have a tendency to center when turning. As you would expect, the opposite is true with a negative caster, the wheels want to turn away from going straight and more in the direction that they are currently turning.
Negative caster was used a lot back in the 70’s and earlier to make the feel of the steering lighter, since less force is needed to turn if the wheels want to go in that direction. The problem there is that negative caster gives some instability when going in a straight line.
Almost all modern cars have positive caster to improve stability and ease of driving at speed. Although the steering wheel will be more difficult to turn, power steering helps that.
August 15th, 2008 |
