So after almost a year of starting this blog, and having gained 1200 page views, I thought it I would eat my own words. I started my first blog saying this would not be a serious blog about technicalities of a car, but considering I am spending the majority of my time working with camber and caster, I thought I would share some of that here.
In my opinion (for what its worth) the car's handling can be divided into two aspects. Firstly, there is the mechanical action of the car; the way the car reacts to driving, how the wheels and tyres move, distort, and where they point at which point in time. Don't worry, I'll explain that in more detail. Secondly, there is the feedback that the car gives to its driver. The weight of the steering, and the self-centering effect, the ability to estimate grip at all four corners mid corner, those things. You will notice that auto-journos are forever banging on about feel and feedback. Its what separates competent cars from the great ones. The Volkswagen Golf from the Ford Focus, the Porsche Boxter from a Mercedes Benz SLK. Mathematically, two cars may be able to corner at identical speeds, generate identical accelerations and g-forces, but they may still have characters that are polar opposites. A good handling car is not merely the product of number crunching. It is the result of fine tuning and optimization. Compromise is the name of the game with vehicle dynamics, finding the perfect balance based on application.
Right, opinions out of the way, lets get down to the terms. What is Camber and what is Caster. Since they are geometric terms, they are best explained with pictures.
According to SAE, camber is the inclination of the wheel plane to the vertical. It is considered positive when the wheel leans outward at the top and negative when it leans inward. Now go to Wikipedia and search for wheel camber. There is a picture of a Formula car with insane amounts of Negative Camber on all four wheels. Now look for a picture of a old tractor. The front wheels will probably have positive camber.
As is demonstrated very conveniently by the Falken/Monster Energy Mustang going VERY sideways, the camber angle on most cars is not static. It changes with the steer angle of the wheel. This effect is caused by the static caster angle in the assembly. In a modern car each wheel rotates about an individual axis in three dimensional space. This steering axis is not usually perpendicular to the ground. It may be inclined in the longitudinal direction (caster) as well as the transverse direction (kingpin) with regard to the chassis.
The caster, as I mentioned earlier, makes the static camber angle change when the wheel is turned about the steering axis. This is obvious once you have seen it in action. But incase you haven't, try this out. Take a pencil or a pen. Cut out a circular piece of paper. Cut out two small rectangular strips of paper. On the circular paper, draw a diameter line. Attach the two strips such that their centers align with the line, and they are perpendicular to the line itself. This picture should explain better:
Apply glue to the areas shaded yellow. Now stick the pencil through the gaps in between. You should be able to freely rotate the circle. Assume that to be the wheel (hub more specifically). The pencil is your steering axis. If the diameter is perpendicular to the ground, the camber of the circle will not change as it rotates about the pencil. However if you tilt the pencil to the ground and then rotate, the camber will change with rotation. This is exactly what caster does on a car's suspension.
When a car is taking a corner at medium to high speeds, there is usually some amount of body roll present. Since the car has camber values of the tyre fixed with respect to the chassis, the body roll will cause the camber values to change by some amount. The amount of change in the camber and whether it increases or decreases depends entirely on the geometric point called the Front view instant center(FVIC). Essentially this is the point about which the wheel traces an arc when moving up or down. If this point is infinitely far from the wheel, then the arc will be a straight line. In that case the camber will remain constant throughout the motion of the wheel. While this is theoretically possible, in reality this rarely happens. Even if the FVIC is at infinity while the car is resting, once in motion, the FVIC may not remain at infinity. In a simple double wishbone setup, if the upper and lower arms are not of uniform length, then the wheel will still gain/lose camber as it travels. This will happen irrespective of the FVIC point at static. This camber change, however, is purely geometric. It is usually designed with a specific objective.
Normally it is preferred having some amount of negative camber, especially for medium to high speed corners. This is because of a phenomenon called camber thrust. I could explain what that is at this point, but then this blog will just become a text book. Instead, I'm just going to say that for less understeer through a corner, the wheels of the car should be cambered negatively on the outside and positively on the inside. Since most cars (with the exception of NASCARs perhaps) have to take turns in both directions, it is not desirable to have the static asymmetric camber. Instead, caster is used to change the camber angle when the wheels are steered.
So now that you have some idea of camber and caster, try observing different vehicles on the road. Most cars, and especially buses and trucks, have very small camber and caster values. They are usually not visible to human eye. However, many modern cars run high caster values on the front wheels. Try looking at the wheels of a Mercedes Benz E-Class when its taking a sharp turn at low speed. The extra camber of the inside wheel is easily visible. In the future, I might go into details of how camber and caster affect handling.
In my opinion (for what its worth) the car's handling can be divided into two aspects. Firstly, there is the mechanical action of the car; the way the car reacts to driving, how the wheels and tyres move, distort, and where they point at which point in time. Don't worry, I'll explain that in more detail. Secondly, there is the feedback that the car gives to its driver. The weight of the steering, and the self-centering effect, the ability to estimate grip at all four corners mid corner, those things. You will notice that auto-journos are forever banging on about feel and feedback. Its what separates competent cars from the great ones. The Volkswagen Golf from the Ford Focus, the Porsche Boxter from a Mercedes Benz SLK. Mathematically, two cars may be able to corner at identical speeds, generate identical accelerations and g-forces, but they may still have characters that are polar opposites. A good handling car is not merely the product of number crunching. It is the result of fine tuning and optimization. Compromise is the name of the game with vehicle dynamics, finding the perfect balance based on application.
Right, opinions out of the way, lets get down to the terms. What is Camber and what is Caster. Since they are geometric terms, they are best explained with pictures.
According to SAE, camber is the inclination of the wheel plane to the vertical. It is considered positive when the wheel leans outward at the top and negative when it leans inward. Now go to Wikipedia and search for wheel camber. There is a picture of a Formula car with insane amounts of Negative Camber on all four wheels. Now look for a picture of a old tractor. The front wheels will probably have positive camber.
 |
| Check out the massive negative camber on the inside wheel. The outside wheel has positive camber. This dynamic change of camber is caused by caster. Picture Courtesy www.dallasnews.com |
The caster, as I mentioned earlier, makes the static camber angle change when the wheel is turned about the steering axis. This is obvious once you have seen it in action. But incase you haven't, try this out. Take a pencil or a pen. Cut out a circular piece of paper. Cut out two small rectangular strips of paper. On the circular paper, draw a diameter line. Attach the two strips such that their centers align with the line, and they are perpendicular to the line itself. This picture should explain better:
When a car is taking a corner at medium to high speeds, there is usually some amount of body roll present. Since the car has camber values of the tyre fixed with respect to the chassis, the body roll will cause the camber values to change by some amount. The amount of change in the camber and whether it increases or decreases depends entirely on the geometric point called the Front view instant center(FVIC). Essentially this is the point about which the wheel traces an arc when moving up or down. If this point is infinitely far from the wheel, then the arc will be a straight line. In that case the camber will remain constant throughout the motion of the wheel. While this is theoretically possible, in reality this rarely happens. Even if the FVIC is at infinity while the car is resting, once in motion, the FVIC may not remain at infinity. In a simple double wishbone setup, if the upper and lower arms are not of uniform length, then the wheel will still gain/lose camber as it travels. This will happen irrespective of the FVIC point at static. This camber change, however, is purely geometric. It is usually designed with a specific objective.
Normally it is preferred having some amount of negative camber, especially for medium to high speed corners. This is because of a phenomenon called camber thrust. I could explain what that is at this point, but then this blog will just become a text book. Instead, I'm just going to say that for less understeer through a corner, the wheels of the car should be cambered negatively on the outside and positively on the inside. Since most cars (with the exception of NASCARs perhaps) have to take turns in both directions, it is not desirable to have the static asymmetric camber. Instead, caster is used to change the camber angle when the wheels are steered.
So now that you have some idea of camber and caster, try observing different vehicles on the road. Most cars, and especially buses and trucks, have very small camber and caster values. They are usually not visible to human eye. However, many modern cars run high caster values on the front wheels. Try looking at the wheels of a Mercedes Benz E-Class when its taking a sharp turn at low speed. The extra camber of the inside wheel is easily visible. In the future, I might go into details of how camber and caster affect handling.
Comments
Post a Comment
Do join in on the fun! Speak out...