car geometry explained

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    psbarham
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    car geometry explained

    Post  psbarham on Mon Jul 12, 2010 2:32 pm

    WHEEL AND STEERING GEOMETRY

    To the majority of people, racers included, the way to get around the corners quickly is about pointing the steering wheel in the right direction. Whilst this is most certainly the case, it is not the whole truth about fast cornering. Different cars will react very differently to the same steering input and some of the reasons can be found in the way that the steering geometry has been set-up.

    TRACKING;-

    The easiest and most obvious part of steering setup is the tracking or `toe-in/toe-out' settings. This is the directional alignment of the front wheels. A common fallacy would be to expect the wheels to be parallel, both pointing in the same direction. In reality the majority of cars, even those on the road, will have the wheels pointing ever so slightly in different directions. Often this amount is so small that it cannot be seen by the naked eye. This measurement is known as the 'toe' and is measured by taking the distance across the car at two points on the wheel, ideally halfway up the tyre. The measuring can be done on either the inside or the outside of the wheels and you can often see different devices being used at the races to do this quickly and easily. When the wheels point outwards at the front, you have a toe-out set-up and the obvious reverse when the wheels point inwards, is toe in.
    Giving a race car a toe-out set-up can be advantageous for a couple of reasons. Firstly, as each wheel is pointing in a different direction, they will be fighting each other when going in a straight line. This will help to keep the car stable in a straight line. If there is too much toe-out though this will cause drag, as the tyres will be scrubbing sideways as the car moves forward.
    The second and more important reason for using toe-out is when the car goes around a corner, as the inside and outside wheels will be travelling in different size arcs. If the two wheels were parallel, one of the two would not be running in a natural arc when cornering and would be scrubbing sideways as before.
    When going around a corner with toe-out the inner wheel will be turned in slightly further than the outer, and both wheels will go round the corner properly. Getting the amount of toe-out right can be a matter of trial and error and if done right should be changed at each track, as the different tracks will have differing corner radii.
    The track surface will also have an affect on the amount of toe-out a car should use. A grippy dry tarmac track will cause more problems of tyre scrub than a wet track. More toe-out in slippery conditions can help to counter the lack of grip, as the wheels can be better aligned for the corners.

    CAMBER;-

    The second part of wheel alignment is the angle at which the wheel sits on the road, which is known as camber. How often have you seen cars at an oval track with the outside front wheel leant right over, with just the amount of camber that is required will depend on many things, the track type, tyre width and side wall strength, and car speed will all have an effect. The amount of sideways force is different on each wheel of the car, the outside front having the most forces, therefore needing the most camber.
    Camber is measured in degrees away from the vertical, but the best way to see if it is set right is to use a tyre pyrometer. This measures the temperature and effectively the amount of use the tyre is getting. By measuring at least three points across the tyre as soon as possible after race conditions, it is possible to see if either the inner or outer edges of the tyre are working harder than the other (the higher the temperature the harder it has worked). By altering the camber, moving it away from the hottest edge, the contact patch should be even, giving a better grip.

    AXLE ALIGNMENT;-

    The final part of wheel alignment is the alignment of wheels in respect to each other across the car. For the majority of race cars there will be a fixed axle either at the front or rear of the car, sometimes both. The exception to this will be cars that have independent suspension all round, which allows the alteration of the fore and aft position of the wheel without any other wheel. With fixed axles though, if you move one wheel, the other is effectively moved in the opposite direction. The axle will pivot about its centre point, affecting the direction the wheels point. Therefore, if you move the outside rear wheel back the rear wheels would point outwards. As the car goes down the straight, the steering would need to be off centre, making the car 'crab' along the track.
    The angle of the axle can be set to aid cornering. By moving the outside front wheel (solid front axle) forward, the wheel will be partially pointing around the corner. By pulling the outside rear (solid rear axle) back the wheels will point slightly outward, but give the effect of rear wheel steering, helping to get the back round the corner. Too much axle angle will have adverse affect down the straight, but some is better than none. It is more important to get around the corners quickly and smoothly than it is to drive quickly in a straight line.
    The above techniques describe how to set the wheel geometry up when the car is stationary. By experimenting with different amounts of each variable, any car's cornering characteristics can be improved. If taken seriously these variables will be different for each track and surface.
    The final thing to note about wheel alignment is that when the car is moving, there are dynamic forces caused by body roll that can change some of the settings. Knowing what these are and whether it is helping or hindering could lead to a breakthrough in a car's performance, but that is for another time.

    CASTER;-
    The final part of wheel alignment is to do with the steering axis. The steering axis is the point around which the wheel pivots when the steering is turned. On some cars it will be around the suspension strut, on others where the upright bolts to the wishbones or front axle. In either case, it is the vertical alignment of the top and bottom of this axis that determines caster. On road cars the bottom end will be further forward than the top, giving positive caster. This creates an effect like the wheels on a shopping trolley. If it is pushed forward the wheels will centre themselves, pointing in the direction of travel.
    The greater the angle of caster, the stronger the centring force, which effectively means heavier steering. With a low caster angle the car will be more willing to go around the corners, but the counter effect is that is will be less willing to straighten up afterwards. If a car is given too much caster, i.e. the bottom of the steering axis is further forward than the top, the steering will be very heavy and the car will be reluctant to turn into a corner. Conversely, if the car were to be given negative caster, with the lower end of the axis further back than the top, there would be no directional stability at all. The car would need constant steering adjustment to keep it in a straight line, not something that is required on a racetrack!
    Therefore the aim with caster is to get a balance between straight-line stability and getting the car to turn easily, without too much effort from the driver. This is achieved by having different amounts of caster on each wheel. The inside wheel will have a low caster angle, though still positive. This gives the light steering into the corner. A higher amount of caster on the outside wheel will give the car the straight-line characteristics that are required.
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