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Fixing the car after the final damage
Fixing the car after the final damage

Greenpower Steering

From CSS's experience the steering geometry is quite critical to a car's stability. The initial idea by new teams is to use a simple go-kart style steering with no caster, inclination or Ackerman angles. Having no caster angle is UNSAFE on a car that is capable of 30 MPH or more. On RR9 we have a caster angle of 5 degrees, an inclenation angle of 18 degrees and a calculated Akerman angle based on all aspects of the steering geometry. Why do we need these ?

  •  Caster angle: This is the side on angle of the steering axis (king pin front/back angle) such that a line drawn through the steering pivot will hit the track at a point that is ahead of where the tyre meets the track. Instead of an angled steering axis, the wheel axle mounting can be behind a vertical steering axis trailing like a trollys castor wheel.  It is CRITICAL to have a reasonable degree of caster. This provides the negative feedback, self centring nature of steering. If the angle/trail was reversed, the cars steering would simply snatch solidly to the left or right and going straight ahead would be very difficult indeed. Having a 0 degree caster angle (vertical king pins/steering axis without any trail) would mean the car can go into an uncontrollable side to side "tank slapper"  possibly leading to a following roll. This would probably occur at some inopportune moment such as when a side wind hits or at a bump on a corner. Higher levels of caster make the car less twitchy and smooth at the expense of light, quick agile steering. It is better to err on the side of smooth steering for new young drivers, but having too much might make the steering to heavy in young hands. An angle in the range 5 - 10 degrees is probably best.

  • Steering Gearing: This is the ratio of the amount of wheel steering movement to the movement of the joystick, wheel or other control. The wheel should only move a relatively small amount for a reasonably large movement off the joystick/wheel/control. If the wheel moves a lot for a small amount og joystick movement the steering becomes twitchy and heavy leading to the drivers careering from side to side. As a rough guide an amount of stick/wheel/control movement should, roughly, turn the wheel by half of this amount so 0.5 gearing. Thus a 100mm movement at the top of the joystick will move the outer edge of the wheel by 50mm. For current Greenpower F24/F24+ tracks a 50mm movement of a 16" wheel is sufficient.

  • Steering Axis Inclination: This is the front on angle of the steering axis (king pin sideways angle) such that the pivot point at the track level is close to where the tyre meets the track. This is less critical than the caster angle but still important. Ideally the pivot point should be close to where the tyre hits the track. When this is so (called centre point steering) then if you hit a bump or object with one wheel the steering will not suddenly pull to one side. Also when applying the brakes, if one wheel loses traction there is not a yank on the steering. However, with exact centre point steering the driver can lose some “road feel” as deviations of the road surface are not felt on the steering. So a pivot point 1 cm or so inside the tyre is often chosen as we have on RR9. Note that this angle also adds to the stabilising self-centering effect as the weight of the car "pushes" the wheels to the centre point.

  •  Ackerman angle: This is important for efficiency and thus for doing well in the GP challenge. When cornering, the inter wheel follows a tighter curve than the outer one. Its turn steering angle should thus be greater than the outer wheels. If not, the tyres will scrub losing precious energy as friction. A rough angle is to point the steering arms from the kingpin to the centre of the rear axle. This however is often quite inaccurate, especially if simple tie rods to a joystick arm are used. The change in geometry due to a rotating arm is significant. There are on-line spreadsheet based calculators that can be used for a more accurate (and thus more efficient) angle to be calculated. See: http://www.eland.org.uk/steer_download.html

There is a good write up on this on CAUC's web site at: http://www.cauc-f24.org/images/TechincalDocuments/Handling.pdf