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Technology Information
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Most of this page contains information about steering geometry, but there are also one or two other things on here.
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Correct steering geometry is particularly important for low powered vehicles, because if the tyres scrub as you turn, the energy wasted can significantly slow you down. It can also end up being expensive in tyres!
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There are several aspects to steering
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WATTS-UP METER
This is a circuit diagram showing how we have wired our Watts-Up meter into Y-Pod?. For more information about what a Watts-Up meter is, please visit: http://www.brchobbies.co.uk/?page=shop&action=additem&item=788
“A” on the diagram represents an ordinary diode; this stops any of the voltage spikes produced by the motor reaching any electronics in the car. We blew up our first motor cooling fans before fitting this.
“B” on the diagram represents a Zener diode, this acts like an ordinary diode until the voltage on the cathode is greater than 15V, when it is, the diode allows the voltage to flow through it, preventing any high voltage spikes reaching the meter.
These diodes are available from RS and Farnell.
The number of the diode we used at A is: 1N5404
The number of the Zener diode we used at B is: 1N5352B
Finally, the trip is fitted where it is so that when the main switch is turned off (stopping the motor from working), the watts-up meter will stay on until the batteries are removed or the trip is switched. This gives the team/user a chance to write down all of the data from the meter once back in the paddock.
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TRACKING AND CAMBER
Toe-in or toe-out is not usually necessary for low powered vehicles, all it usually achieves is to slow you down and scrub your tyres away. However some claim beneficial effects on handling at speed.
To measure toe-in or toe-out (also known as tracking), simply use a tape measure from rim to rim at the front and back, for zero tracking the distances should be equal.
Sometimes a small amount of toe-in (smaller distance at the front) is helpful, so that as any free play in the steering linkages is taken up as the vehicle moves, the wheels will become parallel.
It's occasionally suggested that the wheels be angled inwards at the top, this is called negative camber, the idea is that the outside wheel is then better able to withstand cornering forces, and the wider track will also enhance stability.
However, negative camber is usually only required on vehicles with suspension, upright wheels are the strongest and simplest all round. Tilted wheels are weaker, kingpin design becomes harder if you want zero scrub radius steering, and tyres wear on the sides rather than on the thicker middle section.
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CASTOR ANGLE AND TRAIL
Just like a bike, our steering needs to self-centre if it's to handle well and be stable at speed, we have achieved this by inclining the steering axis so that the tyre's contact point is behind (or trailing) the steering axis.
Around 10-14 degrees of caster angle seems to work OK on most designs.
This angle is in a plane at right angles to the kingpin inclination we've just mentioned: that is an angle seen as viewed from the front of the vehicle: the caster angle is as viewed from the side.
You can see the castor angle quite clearly in the photos, Rotary Racer (above) has 10 degrees of castor while "R" Pod (below) castor angle is 15 degrees.
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"R" Pod 15 Deg. Castor Angle
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KING PIN INCLINATION
In the pictures you'll notice that the kingpin slopes outwards. The idea is that the steering axis meets the ground at or near the contact point of the tyre.
This is called centre point or zero scrub radius steering geometry.
The reason is to reduce tyre scrub in the turns, there is another benefit in that if the wheel hits a bump or a braking force is applied, the forces will be in line with the turning axis, so no torque can be exerted which might jerk the steering.
The kingpin angle used to achieve this should be kept to a minimum to keep the steering from becoming to heavy (the greater the angle, the more the steering geometry will tend to lift the vehicle as the steering is turned).
Rotary Racer's design has the steering axis meeting the ground a little in (5mm) from the exact centre of the tyre contact point: this gives a certain amount of 'road feel'. "R" Pods design puts the intersection of steering axis and ground outside of the tyre contact point in an effort to reduce or eliminate brake steer.
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"R" Pod KPI
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ACKERMANN STEERING GEOMETRY
When a vehicle goes round a corner, it turns around a point along the line of its rear axle. This means that the two front wheels will have to turn through slightly different angles, so that they are guiding the vehicle round this point, and not 'fighting' the turn by scrubbing. The inside wheel should turn through a greater angle than the outside wheel.
Ackermann geometry is simply steering which achieves this by keeping each front wheel at the correct angle, through the whole range of the steering motion.
Even with perfect Ackermann steering, there will still be some scrub, because of other effects like under or oversteer. Some builders 'tweak' Ackermann steering to take account of this, usually by arranging that the wheels remain more close to parallel than exact Ackermann would suggest. Having said that, pure Ackermann works pretty well - and it doesn't have to be perfect.
In the picture above you can see that the tracking arm is not parallel with the wheel it is angled inwards and actually points at the centre of the rear axle. This is only a rough method of obtaining Ackermann geometry, but is usually near enough to work well. A more sophisticated approach is the use a spread sheet like the ones in Pete Elands web site (see links page) to work out the correct angle.
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