Steering Design
- Devon Copeland
- Robin Pearce
- Adam Marchand
Geometry
The maximum angle of our tires (steering angle) is determined using our desired steering radius, 6.2 meters. This radius was chosen because FSGP & WSC require that our car perform an 8 m radius u-turn where all portions of our car less than 200 mm from the ground must remain within the 8 m radius. To ensure we meet this requirement we have set our outer turning radius to be 7 m and therefore our turning radius and inner turning radius to be 6.2 m and 5.4 m respectively. Using this radius we can use geometry to derive that our largest steering angle will be at the inner wheel and will be: Theta = ArcTan( Wheelbase / (radius - track/2)), where wheelbase = 2600 mm, track = 1600 mm and radius = 6.2 m. From this we find our largest steering angle must be 25.71 degrees.
To determine our steering arm angle we use perfect Ackerman principles and draw a line directly from the mounting point of our steering arm on the front axle to the centre point of our rear axle, this is represented by the formula: ArcTan( ((Track/2) - Mounting distance from wheel) / Wheelbase ), where Mounting distance = the distance between the wheel and the steering arm mounting point = 80 mm, wheelbase = 2600 mm and Track = 1600 mm. Therefore our steering arm angle is 15.47 degrees.
The below sketch was used to determine a series of steering geometry equations:
To determine our steering arm length, tie rod length, and maximum rack travel we derived a series of equations using the physical geometry of our car. These equations are as follows:
Therefore,
where R = Tie Rod Length, T = Track, SAL = Steering Arm Length, SAA = Steering arm angle, TR = Rack Travel, MD = Steering arm mounting distance from wheel, Theta = Inner Wheel Steering Angle, Phi & Alpha are angles found through geometry, and b is a length found through geometry.
Since SAL, SAA, R, b, MD, T and Wheelbase are constants we know have Rack travel as a function of Theta (inner steering angle).
A further explanation of this derivation can be explained by asking Robin Pearce (rsgpearce@gmail.com).
The following spreadsheet does the following calculations and takes input as steering arm length, total rack travel and total rack length.
Component Selection
As of right now we will most likely purchase our rack from Stiletto as it meets all of our criteria and is a reasonable price, however no force analysis has been completed to ensure the rack is strong enough so a final decision cannot be made.
Other manufacturers and retailers including, Flaming River, Coleman Racing, Ididit, Summit Racing, Jegs and PitStop USA were contacted, however none were able to provide products close to our needs. The other retailers could only provide racks upwards of 12 lb, far too heavy for our purposes, and were unable to provide custom steering columns.