Steering Design Review for racing vehicle and sports car


Please read previous blogs for following details : - Vehicle Weight, Brake Design, Engine Design and other major system components. 

Please read previous two blogs for following-
Different Types of Weight of Car, front and real calipers, Brake Pad, Brake Fluid Volume, front to rear weight bias, rear traction, lower polar moment of rotation, Skid, Tyre Thread Design, Car Center of Gravity and many more technical terms  



It is recommended to assemble steering rack in line with track rod ends to give Quasi-Ackerman at smaller angles of steering as well as Anti-Ackerman when driver approach full lock. Straight line drag will increase but toe out will give Quasi-Ackerman advantage near corners. Thus Quasi Ackerman will be advantageous at both condition sharp corner and full lock.  Quasi Ackerman is light weight, cost effective, easy to layout and does not require external power. (Bolles, 2010, ch.6), (Alexadender, 2013, pp.82-90)

Basic steering design challenges, cause/effect of specifications and requirements are explained in Table 1 and Table 2.  Unlike city roads, Race track has sharp curves and other conditions such as uneven friction and sudden braking. This cause vehicle to skid and topple therefore a fine steering effectiveness, sensitivity and efficiency are prerequisite. Ackerman is based on geometry however Race Vehicles seldom drive geometrically.

At Formula Ford Circuit tyre can generate appropriate lateral grip only at specific fixed range of optimum slip angle. During cornering the weight shifts to the outer wheels and if braking is applied at same time then major weight will shift to front outer wheel. Ackerman is based on geometry consequently if we consider fluctuation in slip angle, lateral force, weight distribution due to variation in cornering and braking then we must select parallel or anti Ackerman design.

Crash Simulation is done before physical crash test. In Simulation Result the Steering Wheel layout must not damage or penetrate the dummy of driver. For further safety Steering sensor is used to continuously check responsiveness of mechanical steering mechanism. After the physical crash test the steering wheel quick release mechanism is functioning normally. Physical test specification was as per formula ford rules which define speed, size and other environmental condition. Extremity of the cockpit opening is kept 50mm in front of the steering wheel. Roll structures are kept of sufficient height to ensure steering wheel are at all times at least 50mm below a line drawn between highest points of roll structures.

Proper machining and high surface finish is required on components because more than 50% of the force is wasted to overcome system friction. Also as friction increase, wear will increase and more force will be required to operate steering. Mechanical system has greater sensitivity to rod impact and friction. Steering angle depend on steering rack travel subsequently if short steering arms are used then higher forces will be needed to operate steering wheel. Hence to decide steering wheel size we must consider space available for driver ergonomics, accidental safety and comfort.

Bump steer is minimized by locating the rack pivot centers directly in front of the lower control arm pivots. Rack length is governed by its height in the chassis because higher the rack is mounted on vehicle, the longer it will have to be. For quicker steering, steering arm length has been kept short to maximize truing angle however it has increased drivers’ input effort. Benchmarking and optimization is done to finalize steering arm length. Rack ratio is kept 2.5 inch per turn. Rack ratio is optimized by using combination of a short (quick) arm with a small (slow) pinion and the combination of a long (slow) arm with a large (quick) pinion.
Child Part
Requirement
Material
Manufacturing
Cost
Weight
Life
Design Review
Rack Housing
High Surface Finish,

High Strength,

Fine Dimension Tolerance,

Wear Resistance,

Corrosion Resistance,

High Toughness
Steel
Machining
A
C
B
Al Alloy is light weight and has good life. Steel and Aluminum are lesser costly but have high weight. Also Rusting is more for Steel. So Al Alloy will be used for Rack Housing Components.
Aluminum ADC12
PDC Pressure Die Casting and Machining
B
B
B
Al Alloy
C
A
A
Support Tube
ADC12
PDC Pressure Die Casting and Machining
B
C
B
Al Alloy will be used for Support Tube as it has less weight.
Al Alloy
C
A
A
Pinion Housing
Steel
Machining
A
C
B
Al Alloy will be used for Support Tube as it has less weight and lesser wear.
ADC12
PDC Pressure Die Casting and Machining
B
B
B
Al Alloy
C
A
A
Housing Cap + Bush
Steel + Bronze
Casting/Machining
A
B
B
Bronze Bush with ADC12 housing Cap will be used due to light Weight. (HPDC High Pressure Die Casting)
Bronze
Casting/Machining
C
B
A
ADC12+ Bronze
HPDC, Machining
A
A
B
Pinion Housing Clamp
Alloy Steel
Casting/Machining
C
B
A
Driver applied torque is transmitted though the lower pinion gear to the rack gear, So very high Strength is required here.
Al Alloy
HPDC, Machining
C
A
B
Pinion  Helical Gears
Number of
Spiral Tooth
Anti Backlash
ID/ OD
Alloy Steel
Forged, Milling, Hobbing Lapping
C
B
A
Alloy Steel Helical Gears will be used as it has less weight and long life.
Cast Iron
B
C
C
Rack with variable Teeth  Helical  Gear
Rack Speed=60mm / Revolution
Travel Max = 75mm
Carbon Fiber
Molding
C
A
B
Carbon Fiber is very costly.  Both Pinion and Rack should have same material for same wear rate. Alloy Steel will be used for rack.
Alloy Steel
Casting, Machining. 
Gear=Milling, Hobbing Lapping
A
C
A
Al Alloy
B
B
B
Pinion Shaft
High Surface Finish, High Strength, Fine Dimension Tolerance,  Low Wear Rate,
Corrosion Resistance, High Toughness
Steel
Casting, M/c
A
B
B
Aluminum will be used for Pinion Shaft.
ADC 12
HPDC, M/c
B
A
B
Rack Ends
Steel
Casting, M/c
A
C
B
ADC12 will be used for Rack Ends.
ADC12
HPDC, M/c
B
B
B
Steel Alloy
HPDC, M/c
C
A
A
Housing End Supports
ADC 12
HPDC, M/c
A
B
B
Al Alloy will be used for Housing End Support as it has less weight.
Al Alloy
HPDC, M/c
B
A
A