Steering Geometry Flashcards
What are the 2 aims of steering ?
Centre point steering
When cornering all steered wheels must turn about a common centre
What is the scrub radius?
Centre of the king pin to the centre of the tyre
What does a large scrub effect cause?
Large splaying out effect
Heavy steering
Large bending forces on the stub axle and kingpin
How do you overcome the problem of a large scrub radius?
Arrange the wheel and king pin so the centre line of the wheel meets the centre line of the king pin at the road surface
How do you achieve centre point steering
Wheel camber
Kingpin inclination
Dished wheels
What is positive offset?
Wheel centre line and KPI/SIA centre line is below the road surface
What is negative offset?
Wheel centre line meets KPI/SAI above the road surface
What is camber angle?
Angle formed between the vertical and wheel centre line
What is positive camber?
Wheel tilts outwards at the top
What are the disadvantages of positive camber?
Cone effect - roll in circle
Wear on outer edge of tyre
What are the advantages of positive camber?
Lighter steering
Offset is reduced
What is negative camber?
Wheel tilts inwards at the top
What are the disadvantages of negative camber?
Offset increases
Heavier steering
Increased tyre wear
What are the advantages of negative camber
Improved handling
Camber angle reduced
More tyre contact with road surface
When travelling over rough surfaces, small movements are absorbed instead of through steering linkages
Helps keep vehicle in a straight line
How can a dished wheel reduce KPI
Slightly dishing the wheel
What is castor action
Gives driver the feel of the straight ahead position , wheels follow the general direction of the weight
What must be exerted to overcome the self centering or castoring action?
Torque
Too much and too little castor causes what?
Too much- heavy steering
Too little- wandering
What is positive castor?
When the centre line- road contact is ahead of the wheel
What is negative castor?
When the centre line road contact is behind the wheel
What type of castor is required for front or 4 wheeled drive vehicles?
Negative
What type of castor is required for rear wheel drive?
Positive
What forces can cause slip angle?
Cornering, camber, wind and weight
What is the slip angle?
Difference between intended direction of travel and actual direction of travel
What are the 2 ways slip angles can affect a vehicle?
Oversteer - vehicle turns more than intended
Under steer- vehicle turns less than intended
How does oversteer occur?
When the slip angle on the rear tyres are greater than the slip angle on the front
What will occur if the slip angle are equal on the front and back?
Turn the corner uniformly
How can oversteer be caused?
Increase load on the rear of the vehicle
Lower pressure in rear tyres
Large load transfer from inner rear wheel to the outer rear wheel when Cornering
What causes under steer?
The slip angles are greater on the front than the rear
What happens during understeering?
The front wheels are turning a lesser amount than the driver intended
What can be another cause of understeer apart from slip angle?
Increase load on the front
Lower pressure on front tyres
Large load transfer from inner front wheel to outer front wheel when Cornering
What is front wheel alignment (tracking)
Wheels on each axle should be parallel when travelling in a straight line
What is the allowance known as in tracking?
Tow in , toe out
On RWD, forces exerted cause wheels to?
Splay out
On FWD, forces exerted cause wheels to?
Splay in
What is toe in?
Distance on the front of the wheel is shorter than the measurement at the rear
What is toe out ?
When the distance between the front of the wheel is greater than the measurement between the rear of the wheel
Toe setting is determined by what?
Manufacture
How is the setting adjusted?
By altering the length of track rod, activated by LH/RH thread
Toe setting is checked by?
Alignment gauges
Optical tracking gauges
What should you check before tracking alignment?
Tyre size, pressure and uneven wear
Wheel bearing play
Check front suspension for damage
Check run out of the wheels
What must you do before checking alignment?
Rolled backwards and forward to settle the steering and suspension
Allows all play to be taken up
What will occur if the wishbones are the same size?
Road deflections lift the wheel , the track will vary but the camber will stay the same
Make it unstable and increase tyre wear
What occurs if the wishbones are different lengths?
Road deflections lift the wheel but the track stays the same but camber angle alters
More stable will only cause slight tyre wear
What prevents scrubbing when turning?
Wheels must turn about a common centre
A true rolling action must be achieved
What can almost true rolling action be achieved by?
Inclining steering arms towards each other
Line projected from each king pin meet on the centre line of the vehicles ahead of the rear axle
What will distortion of the track arm cause?
Misalignment
Incorrect wheel angles
How is the alignment checked?
Turntable measuring angles
Carrying out a toe out on turns test
What are the possible causes of wander?
Excessive negative castor
Excessive play in steering joints
Tight ball joints, kingpins or steering joints
Incorrect steering box adjustment
What are the possible causes of shimy?
Excessive play in steering box
Excessive play in ball joints or kingpin/swivel pins
What causes the vehicle to pull to one side?
Incorrect front wheel alignment
Unequal track rod adjustment
Incorrect steering angles
