Upload
poppy
View
68
Download
1
Tags:
Embed Size (px)
DESCRIPTION
Chassis System. Chassis is the systems between the body and the road and includes frame/sub-frame, suspension (front and rear), steering system, brake system, wheels and tires - PowerPoint PPT Presentation
Citation preview
Chassis System• Chassis is the systems between the body and the road and includes
frame/sub-frame, suspension (front and rear), steering system, brake system, wheels and tires
• The scope of suspension design is the choice of basic geometry for optimum wheel location, the mounting of suspension members to the body (including the use of sub-frames), the springing medium, and
the provision of damping of vertical wheel movement • The scope of steering design is the optimization of front suspension
geometry for steering, the choice of steering system, the provision of power assistance, the satisfaction of safety requirements
• The scope of brake system design is the choice of friction system, the design of the operating linkage, the provision of servo assistance, the satisfaction of safety requirements, the provision of anti-lock braking and other enhancements such as emergency brake assist.
• The choice of wheel and tire size, choice of wheel material and tire configuration, choice of spare wheel configuration or “run flat”
technology
Suspension System Requirements• Allow each wheel to move vertically to provide ride comfort, while
constraining its movement in other directions to maintain stability and control. Vertical wheel movement from the datum position compresses a spring keeping wheel movement within limits, although bump and rebound stops are provided should the limit – normally set by the space constraints of body design – be reached. A damper ensures that the subsequent spring movement (an oscillation) is quickly reduced to zero.
• The other most important secondary aim of suspension design is to keep all four wheels as nearly upright as possible at all times, not only when traveling across uneven surfaces but also when the body rolls during cornering. A conventional car tire delivers optimum grip for cornering, braking and accelerating when it is upright. In practice it is impossible to achieve this ideal constraint without resorting to extremely costly and space-consuming measures, and current suspension systems are in most cases concerned to approach it as nearly as possible.
• In some cases, as with the use of trailing arms at the rear of front-driven cars, the inevitable camber change and reduced grip during cornering is exploited as a means of reducing understeer – but overall cornering grip is also sacrificed as a result.
Suspension System Requirements• Another important requirement is that the weight of the unsprung mass i.e.
wheel, tire, hub and suspension assembly at the “road” end of the spring, should be as low as possible. The lower the weight is relative to the weight of the body (the lower the ratio of unsprung to sprung mass), the less the body will react to any wheel movement, and the better the tire will be maintained in contact with the road surface, to the benefit of both ride comfort and road holding
• The task of the suspension linkage which attaches each wheel to the vehicle body is to keep the wheel as nearly upright as possible in all circumstances (zero camber angle) and pointing in the desired direction (nominally parallel to the vehicle centre line, except when the front wheels are being steered), regardless of the unevenness of the road surface which causes the wheels to move vertically, and of the attitude of the vehicle body which may move in pitch, roll, and heave (pure vertical movement) according to the forces acting at its centre of gravity.
• The importance of keeping the wheels as nearly vertical as possible is that this gives the tires the best chance to operate efficiently, with minimum rolling resistance. Many competition cars deliberately run positive (top-inwards) camber to achieve maximum cornering grip but the rate of tire wear and the additional rolling resistance when running in a straight line are unacceptable in most road-going cars.
Primary Functions of Suspension
• Support vehicle weight.
• Keep the tires in contact with the road.
• Control vehicle’s direction of travel
• Maintain correct wheel alignment, important in vehicle handling
• Reduce effect of shock loads with the use of springs, dampers and bushings
• Maintain correct vehicle ride height
Types of Front Suspension
Type Usage Cost and Weight
Package Control
McPherson Strut
Small FWD Cars
Light & Not expensive
Compact Ok
SLA or Double Wishbone
Luxury Cars
Heavier & expensive
Not compact
Good
Solid Axle with Leaf Springs
Heavy Trucks
Heavy & not expensive
Compact Minimum
McPherson Strut Suspension
Lower Link
Lower Ball Joint
Stabilizer Bar
Camber Bolt
Strut
Rubber Bushes
Tyre
Top Mount
Link
Brake DiscWheel Rim
Bump Stopper
Spring
Rubber boot
Wheel Bearing
Wheel Cap
Wheel Mounting Bolt
Drive Shaft / Spindle
Lock Nut
Heat Shield for Ball Joint (To protect from Brake Disc heat)
Features of McPherson Strut
Upper control arm in double wishbone is eliminated
Provides anchoring of tie rod on knuckle
Combines the following parts into one assembly to provide wheel control
Spring Seats
Springs, Bump stoppers
Rebound Stopper
Link for mounting Stabilizer Bar
Lower the Forces on BIW-Mountings
Provide Better Space at the side to mount transverse Engine & Gear box
Better Space for Front Crash Members & Crumple zones
Advantages of McPherson Strut
• Advantages
1. Combination of several parts into one assembly
2. Upper transverse link replaced by top mount
3. Occupies less space
1. Transverse engine mounting possible
2. More space for front crumple zone
Disadvantages
1. Less favorable kinematic
characteristics
2. Forces & vibrations transferred
to inner wheel-arch panel which
is relatively elastic
3. Difficult to insulate against road
noise
4. Friction between piston rod &
guide impairs the springing
effect
5. Critical to package [Gaps
between Tyre & damper,
Springs & Wheel-arch]
6. Ground Clearance critical
Double Wishbone Suspension
Lower Link
Lower Ball Joint
Stabilizer Bar
Rubber Bushes
Tyre
Top Mount Upper Control Arm
Brake Disc
Knuckle
Spring& Damper
Wheel Mounting Bolt
Upper Ball Joint
Tie Rod
Brake-Rod
Steering Gear
Features of SLA or Double Wishbone
Has 2 control arms (upper & lower)
connected to the steering knuckle by ball
joints (UBJ & LBJ)
Upper control arm in double wishbone is
shorter than lower arm which helps control
the camber angle to desired level during
body roll
Spring, shock and anti-roll bar are attached
to LCA
Steering arm is attached to the knuckle
Advantages of Double Wishbone Suspension
• Advantages1. Kinematics can be controlled
easily
2. Provides good camber compensation during vertical movement
3. Pitching movements can be
balanced i.e anti-dive, anti-squat
possible
4. Toe-in, Camber & Track change
can be controlled optimally due to
variety of control parameters
• Disadvantages1. More complex than McPherson
Strut
2. Short spindle SLAs tends to require stiffer bushings at the body, as the braking and cornering forces are higher. Also they tend to have poorer kingpin geometry, due to the difficulty of packaging the upper ball joint and the brakes inside the wheel.
3. Long spindle SLAs tend to have better kingpin geometry, but the proximity of the spindle to the tire restricts fitting oversized tires, or snow chains. The location of the upper ball joint may have styling implications in the design of the sheetmetal above it.
Front Suspension Parts
Strut /
Damper
ARB
Spring
Steering Tie-rods
Suspension Bush
Ball Joint
Lower Link
Knuckle
Tyre Wheel rim
Sub-frameCorner Module
Drive Shaft / Bearing
Subframe
Types of Rear Suspension
Type Usage Cost and Weight
Package Control
Twist Beam Small FWD Cars
Light & Not expensive
Compact Ok
Multi-Link Luxury Cars
Heavier & expensive
Not compact
Good
Hotchkiss Trucks Heavy & not expensive
Compact Minimum
TA : Trailing Arms CB : Cross Beam
B : Pivot Bushes S : Coil Spring
D : Dampers E : Top Mount
T : Torsion bar P : Panhard Rod
TA
TACB
B
B
S
S
D
D
E
E
T
P
P
Welded Rigid Connection
Twist Beam Rear Suspension
Features of Twist Beam Suspension
Very compact package
Inexpensive to manufacture, assemble/disassemble.
Eliminates several parts: control arms, anti-roll-bar, etc.
Twist axle acts as a anti-roll-bar
High stresses in the welds
Advantages of a Twist Beam Suspension
• Advantages1. Whole axle easy to assemble &
dismantle
2. Requires very little space, easy to package spare tire, fuel tank, etc.
3. Spring-Damper assembly is easy to fit.
4. Control Arms & Rods are eliminated.
5. Wheel to Spring Damper ratio favorable.
6. Less unsprung mass
7. Cross member acts as a anti-roll-bar
8. Negligible toe-in & track change
9. Low camber change under lateral forces.
• Disadvantages1. Exhibits compliance Oversteer
tendency
2. Torsion & Shear stress in Cross
member
3. High stress in weld seams
Twist Beam Rear Suspension Parts
Strut /
Damper
Spring
Tyre
Drum Drum Brake
Twist Beam
Unitized Bearing
Packaging
Suspension Bush
Wheel rim
Twist Beam Module
3-Link Rear Suspension
Longitudinal Link
Transverse Links Pivot
Bushings
Coil Spring
Damper
Top Mount
Sub-frame
3-Link Rear Suspension Parts
Strut /
Damper
Spring
TyreDrum Drum Brake
Unitized Bearing
Suspension Bush
Subframe with Multilink Suspension
Multi-Link Suspension
Multi-Link Suspension
Features of 3-Link Rear Suspension
Relatively expensive
Requires more space
Easier to control wheel movement with 3 links
Longitudinal link picks up longitudinal loads
Transverse links pick up lateral loads
Advantages of 3-Link Rear Suspension
• Advantages
1. Pitching movements can be
balanced i.e 100% anti-dive, anti-
squat possible
2. Toe-in, camber, track change can
be controlled optimally due to
variety of control parameters
• Disadvantages
1. Costly as compared to twist beam
and other suspensions due to
increased number of components,
links, bushings & bearings
2. Higher production & assembly
costs
3. Higher degree of tolerance control
required to maintain geometry
Hotchkiss Rear Suspension
Hotchkiss Rear Suspension
U Bolt
Suspension Bush
Tyre
Wheel rim
ARB Bush
Hotchkiss Suspension
Shackle
Parabolic Leaf SpringConventional
Leaf Spring
Hotchkiss Suspension
Features of Hotchkiss Rear Suspension
Simple in design
High weight
Easy to assemble
Provides good pay load carrying capacity
Robust in design
Advantages of Hotchkiss Rear Suspension
• Advantages
1. Simple with very few parts
2. Easy to manufacture & assemble
3. Robust design
4. High load carrying capacity
• Disadvantages
1. High weight of suspension i.e high
unsprung mass.
2. Occupies More Space than other
suspension types
Wheel Movements Controlled by Suspension
• Jounce & Rebound• Roll• Toe in/Toe out• Left or Right Steer• Camber• Spin
Axle/Vehicle Jounce & Rebound
At Ride Height In Rebound
y
z
At Ride Height In JounceSpring Compression
Spring Extension
y
zRear View
Axle/Vehicle Roll
y
z
y
z
At Ride Height Axle RollSpring Compression
At Ride Height Body RollSpring Compression
Rear View
Wheel Camber
Wheels with no Camber Wheels with Camber
y
z
Rear View
Wheel Toe in/Toe out
Wheel Toe-in Wheel Toe-out
y
x
Top View
Wheel Steer
Wheel RH Steer Wheel LH Steer
y
xTop View
Suspension Geometry in Wheel Jounce
WheelAssemblyRide Height
WheelAssemblyIn Jounce
Lower Control Arm
Upper Control Arm
Lower Ball Joint
Upper Ball Joint
Body Pivot
Note:
1) Wheel at original position (pink)2) Wheel in jounce (blue)3) Original control arms (solid)4) Control arms in jounce (dotted)5) Note wheel camber
y
z
Rear View
Suspension Geometry in Wheel Rebound
WheelAssemblyRide Height
WheelAssemblyIn Jounce
Lower Control Arm
Upper Control Arm
Lower Ball Joint
Upper Ball Joint
Body Pivot
Note:
1) Wheel at original position (pink)2) Wheel in jounce (blue)3) Original control arms (solid)4) Control arms in jounce (dotted)5) Note wheel camber
y
z
Rear View
Steering Geometry Error in Wheel Jounce
Note:
1) Wheel at original position (pink)2) Wheel in jounce (blue)3) Original tie rod (solid)4) Tie rod in jounce (dotted)5) Note geometry error
Tie Rod
Steering armBall joint
Ideal Location for body ball joint
Ideal path for steeringarm ball joint
New position for bodyball joint
New path for steeringarm ball joint
y
zRear View
Steering Geometry Error in Wheel Jounce
y
z
Ideal centerfor tie rod on body
Center forShort tie rod
Center for long tie rod
Steering armball joint at ride height
Steering arm ball joint at jounce
Ideal path
Short TieRod Path
Long Tie Rod Path
jounce
Ball jointPulled out
Ball joint Pulled in
Rear View
Steering Geometry Error in Wheel Jounce
y
z
Steering armball joint at ride height
Ideal centerfor tie rod on body
Ideal Tie Rod
Center aboveIdeal
New Tie Rod
Ideal path
New PathBall jointPulled out
Steering arm ball joint at jounce
jounce
Rear View
Steering Geometry ErrorsPosition of Tie Road to Body Joint
Steering Arm Ball Joint Position
Steering Geometry Error
At ideal center On ideal path None
Inboard towards wheel Pulled in towards body in jounce or rebound
Toe-in (link ahead ) Toe-out (link behind)
Outboard towards body
Pulled out towards body in jounce or rebound
Toe-in (link behind) Toe-out (link ahead)
Below ideal center Pulled out jounce and pulled in in rebound
right steer (behind) left steer (ahead) in roll
Above ideal center Pulled in in jounce and pulled out in rebound
right steer (ahead) left steer (behind) in roll
Suspension Roll Center
• Roll center is defined as a location at which lateral forces developed by the wheels are transferred to the sprung mass
• Each suspension has a roll center• Lateral forces can be applied to the sprung mass at the roll
center without causing suspension roll• Each suspension has a roll axis about which un-sprung mass
rolls when a pure moment is applied• Vehicle roll axis is the line joining the roll centers of the front
and rear suspensions
Roll Centers
x
z
Roll Center for 4-Link Solid AxleFy
Upper Link UL
Lower Link LL
FyLL
FyUL
ab
FyLL/FyUL = b/a
FyLL+FyUL = Fy
Top View
Side View
x
z
x
y
Roll Center for 3-Link Solid Axle
Top View
Side View
Track Bar
x
z
x
y
Roll Center for Hotchkiss
Side View
Roll Axis
x
z
Roll Center for Positive Swing Arm SLA
Fy
Fy
FU
FL
Rear Viewy
z
Roll Center for Negative Swing Arm SLA
Roll Center for Parallel Arm SLA
Roll Center for Inclined Parallel Arm SLA
Roll Center for McPherson Strut
Assignment
• Determine roll center for your suspension
• Determine suspension envelope in y-z plane for your suspension