Transcript Slide 1

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 frontdriven 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 (topinwards) 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 Package Control
Weight
McPherson
Strut
Small
FWD
Cars
SLA or Double Luxury
Wishbone
Cars
Light & Not Compact
expensive
Ok
Heavier &
expensive
Not
compact
Good
Solid Axle with Heavy
Leaf Springs
Trucks
Heavy &
not
expensive
Compact
Minimum
McPherson Strut Suspension
Top Mount
Bump Stopper
Spring
Strut
Rubber boot
Lock Nut
Link
Camber Bolt
Wheel
Mounting Bolt
Stabilizer Bar
Wheel Cap
Rubber Bushes
Drive Shaft
/ Spindle
Wheel Bearing
Lower Link
Tyre
Wheel Rim
Brake Disc
Heat Shield for Ball Joint (To protect from
Brake Disc heat)
Lower Ball Joint
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
Disadvantages
1. Combination of several parts into
one assembly
1. Less favorable kinematic
2. Upper transverse link replaced by
top mount
2. Forces & vibrations transferred to
3. Occupies less space
characteristics
inner wheel-arch panel which is
relatively elastic
1. Transverse engine mounting
possible
3. Difficult to insulate against road
2. More space for front crumple
zone
4. Friction between piston rod & guide
noise
impairs the springing effect
5. Critical to package [Gaps between
Tyre & damper, Springs & Wheelarch]
6. Ground Clearance critical
Double Wishbone Suspension
Tyre
Stabilizer Bar
Top Mount
Upper Control Arm
Upper Ball Joint
Brake-Rod
Rubber Bushes
Brake Disc
Steering
Gear
Knuckle
Lower Link
Tie Rod
Lower Ball Joint
Spring& Damper
Wheel
Mounting Bolt
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
• Advantages
• Disadvantages
1. Kinematics can be controlled easily
1. More complex than McPherson Strut
2. Provides good camber compensation
during vertical movement
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. 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
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
ARB
Sub-frame
Steering Tie-rods
Subframe
Knuckle
Strut /
Damper
Suspension Bush
Lower Link
Spring
Tyre
Ball
Joint
Corner
Module
Wheel rim
Drive
Shaft /
Bearing
Types of Rear Suspension
Type
Usage Cost and Package Control
Weight
Twist Beam
Small
FWD
Cars
Luxury
Cars
Light & Not Compact
expensive
Ok
Heavier &
expensive
Not
compact
Good
Trucks
Heavy &
not
expensive
Compact
Minimum
Multi-Link
Hotchkiss
Twist Beam Rear Suspension
E
D
E
Welded
Rigid
Connection
S
T
P
D
B
S
TA
CB
TA
B
TA : Trailing Arms
CB : Cross Beam
B : Pivot Bushes
S : Coil Spring
D : Dampers
E : Top Mount
T : Torsion bar
P : Panhard Rod
P
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
• Advantages
• Disadvantages
1. Whole axle easy to assemble & dismantle
1. Exhibits compliance Oversteer tendency
2. Requires very little space, easy to
package spare tire, fuel tank, etc.
2. Torsion & Shear stress in Cross member
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.
3. High stress in weld seams
Twist Beam Rear Suspension Parts
Packaging
Unitized
Bearing
Twist Beam
Suspension
Bush
Twist Beam Module
Tyre
Strut /
Damper
Drum
Spring
Drum
Brake
Wheel rim
3-Link Rear Suspension
Top Mount
Coil Spring
Damper
Sub-frame
Transverse
Links
Longitudinal
Link
Pivot
Bushings
3-Link Rear Suspension Parts
Subframe with Multilink
Suspension
Unitized
Bearing
Multi-Link
Suspension
Suspension
Bush
Strut /
Multi-Link
Suspension
Damper
Drum
Spring
Drum
Brake
Tyre
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
• Disadvantages
1. Pitching movements can be balanced i.e
1. Costly as compared to twist beam and
100% anti-dive, anti-squat possible
other suspensions due to increased
number of components, links, bushings &
2. Toe-in, camber, track change can be
controlled optimally due to variety of
control parameters
bearings
2. Higher production & assembly costs
3. Higher degree of tolerance control
required to maintain geometry
Hotchkiss Rear Suspension
Hotchkiss Rear Suspension
Tyre
Conventional
Leaf Spring
Parabolic Leaf
Spring
Wheel rim
Hotchkiss Suspension
U Bolt
Hotchkiss Suspension
Shackle
Suspension Bush
ARB Bush
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
• Disadvantages
1. Simple with very few parts
1. High weight of suspension i.e high
2. Easy to manufacture & assemble
3. Robust design
4. High load carrying capacity
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
z
Rear View
y
At Ride Height
Spring Compression
In Jounce
z
y
Spring Extension
At Ride Height
In Rebound
Axle/Vehicle Roll
z
Rear View
y
At Ride Height
Spring Compression
Axle Roll
z
y
Spring Compression
At Ride Height
Body Roll
Wheel Camber
z
y
Wheels with no Camber
Wheels with Camber
Rear View
Wheel Toe in/Toe out
x
Top View
y
Wheel Toe-in
Wheel Toe-out
Wheel Steer
x
Top View
y
Wheel RH Steer
Wheel LH Steer
Suspension Geometry in Wheel Jounce
Note:
1)
2)
3)
Upper Ball Joint
4)
Upper Control Arm 5)
Wheel
Assembly
In Jounce
Wheel
Assembly
Ride Height
Lower Ball Joint
Lower Control Arm
Body Pivot
Wheel at original position (pink)
Wheel in jounce (blue)
Original control arms (solid)
Control arms in jounce (dotted)
Note wheel camber
z
y
Rear View
Suspension Geometry in Wheel Rebound
Note:
Wheel
Assembly
In Jounce
Upper Ball Joint
Upper Control Arm
Wheel
Assembly
Ride Height
Lower Ball Joint
Lower Control Arm
1)
2)
3)
4)
5)
Wheel at original position (pink)
Wheel in jounce (blue)
Original control arms (solid)
Control arms in jounce (dotted)
Note wheel camber
z
Body Pivot
y
Rear View
Steering Geometry Error in Wheel Jounce
Note:
z
Rear View
y
Steering arm
Ball joint
1)
2)
3)
4)
5)
Wheel at original position (pink)
Wheel in jounce (blue)
Original tie rod (solid)
Tie rod in jounce (dotted)
Note geometry error
New path for steering
arm ball joint
New position for body
ball joint
Tie Rod
Ideal path for steering
arm ball joint
Ideal Location for body ball joint
Steering Geometry Error in Wheel Jounce
z
Ball joint
Pulled out
Ball joint
Pulled in
Short Tie
Rod Path
y
Steering arm ball joint at
jounce
jounce
Steering arm
ball joint at ride height
Long Tie
Rod Path
Center for
Short tie rod
Ideal center
for tie rod on body
Center for long tie rod
Ideal path
Rear View
Steering Geometry Error in Wheel Jounce
z
y
Ball joint
Pulled out
Steering arm ball
joint at jounce
New Path
Center above
Ideal
jounce
New Tie Rod
Rear View
Ideal Tie Rod
Steering arm
ball joint at ride height
Ideal center
for tie rod on body
Ideal path
Steering Geometry Errors
Position of Tie Road to
Body Joint
Steering Arm Ball Joint Steering Geometry
Position
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
z
x
Roll Center for 4-Link Solid Axle
Fy
Upper Link UL
Top View
Lower Link LL
FyUL
FyLL
y
x
b
a
Side View
FyLL/FyUL = b/a
FyLL+FyUL = Fy
z
x
Roll Center for 3-Link Solid Axle
Track Bar
Top View
y
x
Side View
z
x
Roll Center for Hotchkiss
Roll Axis
Side View
z
x
Roll Center for Positive Swing Arm SLA
FU
Fy
FL
Fy
z
Rear View
y
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