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