Introduction

What is Suspension System?

The Suspension system is a device connecting the body with wheels. The motion is constrained
by the suspension. All kinds of forces and movements between the wheels and the ground passes to
the body through the suspension. The design of suspension system is an important part of the
overall vehicle design which determines performance of the racing car.

SAE Suspension should have following requirements

1) It must have Shock Absorbers.
2) Suspension travel is not less than 25.4mm (1 Inch) for both jounce and rebound.
3) Must have appropriate attenuation vibration ability.
4) Ensure the car has good handling and stability performance

SAE International hosts multiple Formula competitions worldwide each year. The Formula SAE
Collegiate Design Competition is governed by very strict rules and regulations to allow for fair
competitions and the safety of the drivers. The rules state very specific parameters in terms of the
suspension and wheel assembly design and the maximum choice of the engine; but, it remains broad
in other areas such as control mechanisms and aerodynamic design. In general, the rules are tailored
to protect the drivers while ensuring ample space to create one’s own custom designs.

The objective behind the project to overcome the following conditions,

1. Oversteer Configuration for high cornering

2. Light weight and compact assembly
3. Reliable and as per driver safety consideration
4. To achieve good rollover stability
5. Ease for the driver at the time of bump and cornering.

Methodology
This the very important factor by which a planned method from designing to manufacturing the
final prototype. For designing purpose, use of basic hand calculations and research of design
parameters referring design reports which are paper published and also certain figures to
understand the basics by using internet and referring certain books like Carroll Smith’s Tune to Win
and Milliken & Milliken’s Race car vehicle dynamics, etc.
 Drafting and CAD model designing is done on Licensed CAD software’s like Solidworks, Catia, etc.
FEA analysis is done on licensed version of ANSYS. Lotus Shark Suspension Analysis software is used
for dynamic analysis of suspension.
Manufacturing is done by Major machining processes like laser cut, grinding, welding, lathe
operations, etc. and assemblies were done by bolting and press fitting, etc.

General Terms in Suspension System and Wheel Assembly

Camber Angle
Camber angle is the inclination of the vehicle tire with the vertical axis when viewed from the
Front section. In case top of tire leans in towards the centre of the car this is the condition of
negative camber. Positive camber is opposite of this.
Note: Increasing positive camber angle will enlarge the slip angle for a specific cornering force which
will decrease the largest possible cornering force of the vehicle but will also slow down the onset by
Breakaway which is assumed to mean the car starting to slide. On the other hand, increasing
negative camber angle, opposite will occur with a higher cornering force and less time for the car to
Breakaway.

Caster Angle
Caster angle is the angular displacement from the vertical axis of the suspension of a wheel in a
car measured in longitudinal direction. In other words, it’s a line joining the upper and the lower ball
joint of the upright with respect to vertical axis drawn from the center of the tire.
The purpose of this is to provide a degree of Self – Centering for steering the wheel casters around
so as to trail behind the axis of steering. This makes car easier to drive and improves its straight-line
stability.
Note: Improper adjustment will result in steering inputs required both in and out of a corner
resulting in a car which is difficult to keep in straight line. Too much caster (positive) in the front of
the car will understeer more, too little (negative) caster will give oversteer characteristics. A large
positive caster setting (wheel facing forward of axis) is good for high speed stability but can make it
more difficult for turning the steering. Excessive amount will increase tire wear. Excessive caster
angle will make the steering heavier and less responsive.

Kingpin Inclination
The Kingpin axis is determined by the upper ball joints and lower ball joints on the outer end of
the A-arms. This axis is not necessary centred on the tire contact patch. In front view the angle is
called Kingpin inclination and the distance from the centre of the tire print to the axle centre is
called Scrub or Scrub radius. The distance from the kingpin axis to the wheel centre plane measured
horizontally at axle height is called Spindle length. Figure shows the kingpin geometry.
Note: If the spindle length is positive the car will be raised up as the wheels are turned and this
results in a increase of the steering moment at the steering wheel. The larger the kingpin inclination
angle is the more the car will be raised regardless of which way the front wheels are turned. If there
is no caster present this effect is symmetrical from side to side. The raise of the car has a self-aligning
effect of the steering at low speeds. Kingpin inclination affects the Steer camber. When a wheel is
steered it will lean out at the top, towards positive camber if the kingpin inclination angle is positive.
The amount of this is small but not to neglect if the track includes tight turns.
If the driving or braking force is different on the left and right side this will introduce a steering
torque proportional to the scrub radius, which will be felt by the driver at the steering wheel.

Trackwidth
The Distance between centre axis of tire from front view is known as trackwidth.
Note: Generally narrow (small) trackwidth are used at rear to avoid hit cones with the back when
they are already away from this cones with front wheel. Wider track decreases load transfer which is
generally good for getting grip on that end of the car. Only disadvantage is that weight increases
because longer A-Arm, Push or Pull-Rod, Tie Rods and Driveshafts. Also, the moment of inertia yaw is
increased a lot because it is depending on the lever arm of wheels. A wider track will make the
springs feel weaker since it requires longer lever. So, a wider track will make the front suspension
feel softer, promoting a reduction in understeer.

Wheelbase
The Distance between the centre axis of Front and Rear Wheel from longitudinal direction is
known as Wheelbase.
Note: Large Wheelbase causes Cornering issues but increases driver safety. Small Wheelbase causes
easy cornering of vehicle but decreases driver safety.

Instant Center and Roll Center

Instant center is the momentary centre which the suspension linkage pivot around. As the
suspensionmoves the instant centre moves due to the changes in the suspension geometry. Instant
centres can be constructed in both the front view and the side view. If the instant centre is viewed in
front view a line can be drawn from the instant centre to the centre of the tire’s contact patch.
If done for both sides of the car the point of intersection between the lines is the Roll centre of the
sprung mass of the car. The position of the roll centre is determined by the location of the instant
centres. High instant centres will lead to a high roll centre and vice versa. The roll centre establishes
the force coupling point between the sprung and the unsprung masses of the car. The higher the roll
centre is the smaller the rolling moment around the roll center.
If the roll centre is located above the ground the lateral force generated by the tire generates a
moment about the instant centre, which pushes the wheel down and lifts the sprung mass. This
effect is called Jacking. If the roll centre is below the ground level the force will push the sprung mass
down. The lateral force will, regarding the position of the roll centre, imply a vertical deflection. If
the roll centre passes through the ground level when the car is rolling there will be a change in the
movement direction of the sprung mass.

Ground Clearance and Rollover Stability

The ground clearance must be sufficient to prevent any portion of the car other than the tires
from touching the ground. Intentional or excessive ground contact results in higher C.G which
decreases the rollover stability. The track and center of gravity (C.G.) of the car must combine to
provide adequate rollover stability.
Motion Ratio
For packaging damper in the suspension system includes required wheel travel, jounce bump
travel, desired wheel rates, strength requirements and packaging constraints. Most important is
Motion ratio. Motion ratio is nothing but the ratio of wheel travel to spring travel.

Motion Ratio (MR) = Wheel Travel / Spring Travel

Note: Higher Motion ratio requires lower spring rates for the same wheel rate. Lower spring rates
are also lighter and results in less spring and shocks friction as well as lower component load. The
other reasons are greater damper travel and higher shock velocities and wheel displacement are
quite small on a FSAE car, Higher Motion Ratio produces better shock performance.

C.G. Height
Center of gravity, also known as center of mass, is that point at which a system or body behaves
as if all its mass were centered at that point. Where the weight, and also all accelerative forces of
acceleration, braking and cornering act through it.

Centre of gravity location can be defined as:

-The balance point of an object
-The point through which a force will cause pure translation
- The point about which gravity moments are balanced
Note: When making an analysis of the forces applied on the car, the CG is the point to place the car
weight, and the centrifugal forces when the car is turning or when accelerating or decelerating. Any
force that acts through the CG has no tendency to make the car rotate. The center of mass height,
relative to the track, determines load transfer, (related to, but not exactly weight transfer), from side
to side and causes body lean. When tires of a vehicle provide a centripetal force to pull it around a
turn, the momentum of the vehicle actuates load transfer in a direction going from the vehicle's
current position to a point on a path tangent to the vehicle's path. This load transfer presents itself
in the form of body lean.

Anti-Dive
Anti-dive describes the amount of front of the vehicle dives under braking. As the brakes are
applied weight is transferred to the front and that forces the front to dive. Anti-dive is dependent on
the vehicle centre of gravity (C.G), the percentage of braking force developed at the front tires vs.
rear and the design of the front suspension. For the very common double A-Arm Suspension
AntiDive is design to suspension based on the angle of the A-Arm mounting points when viewed
from the side. If the intersection point of the extension lines for the mounting points is located
above the neutral line, there is more than 100 % Anti-Dive. High Anti-Dive values require more
complex suspension design and causes ‘Rattling’ type noise. Typical values are in range of 0 – 50%.
Note: Unless specified, all calculations are based on vehicle at rest on a level road surface. The TCA
(Track Control Arm) is horizontal to road surface and therefore centres of the inner sub-frame
bushing and the outer ball joint are at same distance from the ground. This may not be the case with
shorter load road springs where the ball joint may end up higher than inner bushing. The stub axle
lies on a vertical line, Perpendicular to the load surface that passes through the centre of the TCA
sub-Frame bushing. The tire contact patch is therefore also centred under the TCA. In real life the
axle would be slightly behind the TCA due to Caster Angle.

Anti-Squat
Squat is a term used to refer to the amount the car tips backwards under acceleration. Over
100% of antisquat (AS) means suspension will extend under acceleration. With 100% AS suspension
would neither extend nor compress. Under 100% AS means tendency to compress under
acceleration.
The calculation of anti-squat is similar to that of antidive. Locate the rear Centres of the suspension
from the vehicle’s side view. Draw a line from the rear tire contact patch through the Instantaneous
Centre. This isthe tire force vector. Now draw a line straight down from the vehicle’s centre of
gravity. The Anti-squat is the ratio between the height of where the tire force vector crosses the
centre of gravity plane expressed as a percentage.

Suspension Geometry (Push/Pull)

Push-rod or pull-rod, the difference as the name suggests is the whether the rod push up to the
rocker or pull down to the rocker. The main advantage of a pull rod lie in the possibility to make the
nose lower, assemble most suspension parts lower to the ground and thus lowering the height of
the centre of gravity.
Pull rod set up has a strut from the outer end of the upper wishbones that runs diagonally to the
lower edge of the chassis and "pulls" a rocker to operate the spring/damper. A push rod is the
opposite; the strut runs from the lower wish bone to the upper edge of the chassis.

Types of Load Transfers

Longitudinal Load Transfer

Longitudinal load transfer is the result of the car mass accelerating from the front of the vehicle
to the back or the back to the front under accelerating or deaccelerating (Braking) respectively. It is
important to mention that “The total weight of the vehicle does not change, Load is merely
transferred from the wheels at one end of the car to the wheels at other end.”

Lateral Load Transfer

 In essence the lateral load transfer experienced by the vehicle is the same principle as the
longitudinal transfer only just rotated to 90 degrees such that load is either transferred from the
right to left under left hand corner and from the left to the right in right hand corner.
Vertical Load Transfer
Vertical load transfer is nothing but opposite reaction of vehicle load on wheels and also
fluctuating loads occurs during bump.

Suspension Compartment Geometry

After Fixing all the general parameters 3D sketch of suspension compartment and also the A-
Arms were drafted.

Shocks Selection
The shocks selection is done on the basis of the design requirement and analysis and on the
performance, cost and on the market availability.

Decision
By comparing all the parameters, DNM BURNER RCP 2 was finalized.

Design of Suspension Components

A-Arms Design
A-Arms design started with CAD geometry drawing using suspension compartments and
considering trackwidth, wheelbase, etc. parameters.
Selection of material for A-Arm was done as per Material availability and Machining cost.

Decision
As comparing all the parameters of material, mild steel material for A-Arms, Bell cranks and
pushrods was selected. Afterwards cad modelling and simulation of A-Arms by applying material
which was selected to get proper results before manufacturing the actual prototype is done.
Firstly A-Arm pipe dimensions as 16mm OD x 3mm Thick is taken considering the design and
simulation results. The pipes are cut on manual cutter. Then milling of A-Arm pipes is done to get fit
properly in Bearing Wafer. The entire A-Arm is welded and mounted on chassis using fasteners.
On manufacturing, no large variations occur due to machining accuracy.

Bearing Wafer
Simulation of bearing wafer with 8mm thickness is done with A-Arm pipe and after getting results
it is reduced to 5mm as per weight reduction concerns and easy mounting of spherical bearing.
For the precision in Manufacturing, Bearing wafers are laser cut and made bracket slots in lower A-
Arms bearing wafers for brackets of pushrods.
T-Section
T-Section is basically an extension to A-Arm in which Rod End is fixed by threading and jam nut.
The advantage of T-Section is A-Arm pipe is not directly contact with Rod end and also because of
some heavy loads if rod end fails and its thread stuck in A-Arm so, no need to change full A-Arm.
For Manufacturing T-Section 16mm OD solid Bar is bought and manufactured by machining it on
Lathe machine and also by Tapping process. The manufactured part is very precise and tapping was
also good. Only slight difference in mm (Around 0.5mm) due to manual lathe machine operations.

Bushings
Bushings are used to give certain clearance between Rod Ends/Spherical Bearings and Mounting
Brackets. One more use of bushing Is to limit the total vertical movement of A-Arm. Bushings are
used to separate two different material contact to avoid wear.
For car 3 types of bushings with same diameters were used, only changes are in length of 2mm,
4mm, 6mm length respectively. Keeping bushings in even multiples so that it can be easily
manufactured and easily separable like for bearing Wafer mount on upright 6mm bushings on both
sides were used, and for A-Arm mount on Chassis 4mm bushing and for pushrod to AArm 2mm
bushings were used.
The quality and precision were quite good also the thickness of bushing was enough that it will not
squeeze easily.

Fasteners used in a-Arm

Fasteners Selection
Bolts are selected as per required length, Calculations, safety concern and by make/buy decision.

Decision
Comparing the all parameters it is beneficial to opt for buy components.

Bolts
M8 bolts at mountings of suspension A-Arms on Chassis and upright were used and then
calculated the loads on bolts and by using some formulation calculations of the bolt size were done.
High grade bolts are selected for safety concerns.

Nuts and washers

As per bolt size M8 Nyloc nut and washers were used. Nyloc nut are used for positive locking
purpose.
Jam Nut
Rod end locking in T-Section of A-Arm are locked using Jam nuts.

Rod Ends and Spherical Bearing

Rod ends of POS G-8 Male as per bolt size are used for A-Arms mounting on chassis and spherical
bearing of LS GE – 8E for A-Arms mounting on Upright.

Bell Crank Design

Bell crank design is quite simple and easy to manufacture using laser cut. In bell crank design
firstly geometry diagram is started by which the angle between pushrod and shocks can be checked.
The angle between pushrod and shocks is from 80° – 120° for better load transfer.
As one can do good weight reduction in bell crank; bell crank is first drawn by checking the angles
between pushrod and shocks and geometry.

Decision – By comparing all the parameters of different machining process, it was beneficial to opt
for Laser cutting.

Spacer
Spacer is nothing but a circular pipe of certain dimensions to keep distance between two bell
crank plates.
The final Assembly is done by bolting Bell crank on Chassis.

Fasteners used in Bell Crank

Bolts
Selection of M8 bolts at mountings of suspension Bell crank on Chassis is done and then
calculated the loads on bolts and by using some formulation calculations of the bolt size were done.
High grade bolts are selected for safety concerns.

Nuts and washers

As per bolt size M8 Nyloc nut and washers were used. Nyloc nut are used for positive locking
purpose. Here Washers were also used to separate the Bell crank plate from chassis bracket to avoid
wear.

Pushrod Design
 Firstly, drafting the geometry of shocks connected to bell crank, bell crank to pushrod and
pushrod to lower A-Arm and decided the length of pushrod as per designed geometry.
Material selection for manufacturing of pushrod is as same as A-Arm. (Refer Table 6). CAD model of
pushrod is done by taking same dimension pipe which is used in A-Arm and T-Section.
Then pushrod is manufactured by cutting the pipes manually as per required length and then
welding the T-sections on both ends to fix the Rod-Ends in pushrod. Finally, the Pushrod is fixed by
fitting bushings and bolting on Bell crank and Lower A-Arm. Final manufactured Pushrod is as same
as design because the pushrod distances are complete decimal values which is easy to manufactured
and welding is done properly.

Fasteners used in Pushrod

Bolts
Selection of M8 bolts at mountings of pushrod on Bell crank and lower A-Arm is done and then
calculated the loads on bolts and by using some formulation calculations of the bolt size were done.
High grade bolts are selected for safety concerns.

Nuts and washers

As per bolt size M8 Nyloc nut and washers were used. Nyloc nut are used for positive locking
purpose.
Jam Nut: Rod end locking in T-Section of A-Arm are done using Jam nuts.
Rod Ends: Rod ends of POS G-8 Male as per bolt size for Pushrod mounting on Lower A-Arm and Bell
crank were used.

Bushings
In pushrod 2mm length bushings in bell crank mount as well as Lower A-Arm mount are used.

Tie Rod
Tie rod is nothing but a rod which olds the rear wheel to keep it position properly. In this case tie
rod is directly welded to Rear A-Arm which reduces extra bracket and bolting cost and other is
mounted on Rear upright via bolting of Rod end of Tie rod on upright bracket.
As the Tie Rod is directly welded to rear lower A-Arms therefore assembly takes slight more time.

Fasteners used in Tie Rod

Bolts: Selection of M8 bolts at mountings of tie rod on upright is done and then calculated the loads
on bolts and by using some formulation calculations of the bolt size were done. High grade bolts are
selected for safety concerns.
Nuts and Washers: As per bolt size M8 Nyloc nut and washers were used. Nyloc nut are used for
positive locking purpose.
Rod Ends: Rod ends of POS G-8 Male as per bolt size for Tie Rod mounting on upright are used.
Nuts and Washers: In Tie Rod 4mm length bushings in upright Mounting are used.
Damper Springs
DNM Burner RCP 2 shocks were selected as the DNM company have only specific spring rates
and they were not as per our design. So, springs for the shocks were customized.

Decision
After comparing all the parameters, it was decided to manufacture springs from local market as
per time concern. The manufactured springs free length is slightly more than actual design so that it
gives rebound easily and bending will not occur

Calculations
FRONT REAR
FREQUENCY fn 2 2.2
RIDE RATE Kr(2×22.7×fn)×M 9790.647 13852.97
SPRING RATE Ks 123N/mm 115N/mm
MOTION RATIO Mr 0.95
WHEEL RATE Kw=SR(Mr^2) 116 109.23

1/Ksf=1/Kr - 1/Kt = 1/9790.647 - 1/100000

= 1/9.2×10^-5
= 10869.56
1/Ksr=1/Kr - 1/Kt = 1/13852.97 - 1/100000
= 16080.612

K@f = 0.4 × 10869.56 × 1.210

= 5260.867
K@r = 0.6 × 16080.612 × 1.345
= 12977.053
R@ = (W/g × hi) / Kff + Kfr - Whi)
= (300 × 9.81 × 0.215) / ((5260.867 + 12977.053)
-(300 × 9.81 × 0.215))
= 632.745/17605.175
 = 0.0359 rad/g
R@ = 2.056 deg/g

Simulation Results

Front Bellcrank

Front Bellcrank Deformation

Front Inboard Clamp
Front Inboard Clamp Deformation

Front Outboard Clamp

Front Outboard Clamp Deformation

Rear Bellcrank
Rear Bellcrank Deformation

Rear Inboard Clamp

Rear Inboard Clamp Deformation

Rear Outboard Clamp

Rear Outboard Deformation