hybrid vehicle suspension design

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Introduction: Suspension is the term given to the system of shock absorbers and linkages that connects a vehicle to its wheels. The job of a suspension system is to contribute to the car’s handling and braking for a better safety driving, and to keep the driver as isolated possible from bumps, vibrations, etc. It is important for the suspension to keep the wheel in contact with the road surface as much as possible, since all the forces acting on the vehicle do so through the tires. The suspension also has the important task to protect the vehicle itself and offers comfortable to the driver. Objectives: To provide good ride and handling performance such as ensuring the wheels follow the road profile and very little tire load fluctuation. To ensure that steering control is maintained during maneuvering,wheels to be maintained in the proper position with respect to road surface. To ensure that the vehicle responds favourably to control forces produced by the tires during longitudinal braking, accelerating forces, lateral cornering forces and braking. And resist squat, dive and roll of the vehicle body. To provide isolation from high frequency vibration from tire excitation.

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Introduction:

Suspension is the term given to the system of shock absorbers and linkages that connects a

vehicle to its wheels. The job of a suspension system is to contribute to the car’s handling

and braking for a better safety driving, and to keep the driver as isolated possible from

bumps, vibrations, etc. It is important for the suspension to keep the wheel in contact with

the road surface as much as possible, since all the forces acting on the vehicle do so through

the tires. The suspension also has the important task to protect the vehicle itself and offers

comfortable to the driver.

Objectives:

• To provide good ride and handling performance such as ensuring the wheels follow

the road profile and very little tire load fluctuation.

• To ensure that steering control is maintained during maneuvering,wheels to be

maintained in the proper position with respect to road surface.

• To ensure that the vehicle responds favourably to control forces produced by the tires

during longitudinal braking, accelerating forces, lateral cornering forces and braking.

And resist squat, dive and roll of the vehicle body.

• To provide isolation from high frequency vibration from tire excitation.

Design methodology

Designing the suspension system is done in two parts:

Part A mechanical design of the system

Part B Analysis of system.

Following steps were followed while designing the system

Part A: Mechanical design

1. Decide type of suspension system to be adapt.

2. Decide basic dimensions such as wheelbase, track width, centre of gravity, height, tyre,

wheels and other suspension parameters.

3. Selecting the coordinates for wishbone and damper mounting points using OPTIMUM

KINEMATICS SOFTWARE.

4. Design wishbones and knuckles.

5. Design springs and damper system.

Part B: Analysis the system

1. Using ANSYS WORKBECH analysis software to check the feasibility and safety of

system

2. Using OPTIMUM KINEMATICS the analysis of the suspension system is done.

Selection of type of suspension system

It is very important to choose suspension type according to the use of vehicle. Most of the

hybrid vehicles are used in the cities, considering the use of this vehicle on roads in cities

double wishbone suspension is selected. This is an independent suspension system, so it

increases the ride comfort, stability of vehicle, traction and also reduces the un-sprung mass.

Also double wishbone suspension system is light, offers easy packaging with high degree of

freedom in design of suspension geometry such as very easy to alter the camber, toe angle

etc. of the vehicle according to the requirements. And nonparallel unequal arms were

selected over nonparallel unequal arms to reduce the height of roll centre.

The basic dimensions

The dimensions are as follows:

1. Wheel track(front) = 1145mm

2. Wheel track(rear) = 1480mm

3. Wheel base =1525mm

4. Camber Angle = -2.5degree.

5. Toe angle = 4 degree.

6. Castor angle = 0 degree.

7. Kingpin angle = 0 degree.

8. Sprung weight =160 kg

9. Weight bias(front : Rear) = 39:61

Suspension system analysis:

Suspension system analysis is done using optimum kinematics software, the co-ordinates

are given in such way that the difference between maximum and minimum kinematic roll

centre must be least in order to avoid the rolling of the vehicle, the negative camber of -2.5

degree is set that will help to have the greater tire contact patch while the vehicle negotiating

the turn, and it is set to toe-out of angle 4 degree to improve cornering properties. For both

front and rear the double wishbone suspension system is used, Thebasic geometry of vehicle

front suspension shown below is created in optimum software with respect to chassis co-

ordinates.

Fig.1 front suspension system

3.1 Suspension analysis results table:

The maximum value of kinematic roll centre is =177.177mm

The minimum value of kinematic roll centre is=167.250mm

The variation in the position kinematic roll centre is=9.927mm

Fig.1 front suspension system

nalysis results table:

1. Analysis table

The maximum value of kinematic roll centre is =177.177mm

The minimum value of kinematic roll centre is=167.250mm

The variation in the position kinematic roll centre is=9.927mm

The scrub radius is=85mm

Positive scrub radius increases the steering effort

rolls as the wheel is steered, which reduces the effort when parking and it provides

greater road feel. This also allows greater

some compact sports cars this is why

3.2 Rear suspension system analysis:

Rear suspension system analysis r

The maximum value of kinematic roll centre is =197.207mm

The minimum value of kinematic roll centre is=187.063mm

The variation in the position kinematic roll centre is=10mm

The difference between the position of front and rear kinematic roll centre is =20mm

the steering effort but the advantage of this is that the tire

rolls as the wheel is steered, which reduces the effort when parking and it provides

. This also allows greater width in the engine bay, which is very important in

some compact sports cars this is why itis often the set-up of choice on race cars

Rear suspension system analysis:

Rear suspension system analysis results table

The maximum value of kinematic roll centre is =197.207mm

The minimum value of kinematic roll centre is=187.063mm

The variation in the position kinematic roll centre is=10mm

The difference between the position of front and rear kinematic roll centre is =20mm

the advantage of this is that the tire

rolls as the wheel is steered, which reduces the effort when parking and it provides much

width in the engine bay, which is very important in

up of choice on race cars

The difference between the position of front and rear kinematic roll centre is =20mm

3.2a Camber Angle:

It is the angle between the vertical axis of the wheels used for steering and the vertical axis

of the vehicle when viewed from the front or rear. The negative camber will help in

cornering of the vehicle to have the greater contact patch so the team set a negative camber -

2.5.

3.2b Toe angle:

If the leading edges of the wheel are pointing away from each other then it is called toe out

and if the leading edges of the wheel are pointing towards each other then it is called toe in.

In our vehicle toe out of angle 4 degree is set so as to enhance the cornering properties of the

vehicle.

3.2c Motion ratio:

The spring motion ratio is defined as the ratio of the spring displacement to the wheel

displacement, since the arm rotates around a common pivot, the length between the pivot and

the points of interest also determine the motion ratio. The amount of force transmitted to the

vehicle chassis is reduced when the motion ratio increases. This implies that the wheel rate

will increase as the motion ratio increases so the team has decided to fix the motion ratio as

0.6 in our case distance is measured as d1&d2 as shown in fig 2.

Fig.2 Mathematical model

Mathematically the motion ratio (MR) is defined by the displacement ratio or the length ratio

as below equation.

MR=����

MR=������

MR=0.61

Angle A=23 degrees

And cos (A) is called angle correction factor, cos (24) =0.914

3.2d Wheel rate:

Wheel rate is the actual rate of spring acting at the tire contact patch can be calculated as

WR=suspension spring rate ×(MR) 2×angle correction factor

=35×0.612×cos24

=12N/mm

Design of wishbones & knuckle

Wishbone and knuckle is most important part of the suspension system. Wishbones give

support to spring and help in vertical movement of the system while the knuckle supports the

axel of wheel. The wishbones and knuckle was designed in such a way that the roll centre is

located near the centre of gravity of vehicle and the distance of roll centre from centre of

gravity in minimum when the vehicle undergoes jounce or rebound. The team has decided to

use the front knuckle of fiat palio at front which is well suited with wheel and steering arm

dimensions. At rear we have designed the knuckle for which we have selected the aluminium

6066 material which is light in weight and have very good mechanical properties with good

corrosion resistance.

Mechanical properties of Al6066 are listed in table below

tensile strength 150 Mpa

Yield strength 83 Mpa

Shear strength 97 Mpa

fatigue strength 110 Mpa

Elastic modulus 80 Gpa

Poisions ratio 0.33

Hardness 43

Table . material properties of rear knuckle

Material Selection of A arms

Material consideration for the wishbone becomes the most primary need for design and

fabrication. The strength of the material should be well enough to withstand all the loads

acting on it in dynamic conditions. The material selection also depends on number of factors

such as carbon content, material properties, availability and the most important parameter is

the cost. Initially, three materials are considered based on their availability in the market

AISI 1018, AISI 1040 and AISI 4130, finally we have selected AISI 1040. And mechanical

properties of AISI 1040 is listed below

Carbon content 0.4%

Tensile strength 620 Mpa

Yield strength 415mpa

Hardness 201 BHN

Cast 425/meter

Table.

Dimensions of A arms

Outer diameter 25.4mm

Inner diameter 20mm

Left and right lower A arm oft

link length front

360mm

Left and right lower A arm fore

link length front

312mm

Left and right upper A arm fore

link length front

309mm

Left and right upper A arm fore

link length front

357mm

Left and right lower A arm oft

link length rear

441mm

Left and right lower A arm fore

link length rear

266mm

Left and right upper A arm fore

link length rear

282mm

Left and right upper A arm fore

link length rear

452mm

Angle between two links of A

arm

35degrees

Lower arm angle with horizontal 17degrees

Selection of spring and damper:

Team has decided to adapt passive suspension system because it is always a compromise

between comfort and safety for any given input set of road conditions and a specific stress.

The passive suspension system consists of passive elements, a spring and parallel damper

placed between the vehicle body and each of the wheels as shown as Figure1. The damper

contributes to both driver safety and comfort its task is to damping of body and wheel

oscillations. Spring and damper is designed based on the sprung as well as the unsprung

mass of the vehicle. The value of spring rate of suspension is obtained by considering wheel

ride rate and the tire stiffness of the vehicle.

Fig.1:Spring mass damper system

By the value of wheel ride rate which is vertical force per unit vertical displacement of tire

from ground with respect to chassis and the tire rate we can able to calculate the suspension

spring rate using the following relation

Kr =180lb/inch (31.52 N/mm)

Kt =1855.79lb/inch (325 N/mm)

Kr=���

���� � �……………………………..Eq. (1)

Putting the Kr&Kt in Eq(1)

180 = ������.����������.���

180 Ks+3340142.42=1855.79

Ks =200lb/inch

Ks =35 N/mm

2.1 Chassis Natural Frequency:

Mass of the chassis mc=160kg

Taking factor of safety=2

mc=160kg× 2

=320kg

mc=���

=80kg

=176.18 lb

wc= � ���� …………………Eq. (2)

substituting in Eq(2) we get

=� ������.� !�

��"

=20.88 rad/sec

W=2#fn………………………………..Eq. (3)

Fn=$�%

=��.��

�%

fn=3 Hz

2.2 Shock absorber for spring mass:

Chassis will oscillate about the zero reference but with a decreasing amplitude and

eventually reach steady state x = O if we take slightly under damped condition by setting

damping factor § as 0.9.

§=1/2('

√��×��)

C=2× § × √Ks × mc)…………………..Eq. (4)

Putting the values in Eq. (4)

C=2× 0.9 × �200 × ���/.��/���� �

C=2× 0.9 × √200 × 0.46

=18 lb-sec/inch

C=4 N-sec/mm

2.3 Material property of suspension spring:

The material of spring is AISI 1065 and this material can be used upto wire of diameter

16mm

s1. Ultimate tensile strength 635 Mpa

2. Shear stress 190.5 Mpa

3. Modulus of rigidity 79000

4. Young’s modulus 207000

5. Density 7.86 g/cm3

6. Tensile strength yield 490 Mpa

The diameter of the coil spring wire can be calculated using the formula

T= �34 5%�! …………………………….Eq. (5)

Where

D=mean coil diameter in mm

F=force acting on spring in N

K=stress correction factor

d=diameter of the wire in mm

T=shear stress in Mpa

Substituting values of D,F,K,T in …………………..Eq. (5)

6�=�×7×8×�

%×9

=�×���×/�×�

�%����

6�=347× 4

d=12mm

Number of active coils in spring is calculated as

i=:;�<�34! … … … … … … … … … … … … … … … … … … … Eq. �6�

Where ,

i=active number of coils

y=linear deflection in spring under static condition

=�����������<

�����/��!

Number of active coils=18

Number of inactive coils=2

Total number of coils Nt=18+2=20

F=K@

=�����

y=11.42 N/mm

Total gap between adjacent coils can be taken as the 15% of the deflection

=15% of x

Gap=0.15 ×11.42

=1.713

Solid length =Nt×d

=20×12

=240 mm

Free length = solid length +total axial gap+x

= 240+1.73+11.42

=253.15 mm

Eye to eye distance of the front spring and damper assembly is =310mm

Eye to eye distance of the rear spring and damper assembly is =350mm

The spring and damper we have calculated as 35N/mm and damping coefficient as 4N-

sec/mm

But for our suspension system we have selected 45N/mm fluid type damper which is

supplied by republic motors.

Ansys result

Static analysis results of front A arms

Fig total deformation Fig Equivalent stress

Fig factor of safety Fig 3D model

Static analysis is done using Ansys software the results are listed below

Number of nodes 18432

Number of elements 9296

Total deformation 0.28179mm

Equivalent stress 37.92Mpa

Factor of safety

(minimum)

2.27

Static analysis of rear A arms

Fig total deformation Fig Equivalent stress

Fig factor of safety Fig 3D model

Number of nodes 22843

Number of elements 12637

Total deformation 0.11256mm

Equivalent stress 53.251Mpa

Factor of safety

(minimum)

2.0234

Front knuckle static analysis results

Number of nodes 22843

Number of elements 12637

Total deformation 0.11256mm

Equivalent stress 53.251Mpa

Factor of safety

(minimum)

2.0234

Rear knuckle static analysis results

Number of nodes 22843

Number of elements 12637

Total deformation 0.11256mm

Equivalent stress 53.251Mpa

Factor of safety

(minimum)

2.0234