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Vyankatesh Madane, Akshay Baviskar, Anil Gaikwad and S S SanePiaggio Vehicles Pvt. Ltd., India

Design of Leaf Spring Rear Suspension for Rear

Mounted Engine

ABSTRACT

Light commercial vehicles are extension of three wheelers

due to their need for simplicity and load carrying capacity.

Smaller vehicle being simpler, have engine mounted at the

rear. This give an added advantage in term of simple and

light weight design and thus cost effective and have low

engine noise and vibration in cabin.

In many of the light commercial vehicles, which have been

downsized from the bigger vehicles like trucks, have the

aggregates designs similar to those of trucks like the driveline extending from the mid engine to rear axle having

integral differential. The axle carries the leaf springs for

giving robust look as well the load carrying capacity.

In the new rear suspension design of the light commercial

vehicle, advantage of the mid position engine concept and

rear engine concept have been captured.

This paper is discusses the design philosophy and the

packaging of the same along with criteria for design.

Keywords: Rear Suspension, Rear Axle, Handling, Finite

Element Analysis.

INTRODUCTION

Traditionally, for light commercial vehicles, Engine is placed

at front/middle giving huge space for traditional rear axle

with differential inside as shown in Fig 1. Propeller shafts

are used to transfer drive from power pack to axle housing.

This paper describes complete design of rear suspension with

leaf spring application for rear engine vehicle as shown in

Fig. 2. In new light commercial vehicle development, Engine

is mounted at rear to have low engine noise and vibration

inside cabin. At the same time there is a need of high

load carrying rear suspension to suit market requirement so

necessary to use leaf spring type rear suspension.

Copyright 2013 SAE International and Copyright 2013 SIAT, India

2013-26-0148

Published on9th-12thJanuary2013, SIAT, India

Figure 1. Truck Rear Suspension for Mid Engine.

Figure 2. Rear Suspension Concept for Rear Engine.

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Symposium on International Automotive Technology 2013

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Following are the major differences in this concept:

Differential housing will be part of engine power pack

Central tube need bend to have room for engine

Central tube axis offset from wheel axis

Engine to wheel drive by two axle shafts with CV joints

Vertical suspension movement cause horizontal axle shaft

movement inside differential Propeller shaft is not required

Less joints so less frictional losses

Detail suspension design is described in the following

sections.

METHODOLOGY

Suspension design for rear engine, consist of following steps:

Concept Vehicle Layout

Rear suspension layout made by two point deflection methodusing three link mechanisms. Calculations are made to get

front and rear suspension roll centers and corresponding

roll axis. In this concept level stage, different suspension

parameters like, spring stiffness and camber, spring mountings,

bump stop stiffness etc are fine tuned and finalized.

Rear Axle Design

Due to rear engine mounting, engine was coming in between

rear suspension making the design complicated. Considering

all packaging constraints, rear axle was designed by bending

center tube at offset position with wheel center. Spring design

for target load and stiffness is also completed.

Finite Element Analysis (FEA) of Rear

Axle:

Rear axle analysis done to get optimized size and thickness

of different parts to withstand overloading. Initially analysis

was done with complete vehicle model of rear axle, spring,

spring bushes, frame etc to get effect of leaf spring and bush

stiffness on stress level of rear axle. Based on this, small

reduced model was developed to make different iterations to

design, cost effective rear axle with minimum time.

Rear Suspension Design Considerations/Aspect

Following different overall design calculations were done to

get different performances:

Roll center analysis.

Ride comfort analysis.

Acceleration/brake analysis.

Camber compliance.

Design Verification

Strain data at critical locations are acquired on prototype axle

while vehicle running on different roads and track. Estimated

stress levels and measured stress levels are found matching

Acceleration levels on axle and frame was acquired and

found reasonably matching with calculated.

ValidationTo reduce testing time, sub system level testing was done

in actuator lab. Test setup consist of rear axle, spring, frame

etc. was made. Frame fixation and actuator loading point

in this sub system was finalized using FEA to match the

stress levels on rear axle between complete vehicle and sub

system. Only damaging load was extracted from pave track

and applied to actuators in test lab to reduce testing time

Testing was successfully completed for targeted kilometers

Due to FEA and design verification activities no failure

occurred during testing.

Details of each step are described in following sections.

DESIGN OF REAR SUSPENSION

Initial concept phase started with vehicle layout making to

finalized different suspension parameters.

Concpet Layout

First step done was finalizing spring mounting location on

frame, which further freezes wheel base and leaf spring

length. Based on this wheel base, rear axle loads at differen

conditions like Gross Vehicle Weight (GVW), overload etc

was calculated. Semi elliptical leaf spring was used for

the layout as shown in Fig. 3. This type of spring can beconsidered as two cantilever springs, and the resulting spring

action can be determined by considering the spring as a

three-link mechanism. Geometry of spring action, including

wheel movement at different conditions and shackle effect

was estimated by two-point deflection method layout for

selected wheel displacements.

Figure 3. Vehicle Layout at Un-laden Condition.

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Rear Suspension Layout by Two-Point Deflection

Method

This method has the advantage that all the layout work

can be done within the overall length of the spring. The

unsymmetry factor is small so possible to use this method

to construct the spring layout.

The principle of this method was based upon the use of

the two cantilever deflections corresponding to a given

deflation at the center of the spring seat. These deflections

may be computed for vertical positions of the spring seat,

for maximum compression (metal-to-metal), curb weight and

maximum rebound. When they are applied to the three-link

equivalent of the spring with main leaf in the flat position,

the path of the axle and the angle of the spring shackle can

be determined entirely by construction the layout as shown

in Fig. 4.

Nomenclature for above layouta = Fixed cantilever length called front

length

b = shackled cantilever length called rear

length

L = total spring length

e = Eccentricity =0.5 (eye ID +t)

u = Distance between points B and G

Ff, Fc and Fr = Deflection of point F

Gf, Gc and Gr = Deflection of point G

During vertical movement of wheels, axle shaft will slide inand out inside differential housing which will causing shaft

falling or hitting with differential pin. To resolve this problem

axle shaft layout was made.

Axle Shaft Layout

The axle shaft layout made in the flat condition and decides

the length of the shaft. To avoid shaft falling or hitting with

differential pin, axle shaft layout made at different vehicle

condition i.e. rebound, curb weight and full bump condition

as shown in Fig. 5.

Rebound occurs when the wheel hits a dip or hole and

moves downwards. By using this layout, the axle shaft falling

clearance with respect to gear box cage and maximum axle

tilt angle are obtained.

Curb weight condition happens when the vehicle loaded

with its own curb weight. In this condition, the axle shaf

falling clearance and shaft angle in curb weight condition

are obtained.

Full bump occurs when the wheel hits a bump and moves

up. It is upward displacement of the axle relative to the

body. By this layout, the axle shaft hitting clearance with

respect to differential pin and axle tilt angle in full bump

condition are obtained.

Based on above studies, wheel vertical displacement is

finalized which satisfies all the layouts.

Now as load coming on single rear wheel at all condition

as well as wheel displacements are finalized, spring camber

and bump stop height was finalized to have desired springstiffness and bump stop stiffness as shown in Fig. 6. This

stiffness was further studied and fine tuned by doing handling

and ride comfort calculations. Detail design of spring, bump

stop and rear axle are in following sections.

Figure 4. Suspension Layout: Two point Deflection Method.

Figure 5. Axle Shaft Layout in Rebound Condition.

Figure 6. Spring and Bump Stop Stiffness Finalization.

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Rear Axle Design

Based on layouts and packaging studies as shown in Fig.7,

center distance between left and right spring and wheel

mounting was finalized. Central tube was bend to avoid

interface during vertical travel of axle. Tube axis also designs

at offset position from wheel center line to have room for

axle shafts.

Leaf Spring Design

Based on stiffness finalization as shown in Fig. 6, numbers

of leaf for first stage and second stage with thickness and

width to give required stiffness are finalized as shown in Fig

9. Due to packaging constrains, 50 mm width was selected

Once spring design for stiffness, stress levels on each leaf

are calculated at minimum (curb load) and maximum (meta

to metal) loads. From these stress levels, estimated life iscoming 62,000 based on SAE spring standard [1], which was

equivalent to 160,000 km on road, which is more than target

Figure 7. Packaging Study to Finalized Rear Axle.

Figure 8. Rear Axle.

Figure 10. Bump Stop Design.

Bump Stop Design:

As shown in layout in Fig 5, axle shaft moves inside

the differential when wheel move upward. If this vertica

movement is not stopped, axle shaft will hit inside differentia

pin so in this concept, bump stop design become critical

Lot of iterations is done to get required non liner stiffness

as well as required life as shown in Fig. 10.

Concept rear axle was consists of wheel support plate,spring support plate and central tube as shown in Fig. 8.

Once concept was finalized, cross section and thicknesses of

different plates and tubes were finalized using finite element

method.

Figure 9. Leaf Spring Design.

FINITE ELEMENT ANALYSIS OF

REAR AXLE

Model Creation

Initially analysis was done with complete vehicle model

as shown in Fig. 11 consists of rear axle, spring, spring

bushes, frame etc. to get effect of all components on stres

level of rear axle. Based on this, small reduced model was

developed and correlated with full vehicle model to give

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similar values of stresses to make different iterations to

design, cost effective rear axle with minimum time.

Figure 11. Full Vehicle Model and Reduced Model.

Table 1. Stress Comparison between Full Vehicle Model

and Reduced Model.

Figure 13. Graph of Stress in Wheel Support Plate forDifferent Thickness.

Figure 14. Graph of Stress in Centre Tube for Different

Thickness.

Inertia relief analysis is done in full vehicle model whereas

proper constrains are finalize in reduced model.

Table 1 show, stress levels at full model and reduced model

for critical locations. As stress levels are matching in full

model and reduced so possible to use reduced model to

save analysis time.

Optimization

Finite element analysis was done to find out cross section

and thicknesses of different plates and tubes of rear axle.

As material thickness increases stress on the component

decreases but the weight of the component increases, same

can be verified by the graphs shown in Fig. 12 to 14, for

spring support plate, wheel support plate and central tube.

Hence, optimized design was selected among all the design

proposals based on material endurance strength and minimum

weight criterion. Selected design proposal for the particular

component is shown by bold dot on graph.

Stiffness Variation of Suspension Rubber

Bush

FEM analysis clearly shows the sensitivity of spring

mounting rubber bush stiffness for stress level on central tube

(as shown in Fig. 15), which forces to finalize the optimum

value of stiffness which will reduce the stresses coming on

the rear axle components.

Figure 12. Graph of Stress in Spring Support Plate for

Different Thickness.

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Graph in Fig. 15 shows, stress levels on rear axle tube at

critical locations for different spring bush stiffness. Results

shows, stress levels get increase for lower bush stiffness due

to flexing of leaf spring from shackle bracket; hence higher

bush stiffness is finalized. However due to manufacturing

constrain it is not possible to make bush with stiffness 3000

N/mm; hence cup washer (spacer) is designed to give higher

assembly stiffness in lateral direction even with lower shore

hardness rubber bush.

Cup Spacer

Cup spacer as shown in Fig. 16, was used to restrict flexing

of spring and it eventually restrict lateral movement of

shackle plates which gives reduced stress level on rear axle

components as can be seen from the bush stiffness variation

in Fig 15.

SUSPENSION DESIGN

CONSIDERATIONS/ASPECT

Roll Center Analysis

CAE was very effectively used for giving design input for

finalizing the stiffness of leaf spring by doing dynamic

calculations for vehicle handling capabilities.

Complete vehicle model as shown in Fig. 17, is used to

study dynamic behavior of the vehicle in rolling by giving

lateral acceleration to the vehicle.

Figure 16. Cup Washer (spacer) Design.

Figure 17. Model for Dynamic Calculations.

Figure 18. Graph of Roll Angle vs. Lateral Acceleration.

Figure 15. Graph of Spring Rubber Bush Stiffness vs.

Stress on Central Tube.

Graph in Fig. 18 shows that, decrease in stiffness of rear

leaf spring increases the vehicle roll for same value of latera

acceleration (keeping front suspension stiffness constant)

also lateral acceleration at which side rear wheel leave road

surface goes on increasing. Up to the stiffness of 120 N

mm, one of the rear wheels will leave the road surface when

lateral acceleration reaches to its toppling value (1.3 g); bu

below 120 N/mm, at 80 N/mm, front wheel will leave the

road surface at its toppling acceleration value (1.2 g).

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Figure 19. Ride Comfort Analysis.

Based on Fig. 18, 120 N/mm was the best handling rear

spring stiffness as it give highest lateral toppling acceleration

(1.3 g) however slightly higher stiffness (160 N/mm) is

selected to reduce wheel vertical movement to avoid hitting

of shaft with differential pin.

Ride Comfort Analysis:

On complete vehicle finite element model, rear axle hadgiven input excitation acceleration as a sine swipe from 10

Hz to 20 Hz at constant amplitude 10g and response at frame

for different rear spring stiffness was extracted as output.

Based on Fig. 19, for same input excitation, response at

frame start reducing as spring stiffness reduced. Lowest frame

acceleration (0.3 g-RMS) was for 120 N/mm rear spring

stiffness however slightly higher stiffness (160 N/mm) was

selected to reduce wheel vertical movement to avoid hitting

of shaft with differential pin.

Figure 20. Camber Compliance Analysis

Fig. 20, analysis shows that camber change is 0.250 per 1

payload. To have zero degree camber at overload condition

+0.50 camber was kept at part level.

Acceleration/Brake Analysis

In this type of rear mounted engine vehicles, risk of front

wheel lift during sudden acceleration as engine weight was

at rear. Complete vehicle model as shown in Fig. 17, was

used to study dynamic behavior of the vehicle in acceleration

by giving longitudinal acceleration to the vehicle. Force data

extracted by beam elements at wheel locations shows, at

1.5g acceleration, front wheels will leave road surface. Based

on Vehicle configuration, maximum sudden acceleration of

vehicle was around five times less than 1.5 g acceleration

which confirms front wheels will not leave road surfaces.

Camber Compliance

Camber is important parameter for tyre wear. Camber

should be zero degree to get good tyre life. Based on

Design Verification

Strain gauges were pasted at critical locations as shown in

Fig. 21 and strain data were acquired on prototype axle a

different events. Estimated stress levels and measured stresslevels were found matching. Similarly, acceleration levels on

axle and frame was acquired and found reasonably matching

with calculated, which verify that input loading taken in

analysis were correct.

Figure 21. Strain Gauging on Rear Axle.

Fig. 22 shows, difference between analysis and actual (strain

gauging) results were within 12% which is within acceptable

limit.

Figure 22. Strain Gauging vs Analysis Results.

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Validation

To reduced testing time, sub system level testing is done

in actuator lab. Test setup as shown in Fig. 23, consist of

rear axle, spring, frame etc. was made. Frame fixation and

actuator loading point in this sub system was finalized using

FEA to match the stress levels on rear axle between complete

vehicle and sub system. Only damaging load was extracted

from pave track and applied using actuators in test lab toreduce testing time. Testing was successfully completed for

targeted km. Due to FEA and design verification activities

no failure occurred during testing.

Differential oil was not required so less service cost.

Less joints so less frictional losses.

Less noise and vibration in cabin as engine was a

rear and propeller shaft was not present. Sometime, if

propeller shaft was not balanced it causes vibrations.

Being simple and less parts, this rear suspension system

was having approximate cost saving of 10%.

However one disadvantage was rear axle load is higher being

Engine at rear.

CONCLUSIONS

This paper gives an overview of the design, developmen

and validation of the leaf spring rear suspension for a ligh

commercial vehicle for rear engine.

The process of development also considers the design of

other aggregates components like leaf spring, silent bush

assembly, rubber bump stop and more importantly the rear

axle.

Aspects of the structural durability of the components have

been evaluated by using finite element analysis and actuato

testing.

Last and least, vehicle dynamics stability and overall

suspension design parameters have been verified by using

finite element analysis and running vehicle on road.

This system was having approximate cost saving of 10%.

This system has been implemented on the vehicle successfully

REFERENCES1. SAE spring committee, AE 11: SAE Spring Design

Manual.

CONTACT INFORMATION

Mr. Vyankatesh Madane

Manager,

Piaggio Vehicle Pvt. Ltd., India.

Email: [email protected]

Figure 23. Test Setup of Rear Suspension System Testing.

ADVANTAGE OF LEAF SPRING REAR

SUSPENSION FOR REAR ENGINE:

Following were the different advantages;

Simple frame design being engine at rear.

Propeller shaft (link between front gearbox to reardifferential) and separate differential housing was not

required.

The Technical Paper Review Committee (TPRC) SIAT 2013 has approved this paper for publication.

This paper is reviewed by a minimum of three (3) subject experts and follows SAE guidelines.

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or

transmitted, in any form by any means, electronic, mechanical photocopying, recording, or otherwise,

without the prior written permission of SIAT 2013.

Positions and opinions advanced in this paper are those of the author(s) and not necessarily those of

SIAT 2013. The author is solely responsible for the content of the paper.

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