1. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
6480(Print), ISSN 0976 – 6499(Online) Volume 5, Issue 5, May (2014), pp. 141-148 © IAEME
141
FAILURE OF FRONT SHOCK ABSORBER OF A MOTORCYCLE
Yogesh Mahajan, A.A. Likhite, D.R. Peshwe
Department of Metallurgical and Materials Engineering, Visvesvaraya National Institute of
Technology, Nagpur, Maharashtra, India
ABSTRACT
In vehicle shock absorber is a critical component and reduces the effect of traveling over
rough ground, leading to improved ride quality and vehicle handing. Front Fork pipe is a vital
component in the suspension assembly and failure can cause fatal incidences. This paper presents the
classical failure analysis of a front fork pipe. The fractured surfaces as well as the surface of the fork
pipe were examined in a scanning electron microscope at suitable magnifications. Optical
microscopy was performed to evaluate the basic microstructure of the as received material. Detailed
electron microscopic studies have indicated that the failure was due to the single overload. The
presence of inclusions is responsible for lowering the strength of the steel.
Keywords: Failure, Front Fork Pipe, Inclusion, SEM, Shock Absorber.
I. INTRODUCTION
Motorcycle fork connects a motorcycle’s front wheel and axle to its frame. It typically
incorporates the front suspension and front breaks [1]. A front suspension is a mechanical device
designed to damp shock impulse, and convert kinetic energy to another form of energy
usually thermal energy which can be easily dissipated inside the viscous fluid. Various shock
absorbers are designed so as to damp the shock impulse in effective manner. Pneumatic, hydraulic
and electromagnetic shock absorbers are available in the market.
The motorcycle is a spatial mechanism composed of four rigid bodied: the rear assembly,
front assembly, the front wheel, and the rear wheel [2]. In front assembly of a vehicle shock
absorbers reduce the effect of traveling over rough ground, leading to improved ride quality and
vehicle handing. An automobile shock absorber contains spring-loaded check valves and orifices to
control the flow of oil through an internal piston. This provides a cushioning action so road shocks
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2. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
6480(Print), ISSN 0976 – 6499(Online) Volume 5, Issue 5, May (2014), pp. 141-148 © IAEME
142
have a minimal effect on the occupants and load in the vehicle. Mono suspension was introduced in
the year 1970’s by YAMAHA motors [3]. Shock absorbers help cushion vehicles on uneven roads.
Front fork pipe has an application in shock absorber of motorcycle or scooter. In motorcycle’s fork
tubes are employed in pairs. Usually Cam drum testing machines are designed with the global quality
standards to test the front shock absorber of a motorcycle or scooter. Advanced machine can test four
set of shock absorber at one time. The operation carried out on cam drum testing machine is to
perform the endurance test on the shock absorbers for specific period of time at a given revolution
per minute (RPM).
The objective of this study was to examine the fracture surface and to carry out fractographic
analysis to find out the reason for failure.
II. FAILURE HISTORY
The customer returned the fractured fork pipe samples after premature failure during testing.
Fig. 1a shows the mechanism of testing the front shock absorber of a motorcycle. The shock absorber
experiences repeated bumps or shocks while testing by a typical mechanism fitted at the bottom.
There is an upper clamp bracket which holds the fork pipe.
The design of the system as indicated in fig.1 shows the nature of loading and stress
experienced by the system. The load experienced by the front fork is purely shock load. In one
revolution it experiences the four bumps/shocks. During inspection the speed of revolution used was
120 RPM. To pass the test it has to run for 100 hr minimum cycle without failure or leakage.
Assembly has to withstand 2880000 numbers of bumps minimum so as to qualify the test.
Fig.1: a. Mechanism for testing Front fork pipe system of motorcycle or scooter by Cam drum
testing machines, b. Magnified view of the front fork pipe of location A showing area of the crack
and forces acting it
Failures were reported during testing on Cam Drum testing machine. The details of failed
fork pipes are indicated in the table 1. During testing it is found that one Left hand (LH) fork pipe
was cracked after 20.50 hrs and other after 47.20 hrs. One failure was reported in right hand (RH)
fork pipe after 23.54 hrs.
Under clamp
Mechanism for giving
bumps / shocks
Front fork
Tensile component
Resultant
Compressive component
Crack location
(a)
(a)
Location A
(b)

3. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
6480(Print), ISSN 0976 – 6499(Online) Volume 5, Issue 5, May (2014), pp. 141-148 © IAEME
143
Table 1: Failure history of fork pipe
Sr. No. Description Hrs: Mins
1 LH fork pipe (F 01/LH) 20:50
2 LH fork pipe (F01/LH) 47:20
3 RH fork pipe (F01/RH) 23:54
III. EXPERIMENTAL PROCEDURE
The material under investigation was received after failure during testing. The detailed
metallurgical investigation was carried out on the fractured fork pipe which includes visual
examination of fractured surface and near area of failed components. Chemical analysis and
mechanical tests such as hardness and tensile test were performed on the failed components.
Microstructure analysis by optical microscopy and scanning electron microscopy were carried out.
1. Visual examination
From the visual examination it reveals that the tube fails from 5mm below the bracket
clamping edge. It is also observed that the failure started from the back side of front fork as indicated
in the fig.1b. Dent marks were observed on the fork pipe just below the bracket clamp which can be
seen from the removal of powder coating. The sample analyzed shows that failure occurred by the
crack originating from the welded region. Slight burr is observed on the forging of under bracket. A
white plated layer was observed on the cross sectional area of fork pipe as indicated in fig. 2. Visual
examination of the sample revealed a generally smooth surface with small amount of deformation.
However, the fractured portion of the sample revealed a brighter surface typical of brittle fracture.
Fig.2: Photograph of cross section of the pipe indicating Ni hard Cr-plating on the surface at 23X
magnification
1mm
White plated layer

4. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
6480(Print), ISSN 0976 – 6499(Online) Volume 5, Issue 5, May (2014), pp. 141-148 © IAEME
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2. Chemical Analysis
Chemical analysis was done and presented in the table 2 given below. It contains carbon,
silicon and manganese. Sulfur and phosphorus are within the limits. From the chemical analysis, it is
found that fork pipe material conforms to the specification Gr.SAE 1541.
Table 2: Chemical analysis of fork pipe
% C % Si % Mn % S % P
Specified 0.36-0.44 0.25 Max 1.35-1.65 0.05 Max 0.04 Max
Observed 0.41 0.21 1.39 0.0036 0.019
3. Mechanical Test
Hardness test and Tensile strength were taken on the failed components. Hardness observed
is in the range of 345-395 Hv on the surface of fork pipe. Samples from the failed components are
taken and inspected for tensile test. Tensile strength of the tubes ranges from 87 to 89 Kgf/mm2
conforms to the specification.
4. Microscopic observation
Microstructure shows the cold drawn structure of pearlite and ferrite matrix grains as shown
in fig.3. The microscopic examination indicates the presence of Globular oxide type - D inclusions,
thin series/ thick series - 1.5 as indicated in fig.4
Fig.3: Microstructure of fork pipe Fig.4: Inclusion content in fork pipe
5. Fracture observation
The scanning electron microscopy was performed using a JEOL SEM model JSM 6380A.
Different fractured surfaces are shown in SEM figs. 5, 6 and 7. The SEM image as indicated in fig.5
shows microvoid instability fracture which is fibrous in nature. Area marked by circles in fig.5,
distributed at large on the fracture surface can be seen.

5. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
6480(Print), ISSN 0976 – 6499(Online) Volume 5, Issue 5, May (2014), pp. 141-148 © IAEME
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Fig.5: Fracture indicating Microvoid instability fracture
Fig.6 is the areas encircled at higher magnification of fracture surface shown in figure no 5.
The evidence of entrapment of a nonmetallic particles/ inclusions is seen in SEM image fig.6 while a
void can be seen in the fig. 7 and 8.
Fig.6: Fractrograph at high magnification indicating presence of oxide inclusions
All the three SEM images 5, 6 and 7 though indicative of fibrous fracture are without any
significant evidence of necking or plastic deformation. This confirms that though the material is
ductile, fracture has occurred due to single overload with multiple origins at the locations of
microvoids or entrapments. The arrows indicate the probable stress component at the time of single
overload.

6. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
6480(Print), ISSN 0976 – 6499(Online) Volume 5, Issue 5, May (2014), pp. 141-148 © IAEME
146
Fig.7: Fractrograph at high magnification
Microvoids are observed under Scanning Electron Microscope (SEM) in the sample which
shows the presence of oxides as indicated in fig. 9.
Fig.8: Micrograph showing presence of microvoids

7. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
6480(Print), ISSN 0976 – 6499(Online) Volume 5, Issue 5, May (2014), pp. 141-148 © IAEME
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Fig.9: SEM Micrograph at high magnification
IV. RESULTS AND DISCUSSION
From the nature of loading and above observations, it is clear that the portion of Front fork
pipe near Under bracket is under highly compressive condition while the other portion has developed
equivalent tensile stresses. The conditions when this tensile component of the system exceeds the
fracture strength of material, it leads to failure. The above crack nucleating condition can be created
at voids / entrapments, where the stress concentration at the interface can lead to crack initiation, just
below the under bracket due to single overload.
The presence of Microvoids along with the entrapment of oxide particles in the steel,
cleanliness of steel, weld seam offer least resistance for the propagation of crack. This is the
causative factor for the initiation of crack during single overloading. From the observations it is clear
that the failure is due to the uneven pressure on the assembly. The uneven pressure may be due to the
uneven clamping pressure, or burr at Under bracket location. Dent mark on the under bracket
indicate the excessive/uneven clamping pressure (fastening torque – should be 250-350 kgf cm) on
the assembly which may be the causative factor for the initiation of crack on Front Fork pipe below
the under bracket. Irregular vibration on machine due to improper air pressure in the tyres,
misalignment of tyres with the centre of the drum, fitment problem with fixtures/fixture, etc could be
the main factors which cause such type of failures. In addition to the above observations, it is found
that the route of manufacturing the raw material for the fork tube is not specified on any part
drawing.
V. CONCLUSION
From the above observations, we can conclude that the component failed in brittle manner.
This is a single overload failure. The strength of the material is lowered due to the presence of the
non metallic inclusions. Failure originated as the applied load exceeds the fracture strength of the
material. Uneven pressure during clamping or burr at the under bracket location originate the crack
on front fork pipe below the under bracket.

8. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
6480(Print), ISSN 0976 – 6499(Online) Volume 5, Issue 5, May (2014), pp. 141-148 © IAEME
148
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