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Bicycle Frame Failure Analysis
And
Fatigue Testing
May 8, 2008
Larry Ruff
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Table of Contents
List of Figures 3
1.0 Introduction 4
2.0 Bicycle Frame Fatigue Failure Research 5
3.0 Frame Failure Examples 8
4.0 Frame Model 11
5.0 Frame Fatigue Testing 13
6.0 Conclusions and Future Work 17
References 18
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List of Figures
Figure 1: Lugged Bicycle Frame 4
Figure 2: Frame Testing at Specialized 5
Figure 3: Frame Testing at Trek 6
Figure 4: Frame Testing at Trek 6
Figure 5: EFBe Frame Fatigue Failure Test Fixture 7
Figure 6: Location of Cracks 8
Figure 7: Polished and Etched View of Crack 3 8
Figure 8: Frame Deflection Test Fixture 9
Figure 9: Crack in Chain Stay 10
Figure 10: Transgranular Cracking 10
Figure 11: Chain Stay Loading 12
Figure 12: Rear Triangle Fatigue Test Fixture 13
Figure 13: Left Stay Crack Opened 14
Figure 13: Location of Crack in Right Stay 14
Figure 15: Fracture Surface in Mating Half of Left Stay 15
Figure 16: Another View of Fracture Surface 15
Figure 17: Crack in Right Stay 16
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1.0 Introduction
Bicycle frame failure has always been an issue but in recent times it has become a bigger
problem as manufacturers have been trying to build lighter frames using steel, aluminum,
titanium and carbon fiber. Prior to the early 1980’s most quality bicycle frames were
made from chrome molybdenum steel tubing with the joints reinforced by lugs. Frame
failures did occur but only after many miles were put on the frame. The average weight
for a quality bicycle during that period was about 22 pounds. Present day racing bicycles
at the equivalent quality level weigh 15 to 17 pounds.
Figure 1: Lugged Bicycle Frame
This paper will look at some of the work that is presently being done on bicycle fatigue
and failure and some examples of bicycle frame failures. A test fixture that was designed
to fatigue test the rear triangle of a bicycle along with the results of a test to failure will
be described.
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2.0 Bicycle Frame Fatigue Failure Research
There is very little published research on bicycle frame fatigue failure. Some
manufacturer’s state they are doing it but for business reasons they do not publish the
results. Trek and Specialized both do in-house testing (Figures 2, 3 and 4). There are
ISO and DIN bicycle standards. The ISO 4210:1996 standard covers Safety
Requirements for Bicycles. This standard “specifies safety and performance
requirements for the design, assembly and testing of bicycles.” DIN 79100 does define
bicycle frame fatigue failure testing procedures for three cases: out-of-saddle on pedals,
rider load on saddle and jumping. There is a German organization, EFBe Pruftechnik,
that has also been developing a set of standards based on the DIN standards. Some of the
test equipment they sell is being used by some American manufacturers (Figure 5).
EFBe will do contract testing.
Figure 2: Frame Testing at Specialized (Ref. 1)
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Figures 3 and 4: Frame Testing at Trek (Ref. 2)
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Figure 5: EFBe Frame Fatigue Failure Test Fixture (Ref. 3)
ASTM has been working on a set of standards for bicycle frame fatigue failure testing.
There is a working committee, WK464 New Test Methods for Bicycle Frames, that is
“working to establish procedures for conducting tests to determine the structural
performance properties of bicycle frames” (Ref. 4).
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3.0 Frame Failure Examples
As part of the class Principles and Practice of Failure Analysis offered by Professor
Robert Messler, Jr. in the Materials Engineering Department at Rensselaer Polytechnic
Institute failed bicycle frames are analyzed. During the Spring 2008 semester 6 frames
were analyzed. This report will show two of those studies. Both frames were aluminum
road bicycles.
The first study involved multiple cracks in the bottom bracket area. The crack locations
are shown in Figure 6. Details of crack 3 are shown in Figure 7.
Figure 6: Location of Cracks (Ref. 5)
Figure 7: Polished and Etched View of Crack 3 (Ref. 5)
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The team doing this failure analysis determined that “the combination of (material)
overaging, large rider and corrosion from the environment led to the eventual failure of
this bike frame.” (Ref. 5) The intergranular cracking showed that this was not normal
fatigue failure but rather a corrosion based fatigue failure. The team also felt that the
material was sensitized during welding which contributed to the failure.
Failures in the bottom bracket area of a bicycle are very common. The stresses and
deflections are high. In study two deflection testing was done on a bicycle frame that had
also failed in the bottom bracket area. Figure 8 shows the frame in the test fixture.
Figure 8: Frame deflection Test Fixture (Ref. 6)
Using a 200 pound rider as the basis for the loading this frame saw a vertical deflection
of .004 inches and a lateral deflection of .618 inches. This frame suffered from a fatigue
failure in both chain stays by the bridge. The cracks started at the base of the weld and
went transgranular (Figures 9 and 10). There may also have been a corrosion issue at the
base of the weld (the black area at the start of the crack in Figure 10).
Figure 9: Crack in Chain Stay (Ref. 6) Figure 10: Transgranular Cracking (Ref. 6)
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Besides the two failures described above multiple failures have been observed in the
bottom bracket area in steel, carbon and aluminum frames. Since the manufacturers
usually want the frames returned for warranty reasons it is difficult to gather a number of
samples. It is also difficult if not impossible to get data on the total number of failures.
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4.0 Frame Model
Figure 11 shows some of the assumptions made when analyzing the deflection and stress
in the chain stays of the frame tested in Section 5.0. Bending is assumed to occur at the
bridge with minimal deflection from the bridge to the bottom bracket. Bending of a
constant cross-section beam is assumed. The load is equally split between the two stays.
The numbers used in the analysis and the results are:
The stay is an elliptical tube: major axis = 1.031 inches, minor axis is .701 inches,
wall thickness is .059 inches
The stay is aluminum: E = 10,300,000 psi
I = .00856 inches^4
Load = 103 pounds/2 = 51.5 pounds
Moment = 51.5 pounds X 13.812 inches = 711.32 inch pounds
Stress = 29,126 psi
Max Deflection (calculated) = .513 inches
Assuming the material is 6061-T6, the yield strength is 40,000 psi
The maximum deflection is more than the starting value seen during the test in Section
5.0. Part of this is probably due to the fact that the contribution of the seat stays was
ignored.
The calculated stress is less than the yield point, but the contribution of various stress
concentration factors such as the geometry at the weld-tube interface and the effect of the
welding process on the material microstructure would bring the calculated stress closer to
the yield point. This could explain why the fatigue failure test in Section 5.0 did not
require many cycles for the frame to fail.
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5.0 Frame Fatigue Testing
Figure 12: Rear Triangle Fatigue Test Fixture
A fixture shown in Figure 12 was designed and built to fatigue test the rear triangle of a
bicycle frame. The fixture consists of a welded steel base (A) with mounts for two
pneumatic cylinders (B) used to flex the frame. A PLC (C) controls the activation of the
pneumatic solenoid valves (D). A counter (E) counts the number of cycles and a digital
indicator (F) measures displacement. Not shown is a pressure transducer used to
accurately measure the air pressure supplied to the cylinders to determine force. This
fixture loads just the chain stays rather than the complete frame. The bottom bracket is
rigidly bolted to the fixture. The chain stays are rigidly tied together by an axle as they
would be while riding by the rear wheel.
To test the fixture a frame was cycled to failure. This frame was replaced due to a
problem with the water bottle bosses on the down tube. There is no way to determine the
number of cycles the frame had already undergone but there were no visible cracks in the
chain stays or bottom bracket area before testing started.
There was also a problem with the counter due to some electrical interference. This was
solved after the frame was cycled for a short time.
B A
C
D
E
F
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The chain stays were subjected to an alternating load of 103 pounds (58 psi). Each
complete cycle was 8.2 seconds. Initially the deflection was .386 inches. At 1200 cycles
deflection was .410 inches and a crack was apparent in the left stay. At 1400 cycles
deflection was .505 inches and the crack had grown to the top and bottom of the stay. At
1660 cycles deflection was .545 inches and the crack was working its way further around
the stay. A crack had also appeared in the right stay. The test was then stopped, the
frame was cut apart and the crack in the left stay was opened. The following figures
show the results.
Figure 13: Left Stay Crack Opened
Figure 14: Location of Crack in Right Stay
Crack
Crack
Initiation
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Figure 15: Fracture Surface in Mating Half of Left Stay
Figure 16: Another View of Fracture Surface
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Figure 17: Crack in Right Stay
There was not enough time to do an in depth analysis of the material, microstructure,
crack initiation and propagation but this analysis will be part of future work.
Crack
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6.0 Conclusions and Future Work
This report describes some initial work and results on the topic of bicycle frame fatigue
failure. The bottom bracket area of a bicycle area is most prone to failure and this report
looked at the analysis of and generated failures in that area. Future plans include further
analysis including a more detailed frame model. Rear triangle assemblies of various
materials and configurations will be tested to evaluate stiffness and fatigue failure
resistance.
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References
1:http://www.velonews.com/article/73802/factory-tour-specialized
2:http://www.triathletemag.com/Departments/Features/2007_Features/Inside_Trek_s_Wo
rld_Headquarters.htm
3:http://www.efbe.de/produkte/ermuedpruef/enindex.php
4:ASTM Subcommittee F08.10, Date Initiated 2/7/2003.
5:McGee, H., Munger, A., Crosskey, M., Goldsmith, C., Failure Analysis Lab Exercise
#2: Bicycle Frames. 2008.
6:Jamison, L., Laprade, E., Ruff, L., Tracy, I., Failure Analysis Exercise #2, 2003 Fuji
Professional. 2008.