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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manufacturer’s state they are doing it but for business reasons they do not publish the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.