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Metal Fatigue & Other Mechanical Failures

 MOTORCYCLE ROAD RACING

 

 

 

 

This page shows examples of mechanical failures, in the main due to metal fatigue, lack of lubrication or other imposed abnormal stresses.

(CLICK ON THE IMAGES TO SEE A MORE DETAILED VIEW)

PISTON & VALVE DAMAGE

The following three images show an Aermachi 249cc racing single cylinder piston, exhaust valve and crankcase.

    This is what can happen when the engines designed rev limit is exceeded (for whatever reason) or the piston to valve clearance is too close and the piston comes into contact with the valve head.  Close examination of the valve revealed that fatigue cracks are evident in various areas of the valve head.

CRANKCASE WELDED REPAIR FAILURE

The crankcases on the same engine had cracked in the past and had been welded up as a temporary repair.  Examination of the welded area revealed fatigue cracking emanating from the camshaft bearing housing in the casing.  Immanent failure of the  cases required a renewal.

 

SWING ARM (SUSPENSION) FATIGUE FRACTURE (RESULTING IN TOTAL FAILURE)

The following images are of an Aermacchi 344cc rear swing arm.  The awing arm failed in a road race at Chimay, Belgium.  The resultant crash at around 100 mph, severely damaged the bike but the rider escaped with minor injuries.

Rear swing arm after it was removed from the bike and cleaned for examination.

Close up of the right side arm that parted company with the pivot section.

Close up of the separated arm.  The striated fracture patterns on the surface of the metal surface are typical of a fatigue fracture caused by repeated cyclic events.

This is a close up of the pivot section of the swing arm.  The electrical weld used to join the arm to the pivot section can be seen.  A reinforcing plate has then been welded to the arm and pivot section.  The weld has caused localised hardening of the joined materials.  In the short term this has not been very important.  However, intermittent and hard (racing) use since the manufacture in 1968 has caused fatigue fractures due to cyclic stresses (bending and twisting moments) and the eventual total failure of the component.

Further cracks had started to develop at the wheel spindle ends of the swing arm.  The arms on this machine had been strengthened in the past (as was common on the Aermacchi) by welding reinforcement brackets around the arm ends, as the original component suffered failures in this area.  However, fatigue fractures have started to propagate and would eventually has caused the rear wheel to detach from the machine.

This is the other arm end with similar cracking staring to develop.

None of the cracks (that had not resulted in the total failure) were particularly easy to see before the swing arm was cleaned.  I therefore suspect that the crack that caused the failure and resultant crash may not have been easy to see either.  Moral - Clean machines and examine them thoroughly before using them, especially if they are very new or very old.

 

SWING ARM FATIGUE FRACTURE (FOUND BEFORE TOTAL FAILURE)

The following images are of a swing arm from a Maxton framed Yamaha TZ350.  In contrast to the above swing arm the bike was cleaned and examined at regular intervals.  The cracks were detected during checks of the machine between races and repaired before total failure could occur.

Maxton rear swing arm.  The cracked area has been abraded prior to repair.

Close up of un-repaired cracks in swing arm adjacent to wheel spindle mounting.

 

SEAT FRAME FAILURE

The following images are of a Honda RS125GP aluminium seat sub frame assembly.  The seat frame was found to be loose during checks, dismantling and close examination revealed a badly bent frame with fatigue cracks and badly welded and failed joints.

Alluminium seat frame that has been distorted during crashes, repaired and suffered fatigue failure due to cyclic stress and poor welding techniques.  The lower right side (45) support mounting has sheared off completely.

Close up of the broken support mounting.  The lack of weld penetration is evident, this has lead to the failure of the mounting.

Close up of crack around the seat mounting caused by repeated bending of the tube.

Close up of the left side support mounting.  Cracks are starting to develop due to fatigue cause by repeated bending.

 

PISTON & VALVE FAILURE

  Piston from a 4.2 litre Jaguar XJ6.  The valve timing chain drive sprocket micro spline sheared on the exhaust camshaft at high speed, causing the valve timing to go out of sync.  The result was contact between the exhaust valves, inlet valves and pistons.  The head of one exhaust valve (shown on top of the piston crown) had been broken off and was actually found in the sump of the engine.  There was also major damage to the cylinder head.

 

BIG END BEARING FAILURE

Connecting rod from a racing Suzuki T20.  Note the elongation of the big end eye and damage caused by the big end bearing failure.  The engine was producing normal power at high speed in a race, then within a second there was a loss of power.  The clutch was disengaged and the engine stopped instantly.

 

DRIVE SHAFT FATIGUE FRACTURE

Drive shaft from a 'sit on' large garden grass mower.  The shaft has suffered a torsional fatigue fracture due to repeated hard forward and reverse movements.  There have also been excessive shear forces applied caused by use of the vehicle on rough ground (it has no suspension other than that given by the tyres).

 

PISTON LUBRICATION FAILURE

Piston from a Honda RS125 GP single cylinder two-stroke Grand Prix race bike.  The piston has suffered a severe overheating caused by lubrication breakdown.  It was running on a fuel mix of 25:1 fully synthetic mineral based competition oil, 50% avgas and super unleaded fuel.  The oil was changed to a castor based (Castrol R40) competition oil and no further problems of this nature occurred again.

 

PISTON AND CONNECTING ROD LUBRICATION FAILURE

  Piston, piston pin & connecting rod from an Aermacchi 344cc GP race bike.  This is what can happen when the oil level is allowed to run low!  The rest of the engine damage included, bent valves, bent push rods, cracked barrel, oil pump big end and main bearings.

 

BICYCLE PEDAL CRANK SHAFT FATIGUE FRACTURE

Disassembled pedal crank, shaft and bearing assembly from a child's bicycle.  Note the fractured central shaft.

  Close up of the same assembly.

  The pedal crank shaft.  Zoom in to see the fatigue patterns in the fracture.

  The same shaft viewed end on.  Note that the threaded hole has been drilled and tapped off centre.   This has contributed to the cause of the failure by weakening one side of an already weakened component.  The shaft had been drilled and then hardened to support the bearing surfaces.  This had caused the shaft to be very brittle and unsuitable for its purpose.  The fracture of the pedal crank shaft resulted in an accident involving injury.

 

MORE TO FOLLOW!

 

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Last modified: 12/22/12