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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
RFM lug cracked
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<blockquote data-quote="Diogenes" data-source="post: 17691" data-attributes="member: 91"><p>I don't know if Trev is just trying to be a humourist when he writes </p><p>"forgings bend, castings, snap." but that is just nonsense.</p><p>Castings made of a brittle material like grey cast iron will fail in a brittle manner when subjected to overload in tension.</p><p>Castings made of a ductile material like SG iron will fail in a ductile manner when subjected to overload in tension.</p><p>The crucial thing is whether or not the material itself is brittle or ductile.</p><p>Forgings by their nature are nearly always of ductile material, but can still fail suddenly when subjected to excessive fluctuating load, so a fatigue failure occurs when the crack growth is excessive.</p><p>Con rods of ductile forgings can fail by 'snapping' suddenly, not 'bending'.</p><p></p><p>But you knew all this Trev, didn't you?</p><p></p><p>In some other comments there has been confusion between the loads causing failure of the RFM lugs.</p><p></p><p>The failure mode where the lug axle slot opens up and eventually cracks at the front U end, aided by the stress raising effect of the tapped hole drilled for the chain adjuster, may well be partly caused by use of kickstart while bike is on rear stand, so it is prudent to avoid doing it.</p><p>But it it also partly caused by road bumps pushing the axle up repeatedly onto the upper surface of the slot, particularly when the axle is well to the rear of the slot. These loads and their many applications may outweigh the effect of infrequent use of K/S.</p><p>But heaving the bike onto the rear stand in a brutal way can certainly provide high loads as the stops are crunched, so that practice is also better avoided.</p><p></p><p>The failure mode seen in this recent example is located near where the horizontal tube stops in the drilled hole in the lug.</p><p>More precisely, a circumferential crack starts in the lug where the drilling ends in a 'nice' sharp change of section.</p><p>I don't think that use of K/S on stand has much to do with this failure mode.</p><p>Firstly, most [ not all ] of these circumferential failures happen on Twins and almost always on the drive chain side. [ Yes I do know the K/S is on the same side! ]</p><p>This points to the cause being loads from the drive, which are then combined with dynamic loads from road bumps etc.</p><p>Every time the power goes on or off the stress in the lug changes - an ideal way of getting a fatigue failure when a sharp change of section has been manufactured.</p><p>There are some exceptions, such as failures on Comets and failures on the non-drive side, but there are enough variations in the local wall thickness [ from off-centre drilling ] to make those expected.</p><p></p><p>So don't expect that just changing the lug to a forging will provide an everlasting solution - though it may well outlast most of us!</p><p></p><p>I suppose most owners have solved the fatigue failure problems associated with long distance usage by the simple expedient of only doing a few thousand mile per year.</p></blockquote><p></p>
[QUOTE="Diogenes, post: 17691, member: 91"] I don't know if Trev is just trying to be a humourist when he writes "forgings bend, castings, snap." but that is just nonsense. Castings made of a brittle material like grey cast iron will fail in a brittle manner when subjected to overload in tension. Castings made of a ductile material like SG iron will fail in a ductile manner when subjected to overload in tension. The crucial thing is whether or not the material itself is brittle or ductile. Forgings by their nature are nearly always of ductile material, but can still fail suddenly when subjected to excessive fluctuating load, so a fatigue failure occurs when the crack growth is excessive. Con rods of ductile forgings can fail by 'snapping' suddenly, not 'bending'. But you knew all this Trev, didn't you? In some other comments there has been confusion between the loads causing failure of the RFM lugs. The failure mode where the lug axle slot opens up and eventually cracks at the front U end, aided by the stress raising effect of the tapped hole drilled for the chain adjuster, may well be partly caused by use of kickstart while bike is on rear stand, so it is prudent to avoid doing it. But it it also partly caused by road bumps pushing the axle up repeatedly onto the upper surface of the slot, particularly when the axle is well to the rear of the slot. These loads and their many applications may outweigh the effect of infrequent use of K/S. But heaving the bike onto the rear stand in a brutal way can certainly provide high loads as the stops are crunched, so that practice is also better avoided. The failure mode seen in this recent example is located near where the horizontal tube stops in the drilled hole in the lug. More precisely, a circumferential crack starts in the lug where the drilling ends in a 'nice' sharp change of section. I don't think that use of K/S on stand has much to do with this failure mode. Firstly, most [ not all ] of these circumferential failures happen on Twins and almost always on the drive chain side. [ Yes I do know the K/S is on the same side! ] This points to the cause being loads from the drive, which are then combined with dynamic loads from road bumps etc. Every time the power goes on or off the stress in the lug changes - an ideal way of getting a fatigue failure when a sharp change of section has been manufactured. There are some exceptions, such as failures on Comets and failures on the non-drive side, but there are enough variations in the local wall thickness [ from off-centre drilling ] to make those expected. So don't expect that just changing the lug to a forging will provide an everlasting solution - though it may well outlast most of us! I suppose most owners have solved the fatigue failure problems associated with long distance usage by the simple expedient of only doing a few thousand mile per year. [/QUOTE]
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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
RFM lug cracked
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