RFM lug cracked

ossie

Well Known and Active Forum User
VOC Member
How come your wires go up that side my 49 shadows go up the chain side?
No problem just sumfin i noticed.
Ossie
 

Ken Tidswell

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VOC Member
The lug or what remains of it can be removed by destroying it. It has been done to a Comet I bought with a similar problem.
By carefully milling away the lug ,so it is in 4 bits, it can be heated sufficiently to remove said lug. As my learned colleague states It requies lots of heat.
 

ossie

Well Known and Active Forum User
VOC Member
Ive just put some photos of a rear lug i aquired from somewhere its not off my bike but was broken you can see how the wall thickness varies around the drilled hole.
Look in[ misalain temporary 2010]
 

peterg

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Non-VOC Member
Howdy Chaps,

In answer to some pm's regarding Bob's Shadow it is a high number 53, original with the crate it came in and was on display at the N.A. VOC rally in MN in 2009. Note one of the details not usually seen on restored machines, braided horn wires.

bobccrate.jpg bobcshadow.jpg bobcshadow2.jpg
 
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Tug Wilson

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Non-VOC Member
And I thought I had problems!! Many thanks to all for excellent help. Watch this space for when I get the smaller bits out of the basket.
 

Diogenes

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Non-VOC Member
From what I can glean, the new lugs are forgings, or at least, that is what I am led to believe. So the problem should never again occur, forgings bend, castings, snap.
Rob, you will find it easier to repair than replace. you have no idea of the heat required, both , to remove the existing part, and to replace the new.

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.
 

clevtrev

Well Known and Active Forum User
VOC Member
Blackheart malleable iron is what the lug is made of, not spheroidal graphite, a bit of difference there.

How can starting on the rear stand affect the slot ? One might need to think a little more.
 

notap

Well Known and Active Forum User
Non-VOC Member
How come your wires go up that side my 49 shadows go up the chain side?
No problem just sumfin i noticed.
Ossie

My auto-electrician friend who did the work replied like this when I asked him: "Its the Coriolis Effect - the same reason our Australian bath tub water spins down the plug hole in a different direction from the British water." I suspect the answer was a bit tongue in cheek.
 

notap

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Non-VOC Member
Just a quick update on my RFM problem. The lug ended up worse than anticipated, with a good deal of it being either braze or highly friable - basically beyone rescue. I decided therefore to replace it with a new unit. As everyone said, this was a long and rather painful exercise, and given a choice a repair would have been easier I suspect. But I finished a few hours ago. I adapted some of my work techniques to record the 3D location of the old lug and then placed the new one in the same spot - i knew all those years of uni weren't wasted! Anyway there are some pics at http://jalbum.net/browse/user/album/706932/ . I just like to say thankyou for all the help and suggestions and support. Now to replace the clutch (no doubt I will need some advice here as well!). Rob
 
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