Fatigue Fracture Fix

My suspension arrived from Queensland this weekend (I will review soon), and as I went to fit it I remembered that during dis-assembly I managed to shear the long bolt that passes through the top of the frame and secures the rear sub-frame. I had intended to order a new bolt whilst the bike lay forlornly dormant, but the ol’ “out of play”, “out of mind” rule obviously reared it’s head. So here I am, it’s late Saturday arvo and I’m miles from town and anxious to check out that suspension, I hunted in the nuts n’ bolts tubs and turned toolboxes upside down, it’s amazing what long lost friends you come across doing that, like the cable-tied together clutch perch that gets you to the finish flag of an enduro,  or the old BSA fork top-nut I once fashioned a puller out of because it happened to match the thread of the magneto on an Ossa trials bike I once owned. Well you never know, I may buy another old 1974 Ossa one day!

Anyway no luck with a long bolt so the next option was to fix the broken one.

Classic example

Now here is a classic example of a Fatigue fracture. Looking at the surface of the break at around 6:00-7:00 pm (clock wise as you look at it), it appears as if a large load has, most likely, propagated the initial crack which started right in the groove of the last thread where it transitions to the bolt shoulder (a common stress riser). The bolt, now weakened, has continued to crack in increments whenever a larger than average load was placed on it (my fat ass on the seat). This is evident by the succession of parallel lines in area “A” which have worn smooth as the two semi attached parts would have been continually rubbing together. As the crack increased over time or number of load and unload events (Dynamic/Cyclic Load), it progressed to a point whereupon, as I went to undo the bolt, which had the threaded part corroded and well fixed into the sub-frame, it sheared. This is evident by the rough fractured surface seen in area “B”. This is the sort of clues relating to failure that air crash investigators examine.

broken bolt from a wheel of a lorry / gebroche...

broken bolt from a wheel of a lorry / gebrochener Radmutternbolzen von einem LKW (Photo credit: Wikipedia)

English: image of fatigue of material (crankshaft)

English: image of fatigue of material (crankshaft) (Photo credit: Wikipedia)

I had soaked the sheared part in penetrating fluid when it broke off and then promptly forgot all about it. I expected the extraction of the threaded part from the sub-frame would be a tricky operation. I decided to drill it out and fit a Heli-coil, as I started drilling the threaded slug simply unscrewed itself with the twisting action of the drill. It was obviously the four week soaking in penetration fluid that did the trick!

I delved into the nuts n’ bolts tub and found a Stainless steel countersunk screw with the same thread diameter and pitch.

I then cut the original bolt down a bit, so that if it were to shear again, at least I would have a stub left sticking out to get hold of to assist in removing the threaded part.

I then prepared the ends for welding by grinding a bevel to each part. this gives somewhere for the weld metal to sit.

I Prefer to TIG weld this type of repair. TIG welding resembles Oxy-Acetylene welding in principle, it just uses an electrode to create an arc to melt the part as opposed to a flame. I used a steel welding rod, a lot of people will say you must use a Stainless Steel filler rod if you are welding Steel to Stainless Steel.  But, I say why waste money when the weaker material will mostly be the steel part anyway?

Glowing Red Hot. Just how welders like it.

Light grind

I gave the weld a light grind to remove excess weld metal taking care not damage the thread.

After I cut the old screw head off I then feathered in the start of the thread, I always prefer to use a file for this as it has more control and feel. If you perform this operation with a grinder one false move can stuff up your work thus far!

File rather than grind a chamfer

Wallah ready to install

Now to get that bike together and haul’n.


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