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Fractures of mudguard supports

I received this e-mail from Mike Meakin

An update on the "fracturing rear mudguard support tubes". My daughter (based on her experience as an apprenticed Rolls Royce engineer, Metallurgy element to her Brunel degree and currently Senior Engineer at Nissan R 'n D) has suggested that both of my tubes suffered crystalline/fatigue fractures immediately at the end of the axle mount plate. She suggests that the heat from the localised seam weld of the plate weakened the thin-wall tube at that point. Interestingly, Alan, Duncan and I have all had similar occurences AND have all had SVC's combined rear light units and number plate mounted on the end of the mudguard.
Speaking with Phil G, I understand that he has made a modification which will appear on all kits supplied. This involves mounting the rear lights and number plate in the same relative position as before, but solidly fixed to the body tub, instead of the oscillating mudguards.An extra bonus is having the rear wiring static.
As yet, I haven't seen any details of exactly how the lights/ plate are attached, but I expect Ian Cameron might be able to enlighten us, once he gets started with bodywork.

A follow-up from Mike

Lucy suggests the stays' welding problem is using thin-wall tube and a heavier guage plate, with weld temperature enough for the plate, but as the weld is done with plate and tube cold, weakness is created at the end of the weld bead in the tube. That then results in a crystalline/ fatigue fracture accelerated by the flexing of the tube, carrying, as mine did, a heavy lamp unit (effectively a pendulum). Phil's answer is to fit a vertical brace between mount plate and centre of the mudguard and remove the "heavy bits" from the end of it.

I asked Mike about the examination of the failure by his daughter

Lucy's examination of the struts was just "eyeball", but the light greying of the tube and dull, crystalline edge of the start of the fracture was quite different from the fatigue fracture that tracked round in an almost perfectly straight line.
I've repaired both of my stays by inserting mild steel tube turned down to be a tight fit inside the stay, extending forwards beyond the mount plate and rearwards all the way to the end.(Obviously you can only do this with a fractured tube). Pre-drilled holes in the original stay are then welded up "stitching" the inner tube to the stay (already a tight fit - I felt this close contact was important) effectively increasing the wall thickness.
Both original stays fractured in the first 200 miles. I've done 1,000 miles since with the repaired stays with no sign of further fracture so far. If they DO go again, I'll probably go the "mount lights and plate on the body tub" route.
regards,
Mike
Click to enlarge Click to enlarge Click to enlarge
David,
Front mudguard supports: lower - addition of an "L" strip, fastened by crossmember and spat bolts and bonded to the back of the original strip. Front tubular strut: tapered, stepped, gusset strip added behind bottom mount plate using same 2 bolts, seam welded up the tube centreline. Tapering triangular plate welded to the end of the existing top strip, then seam welded down the tube. Mild steel 25 X 3mm strip used for the braces. Intention is to prevent fatigue fractures at the ends of the tubular struts and the bend of the lower supports
regards
Mike.

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