Wildguzzi.com
General Category => General Discussion => Topic started by: Roebling3 on November 09, 2020, 10:45:14 AM
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This posted by a friend on the 'small block' list.
Her '84 MG V650SP 'disintegrated its cast front wheel'. There was no warning and no reason. Another well regarded correspondent recalled a recent article alleging powder coating cast wheels (high temp. bake), causes molecular changes leading to failures.
Seems plausible. But why does it show up after so many years/miles?
IIRC. Early Carbon Fibre wheels, mostly on race bikes, had some shattering, as if brittle. R3~
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I doubt the powder curing oven, typically 350 to 400 degrees Fahrenheit For twenty minutes would cause a caAt wheel to fail. More likely a poor alloy that is unstable and swells with age. There is plenty of documentation on some early cast ducati wheels cracking and failing.
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Aluminum equals fatigue failure. Given enough cycles it will always happen. Also possible a casting defect. Temperature affect from coating is doubtful and I have no idea what "molecular change" means.
-AJ
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I guess as old as these wheels are, anythings possible. I am sure aluminum casting technology has advanced alot since the day those were cast. I have found a couple of Guzzi wheels with porosity leaks in the casting, but generally they seem pretty strong and overbuilt.
If in doubt, or worried about it ,could the wheels be tested some way ,like magnafluxed? There was a method for inspecting, and testing aluminum oxygen cylinders, right now,I cant remember what it was. :azn:
Rick
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The molecular structure of any metal changes with enough heat causing it to become brittle , solved by quenching which forces the molecules to line up again . No idea at what temperature the aluminum alloy used in cast wheels goes woogity , but it might be possible that long exposure to 400F might do it .
Dusty
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Typically, aluminum castings can operate at up to 500° F continuously, but weaken at over 400° F - so if the part is stressed in service at that temperature it may crack in a reduced number of cycles if operated over 400° F. No stresses including cyclical stresses are applied to wheels being powder coated so there should be no issue with a single unstressed exposure to over 400° F.
In the 70s racers used to crack check cast wheels before use and many were poor quality parts with unacceptable cracks when new and unused. These will grow and may grow significantly in service, depending on the stress.
Much of the stuff you read about this and similar subjects on motorcycle websites is completely uninformed.
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Aluminum can be checked for cracks with a product called Zyglo. Spray it on wait a few minutes, wipe it off and then put it under black light. Cracks will show up readily. I used it all the time on aircraft engine castings.
kk
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Thank you to all respondents.
ZYGLO, you say? Thanx Moparnut72, for the wake-up call.
Would a ZYGLO test be wise @ say 30 years old? Are most heavy touring bikes on wire wheels for other than aesthetic reasons?
Most things considered - failing cast wheels are likely way down the list of things that could happen - on the road.
Good fortune, R3~
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I rode a 1978 Ducati Darmah with Speedline wheels for years. I was told early on that the wheels were "porous" and would not take a tubeless tire. On a group ride with my girlfriend on back, about 9" of the rim broke away. It sounded like a shotgun going off. Immediate flat tire. Luck we didn't crash.
I found another Speedline. I mounted both front and back tires tubeless, no problem. One day I went to back the bike out and it had a flat rear tire. Aired it up and soaped the wheel, it had a crack in a similar place to the failed rim..
Took it to an excellent welder, who determined it could not be repaired because of the high magnesium content. It turns out the "porous" wheels had cracks and that is why they would not hold air.
I still have that bike, and the tires are tubeless.
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Could have been cracked in the tire changing process in its life time.
I have seen a rim break while changing tire.
Both hub break and outer rim break.
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The early "Snowflake" cast wheels on BMW's were subject to a recall because of problems with cracking. I don't know if it was bad material or bad design but they replaced them with a redesigned wheel. Also some of the early aftermarket cast wheels (Lester) for BMW's were supposedly bad and cracked in the hub aria, I personally think it was due to improper bearing installation but that's a different subject. So yes there can be bad wheels out there.
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Almost every picture I pull up of a V650SP has an even number of spokes on the wheel. That's bad casting design. Proper design calls for odd number of spokes to reduce residual stresses during solidification. An odd number is also stronger under applied load.
More than likely someone at some time accidentally cracked it and it failed. Maybe tire change. Maybe just dropped it. And aluminum fails catastrophically. I don't like aluminum.
-AJ
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Zyglo is more genetically known as dye penetrant crack testing, a common industrial practice for aluminum.
Speedline wheels on circa 1978 bevel Ducatis were well known for cracking and coming apart. In some countries they were recalled by government mandate. I bet if they were dye pentrant checked when new they would have been thrown in the trash as bad castings. FPS wheels were the eventual solution for Ducati, and the Campagnolos the preceded Speedlines in 1977 were not as bad either.
My memory of BMW Snowflake wheels is that they were soft, versus being porous, and bent remarkably easily when hitting potholes etc, versus cracking. Maybe that was the replacement type, I do recall a recall :laugh: Same soft and easily bent wheel issue on many Ducatis of the 90s, although maybe not quite so bad.
I can’t believe I’ve memorized all this trivia over the years.
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The BMW Snowflakes from 1978 to about 81 or 82 were recalled. In fact, you can still turn in one of the originals for a brand new updated version. The originals could crack at the spoke hub junctions. Actually pretty rare for that to happen. The most plausible story was that one of the authorities bikes in Europe developed some cracks - most likely riding spiritedly over cobblestones. If you do exchange an original for new, be sure to remove the bearings (requires wasting the seals getting them out) because they don't come greased. I personally know of a couple of axles and fork legs ruined when they froze up.
Yes, BMW wheels are soft. I'd much rather have one bend than break. But they're also HEAVY, one of the reasons I prefer wires.