Alloy Wheels - is it just fashion?

Cheap alloys ben remarkably easily, but most OE ones are pretty strong.

That'll depend on the road, there's a lot of road corners the 325 will win on, but I don't give it much hope atnSilverstone.

Cast alloy wheels are pure fashion and will weigh more than a steel wheel.

Forged alloy wheels cost a packet but are about same or lighter than steel.

Magnesium are useless for road use as they suffer from fatigue cracking and will in time FAIL - no doubt about it, an accident waiting to happen. It's no good fixing one crack by welding, once it cracks the whole wheel is past it's use by date. You can have them X-rayed and crack tested at some expense and 2 weeks later they can crack. It just proves they weren't cracked on day of test, once initiated crack propagation is quite rapid - if not by every revolution then every bend. Any old race wheels should be recycled.

Even a die cast mag steering wheel has definite and limited life of

110,000 cycles - required life 100,000 cycles. cars are full of this stuff will given time quite literallyfall to bits. Steel won't fatigue if the cyclic stress is kept below690MPa. and Mag alloys don't have an endurance limit, the Fatiguecurve never flattens out so ALL will fail eventually, even if onlyexposed to low stress reversals. Feck knows how they are going to on with MOTs for alloy cars once a few alloy suspension arms or major body joints hidden inside panels go. It's not progressive like rust on steel, it's sound today fecked tomorrow. Welding won't fix it, it's not like a cracked head, the whole structure has been used to the life limit.

Then there are built up alloys 2 or 3 piece. Forging deep wide rims is difficult and expensive so they forge or cast a thin centre with spokes. For 2 part it will have one rim included. A spun alloy rim or for 3 piece both rims are bolted to the centre. The spun alloy is in a wrought condition and is better than a deep forging for strength. Cheaper to fix and as good as new as usually only the dented rim is replaced. Many fashion wheels ape this style by having fake bolts round the rim.

In message , Rockingrabbit writes

Oooh look, shiny shiny..

Cobblers, most alloys weigh more than steels.

Nope.

Surely that "eventually" can be extended to a very long time with good design? Isn't the reason that the Dakota is still flying down to the fact that the airframe isn't stressed beyond its elastic limit at landing?

Not sure how this relates to wheels but heck, not knowing anything has ever stopped me sticking my oar in on USENET. ;-)

Tim

You get fatigue below the elastic limit, in fact at _any_ stress level, however low, in aluminium as Peter Hill says.

But you're right that with good design it doesn't have to be a problem in practice-- if you can make the fatigue life longer than you're planning to use the plane by making it a bit thicker in all the right places and so on then you're fine.

In any case, high strength steels (like the kind they make aeroplanes out of) don't have fatigue limits either.

But car wheels are probably mild steel so they will have a fatigue limit.

Aircraft are subject to maintenance schedules that can and do include regular inspection, removal and replacement of the bits that are known to crack or otherwise fail. And then they add extra gussets, brackets, double skins whatever it takes to stop it failing so often. As problems are found in service makers issues tech bulletins, tech variances, updates to repair manuals/schemes and if really serious the certification authorities of each country issue Airworthiness Directives.

++++++++++++++++++++++++++++++++++ DCA/DC3/137C Aileron Outer Hinge Assembly - Inspection Applicability: All model DC-3 and C-47 series Requirement: To detect fatigue cracks and prevent possible failure:

1. Remove cover from aileron outer hinge brackets and visually inspect hinge assembly and associated structure external to the wing tip for cracking, in the web, in the flanges, and at rivets securing brackets to ribs.
2. Cut two 19mm (0.75") diameter holes located either side of aileron hinge rib at station 383 in wing tip upper or lower skin approximately 25mm (1") clear of rib flanges and

63.5mm (2.5") forward of wing tip trailing edge.

1. Insert a borescope or similar optical aid through holes and inspect rib between rear spar and trailing edge member for cracks in web and flanges.
2. Repair any defects found before further flight.
3. After inspection, holes must be appropriately sealed. Compliance: At intervals not exceeding 260 hours TIS Effective Date: 30 December 1988

+++++++++++++++++++++++++++++ DCA/DC3/154 Principal Structural Elements - Inspection Applicability: All model DC-3 and C-47 series Requirement: To ensure continuing structural integrity, introduce an inspection programme which provides for inspection of the Principal Structural Elements (PSE) defined in Chapter I, Section 6 and Chapter III of McDonnell Douglas Corporation Report No. L26-013, "DC-3 Supplemental Inspection Document (SID)," Revision 1 dated January 1990. The non-destructive inspection techniques detailed in the SID provide acceptable methods for accomplishing the required inspections. All inspection results (negative or positive) must be reported to the McDonnell Douglas Corporation as prescribed in the SID and copied to CAA. Cracked structure must be repaired or replaced as prescribed in the SID before further flight. (FAA AD 90-05-08 refers) Compliance: Inspection programme must be in place by 30 April 1991 Effective Date: 26 October 1990 ++++++++++++++++++++++++++++++

As you can see some are quite recent, just because the plane was made

50 years ago doesn't mean all the engineers packed up and went home. They keep a Tech Service staff going - that costs money. Many planes as they get older have more restrictions placed on operation, service ceiling, service speed, max bank angles, climb rate, loading - every conceivable operational parameter can be and is de-rated to ensure safety. Ultimately the certification for civil flight / general aviation can be removed and they need a special dispensation / inspection for each movement or display.

Some aircraft have been repaired after crashing. It's common for military stuff to be put it in a flyable condition at crash site and have a test pilot fly it back in short slow low hops to the maker for full certified repair. At each hop it will be inspected to make sure the temp repair is holding up. Civil usually operate at airports with better service and repair facilities, so get fixed where they get dinted (or cut up and removed).

Doesn't just apply to the aircraft but to how it's done, who does it and the tools used. But one day they will find cracks in the main spar or keel and then it's pretty much over. Unless someone is willing to go the lengths they go to restore precious things like Spits and be subject to severe restrictions on operation.

Err it's just like a cracked head or crank, the entire body isn't stressed evenly. Either it'll be economic to fix or it won't. Cast irons hardly the best material from a fatigue point of view either & it seems to survive in suspension components, brakes etc.

Suggest you look at chart on page 5, looks like only 5-12% to me.

only gets used for high stength or high vibration parts likeengine mounts, hinges and some anti frettage liners/facings. Thesedays more and more are made out of glue. and on page 6 you will see R-R Trent 800 only has 25% steel. Shafts, drives, bearings and a few bits of IP compressor.

Only the Russians use steel extensively.

and only USA Military can afford to make planes out of titanium like SR-71.

I'm sure you're right that not many planes are made of steel these days. All I meant to say was that those that are (or were) used cromoly tubing which also doesn't have a fatigue limit.