One thing I'd like to bring into this interesting discussion is the idea of hub centric wheel spacers. This is certainly an issue with cars, and is often the cause of failure of wheel spacer systems. I don't know if it's a factor on trucks?
On most modern FWD cars, the vertical load of the weight of the car, as it passes from the hub to the wheel, is not meant to be borne by the clamping force of the wheel to the hub face, nor is it meant to be borne by a shear load on the lugs. Rather, there is a lip machined on the face of the hub, which keys into a recess bored into the mounting face of the wheel. The vertical loads are transmitted through this keyed fit. The lug nuts are therefore simply required to clamp the wheel to the face of the hub.
Cheap car spacers often do away with this "hub centric" design, so that one spacer can be cut which will fit many models with varying wheel bore sizes. The loss of the hub centric design means the vertical loads are passed on to the studs, which were not sized for this, and eventually the studs fail. The same is also true of cheap aftermarket wheels, machined with an oversize bore but meant to be installed with adaptor rings. Often times the rings are not installed, and the result is the same.
Somebody early on mentioned spacers being used in racing, and therefore it must be ok. Spacers are often banned from racing, because of all the problems they cause. I have to use 4mm spacers on my Focus to get the wheel spokes to clear the 4 piston brakes. It hasn't been an issue, because the spacers are only 4mm, and I also machined custom hub adaptor rings which reach through the spacers to key into the wheels.
I was surprised when changing the wheels on my Discovery the first time, to find that the wheels are not hub centric. The vertical loads are taken up by the studs. Is this the case on all trucks?
Riddle me this. What makes aluminum 'aircraft grade'?
I'm glad you brought that up, because it is a pet peeve of mine. It is a marketting term, in common usage. 6061-T6 is a grade of aluminum used in aircraft. Therefore the term "aircraft grade". The reality is, it's pretty much the most common type of alumuminum available to small volume fabricators, and is the defacto standard used in ANY custom fabrication. Walk into any metal supplier, and tell them you "need 6" of 6" dia Aluminum". You could be building wheel spacers, or you could be using it as a door stop. 6061-T6 is likely the grade you'll get. If you tell them "I'm just building a billet ashtray for fun, do you have anything cheaper?" No. That's as low as they'll generally have.
So if you wanted, you could make yourself an ashtray machined out of billet "aircraft grade" aluminum.
Where am I going with this... basically, the term is overused, and completely meaningless, as just about anything custom machined is going to be "aircraft grade". And "aircraft grade" really means "nothing special". But it sounds good.
