Holy shit, there is some ignorant posting going on here! There is nothing inherently unsafe about properly-implemented wheel spacers, and any "engineer" who tells you differently is a terrible engineer...whether he can spell or not. Everyone should read a
primer on the topic from an engineer with, you know, actual knowledge and experience on this topic.
There's so much bullshit in this thread to debunk! Let's start with the loads and load path junk. The hub doesn't see a different load path just because there's a spacer there. The load path
does change a bit when the net offset (wheel offset minus any spacer thickness) changes, because the center of the contact patch (where the vertical load's origination point is) changes. However, within reasonable limits it's not critical at all: after all, Chevy produces Camaros with front and rear offsets all over the place (almost an inch of variance for different models), and they don't change the bearings or hubs between different models. So it's not a problem.
There's no weakening of the wheel's interface with the hub. For a given lug nut torque, the clamping force that locates the wheel on the hub is the same as that which locates the wheel to the spacer and the spacer to hub. If you don't think this is true, prove it mathematically. Good luck with that. And it's that clamping force that actually holds the wheel in place, both axially and radially. Period.
You also don't need hubcentric anything. Someone already mentioned this, but if you think "hubcentric" bores and pilots actually locate the wheel, then you don't understand tolerances and you don't understand how tapered lug nuts and seats work. Once you tighten your lug nuts, your wheels are located
only by them and
not by the hub pilot and bore. There have been millions of cars and trucks produced over the decades without hubcentric wheels, and there have also been plenty of race cars without them. The only reason most manufacturers use hubcentric setups now is to make it easier to install the wheels properly on the assembly line or service station.
Statements about dissimilar metals expanding/contracting at different rates are just ridiculous. Do you all not realize that your aluminum wheels are held to an iron hub by steel studs and (usually) steel lug nuts? There are aluminum and steel spacers: use whatever floats your boat.
Oh, and BTW there are often iron or aluminum brake rotor hats already lurking between your wheels and hub faces (there certainly are on our Camaros). So they more or less work exactly like spacers already. Clearly they aren't a hazard!
As for "consult a local Tire Kingdom or your favorite mechanic," do I really need to point out how ignorant most of those people are about how any of this works? I mean, most of the people that work at these shops can only barely grasp how to change a tire, much less the physics of wheel and hub interfaces.
Any spacer is safe as long as it is flat and stiff and you have enough threads remaining on the studs to properly torque the lug nuts. The adapters (which is actually what the OP was asking about) are heavier and have far more failure paths, but again as long as they are properly designed and the studs clear the backs of the wheel faces, they are safe also.