How safe are wheel spacers ?
 

From your description I am going to take a guess and say no. The only safe wheel spacers I know of for our cars, are part of the BroFab e30 wheel bearing kit. The BroFab kit actually sandwiches the spacer between the wheel and hub with studs through the whole assembly.

Do you know the thickness of spacers or have pictures?

Every time the wheel rotates the studs bend a tiny fraction.
It won't happen immediately, but eventually metal fatigue will win.

Big difference between a big static continuous load, as in constant bolt or stud tension, and continual back and forth flexing.
You could probably lift your car right off the ground with a brand new coat hanger.
Try bending it ten times in exactly the same spot with your hands, and see what happens.

You should probably use longer and stronger wheel studs if you put any wheel spacer on.

I don’t buy the coat hanger comparison. Studded wheel spacers are certainly different than the slip-on type. A concur that studded ones add more potential failure points (i.e 2x the lug nuts) and puts the torque arm a greater distance from the bearing and thereby more force and wear on it. There’s a few of us that grew up in the muscle car era (we just called them real cool used cars) with spacers and wide tires and the bearings wore out much sooner than otherwise and this is well documented. Weaker and inferior materials should always be a concern for all of us, particularly those that are safety related. Categorically saying studded wheel spacers flex and break like a coat hanger is not accurate.

I'm not sure what the heck is even being discussed here. No, wheel spacers are not categorically bad or dangerous. End thread.

Citations:

• This is a complete shift in context, but even frikkin Emillio suggests they'll be needed on their forthcoming track-oriented brake kit. Now, that's in regards to a 5mm spacer which is a world of difference from lug adapters or enormous 25mm hard-parker BS.
• Flyin' Miata sells Miata-specific wheel spacers in three different thicknesses.

@Warpspeed - Unless you're putting individual washers on three out of your four studs and then tightening down the wheel on top of them, or you just slap the nuts on and don't bother to try to tighten them evenly, you have a static clamping force of the wheel against the hub. Where do you get bending and flexing?

^ what he said.

Ultimately it comes down to the use conditions. Miata hubs and bearings can last the entire life of a street car, but they've been known to break in only a few sessions on race cars with wide/sticky rubber. At a minimum the addition of wheel spacers will make longer moments and potentially multiply forces acting upon the hubs and bearings.

Like all other things, once you start deviating from OEM design or geometry, you should be prepared to apply some engineering thought if you want safety and reliability.

You guys might get a kick out of watching the BONOSS "active cooling" wheel spacer video put out by the manufacture...

Sort of funny.
No comments or questions AT ALL!
Hard PASS from my viewpoint...

Normal wheel spacers as Emillio suggests (using the stock location for studs, just longer studs) are perfectly fine.
I would not trust any "studded" spacer that changes the mount points from the stock location at all.
I believe such spacers are banned from most motorsport events.
The "normal" reason for a studded spacer is a change in lug pattern or diameter. Big no-no in competition.
The "abnormal" reason for a studded spacer (with air channels built in!) is to improve braking performance.

I want to see him prove his claims. Sounds like COMPLETE BS to me...

There is always an exception to "the rules".
After posting the above I realized I do deal with studded spacers in my shop quite regularly.
1 ton trucks with dually rear wheels often have a huge heavy studded spacers on the fronts to adjust the track of the front tires to better mimic the track width of the rear wheels.
These make front brake work more difficult, especially if rust has set in.

So even the OEM manufactures use studded wheel spacers.

But, they're not trying to "cool" the brakes with them either...

Yes, very true.

I don't know anything about Bonoss, except they are a Chinese company with some pretty impressive advertising claims.
The Chinese also make a lot of fake sub standard counterfeit parts, as well as some pretty good stuff.
Let the buyer beware.

Personally I would be a lot more confident in buying US, Japanese, or European manufactured products that are safety critical.
Greedy litigation mad lawyers on the loose, will quickly bankrupt any manufacturer of non complaint or dangerous products, but that is not the case within Communist China.