Stacking adapter style spacers

[Coletrane]

Okay so I have 4 adapter style 4x100 spacers that are hub and wheel centric. In theory it would work to stack them, and it would be just as safe, but I need some reassurance. Am I insane? Anyone done this?

[Beware]

Yeah, I wouldn't. What thickness are they?

[Coletrane]

20mm, no way I'd even need to stack anything bigger, taking offset down 40 already

[Oh Damn, it's Sam]

That's a lot of failure points to be adding. I wouldn't.

[BenFenner]

I've recommended stacking spacers in a pinch on numerous occasions. As long as you have enough stud or bolt length you will be fine.

Having spent 8 years with 12mm worth of mis-matched, stacked washers in lieu of a real spacer on a daily driver that I tracked on numerous occasions has really changed my mind as to what is acceptable regarding spacers.

People are too scared.

[Coletrane]

People are too scared.

I always looked at adapter style spacers as doubling the amount of nuts that have to magically unwind themselves in order for your wheel to fall off. Also, its more metal there to deal with the extra stress put on the hub/spacer/wheel/nuts, so how could it not be safe? Wish some engineering boffin could prove this in equations.

[BenFenner]

When I made my reply for some reason I assumed you were stacking normal spacers.
If your plan is to stack bolt-on spacers I would not do that. Bolting one spacer to the hub, and then another spacer to that first spacer, then bolting the wheel to the second spacer?

I would be very wary of attempting that. It is true that you've doubled your failure points. There is also something about the idea of not having a continuous clamping load from wheel to hub that makes me nervous. The forces from the wheel are transfered to the hub via friction, don't forget. So having a single clamping location for each lug is important in my mind. I don't even like a single bolt-on spacer for that reason already.

[Oh Damn, it's Sam]

I would be very wary of attempting that. It is true that you've doubled your failure points.

Tripled, even. Twin bolt-on spacers is 12 lugs per wheel for a four-lug application.

I've had lug nuts work their way loose in severe-service applications. Get an aluminum wheel brakes-on-fire hot, and things start stretching in ways you wouldn't anticipate. That's considerably more severe than anything a street car is ever going to see, but having had a wheel come off at speed, it's not a super fun time.

I remain in agreement with Mr. Fenner's updated conclusion. I'll stack the shit out of slip-on spacers, but I don't even like to run a single layer of bolt-on spacers if I can avoid it, much less two.

EDIT: Also, for a good look at why distributing a load through compound failure points can be dangerous, Wikipedia yourself the 1981 Kansas City Hyatt Regency disaster. It's not a precise analogue, since the bolts there were placed in tension rather than shear, but it's illustrative nonetheless.

[BenFenner]

Tripled, even.

Good point.

[Coletrane]

Alright alright, I'll bite the bullet and spend the extra cash on my safety. Whats bad about bolt on spacers though? I figured they would at least be perceived as safer, since I sell slip on spacers to idiots at Autozone WITHOUT any extended studs at all. It blows my mind. I would still figure they'd be "safer" because the spacer's studs and your stock studs will share the force put on them rather than put it all on the studs in the rotor.

[Oh Damn, it's Sam]

I would still figure they'd be "safer" because the spacer's studs and your stock studs will share the force put on them rather than put it all on the studs in the rotor.

That's not how it works when you're loading something in shear. You're placing the full shear load on both (or all three, in the case of stacked spacers) sets of lugs. Hubcentric spacers help this, but you're still placing the full shear load on the centering bosses. The bigger issues, however, are threefold. First, you're adding an extra set or sets of lug nuts that can get loose or strip. Worse, however, the centering bosses and center holes on the hub, spacers, and wheel have their own tolerances and runout. Each set of spacers stacks that runout and tolerance. This can also be an issue for slip-on spacers, but in that case, the lugs (particularly if they're cone seat) can do some of the centering duties. This is not the case with bolt-on spacers, where the lugs themselves have their own tolerances in terms of how centered they are to the hub. The last issue is that you are placing an interloper in the way of the clamping force performed by the lug. With a bolt-on spacer, the line of force for the lug no longer makes a straight line into the hub, it has to make two 90º bends and you're relying upon the material properties of the spacer to distribute the force from the lug into the hub.