Originally Posted by Cspence
Sadly i cant find the strength or tension of the stock miata bolts
Left is with spacers right is without.
To make this easy i rounded it off to 1000lb
I used 1" spacer, the other 1/2" is simply the distance from the mounting face to the distance of the nut it self on the rim.
The basics of bolts, bolts are good for tension however they are not too good about side flex so moment is bad, too much moment on a part thats not made to take it ends up looking like this
The way the studs are inserted, Motion is created naturally, in other words the bolt cannot move around up and down, it stays straight out to the side. This is why you see the half circle with the arrow there.
The basic behind the above picture is you have to have enough tension on the bolts to keep the force of friction high enough to hold that 1000lb (when racing that 1000 will be much much bigger).
Assuming the bolts have enough tension and the force of friction is in control of the entire situation. The aluminum will flex a tiny bit, the spacer it self will also flex a tiny bit and same thing with the rim. Its a small flex but its still there. The closer the nut is to the beginning of the rim.. the less flex you have the less moment the stud will be forced to take.
You can clearly see that the distance increases as the rim gets further from the hub. In the not to scale image above exaggerated to show the angle, lets call distance A 1/2" (first example rim measurement) and distance B 1" and a half... The next thing is the Tension. If the stud is offset by 1/16" the tension increases since the stud was originally 1/2", now its
in other words the stud is now stretched by .003891.
With the bigger spacer you got
In other words the stud is now stretched by .02069...thats more than 5 times more than without the spacer.
The next on the list would be the tolerance of these things. The hub has a lip in the middle thats there to keep the rim centered if force of friction is not enough to hold the rim.
The last time i got a set of spacers in my hands, i could tell they were different by just looking at them. One hole was bigger than the other to the naked eye. Thats not something i want holding my rim positioned. 75% of the spacers dont even have those lips in place, they are just a flat cylinder with 4-5 holes in them and then the big one in the middle. dont forget that most spacers are universal and not many cars have the exact same size lip.
next on the list is the spacer with its own studs.
If its made by a reputable shop (good tolerances) its your best bet of a spacer. In my opinion if you are going to use a spacer use this type of spacer. The only negative to this spacer is that you have a set of nuts that are hidden by the rim.
This is the best explanation with the information i have. Hopefully after this oncoming semester of engineering i'll be able to explain it better.