gap between swing arm and disc brake -- suggestions?

[serious_sam]

Lots of misunderstanding in this thread.

I don't know if proposing an increase in shear load on the screws is just included to sound technical, but if you're loading the mount screws in shear, you've already failed.

"just included to sound technical"..."you've already failed". Arrogant much ?

I admit that I mis-spoke. The shear load is in the joint, not the screws. And the shear load doesn't increase with an offset load due to a spacer. There you are correct.

However, the basis of my argument is sound. The shear capacity of the joint does decrease with an offset load, because the offset is out-of-plane (i.e. the joint is no longer purely in shear), and that moment reduces the preload on one side of the joint. Reduced preload means reduced shear capacity. Increased shear load vs reduced shear capacity - basically the same end effect.

This is entirely spurious. There is also no increase in tensile load.

I hope you're not an engineer.

When the disc is loaded with no spacer (tangential in-plane load), the load on the joint is basically pure shear. With a spacer, the load applied to the disc by the pads is now out-of-plane to the joint at the hub. The offset force applies a moment which causes tension and compression across the joint. Tension externally applied to a joint opposes the initial bolt preload (as the joint extends, the tension on the bolt is increased), and hence reduces the shear capacity of the joint.

Simplifying, the only force on the brake hub is rotational -- this isn't like a wheel spacer that experiences force normal to the ground.
Since the distance between the axis of rotation (axle centerline) and disc mount holes doesn't change with spacers, the moment likewise doesn't change.

Wrong. The pads apply a force to the disc, which creates a moment, but for the assembly to be in equilibrium, there is an equal and opposite moment and force reacting at the disc/hub joint. Draw a free body diagram.

So you actually could use a foot high stack of those spacers, and as long as the joint remains sufficiently clamped to transmit force between the two surfaces via friction rather than loading the bolts in shear, it would make no difference.

"as long as the joint remains sufficiently clamped to transmit force between the two surfaces via friction". Unfortunately, this assumption is not a constant. By changing the geometry of the joint, you're changing the ability to remain sufficiently clamped. The more the load is offset (thicker spacer), the larger the applied moment. At a certain offset distance, that moment will create enough tension in one side of the joint to cause it to fail - initially it will slip, and as the load is increased further the bolts (or female threads, depending on hub material and depth of engagement) will fail. Hence the flaw in your argument.

Also, the longer the bolt (the thicker the spacer), the lower the bolt stiffness, so the preload tension of the bolt increases less (than that of a short bolt) as external tension is applied to the joint. A short bolt increases in tension more rapidly as external tension is applied to the joint, and so maintains a higher preload as this occurs. Draw a bolted joint diagram. Note the short bolt has a steeper gradient, and hence greater reaction to externally applied tensile forces.

In fact, you can see this in Youtube videos stacking absurd numbers of wheel spacers: such a driven wheel, when jacked up, will spin up normally (excepting added rotational mass), but when lowered to the ground, the joint will quickly fail since the distance from the normal force at the wheel to the hub is many times greater, increasing the moment.

This is the exact reason that it's not ideal to add spacers to a rotating joint like this. A perfect example. The wheel is almost exactly the same condition as the disc. The disc has a tangential load, whereas the wheel has a radial force (vertical load) in addition to the tangential force (tractive load). But when offset, both forces are out-of-plane and resolve to apply tension to the joint.

There's a reason many fatigue test standards specify rotating shafts -> cyclic loading. The caliper joint sees effectively one load cycle per application of the brakes. A disc joint sees one load cycle each time the wheel rotates (during braking). In the context of the OP, given the opportunity to compromise the rotating joint of the disc, or have a beefed-up fixed canterlevered joint of the caliper, the choice is fairly obvious. Particularly when connecting to the 1/2" (or whatever it is) plate of aluminium of the swing arm.

 

[fatty]

"you've already failed". Arrogant much ?

I think we had a genuine misunderstanding in our last debate you bailed on, so I'll likewise clarify here: I meant "you've already failed" as a generic you, as in the fabricator/fabrication has already failed structurally.