Bushings, deflection, and NVH

What's the difference, or what terms should I be looking for in Googling for it myself, in terms of firming up a bushing by replacing its entire volume with a harder material (e.g. urethane) vs reducing the volume of the original material?

Imagine a 1/4" thick layer of rubber and a 1" thick layer of harder urethane. Both may deflect 2mm if you put 50 lbs on a 1" diameter round rod pressing into the surface.

But these materials have rising rates in deflection, right? So the first 0.25mm travel will take less force on the rubber than the harder but thicker urethane.

The thicker the material, the less quickly the rate rises, and the closer to linear the rate across a small travel.

So could a softer but thinner bushing provide better road noise isolation while still limiting total deflection to similar levels? What other effects would this have?

Well, SCCA won't allow a change in the amount of metal relative to the bushing, so I'm guessing you're correct in your presumption. However, you're effectively "bottoming out" the bushing, which I imagine would wear through the rubber rather quickly. And much like bumpstops, in extreme situations I suppose it could possibly lead to really unpredictable traits, maybe even binding?

Just guessing...

I'm not thinking of foam rubber in tiny amounts so it collapses completely; both examples should be thought of as operating within a normal range. That is, each being partially deflected at full 'usual' forces. Just at different points along the continuum of firmness, with the softer one thinner so as to give the same net deflection under normal use.

My impression is that the thinner a material is, the quicker it's rate curve will turn upward with deflection. It's not just bottoming it out.

I wasn't thinking specifically about the SCCA, but I'm curious about how they define allowable bushings in classes where they allow urethane bushings. You could achieve the same effect without using additional metal by making two-compound bushings with a thinner softish region and a spacer part made of "shore eleventy-billion" which would be nonmetallic but still effectively a rigid solid.

IIRC, the Street Prepared legal urethane bushings on my 2002 had steel sleeves over the bolts for the urethane to turn on. I'll have to look up the pertinent bits, but if anybody has the spot in the SCCA rules bookmarked and would like to save me the digging...

Well, this is from 2012:

"In a replacement bushing the amount of metal relative to the amount of non-metallic material may not be increased."

In reply to petegossett:

That's terrific, thank you!

Do you happen to know the location in the rules of that section? Is that from the GCR?

EDIT: Seriously, if you don't know off the top of your head, I can totally look it up. Thanks very much for the info so far!

I just looked in the 2013 Solo rules and section 15.8.C has this same wording for Street Prepared as the 2012 rules. The Street Touring rules are also worded the same.

It's more complex than that. Both solid rubber and urethane basically incompressible (!!) The deflection that you observe is due to deformation of it's shape. Take a chunk of rubber or urethane, toss it into a blind threaded hole, and screw in a bolt, and start tightening. Once all the air has escaped, it will go solid, you will no longer be able to tighten any further. So the shape and mounting (basically how much room to expand) affect the 'rate' that you get.

In reply to erohslc:

True!

I figure the cylindrical shape of bushings in a given application dictates that to a great extent, leaving material properties as the primary way to make any adjustments.

Though I guess you could try using thick washers to partially constrain outward expansion of the ends of the cylinder over some portion of the end's area, but that seems awkward and unreliable (and I'm not a big fan of the idea of using hardware that's holding the control arm on the car to do a job that may load it oddly and cause any issues. I bet that could be done safely, but it's just an approach I don't want to pursue).

In reply to petegossett:

Thanks very much again!

In reply to ransom: Yah, factory bushing (and motor mount) setups always have room, since otherwise they would go solid and 'rate' could spike up really high. Another tuning option based on that is to use a harder or stiffer bushing material, but then drill some holes to soften it up. The position of the holes can make the response directional, stiffer in one plane than the other.