Well I've had my new rims on for about a week, and I've definitely noticed a few things. These are +38mm on an 18x8.5, but the tires are the same (stock potenzas) so I feel safe saying that what I am experiencing is due to the decreased offset.

- Tramlining: It does it a LOT more. My guess is that because the contact patch has been pushed away from the car, the same force exerted against the wheel (bumps, grooves, etc) is exerting a greater torque. I have to concentrate a lot harder on keeping the car straight if the road surface is less than perfect.

- Centering: On a nice, clean, flat surface, the car centers itself more quickly. Meaning, if I am turning and let the wheel slip back to the straight position, it does so more quickly and more forcefully. My theory: same as above, greater restoring torque.

- Rubbing: None. Yet. I don't think I'll have a problem with it the way my suspension is set up now, but it looks like if I wanted to lower it I could run into some rubbing against the front fenders (especially on hard bumps). Guess I'll cross that bridge when I come to it.

In hindsight, while the +38s look great (especially in back), I'd choose differently next time, and go for a +40-43mm.

I have a +38 with an 18x8 wheel, does that stuff apply to me too? Or maybe worse in my case?

not as bad I would think - the contact patch will be moved slightly inwards in comparison to mine. same idea though

Well I've had my new rims on for about a week, and I've definitely noticed a few things. These are +38mm on an 18x8.5, but the tires are the same (stock potenzas) so I feel safe saying that what I am experiencing is due to the decreased offset.That isn't entirely true. The same tire on a wider rim will result in a slightly (0.2") wider contact patch. It isn't much of a change but it is a change.
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I'm currently driving 8x18 ET 40 BBS RX ;) wheels and on bad surfaces the car now requires way more concentration when going fast than it used to. I'm going to try to adjust camber to compensate for this.

I've just come back from a test-drive after changing the setup. A little more camber in the front was all that I wanted to do, but I was surprised at the setup mazda gave the car:
It has way more toe-in in the back than in the front. I changed that, too. Now I get a lot less oversteer when accelerating in corners. The car wasn't too symmetrical, either.

What am I trying to say? Have your setup changed when changing wheel-offsets.

I ordered a 45mm offset but they called me back and said all they had in stock for 4 wheels was a 52mm offset. I am assuming that should be fine even though its over the stock offset by 2mms. Never ordered aftermarket rims.

what does offset really mean...i know it's sad :(

what does offset really mean...i know it's sad :(
http://www.tirerack.com/wheels/tech/offset.jsp

It would be sad if you got rims not knowing the answer.

The Stupid question is the one never asked.

Changing offest will change a car's SAI angle, which is not a good thing. The differences may be small or large, but the spindle, wheel bearing, and tire life will all decrease as well as change the handling characteristics of the car.

Yet I still did it myself for clearance issues on my '84 RX-7.

i did get rims w/o knowing the answer, but i didn't get weird offsets, i pretty much stuck with what other people were getting to be safe.