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I'm not about to read through the entire thread, but I think it's a safe bet to make that nobody has measured the CG location or the front RC location on the Evo, let alone determine at what height the front RC drops below ground level. Which isn't to say that the *theory* on which people are basing their claims is incorrect, but you can theorize until you're blue in the face and never come to a solid conclusion (in this case "Why am I understeering?") unless you hunker down and get some concrete measurements, beginning with the above-mentioned CG and FRC locations. It's not that theorizing is bad, but at some point, someone is going to have to say "Look guys, here is the x, y, and z of the car's suspension. Lowering it a-amount will change your RC by b-amount, therefore resulting in so-and-so handling characteristics." I'll save my pennies on roll-center adjuster kits until then.
As far as my comment regarding suspension geometry, I meant to lead you to the concept that a car's geometry rarely changes, unless a control arm is bent or a pickup point is altered. The geometry stays the same when you change a car's height, you simply change the point at which the suspension starts and stops traveling. Those roll center adjusters aren't fixing anything, because nothing is broken.
I see a lot of people throwing out the "lowering a mac-strut car = bad" rhetoric, without a lot of evidence beyond a crude block-and-line diagram depicting some super-extreme example of a mac-strut car being lowered. RC below the ground? 1.5x longer roll couple? Oh noes! Again, all theory, no solid measurements for the car in question. And again, theorizing isn't bad, but I'm going to need more than that to get scared into buying "fixes" for by "broken" suspension geometry, which is exactly what I think is happening now to others, when it's very possible that no fix is necessary.
case in point: high spring rates countering the effects of an increased roll couple. Say Joe Blow goes out and buys a set of coilovers, with rates of 10k all around. With 10kg/mm at the front wheels, that is a solid amount of spring for the front end of an Evo. Obviously Joe lowers the car on the coilovers, and he reads a few threads like this and goes "Uh-oh, my roll center is out of whack. My car will understeer now!" What Joe knows from reading the boards is that an RC below ground will result in understeer. So one of a few things will probably happen:
1. Joe panics and buys a roll-center adjustment kit
2. Joe raises the car up to stockish height under the assumption that Mitsu engineers designed the car with a desirable above-ground RC, thereby avoiding a perceived poor-handling condition
3. Joe drives the car as it is, understeer be damned.
What Joe doesn't know is that a lower RC will also result in
a. less geometric weight transter, that is load applied directly to the RC, and
b. more elastic weight transfer, that is load applied to the sprung mass of the car at the CG
Elastic weight transfer is controlled by the springs, shocks, and ARB's, which Joe Blow has recently upgraded. And, nose-heavy cars don't like a lot of geometric weight transfer for obvious reasons (if it's not obvious: "Hey, I can't control all this weight with my springs and shocks and ARB's! Now, the only thing I can do is fiddle with my linkages and pick up points, that costs $$$.") By lowering the RC, that nasty geometric weight transfer has been reduced, and now the bigger player in weight transfer becomes elastic, and Joe can control elastic weight transfer with his new, stiffer springs and ARB.
Joe also does not know:
c. A roll center closer to the ground reduces jacking forces.
d. A roll center closer to the ground reduces lateral forces acting at the tire.
Joe only knows the one disadvantage of lowering his mac-strut car as told to him by the internet pundits: lowering the car will cause his RC to drop below ground, which results in understeer which is undesirable. While this isn't untrue, Joe most likely does not know of the benefits of lowering his car, or of the other factors that can offset this effect of lowering. These factors can often turn the tables in favor of lowering the car, as the benefits then outweigh the downsides.
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