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WA/A2 04-07 GD GG STI Springs

Old 10 January 2019, 03:52 PM
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Originally Posted by 2pot View Post
Development of some Bilstein inserts coming along nicely.

Similar to this kind of thing:

Should have something to start assessing, by the end of this month.
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Old 12 January 2019, 07:40 PM
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A quick theory reminder, not for 2Pot, as my man know's his onions, but some other readers might benefit from first principles.

You car rotates around a mythical thing known as the "roll centre". Actually, it does this around an axis passing through the front and rear roll centres. Visualise your car on a spit roast. Then again that might be too painful.

So, the Roll Centre. What is it?

Well, any suspension system has one. It's the effective and instantaneous pivot point about which the car will rotate on the wheels as a side force is applied. On a wishbone system, it's defined by drawing a line from outer to inner pivot bolt on each arm and continuing the line in through the centreline of the car and beyond until the lines intersect. Then from there draw bac kto the centre of the tyre's contact patch and where both sides cross each other more or less on the centreline is the Roll Centre.

Roll torque is the action of the centre of gravity under centripetal force acting on this point.

With a little thinking you can deduce a parallel double wishbone would have an infinite intersection, and acts like a live rear axle. In both the roll centre is ALWAYs at ground level.

On a McPherson Strut, our cars, the upper wishbone is defined as a 90 degree line from the top mount inwards, again with a little thought you can figure out it's a very sensitive system to ride heights and angle of the strut.

Older Mc Pherson Strut cars had short lower arms and angled struts and drove like....well. Prehistoric.

From the Mondeo circa 1990, (and most cars shortly after their designers had driven the Mondeo), you can see a sea change is design, where everyone ( Except the idiots at Vauxhall/Opel) went for more vertical Struts and larger longer arms. Thank goodness for that.

What it practically means for us is this. As you lower your car, the lower arm angle changes. The outer pivot (your lower balljoint) moves out increasing Camber, caster, track and toe out. If you shorten your struts 1", you'll lower your Centre of Gravity 1". Your Roll centre geometry will drop about 3-4".

This increases the lever arm moment between CoG and Roll Centre (RC) so the car will roll more. Yuk.

Here is the thing. The Roll axis on a standard New Age Impreza is slightly tail down. The Spit Roast pole is not level but lower at the back. On rolling, the car sinks into the rear corner and un weights the inside front wheel. This causes predictable understeer. Safe.

The general first fix people try is a stronger anti-roll bar. And yes, it helps. It will support the car's outer rear by trying to lift the inner. Geometry doesn't change so it's marginal, but it also causes more transfer side to side on single bumps and makes you bang your head on the window on speed bumps. I hate too strong ARBs for this reason. Very agricultural choice.

By lifting the rear slightly, 10mm or dropping the front the same, you change the angle of the roasting pole, and the car will now corner better. By not dipping at either end, balance is better and it's win win. Too far though, and the front will dip loading the outer front, wearing the tyre and causing sudden understeer, or oversteer as the inside rear is now lifted up. A Hot Hatch is OK, on three wheels , but if you want to put power down on a AWD supercar, this is not a good idea.....

Hope that helped.
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