In a message dated 97-03-22 17:24:02 EST, you write:
<< he most important thing, IMHO, is not to change the suspension's
"instant centre". You do this if you go to either a higher, or lower
profile tyre. When you change the "ic", you lose the MG's delightful
light handling, and road feel/responsiveness.
>>
What I think you are refering to is the "Virtual Swing Arm", which is used
to determine the "Instantanous Roll Centre". The Virtual Swing Arm is the
imaginary point where lines drawn through the upper control arm pivots and
the lower control arm pivots, intersect. Another line from this point
through the centre of the tyre contact patch extended to the centre line of
the car. At the intersection of this line and the car centre line is the
"Instantanous Roll Centre. The IRC is an imaginary point about which the
vehicle will roll under side loads. AND both the VSA and the IRC move as the
angles of the suspension control arms move in relation to each other as when
the vehicle is subject to body roll in a corner or encountering a bump. Thus
the "Instantanous" in IRC.
Higher or lower profile tyre will not necessarily change the over all
diameter of the tyre. It all depends on the size of the tyre. For example,
my GT calls for 165R14, according to the book. I am currently running 195/60R
14. The over all diameter difference is only about 0.2". Or 0.1" difference
in overall radius.
Changing the overall radius of the tyre can cause the IRC to move up or
down by a small amount but not as much as the difference in the tyre radius
(at least on the majority of LBCs). Whether or not this will result in a
detrimental effect on handling depends on many other factors.
Changing the overall radius of the tyre will have NO effect on the relative
angles of the suspension control arms. Granted, they (the arms) will change
their height above the ground, but that is all.
The real danger lies when the "ride height" is adjusted after installation
of lower radius tyres. When an attempt is made to raise the vehicle back to
the original height, which neccesitates (sp?) shimming the springs in most
cases, the relative angles of the control arms do change and consequently a
major change in roll centre height is acheived, many time to the detriment of
handling.
For an comprehensive discussion on suspension types and their effect on
handling, check out Fred Puhn's book "How to Make Your Car Handle", and Paul
Von Valkenberg's "Race Car Engineering and Mechanics". Either, or prefferebly
both, will help make a bit of sense out of the "black art" of suspensions.
Just thought I'd throw my tuppence in, for what it's worth
Rick Morrison'
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