I am one of the worlds best fans high polar moment cars. There is this
misnomer that a high polar moment car cannot react quickly and therefore
couldn't possibly be quick edge to edge (sorry skiing term) in the tight
corners.
What I find is that high polar moment cars that are balanced like the
928 is balanced can eat uneven surfaces better than any other type. A mid engine
car while technically correct still has a large lump at behind the longitudinal
apex and therefore still has a high tendency to want to exit corners with the
heavy end first when grip becomes a negative influence on stability :) There are
two reasons why the mid engined platform has become iconic. The first is that if
properly setup they can be completely neutral under braking. This means that
there is a sacrifice of stability when not under braking. Meaning you have a
lightly loaded front end which you compensate for by putting less grip (smaller
tires) up front. When under braking you shift weight forward that extra weight
makes extra grip up front and reduces grip in the back neutralizing the chassis.
Only the best of drivers can manage that and transition it in and out while
cornering and do it smoothly enough to not lose control. Most ultra high
performance cars are somewhere in the 40/60 range.
Driving a 928 is another story. Same with Vettes. Same concept. They are
technically set up to corner without braking. So braking in the corners is
supposedly, no pun intended, slowing you down or reducing grip.
However, a car like the 928 has such a good combination of featherable throttle,
modulatable brakes and forgiving suspension geometry that it doesn't sacrifice
much stability under braking while cornering. This is the part that still amazes
me to this day. A 50/50 static car should be badly imbalanced under hard braking
and especially under trail braking.
The 928 is not. The polar moment is in no small part the reason. The polar
moment tends to keep the chassis on or about the tangent of the whatever curve
if you initiate properly. Even if knocked out of whack by braking input or frost
heaves as long as you don't exceed that threshold of stability (sort of like an
upset bandwidth) the 928 will recover and maybe just walk outside the intended
line. The balance of off throttle settling and tightening with braking or
throttle is so subtly designed that most don't notice how good it is when it is
set up correctly. I have been experimenting with this stuff for years. The last
and most important part of the system is that magic rear axle. I am amazed that
every car doesn't have this simple system. I look at that picture of the tech
standing in the back of the test mule with the big steering wheel stuck in the
rear axle pivot with the pencil angles drawn on the tape on the wheel well.
Can't believe what they were able to do with such crude tools. I have always
thought that one could get rid of the simple link and actually put a spring and
shock combination in a lower control arm assembly and actually make the effect
better and tunable. But it is really good the way it is. That other little
engineering detail is the last piece of the puzzle that makes the 928 50/50
balance do so well both under braking and at speed. The car should become
unstable on sharp application of trail brake. That is a huge unsettling force
yet it is a complete yawn in a 928 and in no other car that I know of. In fact
once you know what it is you can use the effect like part of the suspension.
Very kewl!
So That Dog... I would like a mid-engined car mostly because I want down force
but I'll keep my 928 for 90% at least of my warm weather lunatic driving. It's
just too much fun.
jfk