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      07-18-2011, 09:12 AM   #23
tarheel91
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Quote:
Originally Posted by CalRick View Post
That is the classic textbook theory with which we are all familiar. Unfortunately, nature doesn't always read the textbook (and thankfully, neither do quite a few clever engineers, which is why we get 911s and Corvettes that don't quite follow the textbook.) I will take the opinion of the guys doing the tests as having some merit on the issue.

The dynamics tend to get a bit more complicated than basic weight distribution, even in a fairly steady state test as their slalom. Middies and rear enders tend not to turn in as well as front biased cars, for instance, unless you start artificially shifting the weight bias thru techniques such as trail braking, which are beyond the scope of that test. Likewise, tuning for over/understeer balance is beyond their scope - they're R&T testing factory supplied cars, not GRM tweaking for best autocross times.
Magazine tests are worthless. There's hardly any consistency or attempt at controlling variables. They're also not exactly world class drivers either.

It's not "classic text book theory;" it's the way the dynamics of the system work. A low yaw inertia is achieved by having the mass distributed as close to the center of gravity as possible (in the X-Y plane), and a mid-engine helps a lot in the regard. A low yaw inertia means more yaw acceleration with an equivalent yaw moment and easier and quicker transitions.

What makes this harder to see is that yaw inertia is hardly the only variable to consider. There are a variety of other things--most notably wheelbase--that impact how well a car can slalom through the cones (which is always transient, not steady state cornering). The point of slalom is that you never stop moving the wheel. There's never a point where the car isn't undergoing yaw acceleration (i.e. steady state). It rewards quick transitions more than anything.

I was talking about factory tuning before a car is released, not anything done by a magazine.
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      09-26-2016, 10:39 PM   #24
Brian Paul Wiegand
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Transient Condition Dependent On Diy

Just as the skid pad is a test of steady state behavior, the slalom is a test of transient behavior. Transient behavior is dependent on the initial location of the instantaneous center (IC) of rotation. When a turning motion is started the transient state begins and the initial IC is located at some distance from the CG. As turning continues the IC moves toward the CG; when the IC becomes coincident with the CG and stays there then the transient state has ended and the steady state begins. The initial IC location is dependent on the DIY (Dynamic Index in Yaw) which is determined as "K^2/(a x b)", where "K" is the yaw radius of gyration and "a" and "b" are the distances of the front and rear axles from the CG. It is desirable to keep the DIY as small as possible to minimize the duration and magnitude of the transient state.
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      09-28-2016, 08:48 PM   #25
Zyal8r
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An old post. Its the TIRES

Check out Car and driver Lightning Lap at VIR.
Some car is always going to be faster and better. Enjoy what you have. I have tracked several Ferraris, Porsches Corvette etc. They are all getting to be almost too good and too easy to drive very fast. The performance at that level is crazy and frustrating in the real world. I have had a lot of fun tracking and autocrossing cars with 150 hp. Try to maintain that perspective. Cars are fun. Be Happy. Oh and its really the tires and the driver.
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