Miata Mailing List: March 1994, Message #142
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   From: Al Jenab 
Subject: RE>Re: Physics
   Date: Fri, 11 Mar 1994 07:55:40 -0500

Fellow Spin Doctors,
Ken is correct about lifting the throttle as a major cause of spins in RWD 
cars.  I'll add a little bit to that explanation:

When you suddenly lift the throttle in a RWD car at or near the limits of 
adhesion (whether that's 20 mph in ice or 120 mph on a racetrack) there is a 
sudden shift in weight to the front wheels.  If the attitude of the car is 
even slightly at an angle relative to it's direction of motion, this weight 
transfer causes the car to pivot about the front wheel with the most traction (
i.e. the one that got the most weight transfered to it).  .

So you spin around that wheel's contact patch.  This will always happen if you 
keep lifting in corners or in ice & snow.  You can even cause a spin in less 
than extreme conditions by unloading the rear tires too quickly in any kind of 
car.  For example, if you're gently turning left on ice and suddenly lift the 
throttle, the right front wheel will load up, getting the most traction, the 
rear wheels and to some extent the left front wheel will unload thus losing 
traction, and you will spin out with your right rear tire leading the way

Now imagine you add weight to the trunk in this situation.  You've got a nice 
big lever arm stretching from the trunk to the contact patch of the tire you'
re spinning around (right front).  Wheeee!  

Extreme cases of this are why Porsche 911's are such difficult cars to master. 
 You lift the throttle in a turn with a rear engined car and you have an 
enormous amount of mass (engine, tranny, diff, etc.) on a long lever wanting 
to swing around that little bitty front tire patch.  The rear tires unload 
quickly and bingo!  You snap 180 degrees in no time.  This is also why the 
Corvair was nailed by Nader.  Grandma just couldn't handle drop throttle 
oversteer.

It really takes steel nerves sometimes to not lift in a corner when you start 
to lose it at speed.  The right thing to do is  to let out (unwind) the 
steering wheel a little bit an GENTLY back off the throttle if necessary.  
This is not always possible though, since you need a little room to do it.

Finally, lift throttle oversteer is distinct from power induced oversteer.  
This is caused by too much throttle applied which then breaks the rear tires 
loose.  We Miata owners don't have to worry about that in anything but ice and 
snow ;^)
-Al 

--------------------------------------
Date: 3/10/94 8:51 PM
To: Al Jenab
>From: Ken Warren
Re: driving the Miata on ice, and what not to do:

The most common cause of spins in lightweight RWD cars, in my experience,
is people who lift rapidly off the throttle in conditions where traction
is already marginal. A lot of people don't believe this, but a sliding
tire has a lower coefficient of friction than a braking but rolling tire. 
So, what happens whe you lift? Well, your engine slows the rear tires down
*real* fast, so that they are sliding. But, your front tires are still
rolling. Next, any slightest thing that might introduce a rotation to the
car causes you to start spinning.  If you're really good, or driving at a
sane speed for road conditions, you catch it. If not, you eat some weeds.
Generally, whether you catch it or not depends on how far you rotate. If
you get to the point where the front tires are sliding, it's all over. 

BTW, my car does this even *with* snow tires on. It doesn't do it with 
chains, but you can't run chains on a Miata at speeds over about 20 mph 
without beating the wheel wells up.

--
Ken
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           Ken Warren               E-Mail: kenw@netaxs.com
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