Date: Thu, 11 Apr 2002 10:21:13 -0500
Reply-To: Chris Stann <ChrisS@INFORMS.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Chris Stann <ChrisS@INFORMS.COM>
Subject: Re: Benefits of a rear sway bar?
In-Reply-To: <005501c1e157$39403c80$0100a8c0@home>
Content-Type: text/plain; charset="iso-8859-1"
>To those that reminded me of the Vanagon's nearly 50/50 f/r weight
>distribution, I don't disagree. The issue with the Vanagon is the polar
>moment of inertia (I think I have that term right) not strictly engine
>weight in the rear. Though 50/50 it may be, each "50" is at the extreme
>front or rear of the vehicle. Think of holding a barbell with 25 lbs. on
>each end. Try to spin it from the center; hard to get it moving, hard to
>stop it from moving. Now hold the same two 25lb weights together in front
of
>your waist. Try to spin; much easier to initiate and to stop. It's the
>barbell's "hard to stop" behavior that can make Vanagons tail happy.
Actually, I might disagree with you. Not to pick on you personally, of
course!
Let's take the case of Acura NSX. It's a nimble, mid-engined car that to
many represents nirvana of automotive engineering, yet it tends to snap into
a spin at the limit and is not very controllable during power-induced
oversteer. Why? The bulk of car's mass is located near its center of
gravity, which coincidentally is located near the center of the car. How do
I know all this? I drove several of them and, I must say, the reports in
Car and Driver magazine about the car's behavior were founded. The car has
little polar inertia which tends to make it spin-prone.
Now let's take a look at the Vanagon's design. It has nearly equal
front-to-rear weight distribution. It has nearly equal front and rear over
hangs. Hence, the mass acting on the front and rear axles must be equal as
well. If it wasn't, the van would not have such nice weight distribution,
right? Despite the rear-engine design, there's enough mass in the front the
vehicle to counter the relatively light drivetrain in the back. Not only is
the drivetrain light but it's located very low in the van's chassis.
The only way to make the rear of the van step out in a turn is for the
momentum, or force, acting on the rear tires to grossly exceed that
momentum, or force, acting on the front tires. This force would also have
to exceed the tire's ability to hold traction under the given force. The
Vanagon has more polar inertia so due to Newton's laws of physics it takes
more force to overcome that inertia. Yanking the emergency in mid-turn may
do it.
I have driven my Vanagon Westfalia through some very fun and twisted country
roads at a pace that challenged many sedans. I did this literally for hours
while being followed by a friend in his '89 Westy. I experimented with
trail-braking, mid-turn braking, and taking different lines through the
curves. If I over-estimated my entry speed, and application of the brakes
slowed the van down without drama. In no way did the Vanagon exhibit these
purported tail-happy traits.
So, after all this, what is my conclusion? The Vanagon is not fool-proof.
It's not a race car. While adding a rear sway bar will indeed decrease its
tendency to oversteer and it may, during some very severe maneuvers, result
in oversteer. Does this mean they would have been safe without a rear sway
bar? Probably not. In my view, given the benefits, such as reduced
sensitivity to side winds, increased stability on roads and more secure
handling under load, and considering the average uses of our beloved
Vanagons, most driver stand to only benefit from this upgrade.
Whew... where's my lunch!
Chris.
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