New rear stress bar

[jdbubb at ix.netcom.com (Dan Bubb)]

I'm with Larry on this.

Tying the tops of the shocks together is going to have zero effect on stiffening the chassis due to
the rubber mounts.

Stress bars are fine if they prevent deflection of the suspension pickup points. i.e. the locations
on the chassis that actually locate and control the attitude of the wheels. The top rear shock mount
makes no contribution in this sense.
Also, no stress bar will actually increase chassis stiffness in the sense of torsional stiffness of
the chassis between the relavent front and rear suspension pickup points. Chassis torsional
stiffness can have a big effect on role stiffness distribution (i.e. does the car understeer or
oversteer?) and suspension response. Stress bars don't effect chassis torsional stiffness.
As far as front stress bars, the lower a-arm mount is the most heavily loaded and since it is
cantilevered, and therefore relatively flexible, it makes a big difference to tie the two lower
mounts together. Note that if they were both seeing the same lateral load during cornering then
tying them together would make no difference, but the loads are very assymetrical so tying them
together essentially doubles the lateral stiffness. Less deflection means less change in camber due
to cornering force and less deflection steer.
The upper front shock tower sees about 1/5th the lateral load of the a-arm pickup. Lower loads, so
lower deflection although the chassis isn't particularly strong in this area, so an upper stress bar
may make some difference as far as reducing unwanted camber change due to load deflection.
A rear stress bar does NOTHING to constrain the rear suspension pickup points. How it could stiffen
the chassis enough to feel, especially in a street application, is beyond me considering that the
major area of chassis flex between the front and rear suspension is the door opening. Tying the rear
shock mounts together, or even triangulating them to the floor of the chassis, isn't going to make
any difference to chassis stiffness when the rear suspension mounts are well forward of the stress
bar location.

NOTE: stress bars will do nothing to reduce chassis role! They don't make the springs stiffer, they
have no effect on the spring mounts in a vertical direction and they have no effect on the sway bar
mounts!
All stress bars can do is constrain the suspension pickup points and reduce camber change and toe
change due to load deflection.

Other minor comments within.
Dan

----- Original Message -----
From: "David Utley" <mr.utility@highstream.net>
To: "L F" <rocco16v@netzero.net>; "Jeff Toomasson" <jtoomasson@yahoo.com>; "ATS - Patrick Bureau"
<ats@longcoeur.com>; <scirocco-l@scirocco.org>
Sent: Thursday, March 04, 2004 12:02 AM
Subject: RE: New rear stress bar


>   I have to disagree with you here, Larry (much to my peril).  While I can
> only offer my reasoning and my experience, I do stand by both.
>
>   My reasoning is, that these cars are light, and therefore they are
> f-l-e-x-I-b-l-e.

Are you talking overall chassis torsional rigidity or flexibilty of the suspension pickup points?
Sciroccos  were quite competitive in overall chassis torsional rigidity (3000 ft-lb/deg) when they
were designed.

> The guage steel used in them is thinner than any other
> car

Maybe you can quote us the material thickness and let us know the cars you're comparing it to???

> especially lighter than one made with a subframe.  Anyone here that has
> owned or at least driven a MkII Golf or later version, will attest that
> although the structure of the car and the design of the suspension is very
> similar, there is a huge difference between a MkI and a MkII in the way that
> they drive.  The difference can be explained by the addition of the subframe
> itself.  The subframe is very stout, with a gauge of steel much thicker than
> the rest of the car.  That subframe is in effect a large stressbar.  Now
> although the subframes do double duty, having both the engine and suspension
> mounted to them, those cars in stock form handle more flatly than a Mk1.
> However, they also lose the most endearing feature of the Mk1, the feedback
> from the road...
>
>   My experience is, that stressbars help.  I have driven cars with and
> without stressbars, and I can feel the difference, easily.

Interesting! I believe it was Shawn Meze (an acknowledged master of the Scirocco chassis) that said
he could feel the difference of the upper front stress bar if he was paying attention. The rear
stress bar has much less effect on deflection.
Maybe you could tell us the specfic differences that you perceive??

  So much so, if I
> did not inspect a car before driving it, I could tell you whether it had
> them on the car or not, front and/or rear.  I have experimented at length
> with the Neuspeed rear triangulated bar in my GTI.  I could feel the
> difference between just having it installed at the strut towers, just
> fastened at the floor, just mounted at the R/R bumper mount, or any
> combination.  I did finally give up the Neuspeed design though, as the space
> I lost was to valuable.  I will in the future get an Autotech rear bar, as I
> can handle that loss.  It should be noted that that bar uses Poly rear
> mounts instead of rubber (on the uppers at least)...
>
>   Larry, I really think you ought to try a bar, just for shits and
> giggles...  I would bet you that you will feel the difference, if you like
> hard, crisp cornering anyways.  After all, if these cars did not bend in
> tight turns, the Rabbit would have been named something else altogether.
> The reason it got its name is because of it's tendency to lift the inside
> rear wheel in hard tight turns...

Wait a minute! You think the rear tire lifts due to the chassis bending????? Like the front is
staying flat and the rear is bending?? Couldn't have anything to due with stiff rear suspension role
resistance?
As far as the rear tire in the air routine, I'd have thought they'd have named it after a dog then.
Care to substantiate this statement also?

>
> David Utley
>
> -----Original Message-----
> From: scirocco-l-bounces@scirocco.org
> [mailto:scirocco-l-bounces@scirocco.org]On Behalf Of L F
> Sent: Wednesday, March 03, 2004 8:42 PM
> To: Jeff Toomasson; ATS - Patrick Bureau; scirocco-l@scirocco.org
> Subject: Re: New rear stress bar
>
> Since the rear stress bar bolts to the top of the rear shock, it does
> nothing to keep the chassis from flexing.  The rear towers could still move
> around the rubber-mounted suspension units.
>  In effect, the stress bar is rubber-mounted.....and a whole heck of a lot
> of good THAT does.    This is a situation duplicated with all the rear
> stress bars I've seen.
>
> In addition, lateral cornering loads are all absorbed by the rear axle
> mounting points....none of the loads go up through the two flexible shock
> mountings (lower and upper) into the body. It's possible that vertical
> stresses in that area can cause the towers to move side-to-side slightly,
> put that wouldn't affect handling to any degree.
>
> Realistically, rear stress bars serve no mechanical function.
> (looking at the designs of all the front stress bars and the way the A1's
> are built, I have my doubts that any of the commercially-available fronts do
> a heck of a lot either....)
>
> Ergo; Scirocco suspension design doesn't result in rear shock towers moving
> AND the normal bar wouldn't stop the movement even if it did occur.
> Save yer money.
>
> Larry
>
>   ----- Original Message -----
>   From: Jeff Toomasson
>   To: ATS - Patrick Bureau ; scirocco-l@scirocco.org
>   Sent: Wednesday, March 03, 2004 11:41 AM
>   Subject: Re: New rear stress bar
>
>
>   he'd better come up with a better description than that if he wants to try
>   to sell a supposed high performance part that no one/few have ever seen...
>   ----- Original Message -----
>   From: <ATS - Patrick Bureau>
>   To: <scirocco-l@scirocco.org>
>   Sent: Wednesday, March 03, 2004 6:33 AM
>   Subject: New rear stress bar
>
>
>   > Anyone seen these on the web, they look interesting considering the
> shape
>   > (looks to be triangular bar) and the price.
>   >
>   >
> <http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=2462969376>
>   > --
>   > ---
>   > ATS - Patrick Bureau - Web site : http://ats.longcoeur.com
>   > AIM: texasscirocco - Yahoo: atsgtx - ICQ: 32918816 - MSN:
>   atsgtx@hotmail.com