New rear stress bar

[mr.utility at highstream.net (David Utley)]

Quoting Dan Bubb <jdbubb@ix.netcom.com>:

> I'm with Larry on this.

Why am I not suprised?  :-P
 
> Tying the tops of the shocks together is going to have zero effect on
> stiffening the chassis due to
> the rubber mounts.
> 
  If you reread what I wrote, my experience is with using the Neuspeed rear 
bar which uses Poly upper mounts, so I have no experience with the type that 
does not use the Poly mounts.  However, I think that both your and Larry 
dismissal of that style of bar is short-sided.  Yes, they are attached to the 
struts which are isolated from the body by rubber.  We all know that rubber is 
used because of its ability to deaden noise, but you should not discount the 
fact that when in good shape (and hopefully using a harder measure durometer), 
rubber can transmit movement to metal.  If it did not, then would we seek the 
rear TT mount for our cars?  When I put that sucker on, I felt a much better 
improvement in clutch snap, if you know what I mean.  When I drive it hard and 
get on it, I get a much more of a wheel spin with the harder mount.  Now, Poly 
would be a much better stiffener where these types of mounts are used, but it 
is not practical for the street.

  On a side note, I have considered the fact that the rear bar may be more 
effective if the type that actually fastened to the body were used.  I have 
not used this type of bar, so I cannot comment...

> 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.

Why not?  Did you see Tobys' post about the amount of movement within the 
chassis in a hard turn?  Furthermore, the strut is itself extremely rigid, why 
can it not help control the deflection of the rear axle where the strut is 
mounted at the base, not too unlike a link on the end of a swaybar, like they 
use on the Mk4 platforms?  Granted, this is an indirect approach, but it is 
much more common than actually fabricating a bar from the tip of the axle to 
somewhere in the center of the beam.  I have seen these bars fabbed, and in 
fact at one time a company (starting with an H) used to make a kit for this? I 
assume that since these are no longer made and few folks do it any longer that 
it is not as helpful as it might be?

> 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.

I do not argue with that at all.  This is why I was looking into using that 
expanding foam, as it addresses this very problem (in theory).

Something to consider:  Why was the GTI front windshield glued in, instead of 
using a rubber seal?  Because the designers recognized an inherent weakness in 
the design with regard to body flex.  Also, have you ever noticed how often 
you can pop the hood of a car and see the paint chipping away around the hole 
for the upper strut mount?  It is from stress that the upper part of the strut 
is inflicting on the body...

> 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.

What?  Please go through this again...

> 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. 

Agreed, on both points...

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.

Agreed...

> 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. 

I guess an obvious question is, how hard do you drive your car?  I will freely 
admit, that if getting points on your record were a contest, I would prolly 
have the highest score here.  But that aside, why is the relationship between 
the front and rear suspension an issue?  Why not make an improvement where 
possible in this case where mentioned, instead of approaching the problem by 
saying, 'Since you cannot take care of the biggest issue, then anything you 
can do is moot?'

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.

  As you are oft to remind me, they are mounted in rubber.  Well, if you mount 
the two of them so that they cannot deflect under load, then wouldn't that be 
a benefit, although it may be small? I believe earlier you tried to say that 
chassis torsional stiffness can have a big effect on the way a car handles.  
If indeed that is the case, then any attachment point within the car to try to 
stop chassis flex would be helpful, but obvoiusly some would perform better 
than others.  Agreed?
> 
> 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!

Are you refering to front or rear here?  On the front, I disagree as the four-
point type lower bar boxes in the suspension, and keep things in alignment.

> 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.
> 

  I would think both...  However, my experience is limited to the Rabbit, as 
that is the car I have driven and modified.  I have never modified a 'Roc.  I 
realize that this is a Scirocco list, and perhaps I missed the point of 
stating my experience where it was not relevant.  I have driven Sciroccos, and 
have noted to myself how much better they feel in turns and hard manuevers.  
Perhaps my experience here is not pertinent...

> > 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???
> 

Well, while I have no degree in engineering, nor have I cut open enough cars 
and certified my results with DOT, NHTSA, or any other federal acronyms, I do 
not feel that my 'opinion' is too far off base.  Perhaps if you do, you could 
site where you think I have gone astray?  :-)

> > 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??
> 

Read above, and then read--->  The car feels more square, as if I were driving 
a board that does not lift its inside edges as much.  With each attatchment 
point that I added, things were improved.  I did not like Neuspeeds' design on 
the rear, even when space was not an issue though.  I thought it was stupid to 
triangulate to one corner, and not the other?  One could prolly add on another 
bar and duplicate it on the other side, but I am not sure it is worth the 
effort...

>   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?

  Explain this a bit to me...  It does occur to me that I may be thinking it 
was the flex in the body when it is actually the rear sway bar mount that 
causes the rear inner wheel to lift, and Autotech made the case for their rear 
sway bar with the eyelets that allowed the bar to move through it and not 
bind.  I would like to upgrade to a set of Autotech sway bars one day...

> 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?
> 

  Dan, do you have an issue or two with me or what?  I read it somewhere, no I 
do not recall the article, magazine, or book.  I do recall howver that the 
reason they named it the Rabbit was for the benefit of the American market.  
They wanted to make it a smooth transition from a 'beetle' to the little water-
cooled car that is called a game?  
  I thought this list was for discussions.  Are you planning to do a paper on 
the comments I make?  It seems to me that you do tend to take issue with some 
of the things I say, often.  Pleae tell me, if I misreading you or what?



-- 
Regards,
David Utley
-----------
Cable Volkswagen
405-470-3129
1-800-522-6793