IRS discussion: was Re: Noise and vibrations.......

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

First, I'd define independent suspension:
If you can move one wheel/tire assembly from droop to bump without changing the camber or toe of the
wheel on the opposite side of the car, then that would be independant.
I would define a beam axle as the two axles being contrained relative to each other in six
dimensions i.e they can't move relative to each other in 3 orthogonal planes and they can't change
angles relative to each other within those three planes.
(I'm trying to make this easy, but it doesn't seem to be coming out that way!)
Anyway, the Scirocco rear suspension meets the definition above. If you remove the spring/shock
assembly from one side of the car then you can freely move that side up and down and see that the
other side is uneffected.
If the Scirocco rear suspension was a beam axle then moving one side up and down would change the
camber of the other side. So, I guess the point is that just because a beam is used in its
construction doesn't mean it has the characteristics of a classic beam axle.
The Scirocco rear axle is really a simple version of a pure trailing arm suspension. ( as opposed to
a semi-trailing arm like the 924/944 Porsche)  It has no camber or toe change from droop to bump. It
is functionally no different from the twin beam concept that Julie proposed. The neat part is that
VW incorporated both sides into one unit. The "T" cross section is stiff in bending in both a
horizontal plane (to control toe during cornering loads) and in a vertical plane (to control
camber), but because of it's open cross section is relatively weak in torsion. So, VW was able use a
really wide base for the bushings to minimized toe change due to bushing deflection (roll oversteer)
and at the same time incorporate some roll stiffness into the structure. It's a really neat design
in its simplicity except for its complete lack of camber change to improve cornering.
Almost all suspensions have some sort of interconnection from side to side in the form of a swaybar.
That interconnection doesn't make the suspension not independant (at least according to my
definition).
Anyway, if you're looking to replace the "T" beam with a bar or tube there is a good chance you will
increase roll resistance at the cost of a lot less stiffness to control toe and camber deflection
due to cornering and bump loads.
There's lots more to say about how the position of the beam effects camber change if anybody's
interested.
Hope all that was clear.
Dan


----- Original Message ----- 
From: "Calimus" <calimus@techography.net>
To: "David Utley" <mr.utility@highstream.net>
Cc: "Dan Bubb" <jdbubb@ix.netcom.com>; <scirocco-l@scirocco.org>
Sent: Wednesday, March 17, 2004 6:43 PM
Subject: Re: Noise and vibrations on the steering.. Solve one, find thereason for the other !


> I knew this would happen, LOL.  I took the conversation off list, but if
> you want to join in thats cool.  My viewpoint is that it's not IRS and I
> kinda explained that a bit.  My basic idea is to replace the cross piece
> of the rear beam with a torsion bar.  That would achieve IRS wihout
> having to fab tons of new mounting points and all that happy stuff.
> Just have to do some fabbing to the rear beam and bolt it back in the
> same way it came out.
>
> William
>
> David Utley wrote:
>
> >Dan,
> >  Now trust me, I am not trying to incite anything but friendly discussion,
> >but you knew someone would ask...  How is it that it is independent, when it
> >uses a beam?  Trucks (american) have an axle beam, and it is a rigid rear
> >suspension, what makes the A1 cars different?
> >
> >David Utley
> >
> >
> >
>