RE: suspension

["Darren Wall" <dwall@sisna.com>]

I disagree with you about the advantage of a strut type suspension over a
wishbone type suspension.  The main function of a suspension is to keep the
tire contact patch as consistent as possible while allowing the tire to
follow the profile of the road.  A well designed double wishbone suspension
will allow less tire camber change through a greater range of up-down motion
than a strut type suspension.  The advantages of strut type suspensions, and
the reason they are so prevelant on modern cars is:  1) They are cheap to
produce.  The component count is low and they are mostly off-the-shelf
items.  2) They are relatively compact.  The strut takes a fair amount of
space vertically, above the tire, but they leave a lot of space between the
front tires, ideal for front wheel drive cars.  3) They are fairly
lightweight.  A-arm suspensions can be made as light as strut type
suspension, but they require more exotic materials, which drives the cost up
even farther.

If you look at purpose-built race cars, most use a type of double wishbone,
albeit with certain additions, i.e.:  inboard mounted coil springs, remote
upper ball-joints.  Most of these changes are to accomodate air-flow around
the suspension members.  Some of the best production suspensions ever
designed are double wishbone suspension used on Hondas, Mercedes, and
certain Toyotas.  These suspension are double wishbone, but have the upper
ball joint located above the tire, instead on inside the tire.  This design
allows the kingpin angle to be optimized so that the suspension acts through
the center of the contact patch, removing torque steer and bump-steer
problems.

If you really want to play with different suspension designs and see how
they actually work, make some cardboard cut-outs of the suspension pieces (I
know, sounds corny, but this is actually how we did it in my suspension
design classes while getting my Automotive Engineering Degree!) and pin them
to a board at the locating points.  You can then move the tire through its
range of motion and see how the suspension works.  You will be surprised
when you build a strut type suspension, the tire sees some pretty radical
camber angles with very little up or down motion!

If you want some references to read about suspension design, I can send you
a few titles (I'm at work and they are at home, or I would just put them in
here).

Darren D. Wall
dwall@sisna.com
darrenwall@netzero.net
1985 4000S quattro
1969 911 2.7Liter Turbo-Look **For Sale**

-----Original Message-----
From: owner-quattro@audifans.com [mailto:owner-quattro@audifans.com]On
Behalf Of Dave Eaton
Sent: Wednesday, June 09, 1999 7:23 PM
To: quattro@audifans.com
Subject: RE: suspension


yes, but the original point remains.  generally speaking, a wishbone is less
useful in keeping good suspension geometry over a wide range of wheel
movement than a strut.

why does the humvee use double "a" arms?  the humvee uses all sorts of
sophistication to keep good traction (hub spur gears, multiple torsens etc).
and clearly optimum suspension design in extremis (e.g. camber) is much less
important than traction.

struts are a very good (compromise) solution when high rates of wheel travel
are required, and some geometry compromise is acceptable, and performance is
important.  witness the current crop of wrc rally cars which mostly use
struts front and rear.  also the ur-quattro did, of course.  struts also
don't get in the way of driveshafts as much as double wishbones can, which
is a factor in an awd system, particularly when you're trying to drive the
front as well as to steer it.

hth,
dave
'95 rs2
'90 ur-q
'61 mb fintail