RE: suspensions

[QSHIPQ@aol.com]

Darren Wall writes:
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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.
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Absolutely agree.  Strut type suspensions have little to do with function, 
everything to do with ease of install and price, especially in small cars.  
The reason the WRX (and most of the audi rally toys) cars use them, is ease 
of service (btdt).  In fact, strut changes in the audi race hubs can be done 
in a matter of minutes, btdt2.

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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-steerproblems.
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!
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Darren is specifically referring to the phenomenon called Ackermann Effect.  
Simplistically speaking, this has to do with suspension travel arcs 
maintaining a constant arc thruout it's travel.  Almost impossible to do with 
a strut type front suspension.  With a triangulated front swaybar in most of 
the audi line since 1986, it's a huge issue even at stock ride height, to 
camber AND toe AND caster (as a sway bar torques, one arm length is 
effectively longer than the other, pulling toe and caster)

Let's not be fooled by "race car" technology here, the WRX argument falls way 
short.  Even wheel travel is not an advantage, look at what the Baja trucks 
and the offroad 'max wheel travel' enthusiasts use for hints here.  When 
service has a bogey of 45minutes in a rally, the strut type looks really 
good.  From a design standpoint, there is much better.

When I see "sophisticated" associated with a strut type suspension, my ears 
perk.  Not the intent or the design.  When put on most of the audis in the 
last 15years, it falls short in both concepts.

Good post Darren.

Scott Justusson
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