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Dave E writes
>well, now that ackermann has been mentioned, we should remember a fewthings.
>1) there is no ackermann "effect". rather there is ackermann *steering*.
>btw, ackermann was not the inventor, that was herr lenkensperger - ackermann
>was the british rights holder. this arranges the steering linkage so that
>the (extended) steering axes meet on the line of the back axle, towards the
>centre of the corner. now, moving right along...
Glad to see that indeed you own the materials you should be reading. The
effect refers to the concept of steering v. toe and (specifically to a
triangulated front swaybar) caster. The inside tire has a different steering
arc than the outside tire due to track differences. Ackermann steering means
that the steering arms are angled inward so that their intersecting angles is
the rear axle. This allows for track radius differences between the two
front steering wheels without scrubbing the inside wheel. Speaking of
Ackermann effect (Specifically Ackermann Steering effect) without bump steer
or toe (and again in a triangulated swaybar, caster), is hard to do, they are
inter-related concepts. You setup a car with Ackermann Steering effect, you
create a bumpsteer problem. Anti-Ackermann Steering is the use of toe out to
compensate for the non intersection of the steering arms. However, a
triangulated front swaybar creates a toe problem, because the effective
length of the arms of a front swaybar become longer on one side and shorter
on the other (the reason for links, or sliding bushings). If you could
isolate each end of the bar, you could actually have a better Anti-Ackermann
Steering Effect, because you have effectively created more toe out on the
inside wheel, a good thing. Problem. The other arm changes in direct
relation, so the toe out effect is negated, in fact exascerbated by the
triangulated front swaybar.
>i think what you are probably talking about scott, rather than ackermann
>"effect", is bump steer. this is the change of steering geometry which can
>occur with bump. this is an issue however whatever choices of suspension
>technology. for this reason, most suspensions place the hinge points so
>that the steering can follow the suspension.
No bump is not what I am speaking of, with all due respect to your newfound
knowlege. Bump has ONLY to do with the steering arms tendency to pull during
compression (usually associated with a McPherson strut arrangement as well).
A properly designed double wishbone has a distinct advantage here. Remember
Dave, if you pull or push a steering arm during a turn, you have created
Ackermann effect. That is to say, you affect the Anti-Ackermann Steering
effect designed into the suspension. When you change toe by arc, you make
more compromises in the turning capabilities of a car. Rack and pinion front
McP strut type cars, are inherently much more susceptable to both bump steer
and (Anti-)Ackermann effect(s).
>you need to remember that there are compromises involved with all
>suspensions. for a fwd format, packaging and weight are clearly in favour
>of the strut, particularly for transverse engine layouts. as is the
>relative ease of handling the driveshaft. is this the only reason for
>clearly not, or else porsche, bmw and mercedes-benz wouldn't use them in
>their rwd platforms.
Guilt by association does not make a good argument, you used it already with
the WRX car thing. Reread you own posts before you went to your books for
help. They are cheap, they are easily serviced, they are easily mass
produced, and they make for a smaller package. We can find several examples
of small cars that don't use them too. The A4 really proves that clearly
there are advantages to a double wishbone concept (albeit with a strut, but
hardly the revolution audi claimed and the rags drooled about). Glad to see
you correcting your own thinking, cuz it was a little off before you did your
homework in your own library.
>struts not sophisticated? not following you here scott. if you want an
>argument on this point, you'll want to brush up on your german :-)
>the classic advantages for wishbones are of course better control over the
>geometry and the roll centre. however, clearly some pretty significant
>manufacturers think that they can do the same or better using struts ("self
>correcting negative offset steering" anyone?)
Guilt by association does not mean it's better, correct, in engineering or
concept. Understand why negative offset is used before you tout the McP
"advantage" there. The safety gurus made a bigger challenge for the
engineers that's all. Bottom line is this, when you look at all the
"revolutions" in McP design, they are just getting closer to the double WB,
in arc, travel and application. Not sure that really changes the baseline
argument re McP based suspensions. All the "significant manufacturers" are
chasing an old and established design with new hardware. Better?
>why don't we see more double wishbones in the rear? well, as m-b has shown
>with a variety of their layouts, their 5-link designs work better, and
>occupy considerably less space (as i can tell by going out to the garage
>;-) ). ditto bmw with their recent designs which have considerably modified
>their usual semi-trailing idea.
I credit audi for making one of the most sophisticated Chapman concepts in
the rear of the quattros. The multilink rear suspension of the 44 chassis
cars is one technical marvel. Unfortunately, we can't give them too much
credit, it really was a design to handle not only the driveshaft, but to
allow very flat and accomodating rear deck volume. The concept of double
wishbone has been around for many years. If you look at the multilinks that
are McP based, they all are the same basic concept. The keys to multilink is
to control all suspension arcs.
>with regard to the ease of service of struts in rallies, this might be a
>benefit, but don't forget that if you go with a double wishbone, you can put
>the spring/damper on the top "a" arm and then have even easier access.
>witness ford and lancia in their group "b" rally cars, and renault with the
>5 turbo. no, the rally boys go with struts because they make them work,
>they like their control over camber, and they get the performance.
Think about that statement again. Remember homog rules (B, N, A, P and most
open class for that matter). Straying from the roadgoing car is a no-no.
Designing one from the ground up, with minimum homog (ford and lancia) makes
my point. The 5 turbo really isn't worth discussing, IMO.
>that we're talking about top-of-the-line cars with the absolute best of
>technology available. the ford focus is the current state-of-the-art "clean
>sheet" rally car design. when you buy your transmissions for $150k usd, you
>don't compromise on putting the power down. ditto subaru and the others.
Sorry, rally cars don't necessarily have the absolute best of tech. All that
has to be approved by the sanctioning bodies first. Not all is.
>i'm not arguing against double wishbones, simply that there are trade-offs
>with each design. the point of the original question. theory to one side,
>a lot of chassis designers have done the theory and come up with answers
>that say "strut" and not double "a" arms...
Again, guilt by association does not make a better design, nor a better
practical application. Multilink front McP can be good, however, when you
look at how they do what they do, it's not all that sophisticated in concept
or execution, it's a complicated execution of the double wishbone concept.
At some point, you start to think...
QSHIPQ Performance Tuning
'87 4Runner turbo