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well, now that ackermann has been mentioned, we should remember a few
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...
2) the original ackermann steering assumed no kinematics (slip and tyre
dynamics). colin chapman is credited as being the 1st designer to question
these assumptions and design cars accordingly.
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.
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 their
clearly not, or else porsche, bmw and mercedes-benz wouldn't use them in
their rwd platforms.
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?)
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.
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. remember
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.
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...
'88 mb 2.3-16
Date: Fri, 11 Jun 1999 15:25:17 EDT
Subject: RE: suspensions
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.
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
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
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.