[s-cars] Understeer vs. oversteer?

[Mark Strangways]

Well, it seems my 150 lbs sub box over the rear axle is good for something
else.
My car seems to get tail happy ever so often.


----- Original Message -----
From: "Theodore Chen" <tedebearp at yahoo.com>
To: "serge" <serge411 at speakeasy.org>; "Joseph Pizzimenti"
<pizzoman at yahoo.com>; <s-car-list at audifans.com>
Sent: Tuesday, November 26, 2002 7:16 PM
Subject: Re: [s-cars] Understeer vs. oversteer?


yes, that's right - which is why i always laugh when i see riceboys
driving around with super stiff springs, bounding over every pavement
irregularity.  they're convinced the car must handle really well because
the ride is so bad.

you want to run the softest springs you can use and still keep the
suspension in an acceptable range of movement.  unfortunately, strut
suspensions have poor camber curves.  although they still gain some
negative camber in compression, the rate of camber gain drops off quickly
as the suspension compresses, and at some point, the camber gain from
suspension compression isn't enough to make up for the camber lost to
body roll.

sticky tires mean you'll need stiffer springs.

also, you need to make sure the springs are stiff enough to keep the
car off the bump stops.  bottoming out the strut is very hard on
the strut, and causes your spring rate to skyrocket (which means you're
going to lose traction).  it's a major tuning problem for teams that
run at the 24 hours of daytona, because they need compliance for the
infield handling course, along with very stiff springs to keep the
car off the bumpstops on the high-speed banked over.  run soft springs
that work on the infield course, and the car will be bouncing on the
bumpstops on the banking.  run stiff springs to keep it off the bumpstops,
and the car won't stick on the infield course.

honestly, i don't think you're ever going to get the understeer out of
the UrS4/S6 without seriously hacking it up.  the fundamental problem
is the placement of the engine ahead of the front wheels.  the trans-am
and IMSA audis had a lot of work done to them to improve front/rear
balance, like moving a whole bunch of accessories to the back.

you can probably use suspension tuning to get the car to neutral, but
my guess is you'll probably accomplish it by hosing the rear traction
to stick as poorly as the front.

-teddy

--- serge <serge411 at speakeasy.org> wrote:
> Joey, Joey, Joey
>
> Think of it in terms of compliance (soft, rubbery, gooey) vs. resistance
> (stiff, like stone). Those stiff springs don't give you traction (not
> directly), the tires (soft, rubbery, sticky > things stiffer allows you to
keep suspension geometry more consistent as you
> flog that car through turns (transferring weight from back to front, side
to
> side....) Adding sway bars (the correct term is anti-roll bars), further
> reduces body roll, and gives you that lovely flat-cornering feel.
>
> However all this joy comes at a price. Remember, compliance=traction. By
> making things stiffer, you have reduced your compliance. You have reduced
> your traction at the limit (though you may have raised that limit
overall).
> You have increased your ability to carry more load/speed (stiffer springs)
> through the corner and flattened out some body roll (bigger sway bar), but
> once you have exceeded available traction (tires) that baby's gonna break
> loose, and quickly.
>
> To put it simply:
> Increasing the stiffness (reducing compliance) of the front end, will lead
> to more understeer (front breaks loose first). Adding stiffness to the
rear
> will lead to more oversteer (rear breaks loose first). Playing around with
> both to suit your driving style will result in a happier driver.
>
> This is all a major simplification of chassis dynamics on which I am no
> expert. It assumes perfectly smooth pavement, and does not address the
> function of shocks, tire pressures, alignment, sprung/unsprung weight,
> torsen funk..... Anyone willing to shoot holes in my oversimplified
> statement is welcome to do so. Our 4000lb cars can use very high spring
> rates, and would probably benefit from massive sway bars, along with solid
> suspension links and engine/trnsmission mounts. But that would make the
car
> lousy for ANY kind of street driving, and we would all be back to that
"our
> cars make lousy track cars because they are so heavy" thing.
>
> HTH
> Serge
>
> > List,
> > Our cars understeer like pigs (not unlike my date this
> > weekend) because all the weight's on the nose, right?
> > Understeer is caused by front wheels losing traction
> > before the rear, so the car plows.  Adding a Rear Sway
> > Bar should increase the rear traction.  Hap, wouldn't
> > this make understeer worse?
> >
> > Joe, confused as hell in NY, Pizzo
> >
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