rec.autos.simulators

in a time I should be doing completely different things, my mind can't
help but wander back to car physics. Here's two questions that have been
bugging me for a while.

The first is the question of toe out/in at the front. It has often been
said that a bit of toe-out at the front will destabilize the car,
helping on turn in. I can't help but think that this should be exactly
the opposite; when you turn in, the outside wheel begins to weigh up,
meaning that you are making the tyre that points relatively more to the
outside of the turn more ***, making the car turn in less. The way
I see it, it is actually toe-in that should create better turn-in, as
weighing the outside tyre which points more in the direction of the turn
would in this case push the nose even further into it. Any ideas?

The second question concerns oval stock car racing. There's always talk
of inside and outside lines, and even real drivers don't take the
outside-apex-outside line even when there is no cars on the inside. I'm
not talking about restrictor plate races, as the cars don't even run at
the limit there, but super speedways. In, say, Nascar 4, trying anything
else than the geometrically ideal line will make you much slower. Is
there generally more *** on the other lines that may make you almost
equally fast in other lines as well, or am I missing something here?

Enlighten me! :)

-Gregor

As for oval lines though, the key to speed on an oval (on any track really,
but ovals emphasize it more) is to minimize the amount of turning that you
do. There is a good discussion of this either on Richard Nunnini's site or
in the Steve Smith book that comes with GPL - they compare it to sailing
where any movement off straight ahead slows you down.

So on an oval, the goal is to keep the turns as gentle as possible. This
keeps the amount of steering to the minimum. So if you take a smooth curve
around the corner, that is less steering input than if you cut in sharp to
get to the apex. So even though you may be in the corner longer, you scrub
off less speed and are therefore faster overall. Plus, you minimize tire
wear which is the main factor in stock cars anyway: keep your tires in good
shape for more laps than the other guys.

Or I could be completely wrong in which case RAS will politely (or not) set
me straight ;-)

With toe out, you will always have one tire trying to turn more than the
*** tire, helping the turn in, even at a very small wheel lock,
with toe in, it will work against it from the outset, even if it's
lightly loaded, so even if the *** wheel is now pointing in the
right direction, you have an opposing force

So not only do you not have the inside wheel helping, you have it
actually working against you

My theory anyway, I really should be brushing up on this stuff, but al
my books on the subject is neatly packed away in boxes, besides, vehicle
dynamics were never my strong point anyway, I could read and learn what
worked, never could understand much about why it worked, except for
weight transfer and dialing in shocks, springs and anti roll bars :-),
but that doesn't seem to apply much here

Doug ? :-)

Beers and cheers
(uncle) Goy

http://www.racesimcentral.net/
http://www.racesimcentral.net/

"A woman is an occasional pleasure but a cigar is always a smoke"
--Groucho Marx--

When cornering, the outside wheel is the one which is weighted and does most
of the steering, so the amount of steering input you give is directed mainly
to this wheel. If you have toe in, the inside wheel with its limited grip
will try and stop the outside wheel from turning ie. its not steering as
much so is pushing against the outside wheel.
With toe out, the outside wheel is again doing most of the steering, but
this time, the inside wheel is steering more into the direction of the turn
so is in effect helping the outside wheel to corner.

Hope that helps, if you understood any of my giberish ;)

--

Ian P
<email invalid due to spammers>

This general thing has puzzled me for a while (and continues to do so,
to some extent!)

However, one possible explanation is that toe-out makes the (lightly
loaded) inside wheel run at a higher slip angle, forcing it to do its
'fair share' of the work.

Haven't run the thought experiment properly, but you might like to
consider this.

HTH,
Jonny

Hope this helps.

BH

There are many reasons for this... if you remember at New Hampshire, they
put down a sealer. When the sealer wears off, the cars lose grip and the
drivers search around for more grip... and if I remember correctly, some
even put the lefts on the arpon.

Some of it is ***. *** sticks to more *** better I would think.

At my local track, I like to run the middle groove in 1 and 2 just because
it seems to be banked just a few degrees higher. That is one thing too.

At Michigan, I've heard a lot of guys say the bottom 8 or so feet aren't
banked as much as the rest of the track, maybe my 2 or 4 degrees. A lot of
guys run the middle there quite often.

There's so many variables that make up the "groove"... and N4 just doesn't
really have any of them... it was programed that the absolute low groove is
always the fastest, which a lot of times isn't true.

BH

In a (large) nutshell, the stabilising effect of toe in is due to the
rearwards drag of the tyres.  If you consider a small left steering movement
with toe in, the left tyre now has zero slip and a small rearwards (rolling
drag) force.  The right tyre is pointing to the left with a leftwards slip
angle, has rearwards rolling drag and sideways (axial) "slip angle" force.
This axial force points to the left and slightly rearwards.  So, in car
coordinates, the left wheel has only rearwards rolling drag force while the
right wheel has rearwards rolling drag force plus the rearwards component of
the slip angle force.

With toe out the effects are reversed and it is destabilising.

This was gleaned from the Oct/Nov 200 issue with thanks (or apologies) to
Erik Zapletal who wrote the letter and Alan Staniforth who had written the
original article (and approved the letter as a plain English explanation of
Ackerman).

The letter continues to discuss the amplified effect with Ackerman as the
difference between wheel angles reaches several degrees whereas static toe
is usually tenths of a degree.  An interesting conclusion is that while
positive Ackerman (dynamic toe out) is destabilising during turn in,
sharpening dynamic response, it is stabilising "at the limit" because the
rearwards force at the inside wheel is the first to drop.

Hopefully I didn't butcher that too much!

Ben

I've also wondered about the origins of this.  Maybe the toe-in thing
started back when most cars had flexible steering systems and lots of
steering compliance?  This makes a good brain twister-- which way will the
front wheels point, with compliant steering and with the influence of
trail, tire aligning torque, etc?  I have not worked through this myself...
and depending on the front end geometry the answer(s) might differ??

There is one car that I did a lot of development testing on.  It was
a pretty strange car (sorry, no details today...), but it responded
very well to toe-out for straight-running stability.

On oval lines, here are two things that make it different from typical road
race cornering.  Just food for thought, not meant to be a complete
list<grin>:

 - When does the banking start getting steep, relative to the turn?
   Little point in turning in until you get help (download) from the banking.

 - Since ovals are usually run in one gear, sometimes keeping a
   peaky motor up on the torque curve (by going around the outside
   on the high line) may be quicker for the whole lap.

This is probably the first thing that I would suspect, but I'm not at all
much of an expert.

Ryan

Indeed, but on the other hand it makes the outside wheel do less work
than is necessary. If there is no weight transfer present (c.g. at the
ground, impossible, but one can get away with it in a thought
experiment), and let's assume a large radius turn so that Ackermann
effects are negligible, both in the case of the toe-in and toe-out you
are away from the ideal, as in both cases the wheels fight each other
equally, at least in the linear regime. If you introduce weight
transfer, then the more loaded tyre becomes ***, and that's why
toe-in should be more unstable.

-Gregor

Thanks for replying. So, you can indeed confirm that toe-out can be a
stable thing, just as I thought? There is another post by Ben that gives
great insight into the phenomenon a bit more. Could you please tell what
the test car you mention was like in terms of wheelbase, track width and
the center of gravity?

Thanks for the suggestions about oval racing, I might try and test some
in one of my favourite simulators!

-Gregor

Ah, there's an interesting aspect I haven't thought about! Basically,
the torque produced by the turned outside wheel is much different than
for the inside. It's the same reason why sometimes turning into further
the spin will save your car instead of opposite locking.

If the track width is wide enough, and the c.g. low enough, this effect
just might be more than the weight shifting one, and probably there
exists a geometry setting where the ammount of toe-in and out  is
actually irrelevant under some assumptions. I'll need a pen and paper
from this point on, but thanks for the clue!

Cheers,

-Gregor

some interesting suggestions you put there. If the banking is being
altered, that may indeed explain it. On the other hand, Nascar 4 seems
to have constant banking along the width, but due to the curvature of
the track the lower line actually has a smaller 'vertical' radius w.r.t.
car, meaning that the car is pushed down more in the low groove and
hence has more grip.

-Gregor

In a word: Ackerman.

--Steve