rec.autos.simulators

--
Rob Swindells
3rd equal in Goldline Bearing FFord Championship (18 Points);
9 points behind Championship Leader (as of round 4 of 6)
"If you're a great driver, you will get through. It's not harder than
any time before." ~Martin Brundle
"If we could do this every week, we would be more consistent..." ~Bryan
Robson (I think)

Also, it takes a larger decrease in force to go from dynamic back to static
friction (unlocking the tyre) than the increase in force nescessary to go
from static to dynamic friction (locking the tyre).

Jan./putting the engineering degree to work. ;-)
------

Hope that helps!

Neil

:>Why does a locked tyre give less grip than a turning tyre?
:><snip>

: Because the dynamic friction coefficient of vulcanised *** is less than
: the static one.

  Isn't that just restating his question in a mathematical way?
Surely what he's asking is "why is the dynamical coefficient less than
the static one"?

  [Incidentally, could someone tell me approximately (percentage) how
much less.  I was discussing this on some other news group.]

  My _guess_ (and it is a guess) is that when the wheel is turning, the
*** is pressed down into the road as it comes into contact whereas
when the wheel is sliding it is not "digging in" to the surface in the
same way but is skipping over the top.  [Think of a cog wheel turning
along a ridged surface versus a cog wheel skipping over the top of the
ridged surface - in the first case the teeth of the cog have time to
"dig in" whereas in the second they don't.] Also, I imagine that a sliding
tyre heats a great deal - I understand that after a certain temperature,
tyres move from being "sticky" to being greasy and therefore slide more
easily.  This may well have some effect.

--
Richard G. Clegg     Only the mind is waving
Dept. of Mathematics (Network Control group) Uni. of York.

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

Jesse Black

I think you're right on--at the small-scale level, bits of the tire are molding
themselves
around little bumps in the road.  In fact, this is why tires violate the "law" that
friction
is independent of contact area--*** is soft enough that having a larger contact
patch will provide more "handles" to grab onto the road--of course this tapers off
to some extent.  The reason that a tire will actually give peak traction in a
partial
slip condition (e.g., six degrees or so of slip angle, or 90 percent rotation speed
for braking), is that parts of the tire are flexing and gripping, other parts are
sliding,
and as you crest the top of the slip angle curve, more and more of the bits are
sliding.  Contrary to popular belief, the a tire's sliding friction is not that
much lower
than peak--sort of a 90 percent-ish thing.  This is why a lot of driving
instructors tell
people to lock the brakes up in a panic braking situation--for a non-professional
driver, messing around with modulating brake pressure is not likely to decrease
stopping distances, plus a car with both fronts locked up is an extremely stable,
predictable object for other cars to avoid... of course a racing driver not only
wants
to eke out a few extra percent, but also save the tires.  The "greasy" effect of
temperature is not likely to affect the static/dynamic, as it takes a little while
for
the tire temperature to increase significantly.  There was a thread earlier about
the benefits of locking up the brakes sliding into the pits, but I can't remember
what the "consensus" was.
-Dave

As another poster remarked, 90%-ish sounds about right. I don't remember
exactly.

Right on the money there, although it takes some doing to cause melting.

Jan.
--

  Aha - thanks very much - I'd been wondering about that.

: This is why a lot of driving  instructors tell people to lock the
: brakes up in a panic braking situation--for a non-professional driver,
: messing around with modulating brake pressure is not likely to decrease
: stopping distances, plus a car with both fronts locked up is an
: extremely stable, predictable object for other cars to avoid...

  Of course it might be stably and predictably sliding off the road
(like me trying to round Parabolica).  I've also seen driving instuctors
giving the advice to "pump" the brakes so that the front wheels
lock and unlock rapidly (I presume no modern car has the brake bias set
so that the rears will lock easily?  Otherwise it could be a disaster).
That would seem to provide a compromise between braking and being
able to steer.

  I _used_ to brake in GPL like that (don't shout at me - I'm TRYING to
do it properly now) - actually it seemed to have some advantages - when
the nose bucked up and down the weight transfer seemed to have some good
"stopping" effects - I guess that for some of the time you have a really
large stopping force (as the weight goes temporarily to the front).

  Is it possible that by inducing a huge forward weight transfer you can
get better braking temporarily - can this be taken advantage of or is it
too temporary to make much difference.  It sounds silly but if you could
bounce the car between heavy front/brakes locked up/no steering and
heavy rear/no brakes/lots of steering you might be able to get BETTER
braking and control than the standard "not quite locked up" technique.

  If you could make it that you were only braking for 1/2 the time
but in that time there was 3 times the weight on the front axle then
you'd be a more efficient braker.

  [Hmm... maybe this won't work - there's only so much weight there to
transfer.]

  Must just rush home to experiment with "stupidly springy car".

--
Richard G. Clegg     Only the mind is waving
Dept. of Mathematics (Network Control group) Uni. of York.

www: http://manor.york.ac.uk/top.html

Even in the wet ? I could very well be wrong, but it certainly FEELS a lot
less than 90% in the wet.

Regarding locking up the fronts: depending on the road surface and tyres
(S.A. spelling!) some cars seem to have a habit of locking one side before
the other, with the result that the car skews before going into a full on
slide. Front wheel drives, especially, seem prone to "darting about" when
attempting to brake at the adhesion threshold. Anyone else experienced this
?

Ryan

Anyone heard of Anti lock braking? There are a lot of videos from the various
car makers that show two cars side by side one with anti lock and one without.
Mercedes being a good example. The Anti lock braking works every time. As soon
as the tyre stops turning and starts skidding you have lost control! It is no
good stopping sooner if something else gets in your way! An easy way to imagine
it is driving on ice. Bang the anchors on and away you will go with no control
what so ever. Keep the tyre rotating and you stand a chance..
There is another thread in this group about driving/racing sims and possible
improvements to your everyday driving. One word - No. Until someone comes up
with a driving simulator like a flight simulator (ie a hydraulic based replica
like pilots practice on) it will still be just a coordination/reflex challenge
(albeit a ***y difficult one) to beat the PC. Try karting for improving your
driving.

:> sliding.  Contrary to popular belief, the a tire's sliding friction is not
: that
:> much lower
:> than peak--sort of a 90 percent-ish thing.

: Even in the wet ? I could very well be wrong, but it certainly FEELS a lot
: less than 90% in the wet.

  I suppose that, in that case, you have to be much much more careful
about locking up - I imagine it's to do with "hydroplaning" - the
locked up tyre scoots along on a thin film of water with virtually
no friction.

: Regarding locking up the fronts: depending on the road surface and tyres
: (S.A. spelling!) some cars seem to have a habit of locking one side before
: the other, with the result that the car skews before going into a full on
: slide. Front wheel drives, especially, seem prone to "darting about" when
: attempting to brake at the adhesion threshold. Anyone else experienced this
: ?

  Well it's bound to happen that the inside tyre (English spelling!)
will lock up first if you're turning at all.  Otherwise I suppose it
might happen if the road was cambered.

--
Richard G. Clegg     Only the mind is waving
Dept. of Mathematics (Network Control group) Uni. of York.

www: http://manor.york.ac.uk/top.html

More like 75%-ish

Not where I come from.  Instructors have always told me and anyone else
I know that you should endeavour to maintain control of a vehicle at all
times.  When the wheels are locked you have no control whatsoever.
If an instructor told me to do this, I'd have to question his
qualifications.

Dave

Sent via Deja.com http://www.deja.com/
Share what you know. Learn what you don't.

If you do that a lot (brake on the limit, even in a sim) it somewhat
becomes second nature, but otherwise the split-second hesitation could
cause more harm than the locking up.

Cheers!
John

Karting will improve your driving more than sim-racing, but sim-racing
will improve your driving more than sitting in front of the TV with a
beer.

I haven't raced for years, but recently went on a "driving day"
organised by a local BMW dealership. It was one of those "damp" days
so common in the UK, and I was whizzing around the track in an M3 Evo.
Coming out of a corner the car got _well_ sideways, with a ditch on
one side and gravel on the other. All that really registered with me
was a "squeak" noise to my left, I gathered the car up and booted it
off down the track. A couple of seconds later I realised the "squeak"
had been my passenger who was whimpering in shock, that the car had in
fact snapped sideways and that I had corrected it and carried on
without consciously registering the fact. Now I do that every day in
GPL, and I would attribute that reaction far more to GPL than to the
racing I did in the past.

FWIW my passenger drove next, did the same thing at a different point
and got a real tank-slapper going, locked up the brakes and ended up
stopped inthe middle of the track facing backwards - he raced as much
as I did in the past - <rhetoric> what could cause such things to be
more pratised in my mind than his? </rhetoric>

Cheers!
John