rec.autos.simulators

Also, the FWD example is the easiest one to clear the misconception that
adding the throttle directly effects any weight transwer (which Todd
Wasson already explained nicely). Adding throttle in a FWD car under
braking actually increases oversteer.

-Gregor

Also, another thing to consider is that, in the case of outboard brakes
(the usual configuration), when using the brakes the torque is
transmited through the suspension links, while in the engine braking
case this is done via the drivetrain. Any flexibility in the suspension
links may thus lead to a different wheel attitude in the two different
braking cases.

-Gregor

Thanks for all the replies guys, just what I wanted, please keep
em coming.

A few observations.

1) In real-life I used engine-braking (by which I mean taking
advantage of the drag caused by decellerartion from max revs
in each gear) in saloons. In this case, under high speed braking
I could not lock the wheels no matter how much pressure I
applied, so thats probably why it worked for me there.

2) I also used it in FFord type cars (Formula Vauxhall Junior)
I think more out of habit, generally unless it was hard braking from
5th Gear I think the brakes would have been good enough to lock
up under rmaximum pressure although I don't ever recall doing so.

3) I felt I had to use it in Formula Vauxhall Lotus, these are much
more powerful cars with wider, slick tires, plus I felt I also needed
to use gas+brake.

The main reason for doing both of these was that I found it made
the car much more stable under braking, I'd thought that this was
down to this technique negating some of the forward weight transfer,
hence allowing the rear tires to play a bigger part in braking.

I think in terms of pure physics, this has been shown perhaps not
to be the case, or at least not in isolation.

The other thing that occured to me, which I think is also the
reason a car feels more stable going through a corner at high
revs in a low-gear, rather than low-revs in a high gear, is that
I am getting some benefit from the centrifugal force of the
engine spinning, rather like a gyroscope.

Is this true?
I can imagine it might be quite significant, anyone who's
ever "played" with a gyroscope will know that these
things take a lot of force to move when they are spiinning.

That being the case, have Papy modelled that in GPL?
It certainly feels like it to me, unless there are other forces
at work here.

Maxx

Besides, if the gyroscopic effects were important, it would only
stabilize the car in, say, left hand corners, but destabilize it in the
right hand ones due to the different sign of additional weight shift
that rotating the engine axis in different directions would bring.

-Gregor

I've been thinking about this since my initial question. I know what
I can feel and what works for me but I've no real idea why. I can
postulate but they would be uneducated guesses. Thanks to some
of the posters in r.a.s. I have a better grasp of the basic physics
but there are a lot more things in play here, all the various
suspension items, the diff, etc, etc. Certainly the cars "attitude"
will be very different in both scenarios (i.e. late, trailing
throttle (coast) change downs v. the method I describe.

I've posted another possible theory just now to do with centrifugal
force (gyroscope effect).

Much depends on your speed and your brake balance and probably
also your setup, specifically your diff. I BELIEVE I can carry more
speed into Noveau Monde using the method I describe, although
I havent done any comparative tests. I am certainly in 3G as I turn in
and often changing to 2G well before half-way down the short
straight after. Here I am most definitely using the engine revs as
a stabilising force rather than a brake (although as braking is so
tricky here I do think it helps braking also).

I think I'll leave any explanation as to why adn indeed if to the
vehicle dynamacists.

Maxx

Bear in mind the last time I had a gyroscope I was 10 so I
could be wrong. If my feeling is correct, then this gyroscopic
effect COULD make the car more stable in the up and down
direction also .. could it .. boy this is confusing stuff!

Maxx

Lets say that the gyroscope such as our engine is pointing forward in
our car, and if you look at the car from behind, let's consider the case
when the engine is rotating in the clockwise direction.

Now, to move the gyroscope (engine), say, to the left, it would require
a change of it's angular momentum, which in the case considered points
forward along the axis of the engine, towards the left as well. Physics
would tell you that the torque required to do so would point in the
direction of the change of the angular momentum (to the left), and be
proportional to the size of this momentum and the rate of rotation (how
fast the nose of the car is turning). In a left turn, the torque needed
would then point to the left relative to the nose direction.

The torque needed to turn the rotating engine is provided by the car
body, so an equal but opposite torque acts on the car body by the
engine. Therefore, apart from the torques produced by the tires, there
now exists an additional torque on the body that points to the right.

Any torque tries to rotate the body anticlockwise around the direction
where it is pointing. In the case of the torque on the body pointing to
the right, there is therefore an additional shift of the weight to the
rear, stabilizing the car.

In the case of a right hand turn, the above analysis repeated will give
you an additional torque on the body pointing to the left, which, on the
other hand, gives an additional weight shift to the front and as such
destabilizes the car.

For transversally mounted engines, one would get similar weight shifts,
but they would be left to right instead of back to front. Come to think
of it, perhaps this could even be of use for cars racing on ovals to
better ballance the weight? I demand a patent on this solution :)!

I hope all of this was at least half comprehensible :).

-Gregor

It's all immaterial anyway, as the speed of the engine is just a function of
how fast you're going and what gear you're in, negating
clutch/differential/wheel slip.  As you wouldn't be spinning the rear wheels
under braking (I hope!) all this is just a function of what gear you're in
whilst going through the corner, rather than how much throttle you apply.

Andy

As any one that has driven a SAAB knows they're _MASSIVELY_
understeered(esp. pre-9-3). Besides that, they have the handbrake on
the _front_ wheels together with FWD so the only way to get the car
into something reminding a sliding state is to throw the car into the
corner with almost floored throttle and brakes.

Worked great, though. The SAABs racked up victories in the 60's...

/Magnus
GPLRank hcp: -40.04

"The purpose of the engine is to accelerate the car. The purpose of the
brakes is to decelerate the car. The purpose of downshifting is to
select the proper gear for the forthcoming corner, and, sometimes, to
stabilixe the car - not to slow the car. The sequence of downshifting is
brake and then downshift. If you are near max. engine revs on a straight
and downshift, you will over-rev the engine."

and earlier in chapter 4:

"The Other Side of the Coin" -

" A large number of exceedingly competent and accomplished drivers
disagree with the idea of skipping gears. Thier numbers include Mario
and Michael Andretti and for a while, our son, Christopher (Smith). They
feel that particularly in long braking areas - from very high speed to
very low speed - the retardation of the rear wheels by the friction of
the engine exerts a stabilizing influence on the back of the car
(somewhat like a sea anchor) and gives them both better control and more
retardation."

TJ

I think the diff/clutch aspect comes in when we consider gas+brake,
although again, I'm sure the car will "react" differently under engine
braking dependent on the configuration of ramp angles and number
of clutches.

I'm sure the diff guru's will help us out here.

Maxx

Hey Thomas, glad to see you back!

Interesting stuff there. Someone told me that CS didn't "believe" in
engine braking. I think it's more down to the definition of engine
braking than the effect itself. It's a fact that it exist, lift off at
max revs in any gear and you will see it for yourself (versus
dipping the clutch at max revs).

Certainly changing down to a lower gear whilst at max revs in
the higher gear is plain dumb, the trick is to change down at
the speed at which you would be pulling max revs in the gear
you are changing down to, this gives you "safe" engine braking.

Whether it actually helps of not is debatable and certainly would
be vehicle specific. If you can't lock the wheels by applying the
maximum pressure on the brakes then maybe it will help.

I know it's used in all formula, including F1 which I would say is
renowned as having the best brakes there is.

I forgot to put this part in my previous post...this is the bottom part
of the Chapter 4 paragraph "The Other Side Of The Coin":

"The diversity of opinion among the very best practitioners of the art
merely confirms that there are no absolutes in motor racing. My job is
to point out the options and thier possible advantage. It is up to you
to determine what is best for you. Bear in mind that what proves best in
one situation may not be best in all situations." -Carrol Smith- "Drive
to Win"

TJ

Oh, right, I almost forgot-- this is why helicopters have tail rotors.
If they didn't, they'd just spin around and crash.

And on the topic of engine braking, wasn't there a famous British GP
driver (I forget exactly who) who once said "Engines used for br[e]aking
usually do".  :-)

        Marc

TJ