rec.autos.simulators

However, I think most modern single-engine planes are also trimmed to
counter (or at least minimize) this effect.

All I can remember is if you take the front wheel off your bicycle and
hold the axle ends with both hands and have someone spin the wheel, you
can rotate one hand over the other easily but not the other way around
without having the wheel spun in the opposite direction. That, I think
is the essence of the gyroscopic question, but does not get to the
bottom of the engine braking problem. As mentioned earlier, outboard
brakes apply stresses to the suspension which may affect the handling
under braking differently than engine-assisted braking.

A side note: if you mount free-spinning wheels (or flywheels) to your
bicycle and spin them fast in the opposite direction of the standard
wheels, the bike looses all stability and you will quickly fall to the
ground no matter how fast you go.

C.Smith mentions also that engine friction and not compression within
the cylinders accounts for the braking force. But I'm not sure about
that.

Bottom line is the physics are actually quite complex and may have a
lot to do with the specific car, engine and suspension design. But I
stand by the theory that all braking ought to be done by the brakes.

Cheers,

Ray

TJ

Interesting discussion you started.

However, IMHO engine braking does not necessarily involve downshifting
or double clutching or heel & toe.  You obtain engine braking merely by
letting off the accelerator.  The compression of the engine (more than
engine friction) causes the braking effect.  This affect is more
pronounced at higher revs (I think because there are more compression
cycles for a given distance traveled than at lower revs), but still
exists at middle revs and sometimes even at low revs (depends on the car
or truck).

Downshifting, double clutching, and  heel & toe are sometimes used in
conjunction with engine braking, but are not, per se, engine breaking.
In fact some cars/trucks require double clutching for up shifting.

I think C. Smith (as quoted by Thomas JS Brown) explains it well when he
describes

" 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."

I believe engine breaking is just one of the tools used in slowing and
controlling the car.  A very precise tool because you can use the
accelerator to slow, be neutral, or accelerate (move the center of
gravity where you want it using one input device).  

But what do I know, I'm slower than Eldred.  Sorry Eldred, just kidding,
I couldn't resist.  

Claude

  That's probably quite possible.  I haven't done any work simulating engines
at part throttle.  (This should be coming by the year 3050.)

Todd Wasson
---
Performance Simulations
Drag Racing and Top Speed Prediction
Software
http://PerformanceSimulations.Com

[snip]

I'll leave the compression v. friction argument to the experts.

I think there is still some confusion over what I mean by "engine
braking" and I think it's because the term is so broad.

I suspect that what C. Smith is referring to is downshifting in
manner which stresses the engine/transmission i.e. To change
down at such a point that the engine is travelling slower than the
wheels, such that the wheels will then try to "drive" the engine,
which, via friction or compression will resist this, causing a
braking effect at the driven wheels.

I also suspect that this is what most people will regard as
engine braking.

This is NOT what I meant but have not really read about it
or discussed it before so I can appreciate now that using
that terms is confusing.

I think I could rephrase my question as "Optimum Downshifting
under Braking (ODUB) : One for the physics guys I think".

To re-iterate, I am taking about timing ones downshift such that
one arrives in the lower gear at maximum revs in that gear, thereby
taking maximum advantage of the decellerative forces of the
engines "rev drop off".

It certainly makes sense. As Claude says, lift off at 9,500 revs and
you will get a greater decellerative force than if you lift off at
6,000 revs.

My initial point was, if you are braking to the maximum capacity
of the tires already, then does this ODUB have any "additional"
effect/benefit.

I think it does as rarely is a driver able to brake at the maximum
capacity of all four of his tires. It may not even be possible to do
so as it needs to brake bias and weight balance of the car to be
perfect to exatracct the maximum from all four tires.

Whilst this might be possible under heavy braking on a flat
approach, it would not be under lighter braking, not if there
was an incline of if we were slightly off balance from the exit of the
previous corner.

The rule about braking and setting up brakes is that the fronts
should lock up first under normal conditions (of course we
should avoid any lockup). So we are always going to have a
slight bias on the front brakes. I think what ODUB might give us
is a very small window of rear braking bias as we approach brake.
Even to the point of split-second lock-up (almost an ABS/Cadence
Brake effect).

I should stress that IMO ODUB is kind to engines and transmissions,
and puts no more stress on them than the act of accelleration and
change up at maximum revs.

I wanted to try and ascertain whether a decellerative force (I'm
sounding more like a physicist daily :-)) acting on the tires via
ODUB would be different in any way that that acting on them
via conventional brakes. I suspect not at the tire end, but I just
get the "feeling" there is elsewhere, does the suspension behave
differently?, is the weight transfer different?

Certainly by using ODUB you are changing the way the car brakes
as you are dynamically altering the brake bias. I think this is the
reason it works (dependent on setup and driver timing).

Certainly I see many driver laps, and my own where I am changing
down to say 2G, as I brake for a corner, then putting it in 3G to
actually take the corner. I've (They've?) done this to get that
extra bit of ODUB at the end of braking.

The next thing I thought was that the actual centrifugal force of
the engine at high revs was perhaps giving me some extra little
bit of stability under braking. I don't notice it in GPL much but one
of the biggest problems in real-life under very heavy braking is
keeping the car stable, avoiding that slight fishtail effect. You
often see this in F1 under braking for chicances and very much
in F3/FF etc. etc.

OK, sorry for the long post, but theres been some great feedback
to my original post. I just wanted to make sure there weren't those
with a misconception of what I was saying.

I'm still very intrigued with this gyroscope effect and if if does
havae an effect and if I really can go faster adn be more stable
turning left than turning right (or vice-versa).

Interestingly, most oval racing in the UK (short track, none-banked
mostly) is done clockwise, whereas it appears that it is the opposite
in the US.

Maxx

- Matt

Maxx wrote ...

It's the wheel lock that's the key for me.  Maximum slowing force comes just
before wheel lock.  On a number of circuits/turns, I just have not developed
the skill yet to drive as deep as others and not lock the wheels when I
apply brake.  That could be partly a controller issue, but it's more likely
a me issue.  My theory is that there's some ratio between slowing power of
brakes and slowing power of engine braking, such that perhaps on a given
piece of road 80% brake depression without downshifting would be equivalent
to 60% brake depression with engine braking.  I can't tell you why, but I
suffer wheel lockup much less often with the latter arrangement.

I believe the reason a car feels more stable (easier to balance) in the
turns at high revs has to do with throttle response.  When you're in the
upper quarter of the rev range of a gear, the throttle response is fairly
direct and predictable.  It's quite easy to tip the car forward or back with
slight throttle movements.  At the low end of the gear's range, the throttle
response is not nearly as uniform and as a result, much less predictable.
At the extreme low end, for example, when the engine is actually "lugging",
too much throttle can momentarily ***the engine and cause a decrease in
rpm's, and you have to back off and bring it up slowly.  It's very tricky
getting just the right revs at the moment you need them when you're at low
revs, and a fairly simple matter at high revs.

--
Chris "Ambulater" Lee
"Don't Bother Running.........You'll Just Die Tired."
Reviews at www.sim-arena.com