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Subject: rec.autos: Frequently Asked Questions
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[this article is one of a pair of articles containing commonly
asked automotive questions; the other article contains questions
of general consumer interest, and is broken out to facilitate
crossposting to misc.consumers -- rpw]
An web index to the FAQ postings, including this one, may be found at
http://www.wizvax.net/rwelty/FAQ/
(This file is
ftp://ftp.wizvax.net/pub/personal/rwelty/Autos/questions.text)
[last change: 6 July 1996: moved to ftp.wizvax.net -- rpw]
Radar Questions:
Q: Where are radar detectors illegal?
A: In the US, currently Virgina and the District of Columbia prohibit
all usage of radar detectors. New York prohibits their use in
large trucks. In Canada, they are illegal in Manitoba, Ontario,
Quebec, Newfoundland, and PEI (Prince Edward Island). They
are apparently are illegal through most, if not all, of Europe.
Legislation which would make them illegal is pending in many other
jurisdictions; chances of such legislation passing varies a great deal.
Q: Where are Radar Detector Detectors used? Do they really work?
A: Usage is spreading rapidly; initially they were used only in Canada,
but now they are appearing in New York and Virginia. It is unsafe
to assume that they are not in use in D.C. They work by detecting
a certain frequency radiated by many currently available super Het
radar detectors; some brands of detector radiate more strongly than
others, and are thus more likely to be spotted. New radar detectors
are becoming available which may not be detected by the current
generation of detector detectors. Note that a detector may only be
spotted by one of these devices if it is turned on.
Q: What is VASCAR? Is it some kind of Radar?
A: VASCAR is nothing more than a fancy stopwatch and time-speed-distance
computer. It depends on the operator pressing buttons as the target
vehicle passes landmarks. No radar signals are emitted by a VASCAR
system.
Q: What is Ka band radar? Where is it used? Should a radar detector be
able to handle it?
A: Ka band has recently been made available by the FCC for use in the US
in so-called photo-radar installations. In these installations, a
low-powered beam is aimed across the road at a 45 degree angle to the
direction of traffic, and a picture is taken of vehicles which the
radar unit determines to have been in violation of the speed limit.
Tickets are mailed to the owner of the vehicle. Because of the low
power and the 45 degree angle, many people believe that a radar
detector cannot give reasonable warning of a Ka band radar unit,
although some manufacturers of radar detectors have added such
capability anyway. The number of locales where photo-radar is in use
is limited, and some question the legality of such units. Best advice:
learn what photo radar units look like, and keep track of where they
are used (or else, don't speed.)
Q: Do radar jammers work? Are they legal?
A: Quick answer: No, and Not in the USA.
Detailed answer: Cheap radar jammers do not work well at all.
Jammers that work are expensive and usually the property of the
military. Jammers are a major violation of the regulations of the
Federal Communications Commission of the USA.
Driving technique and Vehicle Dynamics Questions:
Q: What are understeer and oversteer?
A: Understeer and oversteer are often misunderstood and misused
terms. The popular press commonly uses them to describe the
behavior of automobiles near the "limit of adhesion", but
engineers working in the field of vehicle dynamics prefer to
only use the terms for low-g cornering (that is, cornering
with lateral accelerations of say, 0.2G, well within the
linear or elastic range of tire operation.) These engineers
generally prefer to use the term "plow" instead of "understeer"
and "spin" instead of "oversteer" when talking about behavior
at the limit of adhesion, where performance is controlled by
the friction characteristics of the tires. The terms "tight"
and "loose" are also used in describing limit behavior.
Simply put, these terms describe whether the car wants to go
straight in a corner (steer `less', meaning `understeer' or `plow')
or it wants to turn more in a corner (`oversteer' or `spin'.)
Understeer is commonly designed into most production cars so that
untrained drivers, inadvertantly traveling too fast, won't get into
trouble. `Plowing' may also be induced by using too much throttle
in a corner. Some performance cars, in a corner near the limit, may
tend to rotate (which is an incipient spin; the Porsche 356 and
earlier Porsche 911s were noted for this); this behavior may
induced by lifting on the throttle (sometimes refered to as
Trailing Throttle Oversteer, or TTO). Sometimes a genuine spin
results.
Some technical details: in a corner at speed, the tires on the car
will develop what are called `slip angles'; slip angles are the
difference between the center line of the tires and the path on
which they are actually traveling. In a turn, there will be slip
angles at all four tires, and they will generally be different
for each corner of the car. Understeer and oversteer are defined
in terms of the "rate of change" of the slip angles; in understeer,
the front wheels have a greater rate of change in slip angle than
the rear wheels, and in oversteer, the reverse will be true.
An historical note contributed by Doug Milliken: Folklore has it
that the term "plowing" came from Barney Oldfield driving an early
Christie Front Drive racer. These cars had huge displacement/slow
revving transverse engines with a clutch on each end of the crankshaft.
The output from each clutch went direct to the front wheels through
Hooke joints (u-joints)! The cars look from pictures to be very
front heavy, and with front wheel drive were pretty successful
in the days before WW1. When he cornered on a loose dirt track the
car did actually plow up furrows!! Walter Christie went on to design
some very good battle tanks and other heavy vehicles.
Q: What is a rev-matched downshift?
A: When downshifting, the engine must be rotating faster in the lower gear
than it was in the higher gear. However, during a downshift, normally
you declutch and lift your foot from the throttle, so the revs drop
rather than increase. In rev-matched downshift, you blip the throttle
before re-engaging the clutch so that the engine will already be up to
the new speed. This results in a much smoother and faster downshift.
Q: What does heel-and-toe mean?
A: Heel-and-toe is a technique used to do a rev-matched downshift while
braking. This is normally challenging, because you need the right foot
for both the brake and throttle. It is called heel-and-toe because you
use one end of the foot on the brake, and the other on the throttle to
match revs for the downshift. In many modern cars this is a misnomer;
often you must use the ball of the foot on the brake and the right side
on the throttle.
Note that some race car drivers will skip the clutch, and just use the
left foot on the brake and the right foot on the throttle, accomplishing
the same thing.
Q: What is double-clutch downshifting?
A: While your right foot is doing the above, your left foot can do one of
three things: nothing, declutch once, or declutch twice. The reason for
declutching twice is to match the speeds of the two shafts in the
transmission to the speed of the engine. This is usually coupled with
rev-matching, so that while the engine is in neutral and the clutch
engaged, the throttle is blipped and both shafts of the transmission
speed up.
The procedure is as follows:
(0) declutch
(1) move gearshift lever to neutral
(2) engage clutch
(3) match revs
(4) declutch
(5) move gearshift lever to next lower gear
(6) engage clutch
This sounds like a lot of work, but with practice it becomes natural.
The problem that double-clutching solves is normally the function of the
synchronizers within the gearbox. In transmissions without synchros or
with very worn synchros, double-clutching makes it much easier to shift.
Basically, if you double-clutch well, you are not using the synchros at
all. This is generally unnecessary on street cars with synchros in good
condition.
Q: What do the numbers for acceleration from 0-60, 1/4 mile, skidpad, and
slalom times in the Auto Magazines really mean? May they be compared?
A: In short, 1) not as much as the magazines want you to believe, and
2) almost never.
In more detail: the acceleration numbers (0-60mph and 1/4 mile times
in the US) may be vaguely compared as long as they all come from the
same source. Testing procedures vary so much from magazine to magazine
that comparing a Road & Track number to a Car & Driver number is quite
pointless. Keep in mind, too, that the same variation applies from
driver to driver on the street; the driver is a major (often *the*
major) part of the equation.
Skidpads vary, and even if they didn't, skidpad figures are really
only tests of the stickiness of the stock tires; they change radically
when tire compounds change. DO NOT make any assumptions about the
comparative handling of, say, two sports sedans based on skidpad numbers.
This is not to suggest that skidpads are without value, however. Skidpads
are an excellent educational tool at driving schools. They are simply
of limited value in the comparison of anything except tires.
Slalom times are slightly more useful; they test some small parts of the
automobile's transient response. However, they are also heavily influenced
by the stock rubber on the car, and they do not test many corners of the
car's envelope. They DO NOT tell you all you need to know before making
a buying decision. For example, they don't tell you what the rear end
of the car will do on a road which suddenly goes off-camber. When a car
has an adjustable suspension, these tests are usually done in the `sport'
setting, which may be quite unsuitable for daily driving. The list of
caveats could go on for page after page.
Q: According to my physics class, you can't ever accelerate or corner at
more than 1.0G. Is this true?
A: the equations for friction used in freshman physics textbooks presume
that the surfaces are smooth, dry and non-deformable, none of which
are the case with tires & pavement.
Pavement is _never_ smooth; it is always irregular to a greater or lesser
extent. Tires, which are not really dry and solid (as rubber is a
substance which in its natural form is liquid, and which has only been
coerced into a semblance of solidity by chemical magic), deform to match
the surface of the pavement which a vehicle is traveling over. The
mechanisms by which tires grip pavement are not well understood, but
are clearly not those described by the "Introduction to Physics"
formula.
A better model for tire grip is offered in _Race Car Vehicle Dynamics_,
William and Douglas Milliken, Society of Automotive Engineers, 1995.
In this model, mu, the constant coefficient of friction, is replaced
by a lateral (xy plane) force coefficient, whose value may vary depending
on the vertical (z axis) force being applied, and on other conditions.
For a modern Formula 1 race tire, the lateral force coefficient may
be as high as 1.8 for light loading in the z axis. See chapter two
of this fine book for an exhaustive discussion of tire behaviour
in extreme conditions.
Misc. Questions:
Q: What does <name or acronym> stand for?
A: Here is a list of some of the names which are commonly asked
about; be careful in soliciting the meanings of other names
as misinformation abounds on the net. In particular, NEVER
ask in rec.humor if you want a useful result.
Saab: Svenska Aeroplan A. B.,
or The Swedish Airplane Corporation
Alfa: Societa Anonima Lombarda Fabbrica Automobili,
or The Lombardy Automobile Manufacturing Company
Fiat: Fabbrica Italiana di Automobili Torino,
or The Italian Automobile Manufacturers of Turin
BMW: Bayerische Motoren Werke,
or Bavarian Motor Works
MG: Morris Garage
Q: Does VW own Porsche?
A: No. Porsche is a publicly held company, controlled by the Porsche and
Piech families. Porsche has extensive business dealings with VW/Audi,
which causes some confusion. Since currently Porsche is in some
financial difficulty, there is a possibility that Mercedes or VW may
be interested in purchasing the company in the near future, but this
is only speculation at this time.
--
Richard Welty
[email protected]
