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From: [email protected] (richard welty)
Subject: rec.autos: Frequently Asked Consumer/Automotive Questions
Archive-name: rec-autos/part5
[this article is one of a pair of articles containing commonly
asked automotive questions; the other article contains questions
more geared to the automotive enthusiast, and so is not crossposted
to misc.consumers. -- rpw]
[changes as of 14 April 1993: revised brake fluid section, as
non-silicone DOT-5 fluids are now apparently available -- rpw]
Commonly Asked Automotive Questions
Tire Questions:
Q: What do the funny numbers on the sides of my tires mean?
A: Typically, you will see something like 195/60HR14. the 195 is the
overall width of the tire in millimeters, the tread is usually
narrower. The 60 is the `aspect ratio'; it indicates the height of the
sidewall of the tire relative to the overall width. Our example tire
has a sidewall height of 0.60 * 195 ~= 117 mm. The 14 is the wheel
diameter in inches; there are also some special tires called `TRX'
tires which have three digit metric wheel diameter designations, like
390, which is in millimeters. The R means Radial, and the H is a speed
rating indicating the highest speed at which the tire, when properly
inflated and carrying an appropriate load, may safely operate. Common
speed ratings are S (112MPH), T (118MPH), H (130MPH), and V (up to
150MPH.)
Recent changes to the method for specifying tire sizes move the speed
rating to a different part of the designation; you may therefore find
designations like 195/60R14 85H; the 85 indicates the per-tire load
associated with the speed rating -- exceeding this load in continuous
operation at the rated speed is dangerous practice. What follows is
a table showing a number of `load indices' and corresponding maximum
per-tire loads:
Load Index 50 51 88 89 112 113 145 149 157
Max Load (Kg) 190 195 560 580 1120 1150 2900 3250 4125
Note that the usual mass vs. weight confusion occurs in this table.
In some cases, the letters P or LT may be found in front of a tire
size; the LT designation indicates Light Truck, and the P designation
indicates Passenger car. If no letter is given, then the application
of the tire is Passenger car usage. As far as I know, these letters
only appear in the US market. The LT designation is prinicipally of
interest to owners of light trucks and other utility vehicles. For
the owner of a passenger vehicle, there is no meaningful difference
between a tire with a P designation and one with no designation at
all.
If the aspect ratio is omitted, it is probably 80, but may be 78.
Tires with an MS (mud/snow) designation may have their speed rating
reduced by 20 km/h (about 12mph.)
There is an additional set of ratings on tires for temperature,
traction, and treadwear. Temperature and Traction are graded
A, B, and C (with A the best and C the worst); treadwear is
a numeric rating. These values are of limited value, as they
are assigned somewhat arbitrarily by tire manufacturers and are
thus not useful in comparing different brands of tires.
Q: My car has tires with a funny size designation: 185/65HR390; can i put
normal tires on the car?
A: Your tires are called TRX tires; they were devised by Michelin.
Because of a somewhat different bead design, they are incompatible
with normal tires; Michelin used a different diameter wheel for them
so that they could not be mounted on the wrong type wheel (and so that
more conventional tires could not be mounted on TRX type wheels.)
You will need to aquire different wheels to put a normal tire on your
car; it is barely possible to fit normal tires on TRX wheels, and horribly
dangerous to do so (the tires may simply peel off the rims in a corner,
or possibly blow out at high speed.) TRX type tires are becoming hard
to find; in addition to Michelin, Avon makes suitable tires. Goodyear
has apparently discontinued their line of TRX tires.
Q: Can I rotate radials from side to side, or rotate them only on one side
of my car?
A: Car and tire manufacturers have differing views on this subject; many
say that swapping radials between the left and right hand sides of a
car is now ok (this group includes Michelin and Goodyear); others
(for example, Pirelli and Nokia) will void warranties if such swapping
is done. The best advice is to read your tire manual carefully before
rotating your tires, and stick to the manufacturer's recommendations.
Q: How many snow tires should I buy, and if I buy 2, which end of the
car should I put them on?
A: In short, 4, and both ends. To explain, many drivers in areas that don't
get excessive snow or who don't drive agressively (or at all) in snow
conditions get away with only two snows on the drive wheels, but there
are circumstances where this can be dangerous practice. With a rear
wheel drive car, you can choose between being able to start the car
going (a function of the rear axle) or stopping and turning the car
(a function of the front axle.) In a front wheel drive car, you start,
stop, and turn with the front end. The primary risk of putting the
snow tires on the front only is that if you have to put on the brakes
while going downhill, you run a serious risk of going downhill backwards.
Radar Detectors and Speed Limits:
Q: Why aren't there any comments on Radar Detectors and Speed Limits
in this Q&A posting?
A: Because questions about detectors and speed limits crossposted between
misc.consumers and rec.autos.* always start long, tedious, and pointless
flame wars. If you want to talk about either of these topics, please
subscribe to rec.autos or alt.flame and keep it there.
Safety Equipment:
Q: Do airbags really work?
A: Preliminary statistics suggest the following: Airbags work much
better than no belts; good 3 point belts alone work much better than
Airbags alone, and AirBags + 3 point belts work slightly better than
3 point belts alone. The con to airbags is that some designs tend
to burn the driver's hands when venting the byproducts of the
explosion that occurs inside the bag, and that some designs (but
not all) may knock the driver's hands from the wheel, making retention
of control of the vehicle after the bag deflates more difficult.
Brake Questions:
Q: Do I always need to get the rotors on my disk brakes turned? Midas
always wants to do this.
A: No. There are specific conditions that mandate turning or replacing
rotors; some shops try and make a little extra money by replacing rotors
more often than is strictly necessary. if the rotors are not warped
warped, and only lightly grooved, then there is no need to replace or
to turn them. Note also that some rotors (the rotors on many Hondas
are a good example) are so narrow to begin with that it is not practical
to turn them; they must be replaced when they become too thin, warped,
or badly grooved.
Q: They tell me I should downshift when braking to slow my car down. Is
this really a good idea?
A: It used to be a very good idea, back in the days of mediocre, fade
prone drum brakes. In modern disc brake equipped cars, use of
downshifting to slow the car is not really necessary, except in cases
of long, steep downhill runs. Otherwise, modern disc brakes are more
than adequate to stop a passenger car in all circumstances, and they
are much cheaper to repair than clutch linings.
On the other hand, many standard driver's license tests in the USA
still specify that the driver being tested downshift under braking; I
suggest that before taking a US driver's test, you either 1) learn to
do this smoothly (which takes some time and practice) or 2) borrow a
car with an automatic to take the test.
Q: How often should I replace my brake fluid?
A: Probably more often than you do. Traditional brake fluids tend to
absorb water; this water can corrode internal parts of the brake
system and also lower the boiling point of the fluid. DOT-3 type
are older fluids; DOT-4 and DOT-5 are newer specifications. The
principal differences are in wet and dry boiling points; the dry
boiling point is important in fresh brake fluid, but the wet boiling
point is important in older brake fluid. DOT-3 fluids have the lowest
wet and dry boiling point _requirements_; DOT-4 fluids have better
boiling point requirements; and DOT-5 fluids have the best boiling
point requirements (but DOT-5 fluids are not clearly superior; see
the next Q&A for more details.) While the requirements imply that
DOT-4 fluids are better than DOT-3 fluids, there may be specific
cases where a DOT-3 fluid is preferable, but these are mostly
competition applications. Otherwise, DOT-4 type fluids offer _much_
improved brake pedal feel. Replacement once a year is recommended for
DOT-4 fluids, although agressive drivers may profit by changing out
fluid more frequently, or at least bleeding a modest amount of fluid
out of the brake calipers fairly regularly.
Q: What about DOT-5 brake fluids?
A: This breaks down in to two parts. The DOT-5 specification looks
excellent for performance, but the first DOT-5 fluids were Silicone
based. Silicone fluids are a tricky proposition. Unlike DOT-3 and
DOT-4 fluids, they do not absorb water at all. While this may sound
like a feature, the problem is that any water present pools up in
such systems, interfering with braking performance and corroding any
metals at the spot where the pooling is occuring. The water will tend
to migrate downwards in the braking system to the brake calipers, where
most of the corrosion occurs.
Because of this phenomenon, it is essential when converting to
Silicone to empty the entire brake system and flush it throughly
beforehand; some even recommend replacing all rubber parts in the
brake system when converting to Silicone fluids.
Two other issues that come up with silicone fluids: 1) they are
difficult to pour cleanly (that is, without air bubbles), which
interferes with getting a good brake pedal feel, and 2) while they
generally have much higher boiling points than DOT-4 fluids, they
do have high temperature failure modes which are indistinguishable
in effect from boiling DOT-4 fluids. SIlicone fluids may make sense
in some street car applications, but they are certainly not
recommended for high performance driving applications, and the
economics are questionable for street use.
I have recently become aware of new fluids that meet the DOT-5 standard
that do not contain Silicones; these fluids appear to be reasonably
compatible with the older DOT-3 and DOT-4 fluids, but I have little
information at this time.
Q: ABS is available on some of the cars I'm looking at, but it costs
more. Is it worth it?
A: This does not have a cut and dried answer; therefore, this answer will
be quite long in order to cover the pros and cons. The short answer
is that ABS costs more, both initially, and to maintain, will generally
work better for the `average driver' (that is to say, a driver who does
not have extensive experience in high performance driving techniques),
and may require the `unlearning' of some obsolete braking techniques
like pump braking which should be unlearned in any case.
Now for the long answer. ABS works by monitering the wheels of the
car, looking for signs of locked brakes. It may or may not be able
be able to distinguish between the different wheels (there are several
systems on the market.) It cannot detect impending lockup (which is
what you would really want in an ideal world), but only the existence
of lockup. The sensors used vary; some of the less well designed
sensors are sensitive to tire size, and to brake pad material, and
may cease to function properly if the owner deviates from original
equipment or OE-equivalent components.
When the sensors detect lockup, the ABS system responds by unlocking
the brakes (either individually, or all at once, depending on the
system.) If the driver keeps their foot firmly planted, the ABS
will end up cycling between the locked and unlocked states (if a
sensor existed that could detect _impending lockup_, then we could
sit right at that point, which is where maximum braking effect is
achieved.) This pulsing can often be felt in the brake pedal, as
the system cycles. The percentage of the time that the brakes are
truly engaged is called the `duty cycle'; typically in an ABS system
this is about 40% On dry pavement, a trained driver can beat this
duty cycle quite reliably using a technique called threshold braking;
on wet pavement, braking is so chancy that ABS will outperform
threshold braking nearly every time. Unfortunately, on mud and on
snow, often maximum braking effect can be acheived with the brakes
locked; only Audi, of the manufacturers producing ABS-equipped cars,
has seen fit to provide a disable switch for the ABS system for this
eventuality.
A particularly important feature of ABS is that it preserves
steering control. This is the case simply because, if you are
braking near the limit and turn the wheel, the ABS will release
the brakes if it sees steering-triggered lockup, and back off
on the percentage of the time that the brakes are applied.
Braking distances will lengthen accordingly.
An important caution: ABS cannot exceed the maximum theoretical
braking force in any given situation; if you start sliding on
glare ice, don't expect an ABS system to help you out very much.
The coefficient of friction is not changed by the presence of an
ABS system in your car.
As far as maintenence goes, in addition to the potential
restrictions I've listed above, you have to worry about the
following: 1) parts costs are much higher; the OE master cylinder
for my obscure european sedan lists for $185, but the OE master
cylinder for the ABS-equipped version of the same car lists for
over $1000. Most manufacturers explicitly forbid use of DOT-5
(silicone) brake fluids in ABS-equipped vehicles. Because of
the potential cost of replacement of corroded brake system
components, regular (I suggest annual) replacement of brake fluid
becomes very important.
Q: What about this threshold braking business?
A: [normally, I'd not put this in the consumers Q&A, but recent
publicity about a number of accidents involving police drivers
in ABS-equipped Chevy Caprices suggests that this section is
needed here -- rpw]
Threshold braking is a technique practiced by all serious high
performance drivers; if made a habit, it replaces the `stab
the pedal and lock 'em up' panic habit entirely, and is much
to be prefered. Basically, the premise is that tires generate
maximum braking force when they have just started to slide, but
just before the wheels lock up entirely. Drivers who threshold
brake learn to feel what this `threshold' feels like, and learn
to search for it and hit it on the application of the brake
pedal. In many cars, you can feel that you are near the
threshold when the pedal starts to firm up as you depress it.
In any case, if you can't hear the tires whine just a bit, you're
not very near the threshold.
In a car with ABS, often there is a twinge in the pedal just
before the system starts cycling; if the driver backs off on
the pedal just a tad when the twinge is felt, then they are very
close to the threshold and they'll probably achieve better
stopping distances than if they just punched it and let the ABS
take over.
Recently, there has been a rash of publicity over a number of
accidents, and one death, involving police cars equipped with
ABS systems. The police departments in question quickly blamed
the new ABS systems, but according to Autoweek magazine, it now
seems clear that the problem was a lack of training; none of the
involved officers had any recent performance driving training.
There is reason to believe that the drivers reacted to the pulsing
brake pedal by `pump braking', an old and discredited technique
of stabbing and releasing the brake pedal, the goal being to
try and get brakes back with a failing hydraulic system. If you
think about it for a minute, you'll realize that pump braking must
cut the effective operation of a working brake system by at least
1/2, so if you cut the 40% duty cycle of an ABS system by that
much, you are giving up most of your brakes for the wrong reason.
Threshold braking has the advantage in that it is an effective
and useful technique regardless of whether your car has ABS; if
you do fear a failed hydraulic system, then one or two stabs at
the pedal will be sufficient.
Gas Questions:
Q: Does High Octane gasoline help?
A: Maybe, maybe not. Some cars have knock sensors, and can adjust the
engine timing or turbocharger boost to suit the gasoline being used.
On most cars, however, you should use the cheapest gas that makes your
car run well. Check your owner's manual for details on what your car
needs.
Q: My car was made for leaded gasoline. Will unleaded gas hurt it?
A: It is possible that unleaded gas may *slightly* increase valve wear,
although the Amoco Oil company argues otherwise. The actual increase
in valve wear will be almost unnoticeable, however, as modern leaded
gasolines actually contain very little lead. You should, however,
check your owner's manual; many cars from the early 1970s do not
actually require leaded gasoline.
Q: Do fuel treatments help? What kind should I use?
A: Some do and some don't. During the winter, it is a good idea to use
dry gas; however, some may be harmful to fuel injection systems.
Never use an additive containing Methanol (sometimes called Methyl
Alcohol); such additives may damage fuel systems in cars with carbs
and almost certainly will damage cars with fuel injection.
Manufacturer's opinions vary on additives containing Ethanol (sometimes
called Ethyl Alcohol); if your car has fuel injection, check the owner's
manual on your car before using these. Most manufacturers consider
10% Ethanol acceptable in gasoline. Additives with Isopropyl Alcohol
(Isopropanol), and Petroleum Distillates are fine in fuel injected
cars.
An occasional bottle of fuel injector cleaner is helpful in cars with
fuel injectors, although many premium gasolines contain detergents that
do the same job. Some off brands of fuel injector cleaners contain
Ethanol or Methanol; always check the ingredients before putting anything
in your gas tank.
There are a small number of particularly good additives; these are
noticeably more expensive that the run-of-the-mill ones, but work
much, much better. Among these are Chevron Techron, Redline SL-1,
Wurth `Lubrimoly Ventil Sauber', and BG 44K. A bottle of one of these,
once every six months, is highly recommended.
Q: What about detergent gasolines?
A: The quality of detergent packages in gasolines varies somewhat; BMW
has instituted a testing program, and gasoline brands which pass this
test may advertise that fact. Stickers indicating passage of the test
are now beginning to appear on fuel pumps at gas stations. If such
gasolines are used, then fuel injector cleaners are probably optional.
Beforewarned that while use of BMW approved gasolines will keep a clean
engine clean, they may not clean a motor with bad valve deposits.
Lubrication Questions:
Q: What do the numbers and letters in a motor oil designation mean?
A: There are several different items encoded. There is a two-letter
code indicating the type of detergent package that the manufacturer
uses in the oil; this looks like SE,SF,CD or such. The S codes are
for gasoline engine applications; the C codes are for diesel engine
applications. The second letter is assigned in sequence as new levels
of protection are developed; thus SF is considered better than SE,
SE is considered better than SD, and so forth.
The more noticeable designation is the oil weight. This is either
a single number (e.g., 30 weight) or a pair of numbers separated by
the letter W (e.g., 10W30.) The latter type is much more commonly
used these days, and are the only type that most automobile
manufacturers specify in operators manuals. The first number in the
designation (10W) is the apparent viscosity of the oil when it is cold;
the W stands for `winter'. The second number (30) is the viscosity
of the oil when hot. There is a trick here; the oil doesn't actually
get thicker (turn from 10 weight to 30 weight) as it gets hotter. What
is actually happening is that when the oil is cold, it has the viscosity
of a cold 10 weight oil. as it gets hotter, it doesn't get thin as fast
as a 10W oil would; by the time it is up to temperature, it has the
viscosity of a hot 30 weight oil.
Note that these numbers actually specify ranges of viscosities; not
all 10W oils have exactly the same viscosity when cold, and not all
30 weight oils have the same viscosity when hot. Note also that the
novel behaviour of multi-grade oils is caused by additives, and it has
been reported that with the sole exception of Castrol GTX, 10W40 oils
do not retain their multi-grade characteristics well over time. 10W30,
15W40, and 20W50 oils work very well, though.
Q: Are `quick lube' places any good?
A: Some do adequate work, but there are quite a few incompetent ones out
there. Let the buyer beware, watch them while they work, make sure
that they don't overtorque the oil drain plug, and keep your hand on
your wallet at all times.
Q: Are oil additives like Slick-50 or Tufoil any good?
A: Slick-50 and Tufoil are PTFE-based additives. Many of these have
come onto the market recently; they are different from the moly-based
additives that have been around since the early 50's. PTFE is the
chemical name for Teflon(TM), a trademark owned by DuPont. In general,
auto manufacturers do not recommend use of these products. Most USENET
responses to questions on these additives are favorable (slight
increase of MPG after application, smoother revving) but long term
results (whether PTFE additives are effective after 5K miles) are
debatable. Some manufacturers (such as Saab) claim that either the
product or the engine flush that preceeds application causes
deterioration of the oil seals and eventual leakage. Some BMW owners
have reported death of valve seals shortly after the addition of
Slick 50 to their cars. This writer has been cautioned by a Slick-50
Dealer (!) that Slick-50 should _not_ be used in Japanese motors, as
it may clog the oil return passages in the engine. Otherwise, there are
no known reports of damage caused by PTFE additives.
On the other hand, there are satified Slick 50 customers in the world.
Q: Do synthetic oils really work?
A: Yes, but. More specifically, most auto manufactuers accept synthetics,
but disagree with the extremely long oil change intervals claimed by
oil manufacturers. Auto manufacturers recommend that you continue to
change oil at the intervals recommended in the owners manual for your
car. Even if you decide to try the longer intervals, at least change
the oil filter at regular intervals, as synthetics get just as dirty
as conventional oils.
Synthetic gear lubricants for manual transmissions are another matter
entirely; Amsoil, Redline, and AGIP are very highly regarded and very
effective. Mobil 1 synthetic gear lube gets mixed reviews, however.
Q: Manufacturers are specifying longer and longer oil change intervals.
How often should I change my oil?
A: It depends on how you drive. If your car always (or nearly always)
gets warmed up, and you don't drive it very hard and keep the revs
down, the manufacturer's recommendation is probably fine. If, however,
you drive it hard, drive it at high revs, or alternatively, if you
only drive it to and from the supermarket so that it doesn't get up
to temperature, then you may wish to change oil much more often,
perhaps at 3000 mile intervals (given that most manufacturers are
now specifying 7500 mile intervals.) If you don't drive your car
much at all (say 7500 miles a year), then you probably want to change
oil every six months anyway. If you are storing a car during the winter,
then change oil before storing it and change oil when you bring it out
of storage.
Misc. Questions:
Q: My car has a timing belt. I hear that bad things happen when they
break. What's the story?
A: It depends on the internal design of the motor. Early Ford Escorts,
for example, will suffer severe valve damage if the belt breaks, but
the newer cars will just require a tow and installation of a new belt.
Some Honda motors will not be damaged, but others will be.
If no replacement interval is specified for your car, then change the
belt at least every 60,000 miles; some cars may require more frequent
replacement. Ask your dealer or independent mechanic. Also, ask if
there are any related repairs that should take place at the same time
(for example, the same Ford Escorts that suffer valve damage also have
a timing belt driven water pump, which has been known to seize,
destroying the timing belt, and which then causes major valve damage
as a side effect. Replacing the timing belt while ignoring the water
pump can be a costly mistake.)
Q: Why would anyone be stupid enough to design a motor so that it
self destructs when the timing belt breaks?
A: For performance reasons. Compromising piston design so that the
valves and pistons will not collide requires that the compression
ratio of the engine be reduced significantly; this is why you are
more likely to avoid valve damage in economy cars than in performance
oriented vehicles.
Q: What causes Unintended Acceleration?
A: The final report of the National Highway, Transportation, and Safety
Administration concluded that unintended acceleration could not be
caused by any mechanical failure of the vehicle in question and at the
same time be consistent with the physical evidence. The NHTSA report
goes on to conclude that `pedal misapplication' by the vehicle operator
is probably the cause.
--
richard welty 518-393-7228 [email protected]
``Nothing good has ever been reported about the full rotation of
a race car about either its pitch or roll axis'' -- Carroll Smith
