A few people have asked me to explain the difference between
"DOT-approved" and non-approved stainless-steel brake lines. This
explanation is pretty long, but I think it'll explain everything.
First, a quick explanation of what stainless-steel brake lines are:
The brake lines we're talking about are the flexible ones that
connect between the hard lines (i.e., the inflexible tubing) in the
car and the brake calipers on the wheels.
They've traditionally been made from rubber tubing, with steel or aluminum connectors crimped onto their ends. Nearly all passenger cars are shipped with rubber brake lines, and they hardly ever fail.
"Stainless-steel" lines are made of Teflon tubing, not rubber.
Teflon has a number of advantages over rubber; the chief ones are
that it doesn't expand under pressure and it doesn't deteriorate
with age. It also resists high temperatures and is chemically
inert, so it's compatible with all brake fluids.
However, Teflon is pretty fragile, so it has to be protected from
physical damage (chafing, flying rocks, etc.). Although some
manufacturers armor their Teflon hoses with Kevlar, most protect the
Teflon with an external sheath of braided stainless-steel wire... So
that's why armored Teflon hose is usually called "stainless-steel
hose".
The ends of the hoses have to be securely attached to the brake
calipers and the hard lines, so each hose is terminated by threaded
hose-ends.
Those hose-end fittings can be attached to the hoses a couple of
ways:
The cheap way is to crimp or swage them onto the hoses, like the
fittings on rubber hoses. The more-expensive way is to use a
two-piece replaceable hose end that captures a portion of the hose
between an inner nipple and a concentric outer socket. These
hose-ends (often referred to generically as "Aeroquip fittings"
because they were invented by the Aeroquip Corporation) are used
EVERYWHERE on aircraft and race cars.
Ok... So what's required for a stainless-steel brake line to be
DOT-approved?
First, I should point out that there may be lines available that
meet all the DOT specs, but are non-approved only because they
haven't been submitted to the DOT for approval.
Manufacturers can't legally say that their lines are approved - even
if they KNOW that the lines meet all the DOT specifications -
without actually submitting them to the DOT.
For that reason, stainless-steel brake lines can fall into three
categories:
"DOT approved" - These lines have been submitted to and approved by
the US Department of Transportation.
"non-approved" - These lines don't have a DOT approval, either
because they don't meet the specs or simply because they haven't
been submitted for testing.
"non-conforming" - These lines are non-approved (and non-
approvable) because they fail to meet the DOT specs.
Ok...
The safety standard that brake hoses must meet is called Federal
Motor Vehicle Safety Standard 106; if you have a copy of the Code of
Federal Regulations handy, it's in Title 49, Volume 5, Subpart B,
Section 571.106.
The section that applies to hydraulic hoses is about six pages long,
and it covers everything from labeling requirements to pressure and
temperature testing.
One important thing to note -this'll come up later when I explain
why the "best" hose assemblies can't be DOT approved- is that each
of the requirements in the Standard carries the same weight; if a
hose fails to meet ANY requirement, it won't be approved.
Hypothetically, therefore, a hose which met all the performance
specs but was labeled in lowercase letters (the Standard requires
block capitals) would fail to be approved.
Also, some of the features required by the Standard provide a
certain amount of "idiot-proofing", but at the expense of absolute
maximum strength or safety... It's the same sort of mandated
mediocrity that forced Ferrari to replace the stock 5-point safety
harnesses in US-spec F40s with those ridiculous motorized-mouse
single shoulder belts.
Anyway...
Most of the "performance" specs in the Standard (i.e., burst
strength, compatibility with brake fluids, tensile strength,
expansion under pressure, etc.) are easily met by all halfway-decent
hydraulic brake hoses, but there are a couple of tests and
requirements that are particularly difficult for stainless-steel
hoses to meet.
Those requirements are:
1. The manner in which the fittings must be attached to the
hose.FMVSS 106 specifies that "Each hydraulic brake hose assembly
shall have PERMANENTLY ATTACHED brake hose end fittings which are
attached by deformation of the fitting about the hose BY CRIMPING OR
SWAGING." [Emphasis added]
The idea is that, since crimped-on fittings can't be loosened, a
stupid end-user won't be able to screw with and weaken them.
This is a good thing from a product-liability point of view, I
guess... But it means that any hose assembly which uses the very
best fittings available -like the nipple-and-cutter Aeroquip Super
Gem or Earl's Speed Seal- is non-conforming and CAN'T be
DOT-approved.
2. The "whip-resistance" test. This test involves mounting the hose
on a flexing machine, pressurizing it to 235 psi, then running it at
800 RPM for 35 hours.
When steel-armored hoses were run through that test, it was found
that the hoses tended to bend right at the junction between the hose
and the hose-ends. After a while, the stainless-steel braid would
start to tear, and the broken wires would cut into the inner Teflon
liner, causing it to fail.
One brake-hose manufacturer fought to modify the whip test, claiming
that their stainless-steel hose could easily comply with the test if
only a supplemental support were used during testing to move the
flexing-point away from the hose-ends.
The NHTSA ruled on the issue in August, 1996, deciding to allow
manufacturers to use the supplemental support... But only on the
condition that the same support was used when the hoses were
installed on a real car.
FMVSS 106 was modified to include the use of the support, and the
new rules went into effect in October, 1996.
"DOT-approved" stainless-steel brake hoses went on sale immediately
thereafter.
So now you know what DOT approval entails. For information on why
you would want (or might not want) stainless-steel brake hoses on
your car, see read on...
There are three reasons to install stainless-steel brake lines:
1. They look racy.
2. They don't swell like rubber lines, so they can potentially firm
up your brake pedal.
3. If you're doing a lot of off-road driving, the stainless-steel
braid may protect your lines from being punctured by rocks or
whatever.
Here's the thing, though: Since stainless-steel lines don't bulge
as they age, and since the inner Teflon lining is hidden behind the
braid, there's no easy way to inspect the lines for warning signs of
imminent failure.
This is no big deal on a race car, since the lines are (or should
be) replaced at least once a season. On a street car, where most
people are likely to let YEARS go by without even looking at their
lines, it can be an issue.
Plenty of people, therefore, warn that you should use rubber lines
instead of stainless steel... They'll be happy to give you anecdotal
evidence of steel lines simply bursting (or, more commonly,
separating from their hose-ends) catastrophically and with no
warning.
I haven't seen any references to this sort of failure that mentioned
whether the lines were:
REAL Aeroquip or Earl's hose,
attached properly to the CORRECT hose-ends, and
installed properly on the car.
I define "correct hose-ends" as Earl's Speed-Seal (the new name for
Fluor-O-Seal) or Aeroquip Super Gem, and "real hose" as Earl's
Speed-Flex (the new name for Fluor-O-Flex) or Aeroquip... uhh... I
forget their brand name.
Anyway, Speed-Seal hose-ends work just like Earl's Swivel-Seal ends;
the hose-end can swivel after assembly. The nipple/cutter assembly
on these ends (and on Aeroquip Super Gem ends) was specifically
developed to prevent blowoff of the hose-end... I'm still waiting to
hear from anyone who has firsthand knowledge of one of these hose
assemblies coming apart, and until I hear from that person, I run
"real" stainless-steel lines on my car and replace them regularly.
THIS IS IMPORTANT: The lines that your performance-parts
distributor will sell you are made with no-name hose from
God-knows-where (probably Taiwan), and the hose-ends are just
swaged-on fittings that are an invitation for disaster. I won't put
these on my car, and I don't recommend that you put them on yours,
either.
There are now "DOT-approved" stainless-steel lines. I have no idea
what they are, but I suspect that they STILL use cheap-ass
crimped-on hose ends. Until my suspicions are disproven, I won't
put THEM on my car, either.
If you do decide to put stainless-steel lines on your car, you need
to be aware of a few things:
1. When you install them, you must make SURE that they can't kink,
twist, or stretch under any combination of wheel droop, bump, or
(for the front wheels) steer.
2. The stainless-steel outer braid will cut through anything
against which it rubs, so you have to make sure that the lines don't
rub back and forth over anything important.
3. Stainless steel lines have been known to fail when dirt gets
between the outer braid and the Teflon lining... As the braid moves
back and forth, the dirt abrades the Teflon and can make it rupture.
If you look at stainless-steel lines on motorcycles, you'll see that
many of them are encased in plastic tubing, apparently in an effort
to eliminate this problem. The tubing also helps considerably with
the abrasion issue mentioned above.