Stainless Steel Brake Lines Q&A
Walker, Jr. of scR motorsports
Why are flexible brake hoses used in the first place?
From the factory, nearly every production passenger car has short, flexible
hoses that run from the fixed, hard metal brake tubes to the calipers (or wheel
cylinders as the case may be). These flexible hoses are necessary because the
wheel ends are free to move relative to the body of the vehicle. Inflexible
tubes would not allow for the articulation of the wheel ends without subsequent
What are OEM hoses made from?
Typically, OEM hoses contain a compliant polymeric inner hose to transmit
brake fluid pressure from the brake tubes to the caliper. While the polymeric
tube itself does a good job of withstanding attack from the brake fluid, it must
be protected from the outside world and is consequently wrapped (overmolded)
with a thick, rubber coating. Hollow fasteners at one or both ends of the hose
provide a direct flow path and a leak-free connection system.
So how are Stainless Steel lines different?
Stainless Steel lines (they are actually hoses, but we'll use the common term
“lines” from this point forward in this FAQ) are similar to OEM hoses in
function, but differ greatly in execution. Unlike OEM hoses, SS lines
incorporate a low-compliance Teflon inner hose. In addition, instead of covering
the Teflon with overmolded rubber a woven braid of Stainless Steel strands is
placed over the hose for protection. As with an OEM hose, the ends are
terminated with hollow fasteners to allow for the leak-free passage of brake
So why is that better than the OEM rubber design?
Stainless Steel lines provide a number of benefits as compared to their OEM
rubber overmolded counterparts.
1. The SS braid provides superior protection from flying roadway debris.
2. The SS braid and Teflon hose reduce expansion during pressurization.
3. They provide the race car look.
I understand the protection benefit, but can you explain the reduced
Any time that an object is subjected to internal pressure, it expands. The
amount of expansion will be proportional to the amount of pressure present and
the rigidity of the holding structure. In the case of brake hoses, we are
subjecting Teflon to internal pressures as high as 3000PSI. Because the Teflon
is relatively flexible (which makes it ideal for the job in one regard), it will
expand under these conditions. This expansion creates additional fluid volume in
the hydraulic circuit which is felt by the driver as a soft or mushy pedal.
Rubber overmolding does little to reduce expansion under pressure, as rubber
is also a relatively flexible material. A woven braid of Stainless Steel,
however, can greatly increase the rigidity of the hose under pressure while
still allowing adequate flexibility for wheel end movement. In many cases, this
reduced expansion can be felt by the driver as a firmer or more responsive brake
In addition, the reduced compliance will result in a faster transient
response of the brake system. In other words, the time from the driver hitting
the brake pedal until deceleration is generated will be decreased by a small
amount. The benefit will vary based on each individual application, but in
general overall deceleration can be attained more quickly, resulting in slightly
shorter stopping distances.
What impacts will SS lines have on my vehicle's P-T (pressure vs. torque)
None. Because brake lines and hoses do not affect the torque generated at the
wheel end, the P-T relationship remains unchanged when SS lines are installed.
Only changes to a vehicle's caliper, rotor, or brake pad coefficient of friction
will impact the P-T relationship.
Well then, will SS lines impact my vehicle's P-V (pressure vs. volume)
Absolutely. Because SS lines are much less compliant than their OEM
counterparts, the P-V relationship will be reduced to some degree (less volume
will be required at a given pressure). This is exactly the reason that a car
equipped with SS lines has a firmer brake pedal.
However, because the P-T relationship remains unchanged with SS lines, the
impact to ABS, TCS, and other brake control systems is typically negligible. Our
own BBK kit testing indicates that most ABS, TCS, and other brake control
systems are robust to the small changes affected by the addition of SS lines. On
the other hand, testing at StopTech (and at major OEMs as well) has shown that
while decreases in the P-V relationship typically are invisible to SS lines,
increases in the P-V relationship are not (as would be found with an
In summary, because SS lines and a properly sized and balanced BBK only serve
to reduce the P-V relationship, we have time and time again demonstrated
appropriate system integration with these products. Our in-house testing allows
us to make this statement for every platform we service.
Will I feel a difference on my car if I install SS lines?
The amount of perceived difference will vary by each car's individual design,
age, and usage. Those cars with a significant amount of flexible OEM line or
those that have seen years or use and aging will typically display a more
dramatic improvement in pedal feel than new cars with shorter lines.
What is the difference between lines that are “DOT compliant” and “DOT
The United States Department of Transportation (DOT) has established numerous
standards for automotive components and subsystems. The regulation for brake
hoses happens to be FMVSS106. In this document, anything and everything
pertaining to automotive brake hoses has been laid out in gory detail – at
least, those things important to the federal government.
If a manufacturer claims their SS lines are “DOT compliant”, it means that
their SS lines have passed all FMVSS106 requirements, and they have submitted
the test data to the government for official certification. This does not mean
they are acceptable for use on your car, but it does mean they pass the
government minimum standards.
Another term you may hear in this context is “DOT approved.” However, the DOT
is not in the business of actually approving or disproving compliance – they
don't typically run any tests on aftermarket components themselves. Under these
circumstances, one can only surmise that these manufacturers are trying to state
that their lines are actually “DOT compliant”, but it never hurts to ask before
So, do I need to use only DOT compliant SS lines on my car?
Not necessarily. The DOT requirements must be met in full for official
government approval, so even if a SS line passes every performance test but is
labeled with the wrong type of tag (or something equally trivial) it would fail
certification. While this might mean something to an auto manufacturer or
assembly plant, it is meaningless to the performance enthusiast.
All the DOT compliance means is that the lines have passed a minimum set of
government standards which may or may not be important to you. Does this mean
that DOT compliant lines are the best for your car? Not necessarily, but the
certification should indicate that the manufacturer understands the product and
is trying to hold itself to a certain standard.
Why do some SS lines have a clear plastic covering?
Under certain conditions, dirt and other abrasive contaminants can find their
way between the SS braid and the Teflon inner hose. Over time these contaminants
can be ground into the Teflon line to the point that a leak can develop.
Naturally, a leak in the brake system is never a good thing.
Some manufacturers have taken the extra step to cover the SS braid with a
polymeric coating to prevent contaminants from working their way into the Teflon
liner. While this coating is not necessary for short-term longevity, hoses
without the coating should be inspected and replaced on a more frequent basis.
Why do some SS lines have plastic molded over the end fittings?
Some SS line manufacturers have adopted the practice of molding a semi-rigid
polymer over the fittings at either or both ends of the line. These features act
as a strain relief for the SS braid where the fitting is secured to the line. In
some cases, lines without these features can fail certain dynamic portions of
FMVSS106, as the SS braid can wear itself into the Teflon line where it is
secured to the end fitting.
Do I need to take any special precautions when installing my SS lines?
In general, no. The most important thing to note is that the routing of the
SS line should match either the original stock routing or the instructions
included for a new routing (if applicable). Because the SS braid will eventually
wear through just about anything (once the protective outer layer is worn away),
be sure that there is adequate clearance to all other moving parts under
conditions of full wheel travel and full steering.
It should also be mentioned that after installation care should be taken to
examine your SS line routing to ensure that the line is not stressed when the
wheels are turned to full lock. This is best done with the wheel hanging at full
droop to amplify any routing concerns. Of course the line should never come in
direct contact with any part of the tire, but the line should not be pulled
radially with respect to the overmolded end fittings either.