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The overwhelming cause of brake vibration is a condition called Disc Thickness Variation (hereafter referred to as: DTV).

 

The Final Word on Brake Vibration and “Warped” Rotors

by Clive Murphy

March 24, 2014

 

In simple terms brake vibration is experienced by the driver of a car as a pulsation through the brake
pedal and/or side-to-side vibration of the steering wheel under braking.  The above symptoms can be

frustrating, disconcerting and result in driver complaints about “warped” brake rotors. The phrase
“warped brake rotors” is used almost universally by uninformed vehicle owners to describe the cause of
brake vibration. The actual underlying causes of brake vibration are probably the most misunderstood
aspect of automotive braking.
While it is possible for a brake rotor to “warp” or distort (i.e. change shape), rotor distortion is
responsible for relatively few brake vibration complaints in our experience. The overwhelming cause of
brake vibration is a condition called Disc Thickness Variation (hereafter referred to as: DTV). In other
words the rotor develops a lack of parallelism between its inboard and outboard friction surfaces. We
say, “develops” because even rotors that are machined to the very tightest tolerances during production
can easily develop DTV due to incorrect fitment or driver abuse.
When the brake pads are pressed up against a rotor that has developed DTV they will pass over a
series of thinner and thicker areas on the braking surface of the rotor in rapid succession. Considering
that the brake pads are being pushed against the rotor surface with substantial force they will be forced
down into the ‘cavities’ or thinner areas only to be kicked back violently when reaching the bumps or
‘thicker’ areas. The frequency of this ‘bouncing’ action is typically over 100 Hertz when braking from
speeds of around 80mph. This results in pulses that are transmitted through the caliper pistons, via the
brake fluid to the brake pedal, where the driver experiences them as brake vibration.

 

Causes of DTV

Once you understand the causes of DTV it becomes clear that brake vibration problems can be almost
completely eradicated by following a few simple rules. So, what causes DTV? DTV can be caused by a
number of factors including rotor fitment errors, driver abuse and poor manufacturing processes. The
causes above are mentioned in order of their likelihood of occurrence.

 

DTV Cause No. 1: Failure to ensure that the mounting surface of the brake rotor and hub are
perfectly clean prior to rotor fitment.

 

One of the most common causes of DTV is the failure of the fitter to properly clean the mounting
surface of the rotor and the vehicle’s hub. The smallest fragment of dirt or rust scale caught between
the mating surface of the rotor and hub is magnified as you move towards the outside diameter of the
rotor. This results in an unacceptable amount of axial run-out (inboard-outboard movement) at the
friction surfaces of the brake rotor (the areas of the rotor that come into contact with the brake pads).
To put this into perspective, the maximum axial run-out measurement that Powerbrake™ will accept on
a newly fitted rotor is 0.003 inches (measured in the centre of the friction surface of the rotor). A single
spec of rust scale measuring only 0.002 inches (50 micron) in thickness trapped between the mounting
surface of the rotor and the hub can easily cause the axial run-out value to exceed this tolerance!
Once you have a brake rotor that is fitted with unacceptable run-out you have entered a downward
spiral that will lead to brake vibration and rotor damage over time. Every time you apply your brakes the
pads will first come into contact with the rotor at the point of maximum run-out. In fact, due to the fact
that your brake pads retract only fractionally when you release your brakes, excessive run-out often
causes a light, continuous rubbing of the rotor against the pads at the point of maximum run-out. This
can happen when you’re off the brakes completely.

To understand what happens next we need to briefly discuss two broad categories of brake pads. The
first category of pads relies more heavily on the concept of abrasion to slow your car down. In simple
terms this involves the mechanical gripping and breaking off of opposing pad and rotor materials at a
molecular level. The materials literally wear each other away in the process. The harder material
(typically the rotor) will wear slower than the softer friction material of the brake pad.
The second category of pads relies more heavily on the concept of adhesion. In this case some of the
pad friction material is transferred to across to the contact surface of the rotor, where it forms a thin,
uniform layer of friction material. Essentially you now have the friction material of the pad coming into
contact with identical friction material that has been deposited on the surface of the rotor. Under
braking, the bonds between the friction material of the brake pad and the friction material deposited on
the rotor are constantly breaking and reforming. Material crosses the pad/rotor interface in both
directions as the molecular bonds continually break and reform.
All pad formulations use a combination of both abrasion and adhesion but some pads (such as
common semi-metallic formulations) rely more heavily on abrasion and others (such as many high
temperature fast-road and race formulations) rely far more heavily on adhesion. Abrasive and adhesive
pads affect the formation of DTV in different ways.
Back to our brake rotor that was fitted without the hub and mounting surface of the rotor being carefully
cleaned. Let’s assume that the rust scale on the hub caused the rotor to have a run-out of 0.004 inches.
On many vehicles this will not be felt as brake vibration by the vehicle owner when he collects his
vehicle from the fitment centre. He drives off a happy customer. However, after a typical period of
between 1500 and 3500 miles of driving the brakes begin to develop a vibration, which gets
progressively worse. The vehicle owner complains to the fitment centre, who assumes that the rotors
have “warped”…right…wrong!

Even the best quality rotors money can buy would have developed the same brake vibration. Here’s
why…. As explained above, the rotor was continually coming into contact with the pads at the point of
maximum run-out first. If abrasive pads were being used this area of the rotor would wear down quicker
than the rest of the rotor surface causing DTV, which is felt as an intrusive vibration. If adhesive pads
were being run then the pads would have deposited more material on the initial contact point (the area
of max run-out) than on the rest of the rotor surface. Again this leads to DTV and brake vibration. (See
Figure 1 and 2)

powerbrakefigure1c

 

It gets worse! Once you have the development of DTV the surface of the rotor will begin to heat
unevenly. The high spots will get extremely hot compared to the rest of the rotor. When the temperature
around these high spots reaches 1200 – 1300°F. the cast iron in that area will change structurally and
transform into a material called Cementite. Cementite is far harder than the cast iron of the unaffected
parts of the rotor and will therefore wear considerably less as the rotor wears down with use. Cementite 

also has very poor heat sink properties and will therefore continue to run extremely hot resulting in the
rapid spread of the Cementite formation deeper and deeper into the rotor. As a result the DTV will get
progressively worse with time until it becomes literally unbearable under braking. Depending on the
pads used and the driving style of the vehicle owner this process could take 1500 and 3500 miles to
develop and, no, the initial run-out will not necessarily be felt by the driver immediately. Hence the
confusion and clinging to the concept of brake rotors that “warp”.
The rotor has now entered a destructive spiral that will very quickly become irreversible. Rotor
‘skimming’ will only help if the formation of Cementite has just begun on the rotor surface. Once the
formation of the Cementite has spread deeper into the rotor material, skimming will only remove the
surface layer but the DTV (and vibration) will return as the areas of Cementite wear less than the
surrounding cast iron. If you consider that few mechanics bother to clean the rotor mounting surface or
the inevitable rust scale from the surface of the hub when fitting new brake rotors, the magnitude of the
problem becomes clear. Of course, the exact same thing will occur if a new rotor is fitting to a distorted
hub that is not running true. Distorted hubs are a reasonably common occurrence after a vehicle has
been in an accident but we find that the overwhelming cause of DTV is a dirty hub!

 

DTV Cause No. 2: Improper bedding-in of new rotors and pads.

powerbrakefigure3c

 

After fitting a new set of rotors and pads (or when running a set of used pads on new rotors) it is
absolutely critical that the correct bed-in procedure be followed! It is important that an even, consistent
layer of friction material is transferred from the pads to the rotor surface before the brakes are run at
high temperatures. Most pad manufacturers issue clear bed-in instructions. If not, you need to read our
Brake Tech article #2: Correct bedding-in of new brake rotors and pads. If the flashpoint temperature
(the temperature at the interface between the rotors and pads) gets too hot, before an even layer of
friction material has been established, material transfer will become random and erratic resulting in
uneven material transfer and the resulting formation of DTV. (See Figure 3). We’ve already established
what happens after that…
Apart from causing uneven friction material transfer, failure to follow the correct bed-in procedure can
also lead to distortion (change in shape) of new rotors. The distortion will once again result in axial runout,
the formation of DTV and brake vibration (see article on Distortion). Let’s be clear on this point:
New brake pads and rotors are not like a new performance exhaust system that you can fit to your
vehicle and then immediately go out and drive hard in order to test your new modification. With new
brake rotors and pads you have a very simple choice - Bed them in correctly or you will destroy them
and cost yourself money!

 

DTV Cause No. 3: Fitting the wrong brake pad compound for your driving style.

 

All brake pads have a temperature range that they are designed to operate within. If you exceed that
temperature range, you will set into motion a chain of events that can ruin your pads and rotors. Even if
you took the time to bed your new rotors and pads in correctly and establish an even, consistent layer 

of friction material transfer on the rotor surface, you still need to keep pad and rotor temperatures within
the operating limits of the friction material that you are using.
If you exceed the pads maximum temperature limit the pad will again begin to transfer friction material
in a random and uneven pattern (See Figure 3) leading to formation of DTV and all the resulting long
term hassles. Although this will happen at a far higher temperature than it would have before the pads
and rotors were bedded-in, it will happen nevertheless. The solution is to run a pad compound that is
designed to work optimally at the temperatures that your driving style produces. In other words: You
need to match your brake pads to your driving style.
We don’t want to get into a lengthy discussion about pad compounds, which can get extremely
complex. What we do want to point out is that there is no miracle “All purpose” pad compound that will
offer perfect friction, no noise and smooth braking for all tasks ranging from taking the kids to school to
competing in club-level races on the weekend. If your driving needs are that diverse you will have to
either compromise or invest in more than one set of brake pads and be prepared to change them out
before your weekend club race events.
Generally speaking, pads that are designed for everyday commuting will offer good friction from cold up
to rotor temperatures of around 660°F. They should offer a smooth comfortable brake action and be
relatively noise free. Pads designed for high temperature street use (over 660°F) can be noisier, less
comfortable and often dust more. In some cases their cold friction levels are not optimal. Some of the
better fast-road pads on the market will offer acceptable cold-friction and remain consistent to about
930 – 1020°F. These pads tend to offer slightly poo rer noise, vibration and harshness (NVH)
characteristics. In most cases they will not withstand the temperatures associated with track days and
club race events though.

Beware of scenarios where you fit standard road pads to your wife’s car, not taking into account that
your 18-year-old son has just got his driver’s license and fancies himself as a budding racer. Also
beware of fitting crossover street/track pads to your car and not telling your wife about the reasonably
poor cold-friction characteristics. If you decide to have two sets of pads, then be sure to fit the
crossover pads a week or two before an upcoming club event or track day in order to re-bed the rotors
and replace the old friction deposits with a transfer layer from the performance pads. Failure to do this
will result in poor brake performance on the day and can damage your rotors.
Beware of choosing a brake pad compound based solely on web forum comments. You would not
believe the amount of misinformation and urban legend that we read on a daily basis with regards to
pad compounds. Find out from the manufacturer of the pads whether they suite your intended
application or not and factor this into your decision. As far as price is concerned, you get what you pay
for when it comes to pad compounds. High-performance street, crossover and race pads cost more to
produce than standard road pads do. We strongly suggest spending the money and getting the right
pads for the application! You’ll save money in the long run and enjoy increased safety in the process…

 

Rotor temperatures - rough guidelines:

Normal urban commuting: 175 - 480°F.
Braking from freeway speeds: 390 - 660°F.
Consecutive heavy braking: 570 - 1020°F.
Continuous consecutive heavy braking without sufficient cooling periods: 1020 - 1475°F.
Closed circuit racing with OE brake systems (Production Car Racing): 930 - 1650°F.

 

DTV Cause No. 4: Severe overheating of your brake rotors

powerbrakefigure4c

 

As discussed above, cast iron will transform into a super-hard material called Cementite at
temperatures of around 1200 – 1290°F. We’ve establi shed that Cementite will wear slower than the
surrounding cast iron and will therefore lead to DTV as the rotor wears. What we see in rotors that are
run regularly at temperatures exceeding 1130°F is t hat Cementite formations typically first start to
appear in the areas of the rotor that fall between the underlying cooling vanes of the rotor. This makes
perfect sense, as there is less material in contact with these areas of the rotor to absorb and distribute
the heat. As a result the areas between vanes run hotter and will start to transform earlier than the
areas that have vanes directly beneath them. Cementite formations are often visible as blue/black
areas on the rotor surface. (See Figure 4)
What are the causes of severe rotor overheating? First and foremost - driving style. We have worked
with numerous car enthusiasts and racecar drivers that are fast but do not destroy brakes. On the other
hand we have come across people who manage to reach rotor temperatures of 1470°F on urban roads.
It is very simple to destroy any set of brake rotors. Your brakes convert kinetic energy into heat in order
to slow your vehicle. A medium size sedan will produce enough heat when stopping from 50mph to boil
half a gallon of water in 3 seconds. A typical driver (mixture of urban and freeway driving) will apply
their brakes about once every mile on average, which means that an average brake rotor is expected to
go through this extreme heating and cooling cycle around 60 000 times in its life. If the driver knows
how to use their brakes correctly this will not be a problem.

powerbrakefigure5c

 

Rotors are designed to cool rapidly between brake applications. Typically, after severe (emergency)
applications the rotors will be allowed to cool. The problem arises when the brakes are used hard in
rapid and continuous succession without allowing the rotors enough cooling time between applications.
This creates a compounding of excess heat that will quickly run over the crucial 1130°F mark.
So the message is simple: By all means, use your brakes hard when you need to but you must allow
enough cooling time between applications for the rotor to cool whenever possible.

All Powerbrake™ rotors feature our unique Max Temp Recording (MTR) heat sensitive paint system
that allows our customers to establish the maximum temperature that their rotors are reaching. (See
Figure 5)
The other common cause for severe overheating is sticking (unlubricated or rusted) guidebolts or pins
on calipers. It is absolutely essential to lubricate caliper guide bolts and pins when changing brake
pads. If the guide bolts are damaged or badly rusted they must be replaced. Sticking guide bolts will
cause your brake pads to drag constantly on the rotor, even when you are off the brakes. Even a light
dragging action can overheat and destroy a brake rotor.

 

DTV Cause No. 5: Pad Etching

Whenever possible you should try not to come to a complete stop and leave your foot on the brake
pedal when your rotors are very hot. Of course it is no problem to leave the brakes applied at
intersections etc. during normal urban driving because the brakes are not excessively hot under these
conditions. Rotors will be very hot after heavy consecutive braking or a long brake application from high
speed. Leaving the brake pads clamped to a very hot rotor can lead to ‘pad etching’ or ‘pad welding’ in
which friction material from the pads will be unevenly deposited on the rotor surface at the point of
contact. This can lead to DTV and will often be visible to the eye as the outline of a brake pad on the
surface of the rotor.
Bear in mind that when you stop the rotors no longer have air flow, which means that they will cool far
slower than when you are moving. People who do track days need to pay particular attention to pad
etching. Never pull into the pits and stop with your foot on the brake pedal after a fast lap! This will lead
to pad etching, among other problems. The reason that racecar drivers have cool down laps is to allow
their brakes (and other components) to cool to more moderate temperatures before parking the vehicle.
So, if you have been indulging in some fast-road driving or have braked from high speed on a highway
off-ramp it is not a bad idea to let your car roll back slightly (if there is no traffic behind you) or gently
edge forwards rather than leaving the brakes applied at the intersection. Also, try to slow down and
allow your brakes to cool slightly with moderate driving before arriving home and parking your car.

 

DTV Cause No. 6 – Manufacturing errors

Up to this point all of the causes of DTV that we have covered have been a result of fitment or driver
error. Can DTV be caused by poor manufacturing practices? Yes, manufacturing errors can lead to
DTV. We regularly buy OE and aftermarket rotors and inspect them for manufacturing quality. Although
we do come across rotors that have unacceptable DTV measurements (a lack of parallelism) from the
factory, it is more common to find rotors leave the factory with excessive run-out measurements. A new
rotor with DTV will result in vibration being felt almost immediately after fitment and getting
progressively worse. A new rotor with run-out may only develop DTV and vibration at mileages of
between 1500 and 3500 miles.
The other common manufacturing fault that we see frequently with aftermarket rotors is severe mass
imbalance due to core-shift during the casting process. Of course this leads to balance problems but
more importantly it can lead to differing operating temperatures on the surface of the rotor due to
differences in material thickness and therefore heat sink. This can lead to rotor distortion and uneven
friction material deposits. All Powerbrake™ rotors are inspected for core shift and machine balanced
prior to packaging and shipment.
Powerbrake™ rotors are machined to tolerances up to 3 times tighter than the industry norm in order to
ensure perfect brake function. In addition to this EVERY SINGLE Powerbrake™ rotor is measured in all 

critical dimensions prior to packaging and shipment. Measuring is performed using equipment capable
of measuring accurately in increments of 0.0001 inches (5 microns).
We will never ship a rotor that has unacceptable run-out or DTV… period! This 100% inspection policy
is extremely rare from a rotor manufacturer and is just one of the reasons that we are extremely
confident in our finished product.

 

Rules for avoiding DTV

As mentioned earlier the development of DTV can be almost completely eradicated by following a few
simple rules. Here they are:

- Ensure that both the mounting surface of the rotor (normally the inside of the hat) and the
vehicle’s hub are perfectly clean and free from rust scale or any other form of dirt. The rotor can
be cleaned using a cloth and suitable solvent. The hub should be cleaned using a soft wire
brush or emery paper followed by wiping with a cloth and solvent until perfectly clean.
- Insist that the fitter measures rotor run-out with a dial gauge after fitting each new rotor. This
takes no longer than 5 minutes per side and is definitely worth the effort! If the run-out value
exceeds 0.003 inches the rotor must be removed and the hub re-cleaned. If the problem
persists then the hub should be measured for run-out using the dial gauge. A hub run-out in
excess of 0.0007 inches can be problematic.
- Always bed-in new brake rotors and pads according to manufacturer’s recommendations. This
allows for an even, uniform layer of friction material to be deposited on the rotor surface, creates
a proper wear pattern and avoids rotor distortion. For more information see the Brake Tech
article #2: Correct bedding-in of new brake rotors and pads.
- Select the correct pad compound for your driving style. If necessary have two sets of pads. One
for road use and one for club events and track days. Always bed-in your high-temperature pads
prior to a track day or club event.
- Avoid running your rotors at temperatures over 1130°F. This can be achieved by allowing the
rotors sufficient time to cool between brake applications. Continuous, consecutive heavy braking
in rapid succession destroys rotors and pads. Ensure that the guide bolts and pins in your
calipers are well lubricated, free of rust and moving freely.
- Do not leave your foot on the brake pedal when you stop with extremely hot brake rotors. Slow
down and allow the passing air to cool your brakes to a moderate temperature before parking
the vehicle.

- Buy brake rotors from a manufacturer that checks every single rotor for DTV and run-out prior to
shipment.

Clive Murphy of Powerbrake USA has graciously given us permission to reprint this important article on warped rotors.  We hope you will find it helpful.  Check out clive's website at www.powerbrakeusa.com.

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