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So, the message is clear. Always bed-in rotors correctly prior to hard driving. This will relieve internal stresses within the rotor and ensure maximum resistance to distortion.

 

Rotor Distortion or ‘Warping’

by Clive Murphy

March 24, 2014

 

Are warped brake rotors the main cause of brake vibration?

 

Absolutely not!  The overwhelming majority of brake vibration complaints are caused by a condition

called Rotor Thickness Variation (DTV). In fact brake rotors actually warp or distort rather seldom.
Having said that, distortion can occur and it is worthwhile understanding the causes of distorted brake
rotors.

Let’s start off with a basic definition for a “warped” (hereafter referred to as “distorted”) brake rotor:
A brake rotor is considered distorted if, during operation or as a result of operation, it changes shape
leading to unacceptable axial runout of the friction surfaces of the rotor in relation to the caliper and
brake pads.
This condition may or may not be felt as a brake vibration of varying levels of intensity.
In other words we are ruling out rotors that have unacceptable runout measurements, prior to use,
when they leave the factory. Runout that results from poor manufacturing processes will lead to DTV
and brake vibration but did not result from use/abuse of the rotor and can therefore not be considered a
result of rotor distortion.

 

Permanent distortion

A rotor is considered to have permanent distortion when the change in shape and resulting runout
remain after the rotor has returned to ambient temperature (cooled completely). In other words it is a
permanent condition and will normally be felt even on the initial brake application when rotor are cool
and is referred to as – “Cold Vibration”.
In our experience the most common cause of rotor distortion is the failure of drivers to ‘season’ or
stress-relieve new rotors by following recommended bed-in procedures. During the rotor casting
process molten iron is poured into sand moulds at temperatures in excess of 2450°F and allowed to
cool rapidly into solid form. This rapid cooling causes internal stresses to be ‘stored’ within the rotor
casting. Gradual and moderate thermal cycling (as encountered when bedding-in rotors properly after
fitment) is a very effective method of relieving these stresses. On the other hand - sudden, dramatic
heating and cooling of un-seasoned rotors can lead to distortion or cracking.
So, the message is clear. Always bed-in rotors correctly prior to hard driving. This will relieve internal
stresses within the rotor and ensure maximum resistance to distortion. (To read more about the correct
bedding-in of new rotors click here). Powerbrake™ rotors all undergo a series of stress-relieving
processes during manufacture. Extensive testing has shown that these processes substantially reduce
the chances of distortion and cracking. Having said that, we are the first to point out that correct bed-in
has numerous advantages and must be completed before any hard driving is attempted.

 

Temporary distortion

A rotor can also exhibit temporary distortion that occurs when the rotor changes shape during
operation, resulting in temporary runout, which then disappears after the rotor has returned to ambient 

temperature (cooled completely). In other words it is a temporary condition and will normally be felt only
while braking when rotors are running hot. It is therefore referred to as “Hot Vibration”.
Hot vibration seems to be most prevalent in rotors that have a wide (or tall) friction surface (or pad
track). It is believed that the differences in heating rates (and the resulting expansion rates) that are
caused by substantial differences in rubbing speeds experienced at the inside and outside diameters of
the rotor friction surfaces can lead to a condition of operational distortion or “Hot vibration”. This cause
seems to be particularly prevalent on many German luxury sedans that feature rotors with wide pad
tracks and tall pad profile’s. The solution often lies in the choice of pad compound that is used in these
applications. We recommend staying with a premium quality low-met or semi-met pads if this problem
recurs on our car. The higher metal content of these friction materials tends to pull more heat out of the
rotor, hereby lowering rotor temperatures and limiting the causes of “Hot vibration”. Hi-friction NAO
pads with very low metal contents will leave more heat in the rotors, possibly making the situation
worse.
Another common cause of temporary distortion is uneven material thickness of the friction surfaces of
the rotor. This occurs as a result of core shift during the casting process. In other words, while the outer
surfaces of the rotor run true from the factory, they end up expanding at different rates during operation
as a result of the different thickness of the rotor at different points. This results in temporary distortion of
the rotor when hot or “Hot vibration”. The cooling vanes of the rotor make it impossible to correct the
thickness variances through machining and the rotor must be replaced.

Finally, you should never leave the brake pedal of a vehicle applied after bring the vehicle to a standstill
when the rotors are very hot. The area in contact with the friction material will not be allowed to cool as
quickly as the rest of the rotor surface resulting in definite temporary and possible permanent distortion.
This is particularly relevant to drivers taking part in track days. You will often notice that race teams will
push a racecar backwards and forwards in the pits for a few minutes after the vehicle comes off the
track. This is to allow the rotors (and other components) to rotate and cool more evenly.

 

Distortion leads to DTV

The most important point to accept is that rotor distortion, whether temporary or permanent, will
eventually lead to DTV. DTV is the main cause of severe brake vibration and results in a downward
spiral that will damage a brake rotor beyond repair.

 

Tips for avoiding rotor distortion:

- Buy premium quality rotors that are measured in all critical dimensions prior to shipping.
- Buy rotors that are stress-relieved by the manufacturer.
- Always follow the manufacturer’s bed-in instructions before driving hard.
- Be careful of high-friction NAO pads on applications using “wide pad track” rotors.
- Slow down and let your rotors cool before parking the vehicle.
- Drivers taking part in track days must do a cool down lap prior to leaving the track.
- Never wash your car or spray cool water onto your rotors when they are hot.

 

(I watched a fellow participant at a track day at Sebring park his Vette on the pit wall to jump in for a ride with his buddy.  While sitting there, we heard two loud metallic pings one after the other.  Those were the sounds of his rotors cracking.  Clive makes an important point- always cool down.  Also, do not apply the parking brake!- Ed.)

Clive Murphy of Powerbrake USA has graciously allowed us to reprint this excellent article on rotor warping.  We hope this is helpful to you. Check out Clive's website at www.powerbrakeusa.com.

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