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Terry Haines

Warped Rotors..not so fast!!!

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I thought I'd pin this part of the other post for future reference for all...

 

 

..this is worth a read,written by Dave Mann, it explains why most who fit new rotors still have vibration!!

 

..AUTOMOTIVE BRAKING SYSTEMS- ROUGHNESS AND VIBRATION

 

One of the most common brake system customer vehicle performance complaints is brake

roughness, pulsation and vibration. As common as this complaint is very few people, including

many service technicians, actually understand the true cause of it and what to do to correct it

permanently. This leads to misdiagnosis, unnecessary repairs and parts replaced and the likely

re-occurrence of the problem a short time later. Brake roughness can be defined as vibration that

is felt in the steering wheel, brake pedal and/or seat during vehicle braking. This does not

include the normal pulsation that occurs when anti-lock brakes are activated during a panic stop

or when on wet, snow or ice covered roads, which can activate the anti-lock brakes.

What typically happens is the customer experiences this type of vibration and pulsation and

thinks something is wrong with their brakes and then takes their vehicle into a repair facility.

The customer explains the problem and the repair facility technician or service writer either takes

the vehicle out for a test drive to confirm the symptoms or pulls it into the shop and starts right in

on the brakes. Then they come out to the customer and inform them that their brake rotors are

"warped" and they need new rotors and pads (and sometimes hubs), or they can machine the

rotors on their bench lathe if there is enough rotor material remaining to meet the manufacturers

minimum thickness requirements.

The problem with this diagnosis and repair procedure is that first of all is that brake rotors do

not warp. The second problem is that replacing a brake rotor with a new brake rotor or

machining the rotor on a bench lathe will only fix the problem temporary. The problem will

almost always re-occur after a period of time, thus necessitating further repairs. Brake rotor disc

thickness variation or excessive lateral runout, as well as drums that are out of round can cause

vibrations and pulsations in the brake pedal and/or steering wheel. Brake lining material transfer

onto the rotor can also have an effect on this as well.

 

Here's what really occurs: all brake rotors and hubs have an associated Lateral Runout (LRO).

LRO occurs when two axes are not parallel to each other, such as the axes of the rotor and the

hub or the spindle and the rotor/hub. LRO may be caused by manufacturing tolerances,

improperly torqued wheel nuts (uneven or excessive torque), corrosion between the brake rotor

and the hub, hub with excessive runout, worn or improperly adjusted wheel bearings or any

damage or wear. This is what is commonly referred to as "warped" rotors. These so called

"warped" rotors do not in and of themselves cause the vibrations and pulsations. Any machined

component, such as brake rotors and hubs, are going to have manufacturing tolerances, which

include runout. Typical original equipment new rotor runout specifications are in the range of

0.0015-0.002 in. while low quality aftermarket rotors can be significantly higher. In addition to

excessive manufacturing tolerances, cheap, low quality aftermarket rotors can have increased

impurities and porosity in the metallurgy. I recommend using either the OEM rotors or a high

quality aftermarket rotor.

New rotors and hubs are machined to precision tolerances from the auto manufacturers.

Aftermarket rotors and hubs are usually not machined to the same tolerances, as the aftermarket

manufacturers do not know the OEM specifications, although some are much better than others.

 

 

 

Auto manufacturers will either match mount the rotor and hubs or machine the rotor on the hub

unit as an assembly. Match mounting is matching up the low spot on the brake rotor with the

high spot on the hub. This match mounting process minimizes the runout ofthe assembled

components, but is only a production process. A service technician cannot effectively determine

where the high and low spots are in order to match' mount the components. Machining the rotor

on the hub (and/or spindle) with the proper on-vehicle machining equipment is the very best

method and almost completely eliminates runout. But, if they do not effectively prevent rust and

corrosion in the joint, over a period of time it will induce runout and eventually brake roughness.

What can occur over a period of time is that whatever runout is in the system coupled with

improperly torqued wheel nuts and/or misadjusted or loose wheel bearings and rust and

corrosion forming between the rotor and hub surface leads to increases in runout. As you drive

your vehicle without using the brakes, such as on the highway, every rotation of the rotor high

spot or multiple high spots contacts your brake linings in the caliper, even when you are not

using the brakes and wears the high spot or multiple high spots offthe rotor which causes a thin

spot or multiple thin spots. Over a period of time this repeated process causes what is called

Disc Thickness Variation (DTV). DTV is when the rotor thickness is not the same all the way

around the rotor. DTV is typically caused by lateral runout. DTV can only be measured with

very specialized laboratory testing equipment or with special on vehicle capacitance probes.

When you apply your brakes, and a brake rotor has DTV, the thick and thin spots on the rotor

cause the brake pads to move in and out. This in-and-out pad motion causes increases and

decreases in brake system pressure, which the driver can feel in the brake pedal. This in-and-out

pad motion causes a varying brake force, which is passed to the steering wheel. As the rotor gets

hot, it is much more likely to increase thickness variation, thus increasing pedal pulsations as

well as steering wheel and other vehicle vibrations. This phenomenon is what many technicians

refer to as "warping", however they actually think the rotor warped and needs replacement.

Typical acceptable values for DTV are around 0.0004 in. That's 4 ten thousandth\9! an inch! As

DTV increases beyond 0.0004 in., brake pedal, steering wheel and vehicle vibrations and

pulsations will almost always occur.

Replacing a rotor with excessive DTV with another new rotor will only correct the problem

temporarily, because eventually the associated LRO with the new rotor will lead to DTV over a

period of time, and the problem repeats itself all over again. Machining a rotor with excessive

DTV on a bench lathe will only temporarily correct the problem because the rotor is being

machined true to the bench lathes spindle and not the spindle on the car, plus the spindle on the

bench lathe has its own runout.

The correct method is to machine the brake rotor on the vehicle using an on-vehicle brake lathe.

For applications where the rotor is separate from the hub (loose rotor) make certain that both

surfaces are free of rust and corrosion and be sure to put a thin layer of nickel anti-seize on the

mating surfaces. This will prevent rust induced increases in LRO over a period of time. Rust can

form in-between the mating surfaces and exert tremendous forces which will cause increases in I

tension of the wheel studs and resulting clamp load variation which creates LRO. The LRO will

then eventually lead to DTV, brake roughness and vibration. For vehicles with adjustable wheel

bearings, be certain to properly adjust the bearing preload prior to machining the rotor.

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When I ran a Ford and Lincoln Mercury Dealership in the 90s we had to use this machine. Technicians hated it since it required a few more settings to be precise and that made the job slower than what they were used to. I liked it because it worked well.

 

The only problem with the on car lathe was they didn't remove the rotor to check for rust underneath.

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As a matter of course, I always check the torque on lugs nuts after the tire monkeys rotate my daughter's tires at Sears. Sometimes they're close, other times I've had to stand on a breaker bar to get them down into the range of my torque wrench. They always claim to use the torque limiting sockets, but the micro-adjusting torque wrench doesn't lie.

 

In any case, the rotors often flop loose when the wheels are removed and here in Michigan that means flakes of rust can lodge between the hub and rotor.

 

Great reading... thanks!

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I insist tire shops put wheel lug nuts on with torque wrench, no impact. They go way past the torque spec with the impact wrench, then put torque wrench on and it clicks without turning nut. A cracked stud is just as dangerous or worse that loose nut.

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I might add that back in the day of SSS - Showroom Stock Sedan, we all ran strickly organic pads on our front disc brakes. The thinking, at the time, was that the absence of metallic particles in the pads reduced the rotor temps. The organic pads were sacrificial, but that was the cost we pad for not having our rotors develop hot spots, and subsequent lateral runout issues and warping. I realize the technology in the area of braking is vastly different today than it was 40 years ago, but we managed, for the most part, to "keep it between the ditches" :nono:

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