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Raybestos Brake Tech School Part Two: Rotor Turning, Lightweight Rotors & Slotted Rotors

“The most neglected piece of equipment in the brake shop is the bench lathe,” said Technical Instructor Dann Ingebritson, Affinia Under Vehicle Group. This opened the second day of the four-day Affinia-Raybestos Brake System Diagnosis & Repair Course, i.e., Foundation Brakes. This course is half classroom-based and half hands-on. It is held at the Affinia-Raybestos headquarters in McHenry, Ill., and is also conducted at locations across the nation. Class sizes run a maximum of 12 because of all the hands-on work.

The Foundation Brakes class covers hydraulic braking theory, brake system dynamics, diagnostics, troubleshooting, repair techniques, preventative maintenance practices, rotor resurfacing techniques, bench and on-car lathe operation, brake (friction) pad materials and noise and dust solutions.

The Foundation Brakes class is not one bit product-oriented. It is not a veiled sales job. It is tech training, and you will get your hands dirty. You will install Raybestos pads, rotors and calipers in this class and Raybestos master cylinders, hoses, lines and valves in other classes. However, brand to brand comparisons are never made, and brand names are seldom mentioned.

Instead, the brake tech school is all about brake jobs, accurate diagnosis, fixing it right the first time and preventing comebacks—not about what brand of pad or rotor is better than another. Police Fleet Manager attended the course and recommends it, without reservation, to all new police fleet managers and maintenance techs, or as a refresher for veteran maintenance techs and new shop superintendents or maintenance supervisors.

Affinia-Raybestos conducts 400 training clinics per year, teaching more than 12,000 techs per year. This article is Part 2 of a multi-part series. For “Part 1: Rotors Don’t Warp,” see the March-April 2010 issue of Police Fleet Manager

or go online to, click Resources, then Article Archives.

Bench Lathe Blues

“I just turned the rotor on the lathe. The rotor doesn’t have any thickness variation or lateral runout.” Not necessarily true! You could have fixed the thickness variation but not the lateral runout! Or you could have fixed the thickness variation but actually introduced excessive lateral runout! The least maintained piece of equipment in the brake shop is the rotor cutting lathe.

The badly neglected bench lathe explains why you still need to dial indicate a mounted rotor, even if you just cut it on the lathe. The lathe cut will remove the thickness variation in the rotor and also the runout (Remember, thickness variation is different from lateral runout). But the lathe cut will only remove the variation and runout if the lathe itself is in spec.

So, when was the last time the lathe was dial indicated to qualify it? That 65-pound truck drum that banged into the arbor last week—could that have bent the arbor? Those mounting cones that are occasionally used to “tap” stuff into place? Those dings in the mounting cone surfaces? The radius cones? The bell clamps? A damaged or poorly maintained lathe may give you a rotor with perfect thickness variation but actually cause a lateral runout! The arbor on the bench lathe can be measured for runout with the same dial indicator gadget.

Lightweight Rotors

Recently, lightweight rotors hit the aftermarket. Watch out for these cheap rotors! They are very different from OE rotors in ways that affect stopping performance! They are certainly not OE-equivalent.

The air gap is the distance between the two plates on a vented rotor. It dissipates and absorbs the heat. The air gap on one OE police rotor measures 0.315 inch. The air gap on a lightweight rotor measures 0.551 inch. This increased air gap can lead to heat-warped rotors, and rotors almost never warp from heat! (See Part 1 of the “Raybestos Brake Tech School” series.)

The larger air gap, but the same overall thickness, means the plates have been made thinner. Thinner plates are more likely to heat-check, are less resistant to heat warping and cannot generally be machined. The mass of the rotor is one of the keys to heat control—and stopping power. This particular GM police rotor weighs 26 pounds. The lightweight, non-OE-equivalent aftermarket rotor weighs 20 pounds.

Rotors act as a conduit to dissipate heat. Overheating of the brake system can cause rotor cracking, heat checking, rotor scorching and uneven pad material wear, in addition to brake fade.

The vane design affects cooling and the harmonic frequency of the rotor, i.e., noise. The vanes on the lightweight rotor are smaller in size. Watch out for cheap rotors! Lightweight is NOT good.

Rotor Balancing

The best way to balance a rotor is to grind away material on the inside of the plates near the internal vanes. All rotors, just like all mounted tires, need balance. Unlike tires and wheels, the rotor is balanced at the factory and not by the brake shop. The cheaper rotors are balanced by clip-on weights or weights hammered between the vanes. The problem is that these weights can come loose and either vibrate (producing brake noise) or come off completely.

By the way, the mill cut goes farther around the circumference of the rotor, not deeper. The better rotors limit the amount of total mill cut. Which is better: a mounted tire with a pair of 6-ounce wheel weights attached to the rim, or one with a single quarter-ounce weight? It’s the same thing with rotors. Less material removed to balance the rotor is better. The need to remove lots of material probably means the rotor has other internal casting problems.

Drilled & Slotted Rotors

Drilled and slotted rotors look cool but are a solution to a non-existent problem. Drilled and slotted rotors, especially slotted rotors, run cooler than standard, internal vented rotors. But cooler running rotors are not a solution to “warped” rotors on police cars.

Police rotors don’t warp like a potato chip from heat. Instead, they wear unevenly. Excessive lateral runout causes thickness variation which results in the brake pedal pulsation attributed to “warped” rotors. A cooler running rotor does not solve the problem of excessive lateral runout. A better quality rotor with less initial runout is half of the solution.

Verifying the as-mounted runout with a dial indicator is the other half of the solution. Drilled and slotted rotors are not any part of the solution. If drilled and slotted rotors happen to work better in police use, it is because the base rotor before drilling and slotting is generally a higher quality rotor to begin with.

Drilled and slotted rotors are not without drawbacks. While offering no advantages, the drilled hole (even if chamfered on the outside) becomes a weak spot. That is where the crack will propagate. Of course, there is no way to chamfer the hole on the inside of the air gap, and the holes cannot go just anywhere. They must avoid the vanes. The trend in high performance rotors, according to Raybestos brake tech instructors, is away from drilling.

Slots are different. They lower the operating temperature of the rotor—the exact contact area between the pad and rotor. The slots give these trapped, hot gases a way to escape from between the pad and rotor. However, the rotor doesn’t need to be slotted to receive the benefit of slots! A slotted brake pad will do the exact same thing. The better brake pads have a deep slot in the pad material. You don’t need the extra expense of slotted rotors, and certainly not drilled and slotted rotors.

Rotor Replacement

One last bit of tech advice: Replace rotors in pairs. Turn or cut rotors in pairs. The mass of the rotor, basically the thickness of the rotor (with an OE air gap), determines how well it absorbs and radiates heat. You want this to be exactly the same side-to-side. Front-to-rear, the rotors are already very different on all cars. However, left-to-right, they need to be identical, i.e., exactly the same thickness.

Published in Police Fleet Manager, May/Jun 2010

Rating : 10.0

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Posted on : Aug 16 at 9:46 PM By Patrick Phelps

I was wording if i have heat spots on my rotors is it better to turn or replace.
Note this is on a Police cruiser.

Rotor Quality

Posted on : Aug 25 at 1:07 PM By Mike DalPezzo

Interesting about drilled and slotted - been hearing the same info on performance car websites. That said, 2001 Chevy Impala 9C1 burned through 3 new sets and 2 trips to the lathe in 38 K miles. The only such vehicle in the fleet of Fords. Picked up drilled/slotted rotors and put the car into an administrators function, and the new rotors went 100K.

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