Auto Service World
Feature   June 1, 2003   by Jim Anderton

Scratching the Surface

How important is rotor surface finish to stopping performance? More than you might imagine


Disk or drum, ABS or not, stopping is simple: squeeze a rotor or expand inside a drum, and you’re there. For years, the complete brake job has included refinishing the surface that new pads or linings bear against, and lots of efficient machinery has been developed to achieve this aim. Every serious shop has a brake lathe, but how many technicians really consider the effect of the fresh surface that the lathe creates? Norman Abbott, president of Mississauga, Ontario-based OE Quality Friction has performed a series of experiments that demonstrate some surprising properties of disk brakes under different rotor finish conditions.

In the mid ‘Eighties, the friction industry in general, and Norman Abbott in particular, had a problem. Working for a major brake friction supplier to a large Canadian installer chain, a number of Chrysler K-cars experienced very poor braking after installation of his firm’s pads. Applying some basic engineering principles to track down the problem, including instrumenting a new car with accelerometers, Abbott discovered that the rotor finish was crucial to adequate brake performance.

“At the time, they were paying their technicians a bonus for turning rotors”, says Abbott, “and they were taking new rotors out of the box and turning them before putting them on the vehicle. We also found that the brake lathes were in terrible condition. We finished up making a video to explain the importance of surface finish.”

In 1997, Abbott started OE Quality Friction and slowly, the “problems” recurred. Its origin was the Asian import rotor phenomenon, states Abbott: “The Chinese started to make rotors and drum in the mid ‘Eighties, and were flooding the market with cheap product. People were saying ‘I don’t need to turn rotors; it’s just as cheap to put on two new ones’. The problem of rotor finish went away, but when the Chinese began to dominate the market and prices went up, we went back to machining. The final straw was our own GMC cube van. Our local garage turned the rotors, and when I drove it away, it had no brakes at all. It scared me.”

Abbott decided to do something about the re-emergence of the finish problem, and in the process, updated the original ‘Eighties test to prove some interesting facts about rotor finish.

“In the ‘Eighties, because of some raw material issues in both the OE and aftermarket, semi-metallic materials developed a bad reputation for scoring rotors. People started ‘juicing up’ the friction to try to raise the friction level. Poor quality brake jobs were a problem. It turned out that the most important issue was contaminated graphite. Twenty-five years later, people still say that semi metallic pads score rotors, even though that isn’t true anymore.”

Abbott’s research determined that a non-directional finish is key to good stopping performance. But what is a non-directional finish? “A non-directional finish typically features a criss-cross pattern on the rotor surface. A directional finish is the finish that results when you resurface the rotor with a cutting tool. It looks like the grooves in a vinyl record.”

“When the rotor rotates, the non-directional rotor has no dynamic effect on the performance of the pad. With the directional finish, however, the spiral grooves are spiraling either outwards or, on the other side of the vehicle, inwards. The spiral has an effect on the brake pad.”

To test this hypothesis, Abbott used a simple stopping distance test designed to keep all other braking parameters constant while varying the rotor finish. Abbott’s method tested the effect of rotor finish directionality using a 2001 Ford Taurus. To eliminate variables in rotor composition, Ford original equipment rotors were used, chosen from a single manufacturer for consistency. New pads were used for each test, and the ambient environmental conditions were consistent from test to test. Abbott chose a test speed of 30 mph with a pedal force of 15 pounds, measured with a pedal pressure gauge. Why 30 mph?

“40 is too fast when you have to take your eyes of the road,” declares Abbott, adding, “plus the ABS is activated at higher pedal pressures.”

Starting from a steady 30 mph speed and at a consistent 15 pound brake pedal pressure, stopping distance was measured for multiple runs.

A baseline was established using both the original equipment set up as installed at the factory, using OE replacement parts. Results were very consistent, and a baseline of 70 feet was established. New rotors were then turned with two directional finishes, a fine pitch set at .003″, and a coarse .020″ finish. The fine pitch test returned a distance of 105 feet, a 50% increase, while the coarse pitch resulted in a whopping 137 foot stopping distance, double the OE rotor performance. The coarse pitch rotors also generated a loud clicking noise during braking.

“Even the finest directional finish increases stopping distance,” reported Abbott, adding, ” that means that you have to press harder to get the same stopping power.”

To achieve a non-directional finish the rotors were finished after turning with a most un-automotive tool: an orbital sander. The result was a 68-foot stopping distance.

“What this shows is that once the directional finish is removed from the rotor the stopping distance performance of the pads is restored,” concludes Abbott.

But is it the grooves, or their spiral pattern that causes the loss of braking performance? Also, as Abbott asked rhetorically, “What if the rotors are slightly grooved. Should I turn them?” To explore that issue, concentric grooves were machined into another set of new rotors on a toolroom lathe. Why? “Wear grooves caused by pads don’t spiral inwards or outwards like the grooves on a lathe turned finish.”

Surprisingly, the concentric grooves produced no difference in stopping distance compared to the OE finish, leaving one question unanswered: Why does the spiral groove cause excessive stopping distances?

Abbott has a hypothesis:

“The pads engage the grooves of the rotor like the needle in the grooves of a record. It acts like a thread. The pads are forced outwards or inwards, depending on the direction of rotation of the threads. Eventually, the pads are restricted by the caliper. They can go no further; they spring back into their original position, and the process starts over and over again. During the time of spring-back, the pads are not exerting force on the rotor, and slowing down of the vehicle is not taking place during this period.”

Abbott’s theory also accounts for the clicking noise heard with very coarse directional rotor finishes. Other effects, besides longer stopping distances, are excessive pedal effort, pad overheating and glazing as well as the potential for rear-wheel lock-up on non-ABS equipped vehicles.

The results are very suggestive that the rotor finish is crucial to good stopping performance, regardless of the friction material or pad brand chosen. And the correct finish is a non-directional one. As Norman Abbott says simply, “it takes extra time, but it’s worth it.”

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Rotors: Replace or turn?

OE Quality Friction president Norman Abbott recommends the following when servicing brake rotors:

– If the rotors are only slightly scored or grooved, LEAVE THEM ALONE

– If the rotors are badly scored or grooved, REPLACE THEM.

– If you have to turn the rotors, make sure you end up with a NON-DIRECTIONAL surface finish.