You've just spent a considerable amount of money having the braking system on your vehicle overhauled. So why, when you apply them, don't they have the same reassuring bite they had when new?

How the disc brake rotors were "turned" or machined to true them (make them perfectly flat on each side) and remove wear ridges to create a smooth finish could be the problem.

Norman Abbott, president and owner of brake pad manufacturer OE Quality Friction Inc. in Mississauga, Ont., says improper brake rotor can double stopping distance.

Brake rotors are the cast-iron discs to which a caliper clamps friction material, the brake pads, using hydraulic pressure generated by a foot on the brake pedal (most often assisted by a pressure-boosting system).

They work by generating friction between the pads and the rotor (which rotates with the wheel) and dissipating the resulting heat. This is a pretty brutal process, with the disc and pad both taking a beating every time they're used.

The pads obviously wear out with time and so do the rotors, often developing pronounced ridges. But rotors can also warp due to the heat generated and develop low and high spots (as little as three-thousandths of an inch can cause trouble). These problems are usually conveyed to the driver as noises, squeaks, grinding, groaning. Or by feel, pulsations or harshness felt through the pedal.

Most brake rotors can be resurfaced at least once, and it is how this is done that has Abbott concerned.

The process is simple. The rotor is set up in a lathe with a cutting tool on either side and rotated to remove material. A fast, coarse cut is often used first, usually followed by a slower, finer cut.

The tools start at the centre and work out to the edge and in doing so create a directional spiral cut much like that on an old vinyl record. This is what creates the problem.

New brake rotors come with a non-directional cross-hatch or swirl, rather than spiral finish. And this can, and should, be applied by the brake repair shop, but often isn't, Abbott says. (Three shops I checked said they didn't do it as a matter of course, only when brake squeal was a problem.)

Abbott's tests were conducted using a Ford Taurus with factory-fresh rotors and pads, a speed of 30 miles an hour and a low, 15-pound brake-pedal pressure.

Using this setup, a baseline stopping distance of 70 feet was established. Then, the rotors were replaced with a set turned with fine and then coarse directional cuts. With the fine cut, stopping distance stretched to 105 feet, and with the coarse to 137 feet, almost double the distance.

With these rotor finishes you would have to press the brake pedal much harder to get the same stopping distance. This can result in overheated and glazed pads.

When the directional finish was removed from the discs with an orbital sander, the stopping distance was restored.

Abbott's theory is the brake pads engage the spiral grooves machined into a disc much like the needle of a phonograph on a record. They act like a screw thread, engaging the pads and forcing them inward or outward.

When they can go no further, they snap back into their original position, but while doing this they momentarily aren't pressing against the rotor and slowing the vehicle. This may also account for the clicking noise brakes can make.

As a result of his tests, Abbott believes rotor finish is critical to good brake performance and that only a non-directional finish can deliver that optimal level of performance.

Abbott also had concentric grooves machined into a set of discs to simulate wear and found this didn't affect the stopping distance.

Abbott recommends that if there is only slight scoring on the rotor fitting, new brake pads are all that is required to restore brake performance. Most of us obviously won't be able to make this judgment call ourselves, so we'll have to rely on a trained technician.

If the rotor finish is deeply scored, they need to be replaced or resurfaced, but he says a repair shop must take the time to do the job correctly, using the coarse setting to remove defects, the fine setting to prepare the surface and then apply a non-directional finish using an orbital sander or a swirl finish attachment for the lathe.

The brake pads Abbott produces may seem like one of the lowest-tech parts on a vehicle, but they're actually rather sophisticated devices. They have to work effectively and consistently within an interior temperature range from well-below zero to more than 50 degrees. (We, quite rightly, expect our cars to stop consistently no matter what the ambient temperature.) They also have to withstand operating temperatures in excess of 250 degrees and are expected to last 50,000 kilometres or more.

The pressure is currently on pad producers to increase the automotive consumer's "comfort," with the braking system by finding ways to reduce unwanted noise, vibration and harshness.

Everybody hates squealing brakes. But a growing problem is that annoying black brake dust that fouls your flashy wheels. Most of this is predominately made up of material worn away from the iron rotor, not the pad.

Not my fault, says pad producer Abbott.