Automatic Car - driving tips

DaveF327 wrote: »

...and release the bloody footbrake!

Richard53 wrote: »

I'm reminded of one of Viz's Top Tips: motorists! Make your neighbours think you have an expensive automatic car by briefly engaging reverse before you drive away in the mornings.

Lum wrote:

Doing this will transfer a thin lump of pad material to the disc which will then grow over time as future braking attempts keep hitting the leading edge of this lump and transferring material. Eventually you'll be able to feel this as a juddering in the discs which will be diagnosed as warped discs and need a replacement.

Stooby2 wrote: »

That's a new one on me and sounds like nonsense. How does the brake material transfer? If it melted on, the pad wouldn't release with a noticeable crack/bang sound. And why doesn't it wear off immediately on the next brake application, if it's made of the same stuff as the brake pad? I've inspected hundreds of brakes and never heard of or seen anything like this.

Often uneven pad transfer is confused for disc warping. In reality, the majority of brake discs which are diagnosed as "warped" are actually simply the product of uneven transfer of pad material. Uneven pad transfer will often lead to a thickness variation of the disc. When the thicker section of the disc passes between the pads, the pads will move apart and the brake pedal will raise slightly; this is pedal pulsation. The thickness variation can be felt by the driver when it is approximately 0.17 mm or greater (on automobile discs).

This type of thickness variation has many causes, but there are three primary mechanisms which contribute the most to the propagation of disc thickness variations connected to uneven pad transfer. The first is improper selection of brake pads for a given application. Pads which are effective at low temperatures, such as when braking for the first time in cold weather, often are made of materials which decompose unevenly at higher temperatures. This uneven decomposition results in uneven deposition of material onto the brake disc. Another cause of uneven material transfer is improper break in of a pad/disc combination. For proper break in, the disc surface should be refreshed (either by machining the contact surface or by replacing the disc as a whole) every time the pads are changed on a vehicle. Once this is done, the brakes are heavily applied multiple times in succession. This creates a smooth, even interface between the pad and the disc. When this is not done properly the brake pads will see an uneven distribution of stress and heat, resulting in an uneven, seemingly random, deposition of pad material. The third primary mechanism of uneven pad material transfer is known as "pad imprinting." This occurs when the brake pads are heated to the point that the material begins to break-down and transfer to the disc. In a properly broken in brake system (with properly selected pads), this transfer is natural and actually is a major contributor to the braking force generated by the brake pads. However, if the vehicle comes to a stop and the driver continues to apply the brakes, the pads will deposit a layer of material in the shape of the brake pad. This small thickness variation can begin the cycle of uneven pad transfer.

Other than proper break in, as mentioned above, never leave your foot on the brake pedal after you have used the brakes hard. This is not usually a problem on public roads simply because, under normal conditions, the brakes have time to cool before you bring the car to a stop (unless, like me, you live at the bottom of a long steep hill). In any kind of racing, including autocross and "driving days" it is crucial. Regardless of friction material, clamping the pads to a hot stationary disc will result in material transfer and discernible "brake roughness". What is worse, the pad will leave the tell-tale imprint or outline on the disc and your sin will be visible to all and sundry.