Going to put new rear pads in my 83 QV. Removed what I believe are the original ATE pads. Noticed the inner and outer pad have different part numbers. Each has a "notch" in the direction of rotation and an arrow pointing up. Not sure what this is for. Also, must the replacement pads be "directional"? One set of pads I examined at the parts store were unidirectional. Searched the posts but could not find an answer. Thanks in advance Image Unavailable, Please Login
You are right - the arrow does go with the direction of rotation. However that does not answer the question of why the ATE pads are directional.
It's a design "tweak" to shift the centroid of the force from the piston to be towards the trailing side of the contact area between the pad and the disc (which does make some sense as the disc can be thought of as being forced into a "V" between the pads and you wouldn't want all the clamping force to be on the edges at the entrance to the V). Other manufacturers do this by grinding a step into the face of the piston (making the contact a "C" shape rather than a ring) and then making sure the piston is clocked correctly into the caliper (although this always hit me as a little iffy if the piston can rotate). So doing it in the pad backing makes more sense to minimize mis-assembly errors, but it still requires correct assembly (and more hassle for the manufacturer to keep track of different part numbers). On the critical scale from 1 to 10, I'd give this about a 2 -- as literally billions of disc brake systems seem to work A-OK without it , and they may have abandoned it as it can cause more pad wear on the trailing side (the side with no relief) vs the leading side (the side with the relief) - do you see any of this effect on your old pads? Can't recall anyone posting that detail before, but most F of the era have already had them replaced.
Shouldn't that be "snake oil"? I think the concept of having less pressure between the pad and the rotor at the entrance makes some engineering sense, but to really accommodate the resulting unequal pad wear would require some sort of very complex mechanical foo to account for the angle between the pads continuously changing over their life -- and complex foo is never a good thing...
Not at all. In fact if you look at multi piston calipers often the leading piston is smaller in diameter than the lagging one. The reason which I have read is that the lagging end of the caliper tends to float due to the accumulation of pad residual and debris from the leading edge of the pad, leading to uneven wear. I didn't believe this myself until I started looking closely at multi-piston calipers, and these it was. And here is a picture of the C type piston used on a 911. Same idea, load the trailing edge of the pad more heavily. Image Unavailable, Please Login
