Caliper Pistons - Set which way?

tcesar888 · 09-19-2007, 11:31 PM

Hello!

I'm in the middle of a caliper rebuild (F/R) and there is a half moon notch cut out on the pad facing portion of each caliper piston. I read, and re-read the instructions and study the 20deg Diagram, and I'm in dyer need of clarification. BTW, I don't have the 20deg tool...

I understand the importance of setting this correctly, but need a novice/nubie explanation... Below is "my" take...purposefully not using tech lingo...

"...As the wheel/rotor rotates in a forward direction do you want the pad to be "receiving" the rotor in a slightly "open" or slightly "closed" position?..."

It appears, when I review the 20deg Diagram; it looks like the pad is being positioned in a slightly "closed" leading edge position... True? False?

Am I making any sense here? Can someone validate? BETTER YET, is there some cool trick to set this up correctly...

Thanks in advance!

Tom

jbrinkley · 09-20-2007, 02:51 AM

i always thought it was neither open or closed, just perpendicular to the rotor surface, does that make sense?
if you drew straight lines down the rotor face in the direction of travel, the leading edge of the piston would meet the rotor at all points at the same time.
it's been a long time since I looked at a caliper, I could be wrong

tonythetarga · 09-20-2007, 04:27 AM

Look at the tech article on brake caliper rebuilds at the link below. Get a piece of cereal box cardboard and make your own 20 degree tool. Hold the caliper with the bleeders up and the front calipers in their normal position, in front of the rotor. Visualize the rotor turning and angle the piston towards the turning rotor at 20 degrees.

http://www.pelicanparts.com/techarticles/911_caliper_rebuild/911_caliper_rebuild.htm

Bill Verburg · Bill Verburg's Garage

The leading edge wants to be open.
The reason is that the leading edge runs hotter than the trailing edge and the trailing edge has a tendancy to lift on the layer of plasma generated by the leading edge. The leading void allows the pad to rock a tad, keeps them quieter too.

tcesar888 · 09-20-2007, 10:31 AM

Quote:

Originally Posted by Bill Verburg

The leading edge wants to be open.
The reason is that the leading edge runs hotter than the trailing edge and the trailing edge has a tendancy to lift on the layer of plasma generated by the leading edge. The leading void allows the pad to rock a tad, keeps them quieter too.

Hi Bill,

When I look at this diagram it appears that the lower part of the piston notch is "leading" and the higher part is after and below. Wouldn't that make the pad lean down and "toward" (ie: closed) the on-coming rotor; not open?

I'm so sorry for my stupidity here...and I really appreciate your help and patience.

Thanks,

Tom

Bill Verburg · Bill Verburg's Garage

I guess it is a matter of semantics, yes, the pad will be able to rock down to the rotor at the leading edge.

tcesar888 · 09-20-2007, 12:05 PM

Quote:

Originally Posted by Bill Verburg

I guess it is a matter of semantics, yes, the pad will be able to rock down to the rotor at the leading edge.

Thanks Bill,

I just returned to my office after having discussion on this... I have FOUND the light; and have been corrected... OMG! The pad, in my terms anyway, will be slightly "open" on the leading edge of the rotor... After my buddy drew the diagram at a different angle (should have done that in the first place) it answered my question. Basically the lower notch(es) should be pointed toward the on-coming rotor...thus enabling a slightly open setup... Whew!

Thanks again man!

Tom