Posted by Ben on June 06, 19100 at 09:58:54:
In Reply to: Brake squeak question posted by KSHEEHAN on June 06, 19100 at 03:05:06:
It is my understanding that the squeal is from the friction material and the cast iron vibrating due to the friction material being consumed. Consumed as in turning into dust and/or vapor. Most of the vapor is from the softer binders in the friction material, though there is metal being boiled off. The dust & vapor act as a lubricant between the friction material and the cast iron. Cast iron is used becasue it has the best coefficient of friction over a certain temp range over other metals (seeing stainless rotors on the market and wonder what their coefficient of friction vs cast iron is...assume it has higher ultimate temp though).
The vapor will cool, phase change back to a solid, depositing onto both the friction material and the cast iron, creating a LESSOR coefficient of friction surfaces. This is called glazing.
Not all of the friction material or the cast iron is of the exact same coefficient of friction from one spot to another. Nor is the surface completely flat, there are hills and valleys.
As the two materials scrap each other off or cause the softer material to vaporize, there is a shock wave that resonates throughout the two. The cast iron is of greater mass and is dampened by that mass. Shoes are better attached to the backing plate (various springs, bolts, and such, where disc pads are only inside of a "slot" with no other holding mechanism and "floats" in that slot), shoe brakes don't squeal as a norm, though they do in certain instances.
There are many arguements on exactly which one thing causes squeal. I think they all do.
There are various materials on the market to solve this noise problem. From spray on plastic to shims. Both applied to the back side of the pads (metal side against the piston surface or caliper casting). The shims work best, as any plastic will wear faster and/or be affected by the heat generated by the brakes. Shims with a "coining" work best of the metal shims. That gives them a "spring" or "preload" to the disc pads. This preload absorbs the vibration and/or not allowing the pad to vibrate at all. Many throw away those shims when they put in new pads, as it's sometimes very difficult to get that pack back into the caliper. I recommend saving all of the shims you come across and use them the next time if the new pads don't come with a set (careful not to put too much into that "stack" as you might cause too much initial load on the rotor).
With the move to harder semi-metallic pads, with more metal, there is an increase of potential. The older, softer materials did not squeal as much.
-Ben
PS With all this being said, I've not figured out why the local buses squeal load enough to drive fillings out of your teeth. Those bus brakes are HUGE shoe/drum brakes?!? Do know that they are semi-metalic. Then again, the 18 wheelers don't squeal and they have even larger dia drum/shoe brakes...