Putting the Brakes on DTV and Pads
As we have discussed before, DTV (disk thickness variation) is what causes brake shudder. Runout ("warpage") doesn't cause shudder - the caliper is mounted on slide pins, which allow it to move so as to follow the surface of the rotor. You'd need a rotor "warped" to the shape of a Pringle before it started causing shudder.
Different types of friction material have different DTV characteristics. The Australian and Asian markets are obsessed with "organic". These are becoming more common in the USA, although their marketroids are calling them "ceramic" formulations.
With all materials, but particularly organics, DTV is generated while driving off-brake. The caliper is released, but the pads only retract as far as they are knocked off by the rotor. Because of this, the rotor grazes past the pad every revolution - that is, the pad makes light contact with the extremity of the rotor's runout.
Over thousands and millions of revolutions, this grazing pad contact causes rotor wear. Tiny amounts of wear - tens of microns at worst. But because this wear is only on one side, and only on one part of the rotor, it causes a measurable thin section of rotor, where the contact surfaces are slightly closer together than on the rest of the rotor.
When the brakes are applied with ~constant pressure, you get ~constant friction at the pad-rotor interface - except for this thin section of rotor, where there's less contact pressure so less friction. Say hello to brake torque variation and shudder.
In order to not get DTV, you need one of three things: no pad contact off-brake, "corrective" rotor wear on-brak, or low (no?) rotor wear at very low contact pressure.
To get rid of off-brake pad grazing, you can try for zero rotor runout (how flat can you get? Typical OE specs are about 50 microns), or use positive pad retraction (springs and clips) combined with lots of piston retraction (which leads to terrible pedal feel).
Europeans take the approach of correcting DTV, by using abrasive friction materials. Driving off-brake down the Autobahn, you generate a tiny amount of DTV. Then you use the brakes around town, and wear a uniform quarter micron of rotor away, so you end up with flat rotors again.
You also end up with short rotor life (European cars typically need new rotors every second pad change), and dusty wheels (most wheel dust is actually rotor steel, worn off the rotor by the brake pads). Ever noticed how European cars always have dirty black wheels?
Reduction of low-pressure wear is the holy grail of high-friction organic friction materials. Everybody is trying, nobody is there yet.
Older style American materials (semi-mets) also have a different form of DTV, caused by corrosion and material transfer. Particularly in salty winter conditions, the metal in the pads reacts with the metal in the rotor when the car is parked.
Depending on the chemistry, the rotor can be pitted (leading to thin parts of the rotor = DTV), the rotor can get an oxide layer (leading to thick parts of the rotor = DTV), or the pad can actually stick to the rotor and leave some friction material behind when the bond is broken (thick rotor = DTV, sticky friction material on rotor + DTV = shudder like a bastard).
In any case, I've ranted enough. The fact of the matter is - shudder is not caused by "warped rotors". Anyone who claims otherwise is uninformed.
tim
Quote:
Talk of "hardness" of pads is folklore too.
True, we measure compressibility of pads (ie deflection under a predetermined load) - but it has nothing to do with wear. It tends to be a good indicator of noise - highly compressible pads tend to isolate the excitation so it doesn't set up resonances in the system.
Extreme limit of absurdity: take two pads, one a lump of solid copper, one a mixture of sand in a rubber-modified resin.
Without a doubt, the copper pad will be "harder". And without a doubt, the sandpaper pad will chew the rotor out.
It's all a matter of the type of friction used. There are two types: adhesive friction and abrasive friction. Generally, all pads use a bit of both.
This is my interpretation of overhearing materials guys talk about it. May not be entirely accurate, but, blah.
An abrasive friction process is pretty easy to understand. The two surfaces dig into each other (on a micro scale), and absorb energy by pulling each other apart. In a disc brake, the pad eats into the rotor...and tear themselves to bits in doing so. Highly abrasive friction materials tend to wear fairly quickly themselves. Not sure if that's by design or nature. On a macro scale, it's like trying to stop a runaway tractor by dropping a plough onto the ground. It will slow you down, but it will leave a bit of a mess...of the road and the ground.
Typical abrasives in brake pads include various mineral dusts and fibres. Not as coarse or sharp as sand, but similar in concept.
Adhesive friction is the tricky one. It's all about filming. Basically, the pad spreads a film of stuff on the rotor. As a section of film passes under the pad, it (obviously) heats up, and turns into a very viscous liquid. By my visualisation, it's kind of sticky - like trying to slide two surfaces across each other on a thin layer of honey. It will be hard work trying to slide it, but it doesn't scratch the surfaces. And the honey will spread itself out while you slide it.
The hard part about setting up an adhesive friction system is temperature range. If the film isn't liquified enough, it can just rub off, and the pad will get stuck into the rotor. If it's too hot and slippery, it just won't work. So the film needs to be robust across a temperature spread, so it need to be chemically pretty complex. And the abrasives need to be fine enough that they don't go through the film and attack the rotor. From what I've seen, it's pretty easy to maintain a good film coverage at high temperatures...it's just a matter of whether it does anything for friction at high temps, and whether you rip it apart at low temperatures.
Obviously, if you're rubbing a filmy pad against a filmy rotor, you're not going to get into the rotor metal much at all. I'd be guessing that a good multi-purpous friction material would be adhesive for as wide a temperature as possible, but would have enough abrasives to keep working when the film gets too wet and slippery at high temperatures.
By my understanding, sulfur is an important part of friction films. Lead sufide used to be a popular filming agent, until lead got banned. Now I hear a lot about antimony trisulfide from that side of the office. I believe it's good because you get lots of sulfur out of it when it breaks apart, and that's good for film.
The rest of the pad is made up of lubricants (graphite, copper, other metal dusts and chips), fibres to hold the whole thing together (glass, kevlar, rockwool, organic fibres, asbestos ), and fillers (which hopefully don't do much except make up bulk). The distinction isn't always clear - some fibres are abrasive, some fillers have a light lubricant effect, etc..
So, that's a cooks tour of why some pads wear rotors and others don't. High performance, high temperature pads are probably quite abrasive, because you'd have a hard time maintaining a good adhesive film across that wide a range of temperatures.
In terms of hardness...some organic pads can be incompressible like a bastard, but it film up like a champion. I've seen standard wear tests predicting rotor life of up around 1,000,000km with an incompressible material.
Some high-performance European materials are nice and compressible...but will get through a set of rotors in about 120,000km of normal use.
So much for hard pads wearing rotors...
Quote:
My big brake setup on the racecar uses DBA rotors which are far harder than the pads
Uh...here's some late breaking news - your rotors are made of cast iron. Your pads are made of a mixture of softer metals, polymers, minerals fibres, and resins.
No brake pad is harder than a rotor. Even sintered metal aircraft brake pads are softer than a cast iron rotor.
Not to say that there's no difference in hardness and durability between different rotors - that's outside my field, but I don't doubt you can get different rotor life on two different brands of rotor on the same friction material under the same conditions. Whether or not it has anything to do with the material's hardness I don't know.
tim