Rust bucket! Extending the life of brake rotors.April 5, 2021
When it comes to disc brakes found on your farm vehicles, trucks, trailers, and tractors, you need to understand the term, brake area swept. It identifies the brake rotor’s surface that the pad (friction material) comes in contact with.
Contrary to what you would assume, not all disc brake systems have the pad interface with the entire brake rotor surface.
The efficacy of a disc brake system is rooted in more than the swept area of the rotor. It is the culmination of the hydraulic pressure applied to the caliper piston and its diameter, the compound of the pad along with the metallurgy of the rotor, and the stiffness of the caliper mounting. These are the major design elements for disc brake performance.
Thus, exceptional braking performance can be gleaned while leaving a portion of the rotor’s surface to be a stranger to the brake pad.
This now begs the question, why would some of the rotor area be left unused?
The main reason is heat dissipation. The non-contact area of the rotor acts as a heat sink. It pulls heat from the swept area and, in turn, decreases potential warpage and, in theory, under severe braking conditions, limits the thermal excursion of the caliper piston and brake fluid.
If the brake fluid were to boil, it takes on a gaseous state and will not function as a hydraulic fluid.
As with most things, there is a compromise.
By leaving some of the brake rotor area un-swept, that region is prone to build rust. The rust can get so robust that when the brakes are applied, you are greeted with a grinding sound like that of metal-on-metal.
What is happening?
During the design phase of disc brake development for any application, a predicted life expectancy for the friction material and rotor is assigned.
In most engineering models, it is determined that the brakes will be renewed before the rust in the un-swept area can build to such an extent that it is an issue.
Suppose you are easy on the brakes or live in a rural area where the number of stops is limited. In that case, the rotors will become problematic long before their useful life is reached.
In most cases, if the brakes are applied gently to scrub speed, such as going down a long grade, the un-swept area will work as intended, pulling heat from the friction surface. This will cause the rusted region to expand and encounter the brake pad and grind.
Once the brakes are released, the rotor will still be expanded until it can cool and shrink. The grinding noise will continue and decrease over a period. If the brakes are applied again during the cool-down phase, the grinding will increase.
You now need to decide on your options.
- The rotors can be removed and cut if you can find a shop to turn them on a lathe properly (just enough to remove the rust).
- You can live with the noise.
- You can replace the rotors only to have it occur again in the future.
Rust is like a deep-rooted weed that has runners. It will eventually migrate into the swept area from under the surface. The rotor that is still in specification cannot be cut and needs to be replaced.
Not only is this an unnecessary expense, but your time is involved, even if you have a shop in town do the job.
With this established, there needs to be a better way.
The first step is to keep an eye on the un-swept area, and as soon as you can catch your fingernail on the rust from the swept region, make your move.
Remove the rotors and, using a wire wheel, clean off the rust as best as you can. I then use good metal paint applied with a brush to the un-swept areas.
You do not need to be overly careful with the paint application. If it drips or you apply paint in the swept region, the first one or two brake uses will remove it.
When it comes time to replace the pads and rotors, just estimate the un-swept area from the old rotors and paint the new ones’ surface.
Put anti-seize on the area where the rotor marries to the hub and the wheel to the rotor, along with the studs for the lug nuts. You will have no trouble removing the wheel if you get a flat tire, and the next brake job will be a breeze.
At first blush, you may think this idea is overkill. With the price of parts today and your time being at a premium, the steps provided here will provide the most significant ROI for anything on the farm. Think about it.