Monday, November 19, 2007

What's wrong with this picture?

Yesterday evening, I looked into the sky and realized that winter was upon Vermont. I then remembered that I hadn't put on my snow tires. So, without further procrastination, I broke out the torque wrench and got to it. I am a firm believer in snow tires. Having driven through many nor'easters, several ice storms and a blizzard or two, I can vouch for the benefits of knobby snow-gripping treads. All-season radials are good in the sun and serve well in the rain but they are no match for the winter road conditions of the North. I am not alone in this regard. One of my co-workers so loves the sense of security derived from his Winterforce tires that he leaves them on year round.

In fact, it was with childish zeal that he announced during this morning's car-pool commute, "I've been waiting for this all summer!" He then tromped on the gas and blew by a line of squeamish drivers - who seemed surprised by the presence of freezing rain in November. Incidentally, their cars were all shod in summer radials. Due to his savvy wheel-handling and those Winterforce tires we were able to obtain speeds approaching 55 miles-per-hour and, consequently, were almost able to make it to work on time.

Anyway, when ever I have the wheels off, I take a moment to look over my brakes and suspension bits. This is good practice as it may allow for early detection of part failure and, therefore, prevent one from becoming stranded on the roadside during a blizzard.

I found nothing wrong with the ball-joints or tie-rod ends. These pieces all checked out, which is reassuring, since the front-end overhaul I conducted last spring included the replacement of all these items. Looks like I got that project right the first time.

However, I discovered that my rear brakes were not working - at all. Yup, it seems that the rear calipers are no longer squeezing like they ought to. You see, the Saturn has four-wheel, single-piston disc brakes with vented rotors up front and solid rotors aft. This is a relatively typical set up in our modern age - even for economy cars - but in 1994 - the vintage of my Saturn - it was a cut above the rest. Many similarly priced cars from that era, such as my '90 Volkswagen Jetta GL, have disc brakes at the front wheels and drum brakes at the rear. And older vehicles, like my '74 Volkswagen Super Beetle, have drum brakes at each corner. Watching a so equipped auto stop from 65 miles-per-hour can be very amusing - assuming your not in front of it.

At each of the Saturn's wheels there are three primary brake-system components: a rotor, a caliper, and a pair of brake pads.

The rotor is a cast iron disc that bolts to the hub and fits under the wheel.
During the vehicle's operation the rotor turns with the wheel. The rotor is flat on both sides so as to provide two friction surfaces. These are the surfaces that the brake pads contact during braking. The rotor is also designed to disperse heat, because that is what the vehicle's kinetic energy is transformed into when the brake pedal is depressed.

Rotors cope with heat in a number of ways. First, the size of the rotor is very important. In general, the larger the rotor the more energy it can absorb. Think of a pot of water: the more water in the pot, the more energy necessary to bring it to a boil. Of course, the size of the rotor is limited by the size of the vehicle's wheels.

The next way that rotors dissipate heat is through air-cooling. Most modern vehicles are equipped with vented rotors at the front wheels. Some vehicles, usually the sportier models, come standard with vented rotors mounted at all four wheels. Vented rotors feature air-channeling veins between the two friction surfaces of each rotor. The veins move fresh air through the rotor in order to facilitate more rapid brake cooling. To enhance this further, some manufacturers equip their cars with air ducts that re-redirect even more air over the rotors via grill-like openings in the vehicle's bumper.

And then there are performance orientated rotors which come drilled with holes or slotted with groves that - not only allow for better evacuation of vaporized brake pad material - but also provided more surface area for metal to air cooling.

The caliper is a sort of
hydraulically-operated pincher. It has a cast-iron body that houses two brake pads and a hydraulic piston. This piston, known to engineers as a slave piston- no, it is not clad in leather - is tied into a dual-hydraulic system with the other brake pistons and a fifth piston called the master cylinder. When the brake pedal is depressed the master cylinder extends. This creates pressure in the brake system which forces the slave pistons in the calipers to also extend. This action closes the pincher-like caliper and forces the brake pads against the rotor thus creating the friction that slows the car.

The brake pads are the real workers in the system.
Brake pads and shoes were once constructed of asbestos - yikes. Modern brake pads are a composite of metallic and non-asbestos organic materials. These modern composites - which include ingredients such as ceramics, fiberglass, kevlar, graphite, metal chips, phenol-formaldehyde (think Bakelite) and a sundry of other fillers and chemicals - are engineered to absorb heat, generate consistent friction and ware at an acceptable rate. As well, most pads have incorporated into their design a soft metal strip that only comes into contact with the rotor when the pad needs replacement. When the strip makes contact with the rotor it emits a squeaking noise that alerts the driver to service the brakes.

If you look in the pictures above, you will see that the rear rotors are rusty. This means that the brake pads are making little to no contact with the rotor face - which, if you recall, is important if you want to stop. As I said before, when the brake pedal is depressed the caliper squeezes the brake pads against the brake rotor and the resulting friction - not only slows the car - but also removes a thin layer of material from both the pads and the rotor. So, under normal circumstances surface rust can not form on the rotor because the rotor's face is being resurfaced every time the brake is engaged.

Funny, I didn't notice this problem until now. Perhaps, it is because I use my brakes sparingly. Light or infrequent use of a vehicle's brakes can lead to this sort of failure. Under braking the weight of the car shifts forward toward the already nose-heavy front. Therefore, the front brakes are required to do most of the heavy lifting and - especially on a small car like the Saturn - the calipers in the rear do very little work. If calipers are not operated frequently or heavily enough they tend to seize. It's the "use it or loose it" scenario. A scenario, incidentally, that should not be broached while in the company of eunichs.

Now, the Saturn has never been a fast car - it wouldn't go 80 if you dropped it out of a plane - but it has always been able to slow down in a hurry. In fact, I have often taken pride in my vehicles ability to de-accelerate (that's not really a word - the proper term would be negative acceleration - but that sounds geeky.) Obviously, I must remedy the current situation. So, you the reader can look forward to a tedious and long winded DIY on rear brakes.

- update - December 15, 2007 - In the spirit of fixing things on the cheap, I seemed to have gotten my rear brakes to work again by merely exercising them. For the last few weeks I have been a bit heavy on the brake pedal - not to mention the numerous e-brake turns in the snow last night - and, as a result, the rear calipers have loosened up. Now, with the thermometer reading below zero and the salt thickening on the roads, this might only be a temporary fix.

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