R&T Tech Tidbits from April...on brakes

[sirwilliam]

http://www.roadandtrack.com/article.asp ... le_id=3406

Tech Tidbits — April 2006
Let's take a brake...

By Dennis Simanaitis, Engineering Editor
April 2006

I've been enjoying Drive On! A Social History of the Motor Car, by the late L.J.K. Setright (reviewed in R&T, April 2005; his death, noted in December 2005). Among the book's wealth of tidbits is a discussion of hydraulic brakes.

In particular, there's the neat etymological aspect. The word "hydraulic" comes from the Greek; it means "water pipe." And, in fact, the operating fluid of one of the first hydraulic brake systems was indeed water.

Braking News, Circa 1920

By the turn of that last century, several inventors had already fooled with brakes actuated by fluid pressure. In 1919, brothers Malcolm and Allan Loughead, apparently tired of people mispronouncing their name, formed the Lockheed Hydraulic Brake Company. By the way, this was about the same time the brothers' Loughead Aircraft Manufacturing Company was going belly-up. Of course, they were destined to do better with brakes as well as aircraft.

These water-actuated brakes were fitted to all four corners of Duesenbergs entered in the 1921 French Grand Prix, held on a 10.7-mile circuit at Le Mans. Power, (relatively) light weight and the hydraulic system's stable retardation ruled the day. Jimmy Murphy's Duesenberg became the first American car to win a major European event. Indeed, it wasn't until Dan Gurney's 1967 victory at Spa that another American won a European Grand Prix in an American car. (Nitpickers, note: The word "European" is there because from 1950 through 1960 the Indy 500 was on the official Grand Prix calendar.)

The Duesenberg Model A, first seen at the 1920 New York Auto Show, first produced in December 1921, had Lockheed hydraulic brakes as well. An apparent concession to climate was their combination of water and alcohol as the operating fluid.

Cable-actuated brakes didn't exactly disappear overnight, however. Amazingly, Ford retained cable actuation until its 1939 models.

So Why Not DOT-6 Water?

Water is certainly less expensive than DOT-specification brake fluids. But the only other thing water has going for it in this application is its essential incompressibility.

Much to water's detriment is its behavior at extremes of temperature. Alcohol eliminates the freezing, but nothing treats the boiling. And, in fact, modern brake fluids are characterized in terms of their wet and dry boiling points.

Glycol or Silicone?

DOT-3 brake fluids are glycol-based. DOT-4 fluids contain borate esters as well. DOT-5.1 fluids have advanced chemistry of similar makeup. All three are related to ethylene-glycol antifreeze. Like this liquid, they can mar a painted surface. Last, all three are hygroscopic ("hygro," Greek: wet, moist), meaning they absorb moisture.

This can lead to two problems: corrosion of brake-system components and, in extreme use, vapor lock as the water reaches its boiling point. Since some moisture absorption is inevitable, corrosion inhibitors are part of brake fluid's formulation. These inhibitors wear out in time, though. And that's why glycol brake fluids, like antifreeze, require periodic replacement. The optimal cycle depends on climate, humidity and other factors, but five years isn't out of line.

DOT 5 brake fluids are silicone-based and are not hygroscopic. Thus, they do not require periodic replacement. They work in a particularly wide range of temperatures as well. Last, they do no damage to painted surfaces.

But silicone fluids too are not without their tradeoffs. Some brake-system components of silicone rubber are attacked by these fluids. What's more, any moisture — and some is inevitable — tends to form globules that seek out a low point in the system. There, these water bubbles can degrade performance even worse than a glycol's absorbed moisture. Last, silicone brake fluids absorb air at a rate three times that of their glycol counterparts. This can lead to a spongy pedal and less than optimal ABS operation.