DeTomaso Mailing List: March 1997, Message #1
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Great info. I switched to 5.1 and have not reached a boiling condition on the track as I did with DOT 4. On Fri, 28 Feb 1997 16:09:04 -0500 (EST) MikeLDrew@aol.com writes: >All, > >Here is an excerpt from an article on disc brakes from the Feb '97 >issue of >Motorcyclist magazine. Although aimed at the biker set, it addresses >all the >questions raised by Steve earlier today. > >Mike (gotta love scanners!) > > > >QUESTION: All those DOT numbers on brake fluid bottles are confusing. >What >do they mean? > >They sure are confusing, and it's partly because they were determined >by a >federal agency (what a surprise). > >Brake fluids are classified by the U.S. Department of Transportation >(DOT) by >temperature range and viscosity. Just about the only ones you'll find >in >motor cycle brake systems are DOT 3, DOT 4, DOT 5 and DOT 5.1. > >DOT 3 and DOT 4 are polyglycol-based and can be mixed with each other. >DOT >5.1 is actually based on a different chemical, but since it can be >mixed with >DOT 3 and DOT 4 it's lumped in with them and all three are commonly >referred >to as polyglycols. DOT 5.1 is also lighter in viscosity-about half >that of >DOT 4- because it was designed for the rapid cycling time of ABS >systems. > >Now here's where the confusion comes in: DOT 5 is a silicone based >fluid. >Regardless of the fact that it's wedged in between DOT 4 and DOT 5.1, >it's >chemically dissimilar to the polyglycols. It will not mix with >polyg lycols, >and should never be added to a system filled with polyglycol fluid. > >There's an added bit of confusion for owners of some European bikes, >whose >master cylinder reservoir caps say "Recommended fluid 3 5," leading >owners to >believe they can use DOT 3, 4 or 5. There are no silicone based fluids >in >common use in Europe, however. There, the "5" designation is for a >polyglycol based fluid, which is what these bikes are designed to >use. > >QUESTION: In addition to the DOT number, there are also two boiling >points >listed. Why two, and what do they mean? > >Each DOT classification specifies two boiling points, one dry and the >other >wet. A fluid's dry boiling point is the temperature at which it turns >to >vapor when the fluid is fresh and uncontaminated by water. The wet >boiling >point is measured when the fluid contains 3.5 percent water. > >Water in the brake fluid is bad because water boils at 212 degrees F, >a very >low temperature by brake standards. When the water in the brake fluid >turns >to vapor, it creates minute pockets of compressible gas in the system, >reducing braking efficiency. > >The dry boiling point is a useful yardstick for choosing a >high quality >fluid. The higher the DOT number, the higher the dry boiling point. >But brake >fluid in a brake system absorbs moisture over time and becomes >contaminated >with water. The longer the fluid is in service, the more important the >wet >boiling point becomes, because few street riders change their brake >fluid at >the recommended intervals. In fact, tests show it only takes a couple >of >years for fresh fluid in a sealed brake system to absorb enough >moisture to >degrade to its wet boiling point. > >QUESTION: How does water get into a sealed brake system? > >Brake fluid is hygroscopic, which means it absorbs moisture. And it >does this >with a vengeance, sucking moisture out of the air in the fluid >reservoir, >through the master cylinder cap gasket, past the caliper seals, even >through >rubber brake hoses. Brake fluid in an opened container does the same >thing, >pulling moisture out of the air at the top of the bottle. To forestall >this, >some brake fluid manufacturers fill the air space in containers of >fresh >fluid with nitrogen instead of air. For that reason, you should avoid >using >fluid from large, previously opened containers of brake fluid. Buy it >in >smaller quantities and cap the container immediately after use. > >QUESTION: What are the pros and cons of silicone based brake fluid? > >Unlike polyglycols, silicone fluid- designated DOT 5-is not >hygroscopic. >Better yet, it doesn't even mix with water. Since silicone fluid >doesn't >absorb water, its wet boiling point is essentially the same as its dry >boiling point. It won't harm paint like polyglycols, either, which is >one >reason it comes standard in Harleys. > >Silicone brake systems can become contaminated with water, however, >even if >silicone fluids can't. Moisture can enter the system through the >master >cylinder vent cap, or after a pressure wash or a ride on a rainy day. >Since >water is the heavier of the two fluids, it sinks to the bottom of the >system, >usually ending up in the caliper. There it sits until the temperature >of the >surrounding fluid reaches 212 degrees F, at which point the water >boils, >forming those pesky pockets of gas that reduce braking. > >Silicone fluid's only real drawback is that as it approaches its >boiling >point, it becomes slightly compressible, resulting in a spongy brake >feel. So >while it works well in bikes that aren't ridden very often-it was in >fact >designed for use in military vehicles that sit for a long time and >then have >to run on a moment's notice-it's not suitable for high performance >applications. > >QUESTION: Can I use racing brake fluid in my streetbike? > >You can, but you shouldn't. Typically, racing brake fluid has an >extremely >high dry boiling point, the better to cope with the heavy braking and >high >heat build up of racing speeds. But racing fluid also tends to be >extremely >hygroscopic, much > >more than street fluids, and usually has a very low wet boiling point, >lower >than comparable street fluids. This isn't a problem for racers, who >change >brake fluid every race-sometimes even between practice and the race. >In a >streetbike, however, racing fluid will rapidly degrade until it's no >better-and sometimes worse than street fluid. > >QUESTION: The fluid level in my master cylinder is down, and I need >to add >some. If I don't know what's in there now, how will I know which type >of >fluid to add? > >The recommended fluid should be indicated on the reservoir cap or in >the >owner's manual. What brand of fluid is in there now, however, can be >anybody's guess. Most brake fluid manufacturers recommend against >mixing >brands of fluid, because some brands contain chemicals that aren't >compatible >with those in other brands. > >As a general rule, you can add fluid of a higher DOT number to a lower >number, but not the other way around. Adding DOT4 to a system full of >DOT3 >adds a better fluid to the existing supply. But adding DOT3 to DOT4 >essentially degrades the system with a fluid of lesser capabilities. >You can >also add DOT5.1 to a system containing more viscous DOT3 or DOT4. The >difference in viscosity will quickly even out as engine vibration and >the >pumping action of the brakes mix the two fluids. > >But don't add anything but DOT5.1 to a system that specifies it. >DOT5.1 is >used in ABS systems, and the viscosity change can affect the ABS's >operation. > >Some riders top off the existing fluid with fresh, high quality fluid, >expecting to improve braking performance. But it doesn't work that >way. By >adding fresh fluid to old, moisture laden fluid, you're only saving >the new >fluid the trouble and time of sucking moisture past the seals. As >soon as >the two mix-which they'll do very quickly-you're right back where you >started. > >Before you top off your fluid reservoir, think about why you need to. >The >more the brake pads wear, the more fluid from the reservoir is drawn >into the >system. If the fluid level in the reservoir is down and the system >isn't >leaking fluid, then the level is down because your brake pads are >worn. A >low fluid level should be an early warning that your pads are reaching >the >end of their useful life. Take the opportunity not only to install >new pads, >but flush out all the old brake fluid with fresh fluid. > >