<HTML><BODY style="word-wrap: break-word; -khtml-nbsp-mode: space; -khtml-line-break: after-white-space; "><BR><DIV><DIV>On Apr 17, 2007, at 9:30 AM, Steve Fujan wrote:</DIV><BR class="Apple-interchange-newline"><BLOCKQUOTE type="cite"><SPAN class="Apple-style-span" style="border-collapse: separate; border-spacing: 0px 0px; color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; text-align: auto; -khtml-text-decorations-in-effect: none; text-indent: 0px; -apple-text-size-adjust: auto; text-transform: none; orphans: 2; white-space: normal; widows: 2; word-spacing: 0px; ">I asked the "big tire" question way back in engineering school, and the explanation was that when a tire slips on concrete it is not a matter of exceeding a friction factor, it is actually tearing the material. Since larger tires have more surface contact area, there is more material to tear which takes higher forces and results in more traction. This is not the classical "friction" scenario.<SPAN class="Apple-converted-space"> </SPAN><BR></SPAN></BLOCKQUOTE><BR></DIV><DIV>Ok, then obviously, if the rubber is being torn, there is an indication that some sort of friction bond was created between the rubber and the asphalt stronger than the makeup of the rubber material.</DIV><DIV><BR class="khtml-block-placeholder"></DIV><DIV>So then, the screeching we hear is the sound of rubber tearing? Does that mean the singing sound produced by my wet finger massaging the edge of a crystal glass is the sound of skin coming off my finger? No. Certainly there is a degree of actual slippage rather than tearing when you hear the screeching tires. But that would be the part of the equation that is not building a friction bond.</DIV><DIV><BR class="khtml-block-placeholder"></DIV><DIV><BR class="khtml-block-placeholder"></DIV><BLOCKQUOTE type="cite"><DIV style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">Any farmboy or stevedore who has worked a load on a rope is familiar with</DIV><DIV style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">this phenomenon, where more contact equals more friction.</DIV></BLOCKQUOTE><BR><DIV>And any South Georgia redneck understands about tires getting traction on a racetrack.</DIV><DIV><BR class="khtml-block-placeholder"></DIV><DIV>Ahem, or back country road :-)</DIV><DIV><BR class="khtml-block-placeholder"></DIV><DIV>John Delacour wrote:</DIV><DIV><BR class="khtml-block-placeholder"></DIV><BLOCKQUOTE type="cite"><DIV style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">No amount of polishing is going to reduce friction.</DIV></BLOCKQUOTE><DIV><BR class="khtml-block-placeholder"></DIV><DIV>I'm no physicist, nor engineer. But I know a brand new polished string will render across the capo better than the old ratty corroded string all other factors being the same. Do we confuse friction with some other physical force?</DIV><DIV><BR class="khtml-block-placeholder"></DIV><BLOCKQUOTE type="cite"><DIV style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">Big trucks achieve greater stopping power by the application of greater _force_, using compressed air. <BR></DIV></BLOCKQUOTE><DIV><BR class="khtml-block-placeholder"></DIV><DIV>I think air brakes are more a product of function than "greater_force". Air is easily transferred to trailer brakes. Air pressure is also easier to regulate via release valves. I believe my father explained to me years ago that air brakes work backwards from hydraulic brakes. When the air pressure is off air brakes, the brakes are locked. Air pressure is required to release the brake rather than apply it. That is why big trucks have to sit a while after start up. They're not just warming the engine. They can't go anywhere until the air pressure builds. And that's why you won't see a truckless trailer rolling around. The brakes lock the trailer down when it is detached from the truck. But I'm thinking hydraulic systems are actually more powerful or you would see air systems on farm tractors rather than the hydraulic systems used to raise and lower heavy implements. Same would apply to industrial loaders and dump trucks.</DIV><DIV><BR class="khtml-block-placeholder"></DIV><BR><BLOCKQUOTE type="cite"><DIV style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; "> It is the increased normal force that results in greater friction and the parts are made big in order to distribute wear and spread the heat produced (which causes a reduction in the coefficient of friction, and thus brake fade) over a larger area.</DIV><DIV style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; "><BR class="khtml-block-placeholder"></DIV></BLOCKQUOTE><BR><DIV>Actually, brake pads work just like the racing tires. They are made of composite material that "tears" as the surface builds a friction bond with the rotor. It would be the strength of that composition applied over a larger area that would be required to perform the same stopping function as the vehicle produces a higher inertia force. A smaller brake pad would disintegrate in no time.</DIV><DIV><BR class="khtml-block-placeholder"></DIV><BR><BR><DIV> <SPAN class="Apple-style-span" style="border-collapse: separate; border-spacing: 0px 0px; color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; text-align: auto; -khtml-text-decorations-in-effect: none; text-indent: 0px; -apple-text-size-adjust: auto; text-transform: none; orphans: 2; white-space: normal; widows: 2; word-spacing: 0px; "><DIV>Jeff Tanner, RPT</DIV><DIV>University of South Carolina</DIV><DIV><BR class="khtml-block-placeholder"></DIV><BR class="Apple-interchange-newline"></SPAN> </DIV><BR></BODY></HTML>