Keith Code Posted January 6, 2015 Report Share Posted January 6, 2015 Traction Science Traction limits are hard to reckon for most riders but there are some things to know about it. Traction results from a brew of chemicals the rubber is compounded with, how cleverly the carcass is constructed and shaped, proper inflation, enough tread depth, and maintaining the tire within its optimum temperature range, which varies with different rubber compounds. Heat up a mounted tire to its operating temperature, tilt it over to 45 degrees and apply ever increasing pressure on it. At some point the tire will slip; that amount of load is 101% of the tire's static grip limit. In motion, achieving maximum traction is quite different. As the tire grips it wears. What 'wears out' are the various chemicals, oils, waxes and pigments which bind together the rubber. Abrasion and heat 'cook' them off. You've noticed the bluish-purplish color of a tire from hard cornering, it's called 'blooming'. That is the residue from the chemicals which have been leached out of the tire from heat. It takes very little abrasion to wear it off, maybe a lap. The oily parts—in sufficient quantity to maintain the rubber's flexible and compliant character—support its ability to mate with the road's surface. When they 'cook off', the tire becomes dry and slippery, like dead skin peeling off a sunburn. That sun-cooked layer must be cleaned off to expose fresh skin, or, in this case, fresh rubber. Cleaning it off requires abrasion. The amount of abrasion needed is provided by tire slippage. Tire engineers agree that roughly 15% longitudinal slippage maintains friction value peaks which includes maintaining peak operating temperature. You'd be mistaken to think this 'slippage' is a 'slide': in a corner, the bike is holding its line. It is what is needed to achieve peak traction; considerably less slippage is needed for cleaning it. Depleted rubber must be scrubbed from both tires. There being no power to the front it relies on three forces: 1) slip angle, 2) side grip friction, and 3) abrasion from braking, to uncover fresh rubber. In the steady state part of a corner (after braking and before acceleration) both tires clean up from slip angle and side grip abrasion. Slip angle is interesting. If you were able to freeze the lean and the turned-in front wheel angle you have while going through a corner, then got off and pushed it, the line would be much tighter than when you were riding. The bike's tendency is to always go straight—until some outside force influences it to turn. The turned-in front wheel is that influence—it creates abrasion resistance which forces the bike to go into and hold its arc through the corner. The tires are actually slipping sideways toward the outside, hence, slip angle. The side-slip in skiing is similar. But that's not the whole picture. Camber Force is another factor. Although it has substantially less effect on tire wear, it plays a part in traction. It works like this: On both tires, the outside of the patch (the chicken stripe side) is on a tighter radius than the side that's closest to the tire's center line. Think of a playground merry-go-round. The outside is traveling further in the same amount of time as the inside and therefore going faster than the inside. Conversely, the side of the contact-patch closest to the middle of the corner, is turning slower and is dragging. This creates rubber-cleansing abrasion and also helps the bike stay on its line. (To find more data look up the technical definition of camber thrust or camber force.) In any corner and at any speed sufficient to keep the bike moving and balanced, the tires are always slipping, at least slightly. You wouldn't get through corners or have to replace tires if they didn't. © 2014, Keith Code, all rights... 2 Quote Link to comment Share on other sites More sharing options...
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