- #31
Wes Tausend
Gold Member
- 226
- 47
I could have missed it, but I don't believe anyone pointed out that technically, the traction is actually higher when the tire is still stopped (at rest with respect to the pavement). That is called stiction. Once the tire begins to slip, it slips more easily, that is with less torque. That is called kinetic friction. These frictional changes are part of Dry friction. The above lesser kinetic friction-after-start occurs even before the tire rubber begins to liquefy from heat and "lubricate" the contact surfaces. This exceptionally slippery liquid condition is then known as Lubricated friction. All tires, once they are rolling, have less friction than they did parked because there is always a tiny bit of slip inherently included. These processes are why a vehicle can cling to a tilted, critical slope when parked, but speedily slip away immediately once it starts sliding. Of course the tires may soon began to melt too.
The heuristic reasoning for Dry friction is that the rubber and pavement surfaces are generally both bumpy in a macro sense, so at mutual rest it is as though the two "toothed" surfaces are mechanically engaged like fine gears meshing. Once the tire starts slipping, it thereafter begins to ride higher (away from the pavement) as the "gear teeth bounce" from tooth tip to tooth tip rather than fully re-meshing. This sort of bouncing kinetic frictional loss also occurs in a micro sense between two sliding surfaces even if they are deemed to be very smooth (asperity). Therefore starting friction is always more than continuous friction. Even at the most extreme case, the rounded outer electron valance shell of the near-perfect smooth surfaces finally supply unavoidable small, gear-like, electrostatic bumps and dips to each "flat" surface, and these engage the rounded "shell" bumps and dips of the adjacent surface, thereby maintaining the initial gear-like "quasi-mechanical" mesh... until the objects move ever so slightly at different relative speeds. This smooth, near impossible-to-achieve condition, gets down to the nitty-gritty and is along the lines of various van der Waals forces.
A whole new category of friction arises in automotive braking. Certainly the anti-lock brakes help prevent tire skid by delaying heat liquefaction, but the brake material friction also comes into account. In auto road-racing, most brake-pad compounds are especially chosen to avoid gassing when hot. That sort of hot gas can lubricate the brakes similar to any other fluid (all gas is a fluid) and cause extreme brake fade during frequent use. Unfortunately there is a tendency for these low-gassing compounds to not stop well when they are cold, so these super low-fade brakes are largely useless on the streets. This fact is well accentuated by heavy freight train brakes that must not fade from heat while descending long grades, but do not work well at all until they are initially well heated. Consequently the train operators must learn to always apply them 15 or more seconds in advance, taking into account a preliminary heating delay. At 60 miles an hour, with a standard application, such a train will typically go up to a quarter mile before the brakes even begin to work to stop the 30 plus million pounds of freight. A hard application emergency stop is just a little quicker. It still takes some planning ahead, but is still much more user-friendly than stopping a ship or aircraft that are entirely supported by pure fluid.
More friction definitions here.
Wes
The heuristic reasoning for Dry friction is that the rubber and pavement surfaces are generally both bumpy in a macro sense, so at mutual rest it is as though the two "toothed" surfaces are mechanically engaged like fine gears meshing. Once the tire starts slipping, it thereafter begins to ride higher (away from the pavement) as the "gear teeth bounce" from tooth tip to tooth tip rather than fully re-meshing. This sort of bouncing kinetic frictional loss also occurs in a micro sense between two sliding surfaces even if they are deemed to be very smooth (asperity). Therefore starting friction is always more than continuous friction. Even at the most extreme case, the rounded outer electron valance shell of the near-perfect smooth surfaces finally supply unavoidable small, gear-like, electrostatic bumps and dips to each "flat" surface, and these engage the rounded "shell" bumps and dips of the adjacent surface, thereby maintaining the initial gear-like "quasi-mechanical" mesh... until the objects move ever so slightly at different relative speeds. This smooth, near impossible-to-achieve condition, gets down to the nitty-gritty and is along the lines of various van der Waals forces.
A whole new category of friction arises in automotive braking. Certainly the anti-lock brakes help prevent tire skid by delaying heat liquefaction, but the brake material friction also comes into account. In auto road-racing, most brake-pad compounds are especially chosen to avoid gassing when hot. That sort of hot gas can lubricate the brakes similar to any other fluid (all gas is a fluid) and cause extreme brake fade during frequent use. Unfortunately there is a tendency for these low-gassing compounds to not stop well when they are cold, so these super low-fade brakes are largely useless on the streets. This fact is well accentuated by heavy freight train brakes that must not fade from heat while descending long grades, but do not work well at all until they are initially well heated. Consequently the train operators must learn to always apply them 15 or more seconds in advance, taking into account a preliminary heating delay. At 60 miles an hour, with a standard application, such a train will typically go up to a quarter mile before the brakes even begin to work to stop the 30 plus million pounds of freight. A hard application emergency stop is just a little quicker. It still takes some planning ahead, but is still much more user-friendly than stopping a ship or aircraft that are entirely supported by pure fluid.
More friction definitions here.
Wes