Pete Millikin wrote:Wow Dan, did you stay in a Holiday Inn last night? Keep it up and we'll start calling you Cliff Claven
I gaining the wrong reputation....
Pete Millikin wrote:So answer me this. Traction in the afternoon sessions is always lower than in the morning. My assumption is that the rising asphalt temperature reduces friction and is the overriding factor in lowering traction despite all the run sessions with cars laying down rubber. The higher surface temp heats tires faster beyond their optimal traction temp. Please submit an explanation citing appropriate scientific support.
Well, now Pete, you ask a great age-old question:
why do I loose traction in the warm afternoons? Some thoughts:
1) Although some minute sand particles may be present on a morning track, for the most part there is very little "veneer" on the course. After a lap or two your tires are hot, the asphalt is somewhat cool, and the relationship between tire-to-road is relatively clean, allowing the softer component of the warm tire to squeeze into the "pours" of the cool asphalt on a microscopic level, and give you really secure-feeling grip. With minimal rubber "laid down" on the course, the relative pourosity of the AM track is high, and cool. Result: lots 'o' grip.
2) As the day wears on, and the temp climbs, two things happens First: a veneer of rubber from lots of hot tires on the same line gets laid down on the asphalt, and squeezed into the micro-pours that were present in the AM. So, the pourosity of the course changes(diminishes) as the day goes by, and the opportunity for a "clean" relationship gets corrupted by all that soft compound rubber being laid down under heat and pressure... and the tire compounds, by the way, are made of similar hydrocarbon compounds as bitumen, the asphalt binder (oh geeze ... not that bitumen thing again
). Second: the data about rubber compound to rubber compound traction coefficients as it might exist on asphalt is still being looked at. But my guess is warn-to-hot tires to asphalt (gravels and bitumen) is a better grip than hot rubber to warm rubber - as in afternoon track conditions. (Afterall, of you take two like compounds and rub them together, their specific hardnesses will tend to abrade each other evenly, and generate a zone of a friction-induced slip plane and heat. Add a little more heat to that equation [climbing temeratures and sun], and more slip-plane development could occur. Conversely, if you take two compounds with dissimilar hardnesses, one surface will be abraded rapidly while the other stays static. Heat will only cause more loss of the softer compound.)
So, here's a whacky thought: drive different lines AM and PM and see if your traction coefficient changes, is the same, or non-detectable.
Finally, there may be a "polishing" effect of ashalt by car tires that transport tiny bits of sand/abbrasive materials. Fast driving over the same line with heat, pressure, and repetition would cause repetitive polishing, creating a more slippery surface as the day goes by. By the end of the day, a slippery, polished surface?
So, those are my theories .... and it's all based on hypothesis. (Boy, I gotta start drinking decaf!
)
Dan (aka Cliff)