X3 35i Sim vs Crashnbrn5 at Island Dragway:
As Crashnbrn provided two of his drag slips in his previous thread 'X3 35i Vs. E55 AMG at Island Dragway, NJ'
http://x3.xbimmers.com/forums/showthread.php?t=1144358 , it seemed like a golden opportunity to validate Matthew Snare's Acceleration model for our X3s against documented data. GoogleEarth shows Island Dragway to be oriented East-Southeast at an elevation of 515 feet (+/- 1 foot) along its entire length, so the grade is essentially 0%.
It is unclear whether the dragstrip has the ability to weigh vehicles, but if the exact weight with occupant is known for each race, that could be inserted into the construct for a more refined calculation. As can be seen, the model actually does a remarkably good job at modeling Crash's runs. It is apparent the takeoff appears a bit slow for the sim compared with Crash's actual starts, but if that were to pan out with more runs documented then the equations for the 35i could be refined to capture that.
For those interested, there are 2 different equations for the
force curves for every vehicle. Because the initial stage of acceleration is highly dependent on the ability of the tires to grip the road without spinning,
the initial force equation remains constant and unique for each individual vehicle and its settings until the power being generated by the engine decreases to a force lower than the 'loss of friction' limit. It is simply the weight of the vehicle, the coefficient of friction of the tires, and a constant. Then the second
force equation kicks in which takes into account the available horsepower and the effects of speed.
As such, acceleration changes minimally while the first equation is being used, decreasing only because of the increasing aerodynamic and roll resistance. After the second equation of Force begins, then acceleration begins to fall off at a much faster rate.
It seems likely, then, that Crashnbrn's X3 is able to utilize more of its power early (without the tires losing friction and spinning) than the model is giving it credit for, and I think if I just adjust the coefficient to allow for this it might be more exact. But is Crashnbrn's faster take-off a result of his tire choice, or has BMW allowed its anti-skid technology to allow a more efficient transfer of the energy, or a combination of both?
Apologies for the lengthy entry, but it seemed a few out there might be interested in how the simulation makes these calculations, and why.
If any others have their drag slips and wish to have a similar graph made for their vehicle, I'm pleased to enter your data when I have the time. I will need to know your vehicle type (28i or ...), what your HP is (if it has been tuned to a higher than stock), and if you do know your pre-race weight that would be even better.