AZ Desert Driver
Rare combination
Thanks ...1.1 g just seems too much. 850 hp is not enough to accelerate? Nothing calculated, just gut feel. Thanks for the calculations - and explanations. My feelings have been trumped by science.
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How quickly? Hasn't happened to me for the 2012 P85 or the 2014 P85D yet.
Just FYI, a bunch of us with vbox and pbox accelerometers do record 1.1Gs from about 17 MPH to 35 MPH.
LargeHamCollider
Battery cells != scalable
This does seem to violate the laws of friction, theoretical maximum mu value is unity which means theoretical max acceleration is 1g. But that is between surfaces where the applied force is perpendicular to the two parallel surfaces which are in contact with one another. IRL tires deform and fill small fissures in the road creating patches of contact where the applied force is not perpendicular to the two surfaces (like the teeth in gears but on a smaller scale and with smaller angles) creating an effective coefficient of friction greater than 1 and allowing accelerations greater than 1g to occur.
Drag radials actually form a temporary chemical bond with the road allowing accelerations greater than 3g to occur.
Just to add some additional information, for those interested how fast an EV (one fixed gear) with a certain power and friction coefficient could accelerate.
The discussed problem was the simplest one, we reach the point of maximum power at 60mph, but we could also assume that we have a friction coefficient of 1.1, but only 500kW. That would be a bit more complicated, but sill solvable.
To get the maximum acceleration, we need to use all the force we can, without slipping.
So we just calculate the speed until we can do that. In our example, 500kW, 2.2 tons and 1.1g, v = 500 kW /( 2200kg * 1.1*9.81m/s^2)
= 21.06 m/s
Now we need the time until we get there t = 21.06 m/s / (1.1 * 9.81 m/s^2) = 1.95s
After we get the time to the point of maximum acceleration, it gets a bit tricky and there are multiple solutions. In my opinion the easies way is using the correlation between energy and power. Power is energy d/dt. In our case we just assume that the power stays constant after that point and we don't have to dial back because of increased proximity and skin effect losses.
So we just need the additional energy needed, to have a car moving at 60mph, or 26.82 m/s , over a car moving at 21.06 m/s.
W = {1/2 * m * (Vend)^2} - {1/2 * m * (Vstart)^2} = 0.5 * 2,200kg * (Vend^2 - Vstart^2) = 303kJ
Now we divide that by 500kW and we get 0.61 seconds. We add both times and we get to 2.558s
So not really a lot less, although we reduced our power by about 25%. If we reduce power by 50%, to 319kW we still get a pretty decent 3.1 seconds. Even with just 200hp, less than a quarter of our original calculation, we still end up with less than 5.6 seconds. If we, on the other hand increase our friction coefficient against infinity, we still can't get less than 5.27 seconds with 200hp. Because thats the time it takes to "fill up" the car's energy from 0 to 791MJ with 150kW.
In realty its always a bit lower, since there are also rotation masses, especially the wheels, which also take up lots of energy to speed up and of course some reaction time, be it your brains switching frequency, or that of a microcontiolller, IGB, CPU or something like that.
You can easily do these calculations with excel, its interesting to see what effect power and friction have on 0-60, 0-100, or 0-155mph times. If you want, cow can also add aerodynamic drag, which I left out, or a reducing power curve.
The discussed problem was the simplest one, we reach the point of maximum power at 60mph, but we could also assume that we have a friction coefficient of 1.1, but only 500kW. That would be a bit more complicated, but sill solvable.
To get the maximum acceleration, we need to use all the force we can, without slipping.
So we just calculate the speed until we can do that. In our example, 500kW, 2.2 tons and 1.1g, v = 500 kW /( 2200kg * 1.1*9.81m/s^2)
= 21.06 m/s
Now we need the time until we get there t = 21.06 m/s / (1.1 * 9.81 m/s^2) = 1.95s
After we get the time to the point of maximum acceleration, it gets a bit tricky and there are multiple solutions. In my opinion the easies way is using the correlation between energy and power. Power is energy d/dt. In our case we just assume that the power stays constant after that point and we don't have to dial back because of increased proximity and skin effect losses.
So we just need the additional energy needed, to have a car moving at 60mph, or 26.82 m/s , over a car moving at 21.06 m/s.
W = {1/2 * m * (Vend)^2} - {1/2 * m * (Vstart)^2} = 0.5 * 2,200kg * (Vend^2 - Vstart^2) = 303kJ
Now we divide that by 500kW and we get 0.61 seconds. We add both times and we get to 2.558s
So not really a lot less, although we reduced our power by about 25%. If we reduce power by 50%, to 319kW we still get a pretty decent 3.1 seconds. Even with just 200hp, less than a quarter of our original calculation, we still end up with less than 5.6 seconds. If we, on the other hand increase our friction coefficient against infinity, we still can't get less than 5.27 seconds with 200hp. Because thats the time it takes to "fill up" the car's energy from 0 to 791MJ with 150kW.
In realty its always a bit lower, since there are also rotation masses, especially the wheels, which also take up lots of energy to speed up and of course some reaction time, be it your brains switching frequency, or that of a microcontiolller, IGB, CPU or something like that.
You can easily do these calculations with excel, its interesting to see what effect power and friction have on 0-60, 0-100, or 0-155mph times. If you want, cow can also add aerodynamic drag, which I left out, or a reducing power curve.
RogerHScott
Active Member
Have you ever actually been in one of these situations without Ludicrous enabled and found the "basic" 90D power insufficient?
As a (mere
) 90D owner I find that a little hard to imagine.As I commented in some other thread a while back, after my first drive in my brand-new 90D I told my wife I only then realized the true
reason that "Ludicrous speed" is called that: the idea that you'd need anything more than what the 90D can do is, just, "ludicrous".
Johan
Ex got M3 in the divorce, waiting for EU Model Y!
ICE engines do better from stand still starts. They revv up and "load" a lot of kinetic energy in the drive train that is released on to the wheels as the clutch engages.
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