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Tesla Model S P85D Sets 1/4 Mile World Record vs Hellcat....

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In the P85, you can spin the wheels with TC off. I haven't heard of people doing that in the P85D because power is distributed to all four wheels, so it tends to grip the dry pavement.

But it's not distributed to all four wheels - which was my point at the end there. The P85D rear motor is supposed to be exactly the same as the P85 drive motor, and it still only drives the rear wheels.

At low speeds, the battery can deliver far more power than both motors/inverters can take (from videos it looks like the D goes power limited at about 25 mph under current firmware.)

So in principle, the P85D should spin the rear wheels exactly as easily as the P85 does - the front motor adds additional torque and acceleration, but doesn't change anything about the situation at the rear.

That's why I'm still thinking the car is pulling back some power, though it apparently doesn't flash the TC light as some expect it to from other experiences.
Walter
 
I'm not an expert in laws of physics, but i think there is a difference for rear wheel behavior when you have front motor
P85D is set to motion from the front motor as well, which in turn, squads the rear even more (applying more weight to the back, or more weight transfer front-to-back) which improves rear traction
Add the fact that the rear is now has to break loose while in motion (contributed by front) it's even harder to get a slip
We know it's easier to spin of the line than at 20-30mph for example
I'm running 285 PSS tires now and a cannot break the rear lose anymore, TC or not

My conclusion is that D might be traction limited, but it's not significant
I just dont see how the same rear motor can break loose the same way as on regular P

P.S. Can D run TC off in insane mode?
Let it spin if it can?
 
That's an interesting point. I'm convinced that a P85D with the front motor disabled would spin, but I hadn't really considered that the front motor induced acceleration would produce additional weight transfer.Does P85 flash the traction light on hard straight acceleration when we know it is pulling power on dry roads?Walter
 
Huh. I have no personal experience with the car. The thing that confuses me here is this:

If the car isn't pulling back power during launches and isn't spinning, why is it that people report they can easily spin the wheels when they disable traction control?

Unless I'm confused - I'm pretty sure I saw several reports of being able to break wheels loose on dry pavement in a straight line with TC off a while back.

I know I've seen videos of forcing a P85 to spin while holding it with the brakes, and the D should be able to deliver the same rear wheel torque...
Walter

My car had 19" Goodyear OEM tires on it and I could easily spin them on public roads with the TC off. However at the drag strip they put traction compound down and the pavement is flatter and in better repair than most roadways. I had no hint of wheelspin at the strip with my P85 and the all season goodyears.
 
That's an interesting point. I'm convinced that a P85D with the front motor disabled would spin, but I hadn't really considered that the front motor induced acceleration would produce additional weight transfer.Does P85 flash the traction light on hard straight acceleration when we know it is pulling power on dry roads?Walter

You are correct in that a P85D with the front motor disabled is essentially the same as a P85 with an extra passenger--so it could spin its tires. However, with the additional front motor, you don't see it.

The coefficient of dynamic friction (the amount of friction when the tires are sliding on the pavement) is lower than the coefficient of static friction (the amount of friction when the tires are not sliding). So once tires are spinning, it requires much less torque to keep them spinning than would be required to get them spinning in the first place.

So suppose a P85's rear torque just barely puts it over the limits of static friction. This barely gets them spinning, but once the tires are spinning, the friction drops and less torque is required to keep them spinning. So they can stay spinning for quite awhile.

But now consider a P85D. The rear torque is the same, and *would* put the rear wheels just over the limit of static friction, but this time you've got the front motor pulling as well. Since the fronts help accelerate the car forward, this reduces the forces on the rear tires, putting them now below the limits for static friction. End result is the rear tires don't start spinning.
 
But now consider a P85D. The rear torque is the same, and *would* put the rear wheels just over the limit of static friction, but this time you've got the front motor pulling as well. Since the fronts help accelerate the car forward, this reduces the forces on the rear tires, putting them now below the limits for static friction. End result is the rear tires don't start spinning.

I was with you right up until this last paragraph. I'm not seeing how the front motor pulling reduces rear ale loads at T=0 in this case - there's no mechanical connection, and plenty of battery power to go around, so it seems to me like there's no reason you wouldn't have the same torque at the rear wheels as the RWD case regardless of what the front motor is doing.

The only factors here at T=0 should be axle torque, wheel diameter, normal force (weight), and coefficient of friction. The front motor doesn't directly change any of that, and I ess therefore initially thinking it had no effect on the equation.

However, someone did make a valid point upthread that the faster acceleration and location of the CG above the wheels creates a greater weight transfer which will increase the normal force somewhat - meaning the loads on the rear wheels stay the same, but the effective increase in weight will allow them to carry somewhat more torque without slipping.

Given the lower power and reduced gearing of the front motor and the car's very low CG, it won't be a large effect, but it could be enough if as you postulate the P85 was barely past the traction limits.
Walter
 
Walter,

I see what you're saying about t=0...but if we're close to the static friction limits on the rear wheels at t=0, the assistance in forward acceleration by the front motor could keep the rears just under the static friction limit. While this may be contributing, my guess is it's not the most significant reason why the D isn't breaking traction.

In addition to the weight transfer, one other thing is that we have no evidence that the rear motor in the P85D is producing the same amount of torque as the P85 *at t=0*. (This may be the main reason why the Ds don't break loose). While theoretically maximum torque for an electric motor can be produced at 0 RPM, and while the rear motor on the D has the same potential torque generation as that on the P85, the interverters may be splitting the power differently such that the highest torque in the rear wheels occurs at, for example, t=0.1sec.
 
Walter,

I see what you're saying about t=0...but if we're close to the static friction limits on the rear wheels at t=0, the assistance in forward acceleration by the front motor could keep the rears just under the static friction limit. While this may be contributing, my guess is it's not the most significant reason why the D isn't breaking traction.

Why? What am I missing here? How does assistance from the front motor change the situation of the rear motor at all, aside from weight transfer, while in the RPM range where the rear motor can deliver full torque?

In addition to the weight transfer, one other thing is that we have no evidence that the rear motor in the P85D is producing the same amount of torque as the P85 *at t=0*. (This may be the main reason why the Ds don't break loose). While theoretically maximum torque for an electric motor can be produced at 0 RPM, and while the rear motor on the D has the same potential torque generation as that on the P85, the interverters may be splitting the power differently such that the highest torque in the rear wheels occurs at, for example, t=0.1sec.

That's a valid point - we're all assuming things here. However, I only picked T=0 to make the situation simpler - I think the same argument more or less holds true up until you hit the battery power limit around 25 mph (my best guess from watching the videos that show the dash during hard acceleration.)

Above the battery power limit speed, the car has to split power - below it both inverters can have as much battery power as they want, and while Tesla certainly could program ramp rates into the inverters, with their remarkable traction control and the car's goal/role as a go fast halo car I have trouble seeing why they would add limits to it that weren't in prior models.
Walter
 
Todd B, I understand your logic, makes complete sense, but after I saw this video, I had a few questions.
So, you are saying that if a P85D had the same traction control fuse removed like the P85 in the vid, it would not do a burnout b/c of the front motor pulling? (b/c of the friction coeff etc) I'll trust your logic but from seeing how easily and immediately the P85 spins I'm wondering if a P85D wouldn't do something similar. If the p85D did do a burnout, then theoretically would drag slicks etc improve a 0-60 time? Again I'm a novice w/this stuff, so bear with me :smile:
 
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My S85 earned respect from a Red with 2 black racing stripes Dodge Charger Hellcat from 0-45 mph coming back to my office from lunch today. :) He even gave me thumbs up and a very friendly wave. Last week it was a Lotus Exige S from 0-45 and then again from 45-70 both the legal speed limits, with no one else around. After using my car as a daily driver for 17 months it seems the sporty cars are looking to challenge the Tesla. I think I'll quite accepting the challenge while I'm ahead. No harm done, no tickets, legal speeds, it was fun.
 
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