Traction on hard acceleration

[mattack4000]

Horsepower is essentially torque at any given RPM. The power drop you see is from either a reduction in power output from the battery (likely) or the decrease in power output from the electric motor at higher RPM.

 

[silentsnow31802]

My guess is that the M3 is set up similarly to my first MS - S85. In spite of all the talk about immediate torque, indeed the S 85 did not put out enough kW to the motor from a dead stop and felt stronger once going at 30Mph or so.

That's in start contrast to my new P100D. I understand the "P" comes with higher rated PM (inverters) to allow for full and immediate power delivery.

Yes I agree. I often compare it to the S85 VS the P85. In the end they feel the same, just the S85 is limited from about 0-20 on how much it can pull from the pack. If there was a way to make it more like a P85 I would be a happy camper.

 

[Zoomit]

How do you calculate this? Seem like it would depend on center of gravity, spring rates, etc.

Yes, it's just a sum of forces calculation. I assumed a rigid vehicle (no axle squat from suspension).

Other assumptions:

• CG height: 18" (est)
• wheelbase: 113"
• 3 LR weight distribution: 48% Front, 52% rear
• CG longitudinal position: 59" [113*.52]

Rear wheel load at +0.6g: (59+0.6*18)/113 = 62%

This video helps derive the equation:

 

[Daniel in SD]

Yes I agree. I often compare it to the S85 VS the P85. In the end they feel the same, just the S85 is limited from about 0-20 on how much it can pull from the pack. If there was a way to make it more like a P85 I would be a happy camper.

It's limited by the motor controller, not the pack. The pack has a power limit (kW) which it is nowhere near at 20 mph (look at the graph). S85, Model 3, etc. don't generate maximum power until 45mph. The original P85 didn't have that much more power than the S85 but it had a lot more torque. That's all through changes to the motor controller and motor but since it only had 414hp vs. 362hp they probably didn't have to change the pack much.

Yes, it's just a sum of forces calculation. I assumed a rigid vehicle (no axle squat from suspension).

Other assumptions:

• CG height: 18" (est)
• wheelbase: 113"
• 3 LR weight distribution: 48% Front, 52% rear
• CG longitudinal position: 59" [113*.52]

Rear wheel load at +0.6g: (59+0.6*18)/113 = 62%

This video helps derive the equation:

Interesting. I guess suspension would only increase the transfer to the rear. Thanks!

 

[silentsnow31802]

It's limited by the motor controller, not the pack. The pack has a power limit (kW) which it is nowhere near at 20 mph (look at the graph). S85, Model 3, etc. don't generate maximum power until 45mph. The original P85 didn't have that much more power than the S85 but it had a lot more torque. That's all through changes to the motor controller and motor but since it only had 414hp vs. 362hp they probably didn't have to change the pack much.


Interesting. I guess suspension would only increase the transfer to the rear. Thanks!

Im pretty sure the motor controller pulls from the pack, which makes my statement valid. You do not need to be so technical. This is a forum not an engineering firm.

 

[Daniel in SD]

Im pretty sure the motor controller pulls from the pack, which makes my statement valid. You do not need to be so technical. This is a forum not an engineering firm.

Sorry, I'm an electrical engineer. It's hard for me. haha.
The limit on the battery is how much power can be drawn from it. You can see from the graphs that the motor is not drawing very much power at low speeds so there is no way for the acceleration to be limited by the pack at low speeds.

 

[silentsnow31802]

Sorry, I'm an electrical engineer. It's hard for me. hah
The limit on the battery is how much power can be drawn from it. You can see from the graphs that the motor is not drawing very much power at low speeds so there is no way for the acceleration to be limited by the pack at low speeds.

No prob. I am a controls engineer myself so I understand where you are coming from.
Im hoping that sometime in the future there is an upgrade we can do to turn it basically into a performance model. That difference down low is huge when your actually driving.

 

[roblab]

And why does it need to spin out? I have never needed to, and have felt it lessens my control. You already know it could, but it seems Tesla figured safety was more important.

I know. I'm no fun. But wifey likes me.

 

[Daniel in SD]

And why does it need to spin out? I have never needed to, and have felt it lessens my control. You already know it could, but it seems Tesla figured safety was more important.

I know. I'm no fun. But wifey likes me.

Carbon offsets. Need to offset carbon saved by EV with carbon from tires.

The stat I've seen is that there are 7 gallons of oil used to make a tire. Personally I'm hoping the Model 3 won't be quite as tire hungry as the Model S.

 

[dhrivnak]

My guess would be that the Model 3 does not generate enough torque to the wheels to spin the wheels. This is because it has a flat torque curve almost all the way to 60 mph and a single speed transmission. RWD ice vehicles with similar 0-60 times can spin the wheels because they have faster acceleration (more torque to the wheels) in 1st gear. While the ICE vehicle would beat the Model 3 off the line the Model 3 would catch up after the ICE vehicle shifted into second gear. Obviously this is in drag race type conditions. The ICE vehicle would have to rev the engine and feather the clutch (or load the torque converter in an auto) to achieve "magazine" 0-60 times. All the Tesla driver has to do is push the accelerator to the floor.

You have obviously not driven an EV or at least a Tesla. I have taken my Roadster to the track and NOTHING beats it off the line, and very few cars catch me in the 1/4 mile. There is no wheel spin due to good traction control, but have no mistake they will launch well.

 

[Daniel in SD]

You have obviously not driven an EV or at least a Tesla. I have taken my Roadster to the track and NOTHING beats it off the line, and very few cars catch me in the 1/4 mile. There is no wheel spin due to good traction control, but have no mistake they will launch well.

I've only driven an Model S 60 for any length of time (The slowest Tesla ever made!). It is nowhere near as fast off the line as my Subaru Legacy GT (hard to beat turbo+AWD). I also test drove an P85D and that was definitely quicker though!
Instrumented testing shows the Model 3 to be quite a bit slower off the line than its competition. In the real world where people aren't doing drag race type launches I'm sure it's plenty quick.

 

[12Pack]

Daniel in SD, what's the source of your graph's? They indicate healthy torque with the plain S model from 0 rpm, but that's not what silientsnow and I are saying. My seat-bottom "feel" of the acceleration more closely matched this graph below, where you needed to be at 6000 motor rpm (30-40mph?) before the torque hit.

 

[Daniel in SD]

The torque horsepower graph for the Model S I found in this thread:
Tesla Physics
The Model 3 acceleration data graph I saw on Electric:
Tesla Model 3 accelerates from 0-60 mph in 4.6 secs – faster than Tesla advertises
Constant torque equals constant acceleration (with a fixed drive ratio). The VBOX acceleration data shows the Model 3 has constant acceleration and torque at speeds below 45mph just like the Model S.
The graph you have there is a chassis dyno graph. I have no idea what's going on there but it doesn't look right to me. We know the Model S produces more than 20Nm of torque at 4000 rpm. It looks like they started their measurement at 7000rpm. Above 7000rpm it lines up with the plot in the first link.

 

[12Pack]

Indeed its a dyno graph. It looks like the car was on and running with motor doing 4000rpm when they hit the throttle. But my point was that the way the torque then rises is just how it felt on my old S 85 - rising quickly, but not instantly. The car felt stronger when punching it at about 40mph (which ties to 7000 rpm).

If that's the way it also is on the M3, then it can explain the lack of wheelspin.

 

[Daniel in SD]

Indeed its a dyno graph. It looks like the car was on and running with motor doing 4000rpm when they hit the throttle. But my point was that the way the torque then rises is just how it felt on my old S 85 - rising quickly, but not instantly. The car felt stronger when punching it at about 40mph (which ties to 7000 rpm).

If that's the way it also is on the M3, then it can explain the lack of wheelspin.

It takes the same amount of torque to spin the wheels at 0mph as it does at 40mph though. So if you're accelerating at 40mph and not spinning the wheels then even if you have the same acceleration (torque) at 0mph you will also not be spinning the wheels.
My theory is that many people think the car is accelerating faster around 40mph because it is accelerating faster than a similar ICE vehicle at that speed. Also there's no downshift. That's been my experience driving electric cars, they feel super at quick midrange speeds because they are relatively quick.

 

[12Pack]

Sure, but only if you were in a vacuum. Wind resistance at 40mph is significant. But the point is that in my old MS85, I experienced non-linear, ramping acceleration - unlike in my P100D. This is not about theoretical Newtonian physics, but about the power being able to be delivered by the drive module over time in response to the throttle.

And BTW there are many ICE's that pull cleanly away from my MS once at speed (they just can't get off the line like the P100D)

 

[Daniel in SD]

Sure, but only if you were in a vacuum. Wind resistance at 40mph is significant. But the point is that in my old MS85, I experienced non-linear, ramping acceleration - unlike in my P100D. This is not about theoretical Newtonian physics, but about the power being able to be delivered by the drive module over time in response to the throttle.

And BTW there are many ICE's that pull cleanly away from my MS once at speed (they just can't get off the line like the P100D)

Wind resistance is not significant to acceleration (it is to efficiency): https://www.tesla.com/sites/default/files/blog_attachments/the-slipperiest-car-on-the-road.pdf
The Model S uses 14hp at 70mph. That extrapolates to 4.6hp at 40mph.
I can't explain what you experienced in your S85. It doesn't agree with the plots in the Tesla physics thread. Maybe your car was different or maybe those plots are wrong. I didn't see any VBOX accelerometer data for the Model S85 in a quick search. The VBOX data and the 0-30 and 0-60 numbers we have for the Model 3 all support that it has a flat torque curve.
Plenty of cars can pull away from Teslas at extralegal speeds because they have multispeed transmissions and can stay close to their HP peak while the Tesla's HP is dropping.

 

[12Pack]

Wind resistance is not significant to acceleration (it is to efficiency):

Respectfully, at 40 mph, that’s just incorrect. Basic physics - and common sense. Put your palm out the window at 40mph. The Model S is indeed otherworldly in some respects but must still abide by the laws of our universe.

Again, while the motor may be capable of producing immediate torque, it would seem the non P modules do not deliver full power immediately on throttle, which explains this behavior on the regular S and likely also on the Model 3.

 

[Daniel in SD]

Respectfully, at 40 mph, that’s just incorrect. Basic physics - and common sense. Put your palm out the window at 40mph. The Model S is indeed otherworldly in some respects but must still abide by the laws of our universe.

Again, while the motor may be capable of producing immediate torque, it would seem the non P modules do not deliver full power immediately on throttle, which explains this behavior on the regular S and likely also on the Model 3.

4.6hp is a lot of power! Horses are a lot less aerodynamic than a Model S and they can run pretty fast. haha. This is from Tesla's own website.
Tesla say the Model S has a range of 380 miles at 40mph Model S Efficiency and Range
80kW capacity / 380 miles = 210 Wh/mi
210Wh/mi * 40mph = 8.4kW
8.4kW = 11.2HP
So the total power used at 40mph is 11.2hp but you have to also count rolling resistance, drivetrain loss/efficiency, display, fans, etc.

 

[12Pack]

This is from Tesla's own website.

Honestly, Daniel, I prefer to rely on on my own thinking and training.
1.It takes significantly more hp to accelerate at the same rate from 40 mph than it does from 0 mph due to air and mechanical rolling resistance, enough to make a difference on when you can chirp the tires or not.
2. And this is the point of my posting in this thread - the power modules on the basic S models use lower rated and less efficient CMOS inverters, and hence take a moment to deliver full torque to the wheels when you punch the throttle unlike the P models. Likely the same for the Model 3, which could explain less wheel spin.
Over and out.