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The real top speed of the Model S

TheAustin

Model X 90D (Former Model S P85)
Sep 12, 2011
266
9
The Hamptons
I posted this in Tesla forum, but couldn't get any definitive answers, so I thought I would ask the enlightened folks over here too..

So, we all know that the top speed of the Model S is software-limited to around 130mph (depending on model/options)...If there was no software limitation, what do you think the real top speed of the Model S would be. I would think pretty high, considering the 416hp/443ft-lb torque, even considering the weight of the car. But if you had to pick a number, what would you guess? (Anyone out there "jailbreak" their S yet?!?!)

And, why do they limit the top speed in the first place? Is it vehicle/passenger safety? Is it to specifically protect the life of the batteries? Would it be too difficult to properly cool the battery pack while going 160, 180, 200mph? Not that you could even drive at that speed for a significant amount of time. But still...

Just wondering. Any thoughts?
 

ChadS

Last tank of gas: March 2009
Jul 16, 2009
3,434
2,877
Redmond, WA
I haven't heard anything for the Model S. But for the Roadster, I heard they did a 24-hour test with the motor at 24,000rpm. It's limited to 14,000rpm.

I didn't hear why they set the limit there, but I would imagine:
- they want a wide safety margin, especially considering manufacturing tolerances
- that takes a TON of power, and especially on the Roadster it would probably quickly overheat. I suspect heat is the limiting factor, at least on the Roadster
- in fact it would probably run out of power soon; aerodynamic drag is a huge force at that speed and increases quickly
- they'd have to put on more expensive tires that may be less capable in some other criteria
- performance would be pretty sucky at that point given the gearing, so there's not much to gain from messing with the above items
 

Johan

Ex got M3 in the divorce, waiting for EU Model Y!
Feb 9, 2012
7,509
10,013
Drammen, Norway
I concur that the limitation is all about the fixed gear togheter with the heating issue.

Also, dont forget this graph, that basically says that with the current setup and gearing torque becomes mediocre at high RPMS and in fact if you sped up the motor even more perhaps torque would drop to a level where there simply wouldn't be enough torque to increase speed over a given treshold (remember air resistance increases exponentially with speed). This particular issue of course would be solved by multiple gears, the heating issue not so much.

attachment.php?attachmentid=2755&d=1317483193.jpg
 

Johan

Ex got M3 in the divorce, waiting for EU Model Y!
Feb 9, 2012
7,509
10,013
Drammen, Norway
The phrase "escape velocity" comes to mind. I know it's a mismatch, but the threshold concept is similar in my mind. It's morning though, so perhaps I'm not thinking straight.

Good analogy. Let's have SpaceX look in to that one on Tesla's behalf, I'm sure they will find a nice after-market solution :)
 

Dave EV

Active Member
Jun 23, 2009
2,141
2,126
Earth
Check this out. Top speed appears to be limited to 133 mph in a P85. notice once the limiter kicks in the orange power bar goes down quite a bit so clearly the car can go quite a bit faster...
Amazing how quickly it hits 90 mph, though you can see acceleration start to ramp down quickly after that. It still hits 133 mph very quick. I imagine that with higher gearing it could easily hit 150-160 mph if not more.
 

Jim B

New Member
May 28, 2013
3
0
Seattle
I posted this in Tesla forum, but couldn't get any definitive answers, so I thought I would ask the enlightened folks over here too..

So, we all know that the top speed of the Model S is software-limited to around 130mph (depending on model/options)...If there was no software limitation, what do you think the real top speed of the Model S would be. I would think pretty high, considering the 416hp/443ft-lb torque, even considering the weight of the car. But if you had to pick a number, what would you guess? (Anyone out there "jailbreak" their S yet?!?!)

And, why do they limit the top speed in the first place? Is it vehicle/passenger safety? Is it to specifically protect the life of the batteries? Would it be too difficult to properly cool the battery pack while going 160, 180, 200mph? Not that you could even drive at that speed for a significant amount of time. But still...

Just wondering. Any thoughts?


Although it could be mechanical limitations of the drive train that people have mentioned I tend to think the reason for limiting the speed is the same reason manufacturers limit many cars. That is, if you go over a certain speed you have to upgrade tires, brakes, suspension, etc. For instance H rated tires are good for up to 130 mph. I don't think it is an accident that Tesla picked 130 mph. At speeds over 130 mph tires become expensive and difficult to find. Plus, let's be honest, how many people are really going to drive that fast in the US? 130 mph on a public road isn't just a speeding ticket, it's reckless driving. Sure, you can take it to a track but I doubt many people are buying these cars for that.

My present Lexus has never been over 90 mph. No need for more and I really didn't need that. The Model S was not designed to be a race car. Yes, it performs well, better than most of us can even appreciate but it is not a Boy Racer.
 

Two words: Back EMF.

Electric motors turn into generators at high RPM. The resistance of an electric motor increases proportionally to RPM. More current is required to get any power out of a motor at high RPM. Once a certain point is reached the amount of current in is equal to the resistance back out and you've reached maximum RPM for that motor. Power (torque) drops off long before that point. This graph is a good example of this phenomenon.

Take all you know about power and peak RPM from gas motors and throw it out the window. Time to learn some new science.
 
Can someone go into more detail on the 7K and 15K rpm inflection points? Also the shape of the curve (why "concave" vs. "convex", or whatever) is interesting. Thanks.

If you pulled the motor, put it on a bench, hooked up a much larger current source with your own drive controller and measured it you'd no doubt see that graph actually curve up and to the left below 7k all the way down to 0 rpm in one continuous arc. I'd wager Tesla's motor is capable of generating upwards of 1200 NM of force at 0 rpm. Real world considerations however require it to be software limited to keep drive shafts from snapping and reduction gears from burning out from that much force. Hence the plateau from 0 to 7k.

The continual slope down from 7k to 14k is described by Lenz's law (back EMF). I don't know why there's a sharp inflection down from 15k to 16k. Could just be a software control decision to gradually back you off as you approach top speed.

Yeah I'd like to know at what speed are you going when you hit 7k?

Top Speed 130mph / x = 16k / 7k
Solving for x = 56.875 mph
 

ElSupreme

Model S 03182
Moderator
Jan 13, 2012
4,303
105
Atlanta, GA
Can someone go into more detail on the 7K and 15K rpm inflection points? Also the shape of the curve (why "concave" vs. "convex", or whatever) is interesting. Thanks.

Sort of expanding a little on early adopter's response.

When you run an Induction motor off of a variable frequency drive (VFD). The motor is limited by the amount of current the coils can handle. This current is turned into torque. So you get a direct flat line for torque. As the motor increases speed the inverter/VFD will increase voltage. At the 7k inflection point the motor is running at maximum current and maximum voltage (and thus peaking in power). From that point downward the motor starts creating back emf, this is the motor producing voltage to counter the voltage supplied by the VFD, thus lowering the net voltage across the motor, and the current with it.

The second inflecting point is probably inherent in the ratings of the components in the inverter/VFD itslef. Or it could be programmed in the inverter/VFD to gracefully hit top speed.
 
Can someone go into more detail on the 7K and 15K rpm inflection points? Also the shape of the curve (why "concave" vs. "convex", or whatever) is interesting. Thanks.

Below about 40 mph (varies by car type), our cars are constant torque.
The car is limited by things like how much AC current the inverter and motor can handle, and how sticky your tires are.

Above that threshold, our cars are constant power, and torque * RPM is relatively constant.
It's mostly limited by how much power you can pull out of the battery.

Above a certain point, the motor is turning too fast to be able to efficiently use the battery power, and you get another inflection point where output power drops at higher RPMs. This threshold is apparently around 15k RPM.


The constant torque to constant power threshold should be below 7k RPM in actual cars. I suppose the graph is for the motor itself which would not be constrained by wanting to not shorten the lifetime of a battery. In that case, the threshold would be how much power the motor can handle, and apparently the motor can handle noticeably more power than our 85 kWh batteries can put out without degradation.

- - - Updated - - -

If you graphed power vs RPM instead of torque vs RPM,
I imagine you'd see it ramp linearly to 7k RPM, where it would stay relatively flat until 15k RPM, and would then drop off from there.
 

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