Thoughts on the new Roadster’s “over 250mph” top speed claim by Elon

[ecarfan]

A couple of issues came to mind when I heard Elon make that “over 250mph” top speed claim for the new Roadster:

1) I don’t see how that speed can be achieved without at least a 2-speed gearbox (or maybe 3-speed). Roadster fans know that way back in 2008 the Roadster debuted with a 2-speed Borg Warner gearbox but locked in first gear because the the gearbox kept breaking under hard acceleration. That gearbox was replaced with a single speed unit. It appears that Telsa has now solved that problem.

2) The new Roadster will rebut German critics who say that real cars have to be capable of extended periods of 150+mph travel on the Autobahn. The massive battery capacity may have been chosen in part to support that capability.

3) And if the new Roadster can in fact do that then the 3 motors likely have special cooling systems that the S/X/3 lack.

 

[DOCAL]

We were talking about this at work today, and came to the same conclusion on the gearbox, possibly it could be some sort of CVT.

 

[electracity]

I thought about the autobahn too. Many aspects are aimed at Porsche. I didn't see any active aerodynamics on the roadster. If they are going 200+ the final car may need to change.

The semi needs a cooling system for high continuous output too.

 

[Olle]

Bigger motors with taller fixed gearing might be lighter and more reliable than multi speed gearbox, perhaps. We are only talking 250/155 taller, so not a massive difference compared to today's gearing.

Big difference in power to hold 250 compared to 155 mph, you need to produce a lot of power still at the top speed.

 

[Pezpunk]

I thought about the autobahn too. Many aspects are aimed at Porsche. I didn't see any active aerodynamics on the roadster. If they are going 200+ the final car may need to change.

it has an active rear wing.

 

[rajram]

One thing to keep in mind is the total torque output of the three motors. At a claimed 10,000 Nm (~7376 lb-ft). This is more than an order of magnitude higher than the torque output of a high performance ICE, and allows for a very aggressive reduction ratio on the gearset. I'll have to do some calculations to figure out optimum reduction ratio, but it is entirely conceivable that a single gear will be sufficient to produce the ~1.5g of peak acceleration that this car will sustain at low speeds while still achieving a top speed around 250 mph.

 

[Xenoilphobe]

Each motor could have different gearing, but combined, give the desired performance attributes.

 

[mongo]

I'm thinking a low ratio single speed gearbox, and that they are compensating on the low end with the large pack to get the torque for acceleration.
Most low end is from the rears, so the front may be geared even lower to pick up at the high end.

10,000 nm mostly from rears, 5k is 3500 lb-ft each side the P100D dynoed at 900+ lb-ft

One thing to keep in mind is the total torque output of the three motors. At a claimed 10,000 Nm (~7376 lb-ft). This is more than an order of magnitude higher than the torque output of a high performance ICE, and allows for a very aggressive reduction ratio on the gearset. I'll have to do some calculations to figure out optimum reduction ratio, but it is entirely conceivable that a single gear will be sufficient to produce the ~1.5g of peak acceleration that this car will sustain at low speeds while still achieving a top speed around 250 mph.

The 10k nm is at the axles, so I think the needed calculation is what is the RPM / power curve of the motor to achieve 10k nm at 0-60 and aerodynamic loading + RPM at 250+ for the top end. Highly dependent on pack voltage and motor back EMF.
Low end power is around 430 kW average net for the 1.9 0-60 with no aero loading 100% efficiency.

 

[e-FTW]

I think they focused on high speed performance, as a smaller and lighter car can probably hit 1.9 with P100D levels of available power. All of the extra power beyond that must be designed for the higher speeds. So, taller gearing than the S, different rations between front and back (like the S, but maybe not as wide of a difference) plus a giant pack that can sustain massive power output for high speeds.
This will be nuts!

 

[e-FTW]

Also, at least one battery researcher thinks the performance figures are feasable: https://jalopnik.com/heres-what-a-battery-researcher-told-us-about-the-tesla-1820558723

 

[Johann Koeber]

I sure hope they include a chill mode for me.

In Germany the auto manufacturers have agreed to limit the top speed of their cars to 250 km/h. Almost all German cars manufacturers have signed this agreement in the 1980s; the obvious exception is Porsche (and Ferrari and Lamborghini). So even the strong German autos are electronically limited to 250 km/h. Some car companies or tuners offer the option to lift this limit - mostly at a price. This is partly due to tire issues.

Too fast for me. But I like the good acceleration. When going from 160 km/h to 190 km/h on the Autobahn the MX does OK. More power would be nice (not complaining here at all).

 

[alfagiulia]

There are hardly anymore stretches of the Autobahn where you can sustain >230kph for more than a few kms. There is just too much traffic and too many baustelle.

 

[rajram]

Based on the 0-60 claim of 1.9 seconds, the average acceleration of the Roadster within this time/speed window comes out to ~1.44g. The same first-order calculation on the Model S P100D generates ~1.09g. In reality, the acceleration is non-linear so one can expect peak acceleration within this window to be even higher, provided the instant torque application doesn't vaporize the MPSC2s! My preliminary estimates using the claimed torque output of 10,000 Nm suggests that a single speed will be more than adequate to reach a 250 MPH top speed while inflicting 1.5g of accelerative punishment on all four wheels from a dead stop. In fact, with anything greater than a 1:1 direct ratio gearset, predictive torque management software will be absolutely necessary to limit wheelspin. More to come...

 

[ggnykk]

Each motor could have different gearing, but combined, give the desired performance attributes.

I agree with you. I actually think that all 3 motors (2 at rear, and 1 at front) all have different gear ratio. So the car has maximized acceleration at all kind of speed up to 250 mph. That's the only way they can avoid putting a 2 or 3 gear transmission in the car.

 

[smartypnz]

What can the average person handle in g's? There must be a point where blackouts might occur.

 

[BerTX]

What can the average person handle in g's? There must be a point where blackouts might occur.

Wikipedia says G-LOC in untrained individuals occurs between 4-6 gs.

 

[BerTX]

My thoughts are a little less technical, so maybe OT.

We have to be nagged to put our hands on the wheel using AP because it isn't safe. We can't replace the side mirrors on a car with cameras because it isn't safe.

The driving skill requirement to drive a vehicle that can go 250+ mph? The same as the skill required to sign a $200k check...

 

[ecarfan]

We have to be nagged to put our hands on the wheel using AP because it isn't safe. We can't replace the side mirrors on a car with cameras because it isn't safe.

The driving skill requirement to drive a vehicle that can go 250+ mph? The same as the skill required to sign a $200k check...

Of course on almost any public road where this car is likely to be available there are speed limits far below that figure. What worries me more is how some people will floor it and lose control. But then the S P100D is almost as quick and hundreds of them have been sold.

 

[Krazaak]

You need over 1000 horsepower to reach 250 mph, which means all 3 motors are going to have to be contributing meaningfully from launch till top speed. Likewise, you're not delivering over 7000 ft lbs of torque without all 3. If there's no transmission, then they've got to have a pretty wide rpm range.

 

[Xenoilphobe]

What can the average person handle in g's? There must be a point where blackouts might occur.

I blacked out when I saw the price.