Two Gear Boxes---Two Gear Speeds

Two Gear Boxes---Two Gear Speeds

For those that get the two motor option on the X.

Would you like to have the rear gear box set to current spec and then have the front gear box set to a higher gear ratio?

This way the front gear box could take over at 40mph+ or so---for sure as you hit 65-75. I "think" this would perform the same effect as an overdrive to get better range per charge and have the capability for a higher top speed than the lower ratio gear box.

All of this can be coordinated via the control systems.

Any Tesla people or transmission/gear box sme's out there that can comment?

Vawlkus | September 9, 2012

Having the front wheels engaged at a different speed than the rear wheels is a recipe for stress on the car frame. I realize you're suggesting having only one set of wheels active based on the speed travelled but there'd have to be an overlap.

Besides, doing this would mean adding some sort of clutch to an electric drivetrain, which a MASSIVELY bad idea give the constant torque output of an electric motor. Without a clutch you'd have one motor moving the car while the other motor would be in regen mode, depleting your battery three times faster.

Quigibo | September 9, 2012

The front drive would be at the exact same speed---mph---as the rear drive --- motor and gearbox would turn at a different ratios. And both drives could be engaged at the same time...or off/on as traction or other driving conditions dictated.

And both drives could start the initial movement of the car---but would be best to use the lower ratio motor/gearbox set to do so.

No clutch of any kind is needed as both front and rear drive sets are independant units with no drive/transmission physical linkage between them. BTW--I and others were against a 2 speed transmission on the original roadster because of the electric motor torque.

Regen would also not be a problem as it would be modulated via control stability systems so if one of the drives wasn't being used it would simply freewheel.

One for the engineers---does the high gear ratio have an overwhelming advantage or not?

Brian H | September 9, 2012

I believe this is exactly what the X Perf does. They are using different simple reduction gears, with the front set having a lower reduction, and engaged in highway driving.

Timo | September 9, 2012

I don't think they are off at any speed. Just that front motor has different reduction gear giving it a bit different torque curve, that's all. Less low speed acceleration, more high speed torque.

space09 | September 17, 2012

I thought of this as well. Think of it like a two stage rocket. Main engine (rear) takes you 0 to 100, secondary engine (front) takes you 100 to safest top speed. Don't get me wrong, it would be a lot of work to get these to smoothly work together, especially since there would be a lot of times both engines would be working together. But certainly this seems doable and a way to achieve a 'more efficient' higher top speed than the current 130 of the Model S Perf.

Brian H | September 17, 2012

I believe this is how the Model X dual engine Performance already works.

jkirkebo | October 3, 2012

Top speed is limited by max motor rpm. So if the rear motor is limited to 130mph because it hits 16,000rpm there, you cannot use the front motor with a higher gearing to reach 155mph as that would also take the rear motor to 19,000rpm and destroy it in the process. Remember, the motors are connected by the road+wheels.

To do this, you'd need a clutch to disengage the rear motor from the rear wheels at >130mph. Too much hassle for little gain IMHO.

Nicu.Mihalache | October 3, 2012

Thanks jkirkebo for stating this obvious fact in a clear way. I was too lazy to get into that.

The only modification from the Model S gearbox in AWD config is probably the ratio (same for both rear / front) in order to optimize the delivery of so much power at higher speeds (example: 9.03 ratio instead of 9.73 would give 140 mph top speed instead of 130 mph).

Brian H | October 3, 2012

Without altering the top speed, it provides a higher efficiency at highway speeds in general.

the bonnie | October 3, 2012

Did I miss something? I thought that so far, Tesla had only shown a prototype of the Model X. Some of these posts sound like they're quoting from a specs page.

Could someone point me to the source that outlines how a Model X dual engine Performance works? (I have a vested interest ... Model X Sig #2 will eventually be parked in my garage :) )


Brian H | October 3, 2012

Scraps from the reveal, main page, etc. TM hasn't pulled it all together yet. And I don't think they want to be committed to too many details. Stuff is in the pipeline; we await a promised big Falcon Door upgrade (?) announcement, e.g.

the bonnie | October 4, 2012

Ah, okay. So not fact. I was at the reveal and definitely didn't hear anything definitive. Some of these posts definitely sounded like the author was 'in the know' and their information was fact.

Thanks for the clarification.

Nicu.Mihalache | October 4, 2012

Brian, I think you did not read what jkirkebo wrote. Before you take into account efficiency, you need to keep the motors / gearboxes intact. There is a hard limit on the rpm, and I don't think it comes from the electrical part (even so there are arguments against different ratios, but it's already excluded).

Timo | October 4, 2012

Well, you could cap the top speed for higher RPM motor (rear), and still have different gearing for motors. Front motor could have higher top speed torque giving system better high speed acceleration.

This could lead the somewhat confusing driving experience though, not steady acceleration with diminishing returns for high speed like single motor systems. I too think that system in reality just has two motors with both having higher top speed.

Brian H | October 5, 2012

I seem to recall the statement that the allocation of electricity to each motor was seamless, under the control of the PEM circuitry. Optimized for best torque per Wh expended.