Gearbox: totally unnecessary.

whitex

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My response was ‘tongue in cheek’. It wasn’t supposed to be taken seriously. But there is a serious point to be made and that’s this.

Porsche build sports cars and although the Taycan isn’t a sports thoroughbred it’s from the same stable and this will guide the creation of their products.

Every Porsche can stop from 60 in half the time that it takes to reach it. Is it necessary for a Turbo S to reach a standstill from 60 in less than half the time that it takes a basic RWD model? No, of course not, but that’s what Porsche do.

So, is it necessary for a Taycan that’s doing 130 mph on an autobahn to be able to reach 160 in the blink of an eye? Of course not, but it’s what Porsche do.

As for the range argument, so what? A Model 3 has a far better range and a superior charging infrastructure. It’s also 1/2 to 1/3 of the cost, so if a few extra miles are that important, buy a Tesla.

I do get where the thread is coming from. But when you talk about ‘optimisation’ all a Porsche engineer is going to hear is ‘compromise’.

The Taycan has a two speed gearbox because it makes it drive like a Porsche.
I think I know what you're saying, but the point is not whether the gearbox is an unnecessary splurge "to make it drive like a Porsche", but whether or not it actually takes away from the performance of the car (due to weight and shift delays), or at the very least is an less than ideal solution to some set of problems.

Trying to find an analogy, if you were trying to dig foundations for a building, and someone company said they will dig the whole thing it with a golden shovel - the argument here is not that the shovel made of gold is unnecessary, but that an excavator would do a much better job at it.
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kempez

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I think the problem is what you mean about the 'Performance of the car' and what Porsche mean by the same phrase. Porsche wouldn't have designed the car with a gearbox (at the time), if they thought it didn't need or shouldn't have a gearbox. They've designed it for use cases that they see their customers using the car and it sounds like it's not the use case you have in mind.

For me the gearbox adds to the experience driving the car. The car is also excellent at estimating range at cruise speeds (motorways etc), as well as holding the range figures it has for those kind of speeds. Is that a function of the way the drive-train works specific to the Taycan, or software? No idea, but the car as a package is excellent at high speed cruising as has been proven in many reviews/video's on the net (out of spec reviews/State of Charge).

I won't say that Tesla drive-train isn't better or more efficient. Tesla have the best drive-train out there and also the best software. But that's not Porsche's fault, it's Tesla's innovation and USP, alongside their excellent charging network/network strategy
 

tchavei

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I swear I read somewhere that somebody asked about the "shifting" feel when going from 1st to 2nd. Why it felt inefficient and noticable. Apparently someone at Porsche answered that they did it on purpose so that the driver would feel a similar experience to ICE cars. That they could have made it imperceptible but chose not to on purpose.

Hearsay but it makes so much sense knowing Porsche
 

Andy0565

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I think I know what you're saying, but the point is not whether the gearbox is an unnecessary splurge "to make it drive like a Porsche", but whether or not it actually takes away from the performance of the car (due to weight and shift delays), or at the very least is an less than ideal solution to some set of problems.

Trying to find an analogy, if you were trying to dig foundations for a building, and someone company said they will dig the whole thing it with a golden shovel - the argument here is not that the shovel made of gold is unnecessary, but that an excavator would do a much better job at it.
Understood. The problem is, if you’re known for digging holes with golden shovels and you turn up with an excavator, everyone will be disappointed because there are people who are much better at digging holes with excavators.

Having run much smaller businesses than Porsche and agonised over decisions and alternatives with teams of people, I don’t believe for one minute that the decision to stick a gear box in the Taycan was taken until every alternative that has been discussed on here was thoroughly debated and dismissed.

Can you imagine the reviews without it?

‘Well Porsche have tried to build an electric 911 but it runs out of puff at 125 mph.

or maybe

Porsche have achieved the impossible with an electric motor that pulls from 0 to 165 but at half a millimetre quid, who’s gonna buy it?
 

whitex

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Understood. The problem is, if you’re known for digging holes with golden shovels and you turn up with an excavator, everyone will be disappointed because there are people who are much better at digging holes with excavators.

Having run much smaller businesses than Porsche and agonised over decisions and alternatives with teams of people, I don’t believe for one minute that the decision to stick a gear box in the Taycan was taken until every alternative that has been discussed on here was thoroughly debated and dismissed.

Can you imagine the reviews without it?

‘Well Porsche have tried to build an electric 911 but it runs out of puff at 125 mph.

or maybe

Porsche have achieved the impossible with an electric motor that pulls from 0 to 165 but at half a millimetre quid, who’s gonna buy it?
I don't know the official complete stats, but based on my conversation with a number of dealers (spent over a year cold calling dealers all over the country trying to find an allocation - often shooting the breeze with the SA's I reached), they tell me most of their Taycan buyers are first time Porsche buyers. A large percentage of those are switching out of other EV's - Tesla primarily. My guess majority of Taycan buyers don't care if it can do over 125mph. I would bet that a quad-motor 1,200hp version Taycan which tops out at 125mph would outsell a current dual motor TTS with a transmission and a higher top end speed. Porsche could offer them both and see, or make the transmissions (4 of them since 4 motors) optional on the 1,200hp car and see if people prefer lighter, faster 0-125 vs. higher top speed.
 


Avantgarde

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I don't know the official complete stats, but based on my conversation with a number of dealers (spent over a year cold calling dealers all over the country trying to find an allocation - often shooting the breeze with the SA's I reached), they tell me most of their Taycan buyers are first time Porsche buyers. A large percentage of those are switching out of other EV's - Tesla primarily. My guess majority of Taycan buyers don't care if it can do over 125mph. I would bet that a quad-motor 1,200hp version Taycan which tops out at 125mph would outsell a current dual motor TTS with a transmission and a higher top end speed. Porsche could offer them both and see, or make the transmissions (4 of them since 4 motors) optional on the 1,200hp car and see if people prefer lighter, faster 0-125 vs. higher top speed.
Except that is not how you build and maintain a brand proposition. If porsche walked away from their fundamental principles every time they came up with a model which they sell to "new" customers who would not necessarily expect certain aspects of the porsche experience "because they have not driven a porsche before" we would end up with quite a different product portfolio today.

With all due respect this debate (not the 2-speed gearbox) is utterly unnecessary. Having worked for an auto company for 10 years, having a bunch of automotive engineer friends, and knowing how some of these decisions are made, I don't think there is any chance that porsche with its optimization freak engineers put a 2 speed gearbox into one of the greatly engineered models, only to realize after the fact that it was completely useless. I am open minded and acknowledge that automotive history is full of engineering faliures. But what is the primary (and only) evidence for this argument? "look at 0-60 time of a Taycan, it is more than a Tesla Plaid, so there you go I proved 2-speed gearbox is unnecessary". And everyone who does not accept this uber-flawed proposition are "missing the point". Well its more like.. we are DIS-missing the point.
 

cityhpper

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Well, I do like the gearbox in the Taycan. I don't expect that it is that heavy either, compared to other options available for these cars. Would be interesting if someone knew the actual mass of it.

My two latest cars prior to the Taycan was a Model S Raven, and a BMW M550d xDrive with some extra tuning. The Model S was very impressive up to 120 km/h, but from there and upwards it felt a bit more "sluggish". The BMW on the other hand, really came alive around these speeds. Driving my Taycan across Europe last summer, brought back fond memories of how it felt to drive the M550 on the same roads. Pretty shure the two-speed gearbox, along with the driving dynamics of the Taycan, contributed fairly much to this...
 

buruburu

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Well, I do like the gearbox in the Taycan. I don't expect that it is that heavy either, compared to other options available for these cars. Would be interesting if someone knew the actual mass of it.

My two latest cars prior to the Taycan was a Model S Raven, and a BMW M550d xDrive with some extra tuning. The Model S was very impressive up to 120 km/h, but from there and upwards it felt a bit more "sluggish". The BMW on the other hand, really came alive around these speeds. Driving my Taycan across Europe last summer, brought back fond memories of how it felt to drive the M550 on the same roads. Pretty shure the two-speed gearbox, along with the driving dynamics of the Taycan, contributed fairly much to this...
According to this article. 70kg for axle + transmission. Transmission by itself is 16kg. Effectively inconsequential weight in relation to the total overall weight of the car.

https://www.drive.com.au/news/porsche-taycan-a-look-at-its-motors-transmission-and-dynamic-chassis/
 


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Tesla engineered for cost cutting boredom, Porsche engineered for passion.
Pick you poison.
 

charliemathilde

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#neverscrollup

the gearbox exists because that was the only way the j1 platform could hit the minimum saleable range targets with the battery tech and power train efficiency they had.

The End
 

Kimbo996

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I think most people are familiar with this:

1657749420936.png


Notice how the 1st gear wheel torque follows the 2nd gear wheel torque in the constant HP region (around the upshift).

We can extend this further to arbitrary gear ratios.

The below graph shows a constant HP torque curve in red.
The X axis is mph.
I've marked the Y axis in gear ratio.

From there. You can draw a straight line out to the constant HP curve and that will be the motor torque curve (the straight line until the constant HP torque curve, and it follows the constant HP torque curve from there).

I've put two lines on here. One for 1st gear (purple) and one for 2nd gear (orange).
But you could put a line on there at any gear ratio and see the expected torque curve.

1657750586286.png


So, the actual torque curves are like this:
1657755090123.png

1657755124529.png


Now, the other thing to consider though is the maximum motor speed.
I've added another green, dashed line here for that.

Wherever the straight line from the gear ratio intersects the green line, that's the maximum speed for that gear ratio.

1657750542131.png


So, our torque curves will actually stop at those maximum speeds here:
1657755741410.png


And we're left with the following two final torque curves within the motor's speed range for each gear:
1657755813243.png


1657755857905.png


So, it is as @JimBob quoted from Jalopnik.
First gear for for low end acceleration.
Second gear is for top end speed.

Neither gear can do the other anywhere near as well.
And anything in-between will be a compromise of both.

So.......

Does the two-speed gearbox improve performance?
For a given motor power and within the limits of traction, YES.
Inarguably.
There's no debate to be had on that point.

Could they have done as well or better with a single speed gearbox?
Changing nothing else, no. There is no way a single speed gearbox would have been better. You are either sacrificing significant top end speed, significant acceleration, or both.

But what about the weight reduction of a single speed gearbox?
The weight for the whole gearbox is quoted at 70kg. Assume half that goes away with changing to a single speed gearbox. The weight difference is honestly negligible.

It’s less than the difference between me or my wife driving.
It's 1.5% of the mass of the Turbo S.

And acceleration equals force / mass, so you are not going to see big gains here.

So, the two speed gearbox is required then?
No. The alternative is more HP*. But that's basically it. You can maybe sacrifice some top end speed if you don't feel ~150-160mph is necessary, but not nearly enough. Not unless all you want is the first gear top speed which is lower than the speed limits of some US highways.

*or significantly less weight, but I don't see that happening anytime soon.


-------------
note: I just plugged in approximate values for the curves, so exact points maybe slightly off, but the relationships stand. I don't need any messages about how a top speed in the graph is 3 mph off or something.
-------------
This is a great post thanks. But what you are really saying is the reason the gearbox improves performance is that the rear motor is (electronically) torque limited, presumably for structural reasons, so by allowing it to rev higher at low road speeds it can delivery more power at its (governed) maximum torque. If the motor was strong enough, the two green lines could just stay on top of each other all the way to standstill. Obviously there has to be a torque limit in the system somewhere (the motor can't draw infinite current at zero rpm) but practically it's not obvious why an electric motor needs a gearbox, structural considerations aside. Do you agree?
 

Jhenson29

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But what you are really saying is the reason the gearbox improves performance is that the rear motor is (electronically) torque limited, presumably for structural reasons,
Thermal reasons. In the constant torque region, torque is proportional to (active) current. Heat (power losses dissipated as heat) comes from current (and resistance) and increases by the square of the current.
So, heat increases by the square of torque.

I’m sure there’s a structural limit on the rotor also, but in every motor application I’ve ever worked on (…a lot) it’s the thermal limit that’s an issue.

I have some motors that run at 300% rated torque momentarily. It’s all about the heat.
Obviously there has to be a torque limit in the system somewhere (the motor can't draw infinite current at zero rpm) but practically it's not obvious why an electric motor needs a gearbox, structural considerations aside. Do you agree?
What type of gearing and how many gears comes down to how you want to use your power band. You can get more power at a lower speed by using a higher gearing. Or by just adding more power (bigger/more motors). Those are your choices.
 
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Kimbo996

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Thermal reasons. In the constant torque region, torque is proportional to (active) current. Heat (power losses dissipated as heat) comes from current (and resistance) and increases by the square of the current.
So, heat increases by the square of torque.

I’m sure there’s a structural limit on the rotor also, but in every motor application I’ve ever worked on (…a lot) it’s the thermal limit that’s an issue.

I have some motors that run at 300% rated torque momentarily. It’s all about the heat.

What type of gearing and how many gears comes down to how you want to use your power band. You can get more power at a lower speed by using a higher gearing. Or by just adding more power (bigger/more motors). Those are your choices.

Thanks. I agree, especially in practice, having melted plenty of RC car motors. What I can't really fathom is why (structural / traction limits aside) given good enough electronics we can't keep the motor in the constant power region (hyperbolic torque) to an arbitrarily low rpm. Aren't heat losses proportional to power output (constant efficiency assumption) regardelss of RPM? At constant power, why should the engine heat up more at lower RPM? The electronic speed controls are obviously super sophisticated so the applied voltage should perfectly compensate for back emf to keep constant power across a pretty wide range. Anyway appreciate your insights.
 

Jhenson29

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Aren't heat losses proportional to power output (constant efficiency assumption) regardelss of RPM?
No. Has nothing to do with power. Or voltage. Just current. This is why power distribution is done at such high voltages. To keep current low.

A motor can overheat and burn up at zero (output) power. Think of unregulated locked rotor current. No rpm means no power. But I can guarantee that motor is going to cook.
At constant power, why should the engine heat up more at lower RPM?
Edit; I wrote something here, but it really needs more time/attention to be answered properly, so I’ll try to come back to it when I have more time.

Edit, the short answer is that because constant HP at lower rpm means more torque means more current means more heat.
 
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