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lagann
Member
I've read the problem ends up being the motor can change speeds so quickly, and the torque rips apart the transmission like tissue paper.
jerjozwik
Active Member
lagann
Member
Its not the engine spin up, its the torque. Its no different than a souped up diesel motor tearing apart the gearbox from an economy four cylinder.
For a passenger car, as Tesla has found, the case to build a reliable and efficient transmission doesn't close. You end up with something too expensive, too complex, and too heavy. For other applications, it makes sense. For instance: http://www.fiaformulae.com/en/guide/car.aspx?page=1334
Fia formula-e rules specify 5 speed gearbox, but it does not make any sense. Gearbox is needed if your engine does not produce enough torque at low rpm.
I believe motor from Tesla M85 is cheaper, more efficient, more reliable and lighter than their gearbox-motor combination. It certainly has enough torque for a 900 kg car.
I believe motor from Tesla M85 is cheaper, more efficient, more reliable and lighter than their gearbox-motor combination. It certainly has enough torque for a 900 kg car.
The gearbox is needed to maximize acceleration at all vehicle speeds. The single speed Model S is very impressive at lower speeds but peters out at higher speeds relative to similar ICE vehicles.
I don't doubt the Model S drivetrain is cheaper, but for that application its not going to be more efficient and reliable over the Formula E spec units. And...I'd bet that the Formula E drivetrain is probably not that much different in mass than the Model S. The forumla E gearbox probably only weighs 25-30kg.
thegruf
Active Member
I dont get this thread.
What could you want for a road car that the Model S doesnt offer?
sub 5s 0-60; 155 mph. Everything and more you could need from a road car.
Tesla's brilliance is the simplicity of engineering.
A gearbox could improve motor efficiency a bit but at the expense of complexity and weight which would absorb the gains anyway.
Sure for track cars and dragstrip monsters other technologies are relevant but with the exception of Germany (even there only on select roads), national speed limits are iro 80mph/130kmh maximum around the world and the MS is plenty quick enough and certainly doesnt "peter out" at these speeds.
For track use, cooling is the primary issue, maybe one day Tesla will release and enhanced cooling pack option for those who want to track the car.
Gearboxes are more about ICE thinking, than Tesla EV thinking, ie about as relevant as gasoline is to the MS imho.
my 2c :wink:
What could you want for a road car that the Model S doesnt offer?
sub 5s 0-60; 155 mph. Everything and more you could need from a road car.
Tesla's brilliance is the simplicity of engineering.
A gearbox could improve motor efficiency a bit but at the expense of complexity and weight which would absorb the gains anyway.
Sure for track cars and dragstrip monsters other technologies are relevant but with the exception of Germany (even there only on select roads), national speed limits are iro 80mph/130kmh maximum around the world and the MS is plenty quick enough and certainly doesnt "peter out" at these speeds.
For track use, cooling is the primary issue, maybe one day Tesla will release and enhanced cooling pack option for those who want to track the car.
Gearboxes are more about ICE thinking, than Tesla EV thinking, ie about as relevant as gasoline is to the MS imho.
my 2c :wink:
A gearbox in a Model S could get you more low speed torque or a higher top speed.
but it's not going to get you more power at 40-80 mph.
At those speeds, you're limited by how much power you want to pull out of your battery.
The electric motor is quite efficient at a wide range of RPMs, so if you're pulling 300 kW out of your battery, your wheels are doing nearly 300 kW of work no matter what gearing you choose.
At low speeds, however, the motor has a torque limit and isn't able to pull 300 kW. If you change the gearing, you can get more torque to the wheels for the same torque in the motor. That would mean that the motor would be spinning faster at any given speed, and therefore pulling more power out of the battery at maximum torque at low speed. The tradeoff is that now your motor is at its RPM limit at a lower speed, and even though Americans rarely drive over 100 mph, I think most of us don't want to think that our car is limited to a top speed of 100 mph.
For both the P85 and the P85D with the stock sticky tires, the gearing is set up so that at low speeds (<30 mph), the torque to the wheels is just on the edge of breaking the tires loose. So if you were to choose a 1st gear with a shorter gear ratio, you'd also probably want to change the tires/wheels to something even stickier than the stock set up. and then you could choose a 2nd gear that goes above 150 mph.
but without new tires/wheels, a gearbox isn't really going to improve the Model S 0-60 or 0-80 times noticeably. You need to pull more than 320 kW in order to do that. Limiting yourself to 430 hp out of the battery on a 5000 pound car is really meh when a Viper is 640 hp on 3500 pounds.
From 80 to 100 mph, a 2nd gear might help slightly because the motor efficiency does start to drop at those RPMs, but I'd be surprised if it's enough to make it worthwhile.
If you like driving over 100 mph, then, yeah, a gearbox could help.
I don't, so I don't want a gearbox.
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Everything you could need, yes. but everything you could want? nah.
Model S's performance from 40 to 100 mph is actually lacking for an expensive car.
It's nice that the power is instantly there, and that you don't have to wait for the gearbox to shift and the engine to whine, but one could certainly want the better passing performance of a M5, S6, Viper, etc.
Of course what I really want is more mundane: quieter climate control, better key detection, a customizable touchscreen view, etc.
Either way, a gearbox isn't the answer to what I want.
Yes, P85 acceleration peters out at higher speeds relative to similar ICE vehicles. That is caused mainly by lack of power. Power = RPM*torque (with unit conversion)
At full acceleration Tesla P85 power meter falls slightly before top speed (from about 200 km/h to top speed). That could be corrected by 2. gear. A lot better fix is larger motor or 2. motor to front.
If I remember correctly M85P motor weights 40 kg. So more powerful motor is lighter than weaker motor + gearbox. It is also more reliable and more efficient.
Electric motor type used by Tesla has poor efficiency only:
1: Low power (3 kW taken from 300 kW motor). Tesla D models are more efficient because of smaller front motor can be used alone when only low power is needed. Multiple gears wont help here.
2: At max torque. Solution: Motor with so much torque that traction control cuts inefficient region out. Lower gear would help, but larger motor is better solution
Well...a great many things if you're asking, though none of them (IMO) have to do with the drivetrain. Tesla definitely settled on the right solution for a road car application.
I brought formula-E in as an example of how a multi-geared transmission can actually be the optimal choice for an EV, at least for a particular use case...the unfortunate misunderstanding of context by some sort of muddied that point, but...its just the internet so no biggie. :wink:
I think they did it to make the cars noisier
Putting straight cut gears into the boxes really makes the cars scream around and as a spectator you get much more of a sense of speed as the cars audibly change gears coming into corners. (Along with a big thunk as they shift).
As it's a single manufacturer series right now, they also probably wanted to base it on existing Formula Renault series suspension elements where the gearbox was a structural member of the chassis.
That seems likely reason!
Will they really change gears or use optimum gear all the time...
roblab
Active Member
Right. Go from two or three moving parts to 150. Or only 100. Makes sense. Cuts costs. Increase maintenance, complexity, breakage. If automatic tranny, change filters, oil, replace clutches.
Transmissions were made to multiply torque. Motors have full torque from just about zero rpm. Gas cars have full torque at about 2500 to 4000 rpm. Gas cars have transmissions because they HAVE TO.
manis
Member
A lot of people don't exceed 65. So I'm sure there would be no problem finding buyers that would be fine with a top speed of 70 or 75 and even more if it saved them $5k-$10k.
thegruf
Active Member
Rasing the fact that Formula E has gearboxes is very interesting.
For a race car it is entirely correct that gearboxes are fitted and peak hp and torque are never achieved across the entire rev range, there requirement for optimal performance overcomes simplicity.
You're spot on that the is a lot to wish for on the Tesla, but for a road car drivetrain to legal limits I really think the drivetrain, given current technology is absolutely spot on.
There is one possible argument in favour of a gearbox on the Tesla - would is permit a smaller (importantly lighter and cheaper) motor to be used, sufficiently powerful enough to provide adequate acceleration at low speeds then change gear to provide worthwhile acceleration at higher speeds. Problem then is the gearbox weight negates the weight benefit. Efficienct saving by operating the motor more in its "sweetspot" are likely to not be discernable in real world use.
As others have said the complexity just renders this a mostly an idea with more negatives than positives, simplicity wins every time.
Oooh, interesting point. Yeah, that's a possibility.
Fluid filled gearshift transmissions are extraordinarily expensive, finicky, and generally problematic: they will never be viable for an electric car. It just won't be worth it. Multiple traction motors with different windings is a much more realistic possibliity.
Nissan LEAF has a top speed of 94MPH. I don't see people complaining about that.
I drove a Ford RangerEV for years with a 74MPH top speed. Once a while I wish it had a few more MPH, but usually it was OK.
People do read about and compare 1/4 miles times, so I think the gearbox should at least offer a respectable 1/4 mile time. For a car like that, the "trap speed" would be ~100MPH, so I think it would be a good idea to gear for at least 100MPH top speed, but not much more.
I don't think Tesla should be designing with the German Autobahn in mind. That is a special case perhaps better handled by some other kind of vehicle.
