Why don't use a Multispeed Gearbox in Electric Cars?

29327 Views 80 Replies 23 Participants Last post by major, Feb 21, 2015

leonheart

Discussion Starter · Mar 3, 2011

Hi
I'm new in this Forum and I don't speak english very well so I apologize if sometimes I'm wrong.

My principal question is the title of this Thread: Why don't use a Multispeed Gearbox in Electric Cars?

I think that the electric motor are better than ICE motor for the torque offered at 0RPM, but, generally isn't possible go over 2'500-5'000RPM.
I take the Tesla Roadster for a simply example:
http://www.teslamotors.com/roadster/specs

So the torque is usefull for the acceleration but the limited RPM's range doesn't guarantee an high Top speed.
In the case of the Roadster we have just one speed with an Extremly long Ratio. Nevertheless, It's possible go over 200km/h and accelerate 0-100 in a time under 4seconds.
So, Why don't use a gear to accelerate in less than 3 seconds and other 3 or 4 gear to go over 250-260km/h(IMHO it's possible 'cause an ICE Vehicle with 288hp can go over 250km/h)?

1 - 20 of 81 Posts

nimblemotors

· #2 · Mar 3, 2011

yeah why not, go ahead and use gears!
You'll just carry around some more weight in your car, and if you are an OEM, having a transmission manufactured to your specs might cost multi-billions extra. In a junkyard a transmission cost $100.

leonheart said:
Hi
I'm new in this Forum and I don't speak english very well so I apologize if sometimes I'm wrong.

My principal question is the title of this Thread: Why don't use a Multispeed Gearbox in Electric Cars?

I think that the electric motor are better than ICE motor for the torque offered at 0RPM, but, generally isn't possible go over 2'500-5'000RPM.
I take the Tesla Roadster for a simply example:
http://www.teslamotors.com/roadster/specs

So the torque is usefull for the acceleration but the limited RPM's range doesn't guarantee an high Top speed.
In the case of the Roadster we have just one speed with an Extremly long Ratio. Nevertheless, It's possible go over 200km/h and accelerate 0-100 in a time under 4seconds.
So, Why don't use a gear to accelerate in less than 3 seconds and other 3 or 4 gear to go over 250-260km/h(IMHO it's possible 'cause an ICE Vehicle with 288hp can go over 250km/h)?

leonheart

Discussion Starter · #3 · Mar 3, 2011

nimblemotors said:
yeah why not, go ahead and use gears!
You'll just carry around some more weight in your car, and if you are an OEM, having a transmission manufactured to your specs might cost multi-billions extra. In a junkyard a transmission cost $100.

So, the cost of production are the only reason for don't produce, in a single prototype too, a specs gearbox? It's impossible to get a gearbox to another car and modify just the final trasmission*?
Correct me if I'm wrong.

*: If it's possible modify the final transmission like in the motorcycle, replacing front and rear sprocket (Crown and pinion in the motorcycle transmission)

Woodsmith

· #4 · Mar 3, 2011

The main issue, I think, is that a multiple speed transmission is only used if it is actually needed.
With an ICE many ratios are needed because the power and torque curve of the ICE is in such a narrow band.
The change points may be from peak torque at 2500rpm to peak power at 4000rpm so only 1500rpm to work with.

An electric motor could be peak torque at 0rpm to peak power at 4000rpm so there is 4000rpm to play with and so fewer ratios needed.

With an electric drive if it can drive, accelerate and reach the required top speed direct drive or with only two ratios then why have more?

Much of the time people doing DIY conversions only use a couple of gear ratios and some even try to remove the unused gears from the box to save weight and drag.
Those who can manage with a single gear ratio will even remove the transmission completely.

leonheart

Discussion Starter · #5 · Mar 3, 2011

Woodsmith said:
The main issue, I think, is that a multiple speed transmission is only used if it is actually needed.
With an ICE many ratios are needed because the power and torque curve of the ICE is in such a narrow band.
The change points may be from peak torque at 2500rpm to peak power at 4000rpm so only 1500rpm to work with.

An electric motor could be peak torque at 0rpm to peak power at 4000rpm so there is 4000rpm to play with and so fewer ratios needed.

With an electric drive if it can drive, accelerate and reach the required top speed direct drive or with only two ratios then why have more?

Much of the time people doing DIY conversions only use a couple of gear ratios and some even try to remove the unused gears from the box to save weight and drag.
Those who can manage with a single gear ratio will even remove the transmission completely.

Yeah, It's true, but i think that an huge torque could be used to have more top speed ('cause we have it from 0 to 4'000rpm) and acceleration too.
I think that a multi speed transmission could accentuate the acceleration and increase top speed than a single ratio do. Generally, the problem in the electric car is the low top speed respect the provided power. This in the ICE motor is the opposite.
In see ICE car go over 170km/h with just 75hp, why don't have an electric car that do that with more acceleration?

rwaudio

· #6 · Mar 3, 2011

leonheart said:
Yeah, It's true, but i think that an huge torque could be used to have more top speed ('cause we have it from 0 to 4'000rpm) and acceleration too.
I think that a multi speed transmission could accentuate the acceleration and increase top speed than a single ratio do. Generally, the problem in the electric car is the low top speed respect the provided power. This in the ICE motor is the opposite.
In see ICE car go over 170km/h with just 75hp, why don't have an electric car that do that with more acceleration?

A transmission is there to multiply torque, it doesn't really have much to do with HP. Since the electric motor can have full torque from 0rpm then you don't need to multiply anymore.

Many electric conversions use controllers with high peak current, but fairly low continuous current, this could have something to do with why they don't all go "fast", you might be into current limiting on a Curtis/Zilla etc. when you really need the power at much higher speeds. I would imagine that a water cooled Soliton1 would have a better shot at making the electric comparable to a gas car. Then it comes down to how well your motor can handle (get rid of) the heat for extended periods of time.

When there are high power, light weight, water cooled AC motors I'm sure we will see a change in what "fast" is in an EV.

MalcolmB

· #7 · Mar 3, 2011 (Edited)

leonheart said:
In see ICE car go over 170km/h with just 75hp, why don't have an electric car that do that with more acceleration?

You're right of course, a two-speed transmission can give you improved starting torque and a top speed comparable with that of ICE cars. BUT, to get 75 hp at the wheels you'll need close to 75 kW from the battery. With a moderate battery capacity of 15 kWh that gives you a run time of less than 12 minutes.

As the energy density of batteries improves I'm sure there will be more demand for gearboxes designed or adapted for EVs. In the meantime a lot of people make do without, because it makes things simpler and cheaper, and because high speeds drain the pack so much faster.

That's obviously a simplification, and there are still efficiency and performance advantages to using a multi-speed transmission. Try doing a search for "powerglide" on this forum.

leonheart

Discussion Starter · #8 · Mar 3, 2011

rwaudio said:
A transmission is there to multiply torque, it doesn't really have much to do with HP. Since the electric motor can have full torque from 0rpm then you don't need to multiply anymore.

Many electric conversions use controllers with high peak current, but fairly low continuous current, this could have something to do with why they don't all go "fast", you might be into current limiting on a Curtis/Zilla etc. when you really need the power at much higher speeds. I would imagine that a water cooled Soliton1 would have a better shot at making the electric comparable to a gas car. Then it comes down to how well your motor can handle (get rid of) the heat for extended periods of time.

When there are high power, light weight, water cooled AC motors I'm sure we will see a change in what "fast" is in an EV.

Yes, but a transmission could multiply the Wheel RPM too. If we have a ratio less than 1 we have more RPM at the wheel than at the motor. If we have 2 or more ratio, we could multiply at first the torque to increase acceleration and finally more top speed. Maybe I was not clear when I explained my question, I know is not NECESSARY but I think is usefull to have more performance from the same motor.
Are here in the forum someone who modified the transmission of a car for his conversion?

leonheart

Discussion Starter · #9 · Mar 3, 2011

MalcolmB said:
You're right of course, a two-speed transmission can give you improved starting torque and a top speed comparable with that of ICE cars. BUT, to get 75 hp at the wheels you'll need close to 75 kW from the battery. With a moderate battery capacity of 15 kWh that gives you a run time of less than 12 minutes.

As the energy density of batteries improves I'm sure there will be more demand for gearboxes designed or adapted for EVs. In the meantime a lot of people make do without, because it makes things simpler and cheaper, and because high speeds drain the pack so much faster.

That's obviously a simplification, and there are still efficiency and performance advantages to using a multi-speed transmission. Try doing a search for "powerglide" on this forum.

Really thanks MalcomB.

Woodsmith

· #10 · Mar 3, 2011

leonheart said:
Yes, but a transmission could multiply the Wheel RPM too. If we have a ratio less than 1 we have more RPM at the wheel than at the motor. If we have 2 or more ratio, we could multiply at first the torque to increase acceleration and finally more top speed. Maybe I was not clear when I explained my question, I know is not NECESSARY but I think is usefull to have more performance from the same motor.
Are here in the forum someone who modified the transmission of a car for his conversion?

I know what you are getting at.
You are suggesting that one could gear an EV so that the motor is spinning at, say, 100rpm and travelling at 100mph because there is sufficient torque in the hypothetical motor to do that. You are supposing that it would equate to 200mph at 200rpm, still running close to maximum torque.

Well, if there was enough torque to overcome the various resistances I guess in theory it may work as a purely mechanical thing (though the gear wheels will be massive to cope with the enormous stresses) but the motor will be drawing maximum current for long durations and running very inefficiently.
It would be a toss up of which bit failed first, the motor over heating and burning out, the controller shutting down, the batteries melting, the cables/breakers giving up.

There are very good reasons why an EV will aim to cruise at near maximum rpm as that is when the current is lowest and efficiency highest for that speed.

If you were to want to drive at very high speed you would work you ratios to get the highest motor speed that just provided enough torque to maintain that speed and no more.

In the real world....
Actually you are not looking at real world are you, so it doesn't matter for now.

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rwaudio

· #11 · Mar 4, 2011

leonheart said:
Yes, but a transmission could multiply the Wheel RPM too. If we have a ratio less than 1 we have more RPM at the wheel than at the motor. If we have 2 or more ratio, we could multiply at first the torque to increase acceleration and finally more top speed. Maybe I was not clear when I explained my question, I know is not NECESSARY but I think is usefull to have more performance from the same motor.
Are here in the forum someone who modified the transmission of a car for his conversion?

What I'm getting at is the ratio's aren't what's holding back EV's from being fast. Many people keep the transmission, me included but that doesn't mean I'm going to have a 160+mph car.

michaeljayclark

· #12 · Mar 4, 2011

not looking to go fast, just 70 is fine for me.

my conversion has a 3.42 rear end. I want to have a one speed transmission as the original poster has said. he was merely looking for someone that is using something else instead of the transmssion.

I have been looking at custom airplane gearbox reducers. these I believe could be adapter to our purpose as well.

http://glasairproject.com/Marcotte/Page2.html

a basic site dedicated to these gearboxes.

If I have a 1.81 gearbox, the motor would be at 74 rpm at 1 mph, then at 5212 rpm at 70 mph.

final gear ratio of 6.19. seems to be just about perfect. perfect would be 3.83 rear end gear but close enough.

I have contacted the site owners for more information.

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dtbaker

· #13 · Mar 4, 2011

...and you really do need some rpm to drive the internal fan for a little cooling.

Duncan

· #14 · Mar 4, 2011

Hi Leon

A gearbox can give you additional torque at lower speeds - good idea -Until you can't use any more torque!
If you can spin your driven wheels more torque is redundant!

The Tesla can do 0-60mph in 3.7 seconds - I don't think it will be able to put any more torque on the ground (with road tires)
Its top speed is 125mph - enough to get a ticket almost everywhere!

For a Tesla a gearbox is unnecessary weight and complication

For the rest of use - most of us are power limited as far as top speed is concerned but most EV's can get off the line fast enough to give an IC car a fright

Duncan

· #15 · Mar 4, 2011

Hi Michael

Which drive are you using?
Can't you find a decent diff ratio to avoid the additional complexity?

My Subaru has a 4.1:1 diff - I think its the same diff as used in some Nissan's

michaeljayclark

· #16 · Mar 4, 2011

I am using this calculator

http://xse.com/leres/ss/calculator.html

I change the 1st gear ratio to 1.81 and all the others to 0.

I change the rear axle ratio to 3.42

my tire size is 235 70 15

then it changes and shows a final ratio of 6.19. it defaults to show at 55 rpm my motor is at 4589 rpm

MPH @ 6000 RPM is 71.9

its a great calculator.

what I am trying to do here is eliminate complexity. If we can eliminate the transmission we are reducing weight. we are also getting rid of 3 other gears we dont need in the EV. reverse is easy with a reversing contactor.

Ive been researching the gearboxes and it turns out electric boats us them. they have calculated that the gearbox is 97% efficient.

imagine looking at the transwarp motors from netgain that have a yoke attached at a 1:1 ratio. Imagine this gearbox attached to the back of the motor with a yoke to ujoint up to the rear end and having a choice of the gearbox ration to give your EV a final ratio of 6.2!

Ive been asking about how many RPM does kostov motors need to have before it sees actual amperage and doesnt melt. I havent been able to get too many answers unfortunately because I dont think the answer is truly known.

the more rpm the motor gets before it seems serious amperage the better! If I use a 1.81 ratio and hit the pedal to the metal my motor will be at 83 rpm when it see 600 amps from a solition jr. thats at 1 mph using the calculator. If I change the calculator to what rpm my motor will see at 2 mph is goes up to 167. at 3 mph its at 250 rpm....

an additional blower to blow air into the motor at lower rpms would only help and hey, Ive got a used blower motor that came out of a 1996 jimmy here that nnoone wants! (Ive listed it on craigslist, noone wants it

)

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Woodsmith

· #17 · Mar 4, 2011

michaeljayclark said:
what I am trying to do here is eliminate complexity. If we can eliminate the transmission we are reducing weight. we are also getting rid of 3 other gears we dont need in the EV. reverse is easy with a reversing contactor.

I found this reversing gearbox that would save the electrical issues of reversing contactors.
Nova Racing - Reversing Gearbox.
It is used on motorbike engined Caterham Sevens, Triking Morgan replica, and other similar cars.

JRP3

· #18 · Mar 4, 2011

Just thought I'd point out that Tesla tried to use a 2 speed gearbox and a number of vendors failed to build one that could handle the shift under full load without breaking. Since most people just don't care about going more than 125mph they decided to simply increase the power to the motor and skip the gearbox.

G

· #19 · Mar 4, 2011

The Tesla also has an AC motor which can run at a much higher RPM than the typical DC motor of the DIY crowd. The DC motor has better torque and we can use the transmission to our advantage and still go fast. You can change the final drive gear in the transmission to better utilize the limits of rpm with the DC motor. Most don't need to go 125 mph. Most are just regular vehicles on the road to get you from there to here or here to there. No need because speed limits are still in effect.

Pete

DavidDymaxion

· #20 · Mar 4, 2011

Most of the successful single gear ratio electric cars have twin motors, or a single huge 13 inch motor. Using the tranny greatly lowers the startup current and heating, so you can use a single 9 or 11 inch motor instead.

If you really want it all, great startup and high top speed, you really need a transmission. The Buckeye Bullet (315 mph!) used a five speed tranny. If you are happy with a mere 170 mph (Killacycle) or 125 mph (Tesla or NEDRA racers) then a single gear can work great, provided you have a stout enough controller, motor(s), and pack.

leonheart said:
Hi
I'm new in this Forum and I don't speak english very well so I apologize if sometimes I'm wrong.

My principal question is the title of this Thread: Why don't use a Multispeed Gearbox in Electric Cars?

I think that the electric motor are better than ICE motor for the torque offered at 0RPM, but, generally isn't possible go over 2'500-5'000RPM.
I take the Tesla Roadster for a simply example:
http://www.teslamotors.com/roadster/specs

So the torque is usefull for the acceleration but the limited RPM's range doesn't guarantee an high Top speed.
In the case of the Roadster we have just one speed with an Extremly long Ratio. Nevertheless, It's possible go over 200km/h and accelerate 0-100 in a time under 4seconds.
So, Why don't use a gear to accelerate in less than 3 seconds and other 3 or 4 gear to go over 250-260km/h(IMHO it's possible 'cause an ICE Vehicle with 288hp can go over 250km/h)?

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