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  #1  
Old 03-03-2011, 02:06 PM
leonheart leonheart is offline
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Default Why don't use a Multispeed Gearbox in Electric Cars?

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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  #2  
Old 03-03-2011, 03:25 PM
nimblemotors nimblemotors is offline
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Default Re: Why don't use a Multispeed Gearbox in Electric Cars?

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.

Quote:
Originally Posted by leonheart View Post
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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  #3  
Old 03-03-2011, 03:46 PM
leonheart leonheart is offline
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Default Re: Why don't use a Multispeed Gearbox in Electric Cars?

Quote:
Originally Posted by nimblemotors View Post
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)
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Old 03-03-2011, 04:02 PM
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Default Re: Why don't use a Multispeed Gearbox in Electric Cars?

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.
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Old 03-03-2011, 04:11 PM
leonheart leonheart is offline
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Default Re: Why don't use a Multispeed Gearbox in Electric Cars?

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Originally Posted by Woodsmith View Post
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?
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Old 03-03-2011, 05:01 PM
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Default Re: Why don't use a Multispeed Gearbox in Electric Cars?

Quote:
Originally Posted by leonheart View Post
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.
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Old 03-03-2011, 05:10 PM
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Default Re: Why don't use a Multispeed Gearbox in Electric Cars?

Quote:
Originally Posted by leonheart View Post
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.
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Last edited by MalcolmB; 03-03-2011 at 05:14 PM.
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Old 03-03-2011, 05:14 PM
leonheart leonheart is offline
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Default Re: Why don't use a Multispeed Gearbox in Electric Cars?

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Originally Posted by rwaudio View Post
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?
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Old 03-03-2011, 05:15 PM
leonheart leonheart is offline
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Default Re: Why don't use a Multispeed Gearbox in Electric Cars?

Quote:
Originally Posted by MalcolmB View Post
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.
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Old 03-03-2011, 05:58 PM
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Default Re: Why don't use a Multispeed Gearbox in Electric Cars?

Quote:
Originally Posted by leonheart View Post
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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