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

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#1 ·
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)?
 
#2 ·
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.

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)?
 
#3 ·
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)
 
#4 ·
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.
 
#5 ·
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?
 
#6 ·
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.
 
#12 ·
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.
 
#14 ·
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
 
#16 ·
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 ;))
 
#17 ·
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.
 
#18 ·
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 ·
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 :)
 
#20 ·
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.
 
#21 ·
still using a transmission, just not the ice transmission.

generally i think most just use 2nd gear in the manual transmission. the gear ratio is usually over 2 and therefore doesnt give the best gear ration band and the person must shift at speeds over 55.

i am suggesting a way to get a single speed gearbox and give us a final ratio of 6.2 and not spend 1000 dollars.

the single speed gearbox is also more efficient, something we need as well.
 
#22 ·
hi guys i was looking at the links you guys posted of the diff calculator and i just used the 24inch dia wheels and the 3.75 diff ratio (which seems to be close to what most cars use). and i found that at 60mph or 100km/h the motor revs at about 3000 rpm.

i then had a look at the stats for the trans warp 9 and saw that at around 3000rpm was when the motor was at its most efficient at 87%-89%.

so wouldn't it be best to keep the final drive ratio at around 3.5 - 4 (most diff ratios) so the motor spins at maximum efficiency?

if you look at the stats the motor produces 15hp with 72V and under 200amps, which isn't that much current.

having a ratio in this range also allows for a high top speed of well over 100mph, going of the maximum revs of the motor.

so why would you want to gear down the motor or have a gearbox at all?
 
#23 ·
so why would you want to gear down the motor or have a gearbox at all?
Torque, torque, torque. From a standstill, you have to overcome the inertia of the vehicle. The heavier the vehicle, the more torque you need. Smaller motors can't take the high current required to get large vehicles moving without overheating/zorching. So something like a Geo Metro could be direct driven with a Warp 11, possibly even a Warp 9 (Look at the "TransWarp" if you want a motor of that size already set up for direct drive). But something larger, like an F250, for example, would need a Warp 13, at least, if not a pair siamesed together, just to overcome it's standing inertia.

If you use a gearbox to gear down the motor, you gear up the torque, at the expense of speed, which makes a smaller motor more easily able to move a larger vehicle.
 
#27 ·
getting this single speed transmission isnt much to make. it will be the same as going to a junkyard and getting a transmission without the worry of it beiong damaged in some way.

no clutch, only 2 gears to run power through, viola, more efficient and less than 1000.

I wouldnt propose such an item unless the cost is down where it makes sense.

he bar is set to keep prices around $10,000.

this single speed transmission is looking good to be made here on a cnc machine, just need to get the drawing made and machine it out.

when done it will be offered to everyone. I would really like to mate it with a kostov motor and it be the transwarp but not 1:1, able to be made with all the different ratios available to choose from.
 
#29 ·
Possible reasons:

  • The particular car's diff doesn't have the needed ratios available
  • You have an AC motor that spins 12,000 rpm -- there are no rearends that'll gear that down enough in 1 step
  • You don't want to spin the driveshaft designed to spin at 2000 rpm at 5000+ rpm (BTW the Porsche 924/944 is the only car I'm aware of that spins the driveshaft at motor rpm)
Michael Why would I use two non shift-able step down gear sets when I could use one??

For a lot less than the cost of your step down I could find the correct ratio diff

That give me one properly engineered step down
 
#32 ·
Hi David

If you fit a higher ratio diff you increase the drive-shaft speed.
However you only increase its average speed - its maximum speed is set by the motor maximum speed

I don't think it will make any difference to drive-shaft life - the vehicle manufacturer will have tested it at max revs continuously

Which is normally an easy test - its the speed/load cycles that are the more strenuous tests

If you could get a 12,000rpm motor and a very high ratio diff to match - then you would have to get a high speed drive-shaft to go with it!!

(Bets it would be a lot cheaper than the motor or the diff-)
 
#35 ·
I look at it this way, I woudln't want to run my car at redline for an hour a day, run at its top speed for hours, or continuously ride the brakes down a hill. Yeah, it's designed to hit redline, top speed, or brake hard briefly, but I doubt designed for continuous redline duty. Likewise I generally wouldn't want to run the driveshaft at its max rpm continuously. Remember the forces go as the square of the rotational speed, so if you are running twice as fast that's four times the forces.

Let's look at a particular case. A 3.0 ratio Ford 9 inch has the driveshaft spinning around 3000 rpm at freeway speeds. If you jump to a 6.5 ratio, it's now spinning, continuously, 6500 rpm at freeway speed. Use this in a car with smaller wheels and a high rpm motor, and you could be spinning it at 7000+ rpm at regular freeway speeds. It might also be an argument for using a racing driveshaft.

The main message is if you are worried about running a driveshaft at or above its max speed, then that's yet another argument for using a tranny over a super low rear gear.

One minor correction, maximum driveshaft speed is set by the maximum vehicle speed for most cars. For the Porsche 924/944/928/Newer Corvette it is set by maximum engine speed, as the driveshaft feeds the tranny input shaft, as the tranny is in the rear.
 
#34 ·
would love to use a CVT transmission but getting a CVT to match up to motors and operate without a transmission computer is a very hard bargain. It has been done but with many hours of R&D.

the motor cant run at just 3.42 ratio. the rpms would be too low. motors getting 600 amps at low rpm = death.

Ive been explained to compare the amount of work when the rpm too low technically but I just say the rule of thumb is to not have max amps to far below 100 rpm. thats why we just dont hook the motor to the rear end. unless youy are running drag races, then trashing a motor to go fast is just fun!
 
#36 · (Edited)
this makes me wonder how you would have an ac system with a 12000 rpm motor and a drive shaft.

here's an interesting driveshaft speed calculator

http://www.wallaceracing.com/driveshaftspeed.htm#modulus

looks like the shorter the shaft the more rpm it can handle.

a 20 inch driveshaft, 3.5 inch outside diameter, material thickness .110 handles 63858 rpm! LOL, 12 inch can handle 177385 rpm
 
#37 · (Edited)
My battery drill has a 2 speed selector on top. I can use #2 to drive screws until the battery goes so low that it just stops. Then I can switch to #1 gear reduction and drive 20-30 more deck screws.

So gear reduction in an electric car can do the same thing as a gas car. It can reduce the power draw needed for take off and increase drive time, not to mention create less heat at lower speeds. You'll use much less juice and take off a tad swifter at the same time.

I would suggest a auxiliary underdrive unit with electric shift, but I think you need to disengage the clutch to shift. You could make the adapter plate to fit the auxiliary unit just as well as the tranny and input spline. But how to disengage the power while the electric shift is completed is the problem. Maybe putting a clutch bell housing right to the auxiliary unit... hmmmm. I'll have to check into the bolt patterns. Aux tranny's are extremly light and even with a bell housing and clutch set up, it would still be a lightweight add on compared to direct drive.

As for the high end of the speedometer, Chevy made a 2:08 rear end that would prolly launch your Hoopty over 1:eek: MPH. Check with your local rear end re-builder for available options for your car. then you could have a low gear for 0-50 and one for getting speeding tickets

But my question is 'can you use automatic tranny's??' I would think you need constant RPM's to run the front pump. I heard someone say 'Powerglide'? When the engine is running, the pump is turning also; at least 500RPM's. If the electric motor stops, how does this effect the tranny? Like you cant tow a car with auto tranny with the wheels on the ground, because it blows all the seals inside the tranny as only the rear pump is turning. How is this overcome?
 
#38 ·
But my question is 'can you use automatic tranny's??' I would think you need constant RPM's to run the front pump. I heard someone say 'Powerglide'? When the engine is running, the pump is turning also; at least 500RPM's. If the electric motor stops, how does this effect the tranny? Like you cant tow a car with auto tranny with the wheels on the ground, because it blows all the seals inside the tranny as only the rear pump is turning. How is this overcome?
There are some people who modify auto trannies for ev's. It seems a bit expensive at this point. The Powerglide I have seen does not have an ideal ratio, as a first gear of about 3.0:1 seems a good compromise for an ev, based on the number of people running in 2nd and 3rd gear in their conversions. In general, I would think, the more powerful your motor the less need there is for a tranny.

Dawid
 
#39 ·
Kansas EV has modified a powerglide with auxiliary pumps to keep the pressure up on the tranny.

http://www.kansasev.com/evglide-powertrain.html

the pump puts fluid pressure through the back and front.

Ive played with all the numbers to try and figure out the best gear ratio to have the best RPMs on the electric motor.

The 11 inch motors can be run 1:1 without an issue, they have more commutator bars to handle the amperage put through with the lower rpm. ONLY issue with less commutator bars available to transfer current, the MORE amperage it takes to take off. More Amps used to launch the vehicle, the less range you get. Major peukart effect in da house!

this may not be the exact technical reason but its how I understand it and an easy way to understand it.
 

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#40 ·
Not exactly. Brush area and cooling are what dictate your current carrying capability. If you run high amps at low RPM's your motor doesn't cool down enough with the internal fan. That's why people add external cooling blowers and look for motors with a large brush contact area.
 
#41 ·
ok, Ill add cooling fan speed to my explanation to why a motor will not like and die when too much amperage is applied at lower RPM. more amperage equals more heat basically.

A blower motor would make an excellent blower to force air on the commutator at low RPMs.

I knew I was close on the technicals! ;)
 
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