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Discussion Starter · #1 ·
Hello! Looking to gain some knowledge on using a transmission vs direct drive for EV conversions. When I say transmission, I mean mounting your electric motor to the existing transmission (or a modified one). It seems that most conversions that are now using Tesla drive units opt to place the entire unit right in the rear, powering those wheels.



I understand that by using the entire Tesla unit you are installing a gear box, inverter and all those goodies, thus why you can remove your previous transmission. My question on this particular conversion, is more when can you do this? I have seen some people saying you can only take this approach with independent rear suspensions. Is this true or is it a case of IRS requires less fabrication to the conversion.



Non-Tesla conversions. By using a non-tesla motor, it seems a transmission is the best way to go. Either the existing, modified or a custom such as a powerglide. I say this as the transmission allows you to use park and then multiple gears which could allow for better efficiency or higher speeds. By going direct drive (power to wheels with no transmission) you are then limited by the rpm or your motor right?
 

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I admire your drive to make a website, but if you don't have at least general knowledge of the topic's you want to publish...your articles will never be worth anything to anyone, more like standard clickbait shizzle.

Please don't try to educate people without knowledge.

Only thing that might work is if you let other people (with the knowledge) write the content.
It would be good if there's a place where the general knowledge is written down, maybe that place exists, I didn't search for it, here on the forums a lot is written, but grinding it down to a couple of days reading vs a couple of weeks would help beginners ;)

The article on battery voltage was of the level you don't want on the internet, please don't try to learn things to people if you don't understand it yourself.
 

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Discussion Starter · #3 ·
thanks for the feedback. learning is a process and I am doing my best. Anything you wanted to share on tranmissions vs direct drive as that is what I was asking about.
 

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Discussion Starter · #5 ·
So I guess that is a no on the sharing information regarding the topic I posted about? To answer your question, learning from people with experience is obviously preferred.
 

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So I guess that is a no on the sharing information regarding the topic I posted about? To answer your question, learning from people with experience is obviously preferred.
I suggest you learn first from the many topics on the forum, and by answering the question you've answered why you shouldn't be writing about these topics before you've mastered the knowledge. And that's not something you can do by asking random questions on the forum.

Using a transmission VS direct drive depends on so many factors you can't just say when what is the best use.

And by using direct drive, the transmission is basically in the differential.

'direct drive' will be used less and less with all the OEM drive units becoming available. Using a complete Tesla drive unit is also using a transmission.
 

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I don't have any issue with someone wanting to learn, and thus asking about a technical subject. In this case, ElectricSpeedShop is gathering knowledge before publishing a blog post on the subject, which is certainly the right order. :)

In this case, it is a frequently-discussed topic, and obviously some of those previous discussions have been read, which is a great start. That research has led to two reasonably specific questions, and while it might have been better to address them in separate discussions, they can certainly be discussed here.
 

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Discussion Starter · #8 ·
I don't have any issue with someone wanting to learn, and thus asking about a technical subject. In this case, ElectricSpeedShop is gathering knowledge before publishing a blog post on the subject, which is certainly the right order. :)

In this case, it is a frequently-discussed topic, and obviously some of those previous discussions have been read, which is a great start. That research has led to two reasonably specific questions, and while it might have been better to address them in separate discussions, they can certainly be discussed here.

Thanks Brian- If I had to get more specific. I am very interested in the use Tesla drive unit. Is an independent rear suspension required to make that work, it seems like that is a majority of the use cases. Secondary to that, how does your speedometer than work? I think most use the rotation of the driveshaft correct?
 

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I understand that by using the entire Tesla unit you are installing a gear box, inverter and all those goodies, thus why you can remove your previous transmission.
All production electric cars (including Tesla) use "drive units" which incorporate either
  • one motor, reduction gearing, and a differential, or
  • two motors (one for each wheel), with reduction gearing for each motor
The housing of the inverter is usually integrated with the drive unit to some extent. The charger is integrated as well in some cases (such as later years of the Leaf).

For clarification:
  1. A differential is a mechanical device that splits the input from one shaft to two output shafts which can turn at different speeds. A car needs some way to allow the left and right wheels to rotate at different speeds while turning a corner, so if there is one input (from an engine or a single electric motor), a differential is needed for each driven end of the car. If the same input drive both front and rear wheels, then a differential is needed to split that input between front and rear as well, because the front wheels must rotate faster than the rears in a corner.
  2. Gear reduction is the system of gears (but chains or belts around differently-sized sprockets or pulleys can be used as well) which reduces the speed and increases the torque (in the same proportion) of an input from a motor to match the high motor speed to the lower speed of the vehicle's wheels.
So yes, using a complete drive unit eliminates the need for a separate transmission to provide gear reduction, and eliminates the need for a separate differential to allow the vehicle to turn while driving more than one wheel.

My question on this particular conversion, is more when can you do this? I have seen some people saying you can only take this approach with independent rear suspensions. Is this true or is it a case of IRS requires less fabrication to the conversion.
The drive unit has two outputs, driving the axle shafts to the wheel hubs. The drive unit is mounted to the vehicle structure, but the wheels obviously go up and down on the suspension... so the axle shafts have joints at both ends to allow the motion.

In an independent suspension, the left and right wheels move up and down independently of each other. If those wheels are driven, there must be jointed axle shafts to whatever is driving them. The electric drive unit just takes the place of the original transaxle or final drive unit (the component which includes the differential).

In a beam-axle (non-independent) suspension, the left and right wheel hubs are carried on a beam that connects the two sides.
  • Usually the axle shafts are in the beam, with no joints, so the differential that drives them is mounted on the beam and moves up and down with it; to do this with an electric drive unit would require mounting the whole drive unit on the beam, which is terrible for ride and handling (and would be hard on the drive unit), so that's not done in cars or light trucks.
  • The alternative is to have the axle shafts outside of the beam, running to a drive unit mounted to the vehicle structure and not the axle beam. This is called a de Dion suspension, and is now rarely used; it was used for the Ford Ranger EV. To build a custom de Dion suspension system would not be trivial, but it could be done.

So it is only feasible for a typical conversion builder to use a drive unit with an independent suspension.
 

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... how does your speedometer than work? I think most use the rotation of the driveshaft correct?
Traditionally a speedometer was entirely mechanical, and driven by a cable (actually a flexible shaft) from the output of the transmission... which is the propeller shaft (also called driveshaft). In a transaxle (which combines the transmission and the differential) that's no problem - it's still driven by the shaft that connects the transmission and differential internally. That works the same for a vehicle with an engine or one with an electric motor.

Now speedometers are electronic, driven by a sensor which looks at the same shaft which a speedometer cable would have been connected to.

It's also possible to connect the speedometer to a wheel hub, even if that wheel isn't driven. That's not done on modern cars, but a traditional bicycle speedometer was connected to the front hub this way. Now that every car has anti-lock brakes and other electronic management systems each wheel speed is measured anyway, so the car can just use those wheel speeds to indicated road speed. That works regardless of how the car is driven.

Most EVs have a special advantage to make speed measurement simple. With an engine you can't just use the engine speed to indicate road speed, because the clutch might be disconnected, the transmission might be in neutral, and even if it's in gear and driving it could be in any gear so the engine speed would need to be divided by different ratios. Because a typical EV has only one gear ratio, and no clutch, the motor speed can be used directly (with a constant scaling factor, of course) to indicate speed... the gearbox doesn't need to be involved at all, and no other sensors are required.
 

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Non-Tesla conversions. By using a non-tesla motor, it seems a transmission is the best way to go. Either the existing, modified or a custom such as a powerglide. I say this as the transmission allows you to use park and then multiple gears which could allow for better efficiency or higher speeds.
It doesn't matter whether the motor comes from a Tesla or some other brand. What you're asking about is using just the motor from an EV, or an aftermarket or adapted non-EV motor by itself, instead of a complete drive unit.

The choice is then whether to use the transmission already in the vehicle (or a transmission from another vehicle), or to connect the motor 'directly' to a final drive unit.
And by using direct drive, the transmission is basically in the differential.
To clarify the use of terminology here, if a conversion starts with a car having a separate transmission and final drive unit, the final drive is the component that the shaft from transmission goes into and the axle shafts come out of. This includes a right-angle turn, using a pinion and ring gear set. Usually, the ring is much larger than the pinion, so this gear set provides substantial reduction gearing (a single-speed transmission)... a ratio typically between 3:1 and 6:1.

So even if the original transmission is left out, there is some reduction gearing between the motor and the axle shafts, which is needed. What's missing by omitting the usual multi-speed transmission?
  • parking feature: this is nice, but not included in a manual transmission anyway, so it certainly isn't needed
  • choice of ratios: a modern high-voltage AC motor can produce full rated power over a broad range of speeds, so one ratio works fine; with DIY conversions the motor often needs a choice of ratios to keep the motor within the speed range that it produces high enough power
  • sufficient gear reduction: the motor generally does not produce enough torque to drive the wheels, and can turn much faster than the wheels, so a gearing matches the two; the gear reduction in the ring-and-pinion set of a final drive unit is often not enough.

By going direct drive (power to wheels with no transmission) you are then limited by the rpm or your motor right?
Regardless of the transmission system, top speed of the vehicle is limited by power available from the motor, and the combination of the maximum speed of the motor and the overall speed ratio between the motor and the wheels, and the outside diameter of the tires. With multi-speed transmission, this limit applies to the transmission ratio which produces the highest speed (which may not be the highest gear, due to the motor speed at which maximum power is available).

Whether keeping (or adapting from another car) a multi-speed transmission is "the best way to go" depends on the motor, the voltage available to the motor, the gear ratio of other components, performance expectations, and various installation factors.... or, as already posted:
Using a transmission VS direct drive depends on so many factors you can't just say when what is the best use.
:)
 

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Discussion Starter · #13 ·
Traditionally a speedometer was entirely mechanical, and driven by a cable (actually a flexible shaft) from the output of the transmission... which is the propeller shaft (also called driveshaft). In a transaxle (which combines the transmission and the differential) that's no problem - it's still driven by the shaft that connects the transmission and differential internally. That works the same for a vehicle with an engine or one with an electric motor.

Now speedometers are electronic, driven by a sensor which looks at the same shaft which a speedometer cable would have been connected to.

It's also possible to connect the speedometer to a wheel hub, even if that wheel isn't driven. That's not done on modern cars, but a traditional bicycle speedometer was connected to the front hub this way. Now that every car has anti-lock brakes and other electronic management systems each wheel speed is measured anyway, so the car can just use those wheel speeds to indicated road speed. That works regardless of how the car is driven.

Most EVs have a special advantage to make speed measurement simple. With an engine you can't just use the engine speed to indicate road speed, because the clutch might be disconnected, the transmission might be neutral, and even if it's in gear and driving it could be in any gear so the engine speed would need to be divided by different ratios. Because a typical EV has only one gear ratio, and no clutch, the motor speed can be used directly (with a scaling factor, of course) to indicate speed... the gearbox doesn't need to be involved at all, and no other sensors are required.

Interesting. I would think if you are buying a Tesla drive unit kit, it would have the ability to do this already. Otherwise you would need to get into the software to adjust those scaling factors?



Thanks for all the information on the IRS and using a whole drive unit as well! Much appreciated!
 

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I would think if you are buying a Tesla drive unit kit, it would have the ability to do this already. Otherwise you would need to get into the software to adjust those scaling factors?
Yes, it's in there somewhere... in the maze of supporting electronics and software. As long as you use the same overall tire diameter as Tesla (or other manufacturer of the drive unit being used) did, the speed indication will be correct, assuming that you can find it as a message on the CAN (Controller Area Network) system. If you change the tire diameter significantly, you will need to multiply the speed by a correction factor (re-scale it) between the source the value and the display device.
 

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Discussion Starter · #15 ·
Sorry to jump back to a previous post in this thread, but since you would be putting the Tesla drive unit in the rear. You are removing the drive shaft now as well correct? Basically the engine back... Engine, transmission, exhaust, drive shaft
 

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I have a VW Beetle with a DC motor mounted directly to the VW manual transaxle. I use 2nd gear below ~30 mph and 3rd gear above. A transmission is needed to get good acceleration if you have a low power motor (mine is 40 hp, about the same as the original VW motor). And the higher gear is good for freeway travel. I understand that some cars with lots of power don't have trannies.
 

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I have a VW Beetle with a DC motor mounted directly to the VW manual transaxle. I use 2nd gear below ~30 mph and 3rd gear above. A transmission is needed to get good acceleration if you have a low power motor (mine is 40 hp, about the same as the original VW motor).
I would say that a multi-speed transmission is needed if the motor power is not available over a wide enough range of speeds.

I understand that some cars with lots of power don't have trannies.
Normal modern production EVs have single-speed transmissions, so they don't shift. They can do this because they have enough battery voltage to drive their motors at full power over a broad range of speeds, so no shifting is required, not because they have powerful motors. Even the Smart ForTwo ED and Mitsubishi i-MiEV have only single-speed transmissions.
 

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... since you would be putting the Tesla drive unit in the rear. You are removing the drive shaft now as well correct? Basically the engine back... Engine, transmission, exhaust, drive shaft
... and final drive (diff), fuel tank and system, cooling system (but you'll need a replacement for a liquid-cooled drive unit)...

But why ask about the driveshaft (propeller shaft)? If you understand what it does - connecting the transmission you are not using to the final drive you are not using - why would there be a question?

When the motor (or engine) and the transmission are in the same location - directly connected motor and transaxle - there's no long shaft needed to connect them. This is true whether it is a one of the common engine arrangements (engine in front with front-wheel-drive, or rear engine with rear wheel drive), or a typical EV (such as the a Tesla with rear motor and drive, or Leaf with front motor and drive).
 

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Regarding the first few posts, I totally disagree. If the only time you could promote a cause was if you knew everything about that cause nothing would ever progress. From my understanding your site does not aim to be the definitive source of technical information. It aims to promote electric vehicles and conversions. It puts forward differing views and approaches on the subject. I think the more discussion the better.


Regarding transmissions:


It all comes down to requirements and constraints.


Cost, platform space, platform layout, acceleration, power, torque, weight, speed, flexibility, reliability, volts and amps are all variables that need to be understood to make an educated decision. These variables are quite often different for different people. Which is great because it is why we end up with so many different solutions.


Even with a DC motor in a home conversion decent results can be achieved with direct drive to a diff. But decent in terms of road car performance versus race/sports car performance are different things. In my case my launch speed is roughly halved if I launch in 4th gear versus launching in 1st gear. 4th gear (equivalent of direct drive) would be fine for road use but would not stack up in a performance situation.


Then there is the layout. In my case I wanted to run my motor in the transverse position. The only realistic options to achieve that would be to use a commercial drive unit like the Tesla or mount the motor to a FWd gearbox which has the diff built in.


The direct drive option is generally going to be a lower total weight. But the flip side is the final drive ratio is set in stone. A gearbox gives you the ability to pick a ratio that suits a given situation.


Introducing a gearbox can impact reliability. Electric motors as we all know produce great torque. So finding a gearbox to handle that torque is important. But even then a gearbox introduces a whole series of extra components which could fail or wear out. (Talk to me about cutches some time :) )


The commercial products compensate for the lack of gears by utilising great current and voltage figures. The resulting acceleration and revs means the lack of ratios is not an issue. When space, budget or weight constraints mean the higher current or voltage is not possible a gearbox helps improve the end result.


Then there is the whole reversing capability. Goign direct drive means you need an electrically controller reverse as there is no mechanical reverse gear. Generally speaking this is acceptable for almost everyone. However in my case it is not acceptable. I run a DC brushed motor. To get decent performance I must advance the the brush timing. Applying electrical reverse would mean the brush timing would be retarded and would result in arcing damage to the brush springs etc. Most DC users accept the limitation and just drive very slowly in reverse. But in my case I need to drive fast in reverse so mechanical reverse seems to be the only answer other than switch to an AC motor.
 

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Regarding the first few posts, I totally disagree. If the only time you could promote a cause was if you knew everything about that cause nothing would ever progress. From my understanding your site does not aim to be the definitive source of technical information. It aims to promote electric vehicles and conversions. It puts forward differing views and approaches on the oil. I think the more discussion the better.
Thanks for your post and Brian's too. I am a brand new member who nearly canceled and ran for the bush when I read the first few posts.
Thanks also for the clearly presented information which is I think what the original poster and myself were looking for. Excellent.
 
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