# Gearing down in direct drive EVs



## DrGee (Aug 22, 2018)

Here's a newbie question. If I chuck out the transmission in my EV conversion project, how do I gear down the revs from the motor to the wheels? Are there off the shelf single speed "gearboxes" with a 3:1 or 4:1 ratio or thereabouts?


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## brian_ (Feb 7, 2017)

DrGee said:


> If I chuck out the transmission in my EV conversion project, how do I gear down the revs from the motor to the wheels? Are there off the shelf single speed "gearboxes" with a 3:1 or 4:1 ratio or thereabouts?


Yes and no. The only suitable unit that I am aware of - assuming that you still have the final drive reduction and need an additional stage before that - is the ev-TorqueBox. It is stronger than most conversions need (because it was intended for trucks), and it is a limited-production product, so it seems wildly expensive to me for what you get... but it works. Apparently there are various ratios available, but I doubt that they are as high as 3:1 or 4:1.


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## Duncan (Dec 8, 2008)

You use a differential - like the one in the back of a Subaru - or a BMW 
Or most rear wheel drive vehicles

You get the gearing + the diff , if you are careful you can get a Limited Slip Diff

Most diffs are about 4:1 - just right for using a DC motor at normal highway speeds


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## brian_ (Feb 7, 2017)

Duncan said:


> You use a differential - like the one in the back of a Subaru - or a BMW
> Or most rear wheel drive vehicles
> ...
> Most diffs are about 4:1 - just right for using a DC motor at normal highway speeds


Yes, the question is from someone who (at least tentatively) is planning on using the front and rear final drives of a production 4WD vehicle, and that's what I meant by this:


brian_ said:


> ... assuming that you still have the final drive reduction and need an additional stage before that...


The reduction of a typical final drive alone is enough if you are using a motor which only goes to perhaps 3,000 rpm. If you want to use a broader speed range of a higher-speed motor, more reduction is required - that's what an additional gearbox would be for.

It is also possible with common final drives ("diffs", "axles") to change the ring and pinion gears to get a different (typically more reduction) ratio, but the choice depends on the specific brand and model.


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## electro wrks (Mar 5, 2012)

DrGee said:


> Here's a newbie question. If I chuck out the transmission in my EV conversion project, how do I gear down the revs from the motor to the wheels? Are there off the shelf single speed "gearboxes" with a 3:1 or 4:1 ratio or thereabouts?



More details would help. Front wheel drive? Rear wheel drive? Lower RPM DC motor(Duncan's specialty)? Higher RPM AC motor(more and more popular)?


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## DrGee (Aug 22, 2018)

brian_ said:


> Duncan said:
> 
> 
> > You use a differential - like the one in the back of a Subaru - or a BMW
> ...


These are great answers people.. Thanks so much. 
I will be keeping the final drives( front axle & rear differential). I hadn't realised that the ratio in a final drive was that high. Should be adequate for the pair of forklift DC motors I plan to use.


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## DrGee (Aug 22, 2018)

electro wrks said:


> DrGee said:
> 
> 
> > Here's a newbie question. If I chuck out the transmission in my EV conversion project, how do I gear down the revs from the motor to the wheels? Are there off the shelf single speed "gearboxes" with a 3:1 or 4:1 ratio or thereabouts?
> ...


So sorry, I forgot to mention - it's for a 4wd using a pair of DC forklift motors. I'm trying to avoid cost and complexity.


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## brian_ (Feb 7, 2017)

electro wrks said:


> More details would help. Front wheel drive? Rear wheel drive?


While it doesn't really matter whether or not it is rear wheel drive, it does matter if the final drive is included in a transaxle (front wheel drive, or rear-engine rear wheel drive, or even some front engine rear wheel drive if they have a rear transaxle), or if there is a separate transmission and final drive.

In this case, the original vehicle would have a transmission on the back of the engine, plus an integrated or bolted-on transfer case for the 4WD system, driving two propeller shafts to front and rear final drive units... so that's separate final drives.


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## brian_ (Feb 7, 2017)

DrGee said:


> So sorry, I forgot to mention - it's for a 4wd using a pair of DC forklift motors. I'm trying to avoid cost and complexity.


The idea of using a motor through a fixed reduction ratio (whether it is just one reduction stage in the final drive, or two stages) is certainly workable... especially with high voltage systems as used in modern EVs. With relatively low-voltage systems peak power is only available over a narrow range of speed; with a lot of motor power for the vehicle weight (like Duncan's Device) that's okay, but it may or may not be acceptable for other motor-vehicle combinations. The advantage of keeping the transfer case (and thus using only one motor for both axles) is having high (1:1) and low (reduction from less than 2:1 to as extreme as 4:1, depending on the transfer case) ratios for normal driving and low-speed situations.

So, simple is good, but there are consequences to every choice.


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## electro wrks (Mar 5, 2012)

DrGee said:


> So sorry, I forgot to mention - it's for a 4wd using a pair of DC forklift motors. I'm trying to avoid cost and complexity.


 DrGee, when you skip to a new thread, on the same build, you need to bring the rest of us up to speed on the basic info from your original thread. Better yet, try to stick to one thread to avoid the confusion. Most of us(except maybe Brian?) have a life outside of this forum and can't read and parse every post that's ever been written.


Another option might be to reference your previous threads or posts.


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## brian_ (Feb 7, 2017)

electro wrks said:


> DrGee, when you skip to a new thread, on the same build, you need to bring the rest of us up to speed on the basic info from your original thread.
> ...
> Most of us(except maybe Brian?) have a life outside of this forum and can't read and parse every post that's ever been written.


 LOL

But seriously, the 4WD scenario is of particular interest to me, I had responded to the main thread, and it was recent, so I happened to remember it.



electro wrks said:


> ... Better yet, try to stick to one thread to avoid the confusion.
> ...
> Another option might be to reference your previous threads or posts.


To be fair, the question in this thread is a distinct and specific topic, which would be reasonable without the whole context - all it needed to include was the use of the original final drive unit (diff).

There is an advantage to keeping a thread to a more narrow topic - it is less likely to get sidetracked and become a massive pile of discussion in which no one can later find anything. A link to the other thread "for anyone wanting more background" would work fine for me.


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## DrGee (Aug 22, 2018)

brian_ said:


> electro wrks said:
> 
> 
> > DrGee, when you skip to a new thread, on the same build, you need to bring the rest of us up to speed on the basic info from your original thread.
> ...


Thanks for your reply Brian! 

I decided to move my questions about the transmission to the other thread - AC Vs DC, in "DIY wiki discussion"


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## MattsAwesomeStuff (Aug 10, 2017)

I have a general question on the topic...

When repurposing a rear-diff as a simple reduction gearbox...

Does that imply just direct driving a RWD car, or, are some of you suggesting taking one and just mounting it for a FWD?

To get a reduction, do you need to be at 90 degrees from the input? (I.E. mount the motor vertically or, I guess torpedo-wise rather than sideways & horizontal?).

I presume for FWD you're lopping off the axle parts and then, somehow joining those to your CVs?


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## brian_ (Feb 7, 2017)

MattsAwesomeStuff said:


> When repurposing a rear-diff as a simple reduction gearbox...
> 
> Does that imply just direct driving a RWD car, or, are some of you suggesting taking one and just mounting it for a FWD?


This approach has been used for FWD, but in this case DrGee is talking about a 4WD vehicle which comes with similar final drive units at front and rear.



MattsAwesomeStuff said:


> To get a reduction, do you need to be at 90 degrees from the input? (I.E. mount the motor vertically or, I guess torpedo-wise rather than sideways & horizontal?).


The input shaft (propeller shaft) to the rear of a front-engine vehicle is obviously 90 degrees to the axle, and in a typical 4WD system with a longitudinal engine the same is true of the front (the front propeller shaft is running forward from the transfer case which is in the middle of the vehicle).

No common final drive unit will be designed for a vertical input shaft, although some DIY builders have apparently planned to - and even built - vehicles with the final drive rotated to place the shaft of an attached motor vertically. This is a bad idea in multiple ways.

A final drive with a transverse input will normally be part of a transaxle, so you wouldn't likely use it unless you were using the the transmission as well. There are lots of 4WD (AWD) vehicles with a transverse engine, and converting them while using the final drives would presumably mean using the transmission as well, and a single motor.



MattsAwesomeStuff said:


> I presume for FWD you're lopping off the axle parts and then, somehow joining those to your CVs?


Front axle parts are similar to rear axle parts from a vehicle with independent rear suspension, although outer joints may be a different type of CV (for greater angular capability at the front), and shaft lengths are usually very different left to right in the front of a 4WD system (because the final drive sits to one side of the engine).

For bonus confusion, the output of the final drive which passes under the engine sometimes goes right through the engine crankcase, and even if it is separate it is usually in a fixed tube so that the inner CV joint is on the opposite side of the engine from the differential.

For DrGee this is not an issue, as he would use the complete final drive and axles that come with the vehicle (as long as the housing can be separated from the engine). If adding this style of final drive to a vehicle which had a different system, custom axles would be needed (a common conversion challenge).


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## mizlplix (May 1, 2011)

Rule 1: If you live in steep or rolling hills, stick with a transmission.
Rule 2: If you are going direct drive with no clutch or transmission, the lower you are below 2500 Lbs the better.

Rule 3: Simple is best. EXAMPLE: My car is 1,900Lbs. I live in a flat area, but still have a powerglide. I can drive around in high gear all the time, but if I want to zip ahead sometime, that low gear is awsome......

Rule 4: Gear your EV as you would a gas motor. Motor RPM / Tire height/ desired road speed.

EXAMPLE: I have an AC50 @ 6,500 RPM. My tires are 32" tall, SO I have a 6.14:1 rear axle ratio. It gives me a normal speed range.


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## DrGee (Aug 22, 2018)

mizlplix said:


> Rule 1: If you live in steep or rolling hills, stick with a transmission.
> Rule 2: If you are going direct drive with no clutch or transmission, the lower you are below 2500 Lbs the better.
> 
> Rule 3: Simple is best. EXAMPLE: My car is 1,900Lbs. I live in a flat area, but still have a powerglide. I can drive around in high gear all the time, but if I want to zip ahead sometime, that low gear is awsome......
> ...


Okay Mizlplix, 
Great details & very relevant points.. Love it! 
Now, 
1.I live where it's flat as a pancake- fortunately.. 
2.The car is a Jeep Grand Cherokee - about 2 tons. 
-but if I use two forklift DC motors as planned, does it change #2? 
3. Can you describe your powerglide? 
Cheers!


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## brian_ (Feb 7, 2017)

There are some huge assumptions being made here, which is usually the case for "simple rules" - they are generalizations which only work for the assumed case...


mizlplix said:


> Rule 2: If you are going direct drive with no clutch or transmission, the lower you are below 2500 Lbs the better.


This assumes a specific size of motor, a specific motor speed range, and some final drive gearing. Obviously with a larger motor, a single-speed transmission works for a heavier car, and with a smaller motor, the single-speed transmission would only work for a lighter car. What is the assumed motor size/power?


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## DrGee (Aug 22, 2018)

In the Grand Cherokee, the central transfer case can be separated easily from the 6 speed automatic box. Here's a consideration - 
I remove the gearbox with the ICE and do a direct drive from a single large motor to the central transfer case. All-time 4x4 will be retained & no need for two motors.. Any comments?


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## brian_ (Feb 7, 2017)

DrGee said:


> In the Grand Cherokee, the central transfer case can be separated easily from the 6 speed automatic box.


I assume that we're still talking about a 2011 (model year) Jeep Grand Cherokee WK2. If Wikipedia's list is correct, this has a Mercedes 5-speed or Chrysler 5-speed (depending on engine), not the later Chrysler 6-speed. The number of gears isn't important, but different transmissions may come with different transfer cases, which may mount differently. All should detach reasonably easily from the transmission.



DrGee said:


> I remove the gearbox with the ICE and do a direct drive from a single large motor to the central transfer case. All-time 4x4 will be retained & no need for two motors.


Fundamentally, it should work.

You gain the simplicity of a single motor and single controller, and don't need to coordinate front and rear motors, but then you must carry the weight and complexity of the transfer case and live with whatever it does for front-to-rear power distribution (which may depend on the transmission controller that you don't have any more, although it may be independent).

Gearing is then fixed (as it would be in any solution not using a multi-speed transmission) to what the transfer case and final drives include. There were at least two different transfer cases, one with a single ratio (presumably direct) and the other with two ratios (presumably a low and direct). The low and high ratios may not be usable as a routine first and second gear, as frequent shifting between them is not expected.


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## DrGee (Aug 22, 2018)

brian_ said:


> DrGee said:
> 
> 
> > In the Grand Cherokee, the central transfer case can be separated easily from the 6 speed automatic box.
> ...


I've decided to use my 2005 WK Grand Cherokee. It has nearly 200,000 miles on it, but still runs flawlessly. (Couldn't justify searching for a 2011 with a blown engine, when I've already got my '05 which I know so well).

You're right about the gearbox - it's a 5 speed.
I believe you're right about the two types of transfer cases too. Mine is the the single speed NV140. It has an electronically controlled clutch pack & splits torque 48/52, front /rear. So both speed & torque are fixed, but I don't mind. 
I wonder if the electronic control will be an issue since the sensors might be unhappy without the ICE.. 

Both front and rear differentials are conventional ( not limited slip). They've got a ratio of 3.07:1. 
I may need to increase the ratio at these final drives, but this all depends on the characteristics of the mighty motor I'll need (rpm at what safe voltage etc). I'm going to enjoy finding one. Let's see if I can better Damien..


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