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.
Posts by brian_
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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:
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.
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.
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.
LOLBut 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.
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.
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.
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.
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).
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...
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?
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.
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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