Maximum Overdrive

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Chavannigans

Scout Community Veteran
Mar 28, 2025
1,208
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East Texas
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This is another “So hear me out” post by yours truly.



Low Speed Overdrive would be a game changer in some situations and would make more sense than a Front Dig.



We had quite a few scale RC rock crawlers in storage and I recently got them all back out now that the kids are getting old enough to play with them. So this weekend I tore them all down and started getting them running again.





Most of them have the same differential ratios which is awesome for trail running, but one of our SCX24 Jeep Rubicons has a set of brass axles with an overdrive gear in the front differential. It is incredibly capable when you are climbing obstacles, but it doesn’t like full speeds for long periods of time.

An overdriven drivetrain has mismatched differential gears with a numerically higher ratio in the front. So basically your front tires always turn faster than your rear tires which has its own pros and cons.

Typically 10%-20% overdrive is acceptable for trails and higher speeds while 24%-30% are used in competition rock crawlers. Having a front overdrive can also make turning with your lockers engaged much easier on you as well as your vehicle.



Am I comparing Scouts to remote controls cars? Yeah I guess I am. But many people have given their lives so I can say stupid things on the Internet, where nobody knows I’m really a dog.



Woof (I digress)



A big benefit of the Scouts having a divorced drivetrain is you can drive each axle separately.



But when you have both lockers engaged in low speed situations, specifically inclines, it would be awesome if the front motor could be manually sped up to a faster rate than the rear. And while I expect this to already be a feature managed by the Traction Control system regardless if the lockers were engaged, I would love being able to adjust it myself within a reasonable range.



Maybe around 10%, give or take with a few levels of adjustment.



Obviously not an ideal feature for higher speeds with any meaningful traction for any period of time, but in some specific low-traction scenarios it may be really useful. Snow, loose rock, wet rock shelf’s. Etc.



It could disengage in speeds above 5mph and when the vehicle is on level or down-sloped terrain to prevent drivetrain stresses and undesirable steering behavior.



Having the ability to pull yourself over an obstacle is a lot safer than having to “bump it” and jab the throttle as often. Slow controlled crawling is the way to go and puts a lot less stress on your body as well as your off-roading Connection Machine.



But it would also be beneficial when you are making sharp turns in low traction situations like switchbacks covered in loose rock, snow, or soil.

That’s because the front motor would be pulling you forward in the direction you want to go, and the rear axles would push at a slower rate to make long vehicles feel even shorter offroad as it drags the rear tires to pivot around obstacles or hazards.



It could be a feature already enabled in specific offroad modes like Rock Crawl Mode or Snow/Mud Mode to varying degrees already and this could all just be a fever dream.



I’m going to go do human things now. Good night.
 
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This final one is interesting. This is the Normal mode, no differential lock, but with traction control and one pedal drive on.

Torque:

For the most part, the rear motor is being commanded with more torque, but not at the same ratio as with sport mode nor off-road mode. These are almost at a 50:50 torque ratio, probably something like 52% rear, 48% front. I could do the math, but precision here is not a big deal.
Note however, there are a few instances where there’s a larger deviation. Just after 11:14:14 (a couple of times), there’s a huge spike in torque on the rear motor compared with the front. And again at 11:14:54.

If you look at the motor speed, you can see that there was slippage of the front tires just after 11:14:14 and that’s why there was more torque sent to the rear motor: to overcome that slippage.

But that spike at 11:14:54 is different. Let’s look at the motor speeds to understand that one.

Screenshot_20250815-211429.png



Motor speed:
For the most part, the front and rear motors have the same speeds. But look at that jagged blip at about 11:14:54. The rear motor not only dropped speed dramatically, it went in reverse. But the front motor did not. What the heck is going on there?!

This is where traction control comes into play. On a conventional ICE vehicle, the tires have to all turn in the same direction. There’s no way to decouple the front from the rear without some fancy differential/transfer case hardware linkages. It’s just not an option in normal driving. But in an EV, the motors do not have to be commanded to turn in the same direction. And when there’s a reason to command the rear to turn in a different direction from the front, the EV can be programmed to do that.

Some kind of slippage happened, but not just wheel slippage. The entire truck slipped down a deep rut in the trail here and the truck tried to recover control by reversing the rear motor while still allowing the front motor to turn forward. This allowed it to regain yaw control since the vehicle was not only slipping forward, it was yawing as well. As far as the sensors were concerned, the truck was out of control in yaw and the way to recover it was to reverse the rear motor and turn the front motor forward.

Fascinating!

Screenshot_20250815-210825.png
 
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Okay, that’s about all the data I have that shows anything new.

Summary: An EV can command each of its motors to put out different torques. It can also command each of the motors to spin at the rate it decides is necessary, including telling one or more motors to turn in the opposite direction from the others.

The thing @Chavannigans was talking about in the OP is relatively simple to make happen: turn the front tires at a higher rate than the rear tires. The reasons for something like this were explained in that OP.

There are other things that can be done with EVs that were simply not even conceivable with a gas vehicle: turning the rear wheels in reverse and the front wheels forward to regain yaw control, for example.

A four motor EV can have even more interesting responses to various situations.

This is the way Ford does this with the Lightning. Other manufacturers very likely have different ways of doing things to accomplish similar goals. The Lightning is NOT advertised as an off-roader, but it does some parts of off-roading quite well. It is NOT a rock crawler. It is too long and too wide. The Terra will also not be a rock crawler. It is also too long and too wide. But the Lightning is a fantastic trail truck and the Terra is likely to be even better. The Traveler, being somewhat shorter will also be a fantastic off-roader (also, IMO, not a rock crawler—I know some people crawl very large SUVs and pickups, but almost never do they do so with a stock, brand-new, $70-80k vehicle).