-
Want to remove ads? Register an account and login to see fewer ads, and become a Supporting Member to remove almost all ads.
Off roading with rear diffs makes a huge difference
- Thread starter everydaychris
- Start date
similar to others! but the torque applied prob with the electric motors help alot
Many serious offroad vehicles have rear and even front lockers, controlled electronically or by air. Many factory vehicles, like van's and trucks can be optioned with a rear limited slip which is different, but can aid in providing power to both wheels.
These systems are nothing new and have been around for decades.
Rear steer has been around, but mostly on sports cars, not offroad vehicles. The 90's Honda Prelude had rear steering when optioned.
seenhear
Member
OK some terminology education is called for here:
All four-wheel vehicles must have some kind of differential acting between driven wheel pairs. ALL OF THEM. Even a simple 2WD sedan like Toyota Celica and a Ford Pinto. The exception now-a-days, is when there is a separate motor for each driven wheel, as is the case for a quad motor Rivian, and for the rear wheels of a tri-motor Tesla (Plaid and Cyberbeast). When each wheel in a left/right pair has its own individually controllable motor, no differential is required.
Differentials are required in every car (quad and tri-motor EVs excepted) to allow a pair of driven wheels (e.g. the rear wheels in a rear wheel drive car or for the front wheels in a front wheel drive car) to rotate at different speeds so that the car doesn't hop, skip, and jump and lose control going around the most benign corners, when the outside wheel must turn faster than the inside wheel.
So the dual motor CT had differentials already, one for the front wheels, and one for the rear wheels (and the cyberbeast has one for the front only). As do all other Teslas ever made from the OG model S with rear wheel drive single motor, to the AWD model Y and everything else.
Most cars (including all Telsas except the CT maybe) have what is called an OPEN differential, which allows different(ial) RPM at each wheel, but supplies equal torque to both wheels at all times. What this means is that when one wheel slips (and torque goes to zero) then both wheels get zero torque, and you go nowhere.
The open differential was invented in 1827. That's not a typo. It's been around for nearly 2 centuries, not just "several decades." It is a simple, classic, effective, efficient, beautiful example of mechanical design.
What most cars and trucks do NOT have, is a LOCKING differential. A locking differential (when locked) supplies equal RPM to both wheels at all times. This means the wheel with traction can use all the available torque to move the vehicle forward.
The locking differential was patented in 1941 by the Detroit Automotive Product Corporation, and first saw wide use on off-road USA military vehicles in WWII. It's part of why the og "Willy's" US Army jeep became so famous for it's off-road prowess during WWII and the Korean War.
A locking differential is mechanically different from an open differential. One cannot be turned into the other. So the CT already had locking differentials. What it lacked (apparently) was the software to control/implement them. That was recently delivered in an OTA update.
A third category of differential is the "limited slip" differential which is kind of a compromise when it comes to the needs of advanced off road drivers. It works well when one driven wheel slips on ice, or peels out at the drag strip. It's not great when a driven wheel is literally not in contact with the ground. It uses clutches to "limit the slip." In other words it adds friction to the differential so that when one wheel slips, the other wheel is not without any torque. But usually the amount of torque delivered by a limited slip diff is not enough to drive the vehicle forward on challenging terrain. "Viscous" differentials, as seen on many high performance sports cars, fall into this category. They are the best of the limited slip category, AFAIK. At one point viscous diffs were the choice for cars like the Subaru Outback and WRX, for their all-wheel drive systems. I believe viscous diffs are still the primary mechanism for most "all wheel drive" systems that are intended for light off-road duty and snow/ice. Advanced off road duty generally prefers a locking diff, or a system of similar functionality.
One way to achieve near-locking diff performance is with selective braking. With an open differential, if you can apply the brake to the slipping wheel, you increase the torque demand at that wheel. This in turn (no pun) increases the torque applied at the non-slipping wheel, since an open differential supplies equal torque to both wheels at all times. By applying the brake to the slipping wheel, you get torque to the wheel with traction, and can usually move forward. Put a slip/spin detector at each wheel, implement a braking system that can brake wheels independently, and a computer system to run it all, and you have a pretty effective off road all wheel drive system without the hassle of a locking differential. This is the kind of system used by many high-end "off road" SUVs from the factory (e.g. Mercedes G-class, Land Rovers, Jeep Grand Cherokee, Toyota Land Cruiser, and so forth). But a true locker is still preferred in the most extreme situations.
All four-wheel drive systems must have at least 3 differentials: one for the front wheel pair, one for the rear wheel pair, and one in between that splits the torque from the main drive shaft of the engine between the front and rear wheels. Each of these can be open, limited slip, or locking, or some combination thereof. Our 2008 Toyota Sequoia has a locking center diff (locking front and rear drive torques) but limited slip on the front and rear. This hybrid design works pretty well in all but the most extreme terrain.
Dual motor EVs do not have nor need a center differential; but they do need diffs at the front motor and at the rear motor. From the videos, it looks like Tesla has used a true locking differential, but it's not 100% known. They could be implementing some fancy brake controls on an open or limited slip diff to get near-locker performance.
Hope this helps for those of you who didn't know what a differential is.
By far, this is the best video explaining the need for a differential
seenhear
Member
Yes! I have that whole series saved in my Youtube play list. They are awesome. The one on suspension is one of my favs.
The main thing I wanted to make clear to everyone is that all cars have differentials. You literally can't drive without one (again save for the quad Rivian). The OP here made it sound like the CT just got a differential. What it got was the ability to LOCK its differential.
seenhear
Member
... and in case there are any "well actually" folks out there, yes, you could design a car to be 1-wheel drive and avoid the need for a differential. That was abandoned after (I think) the Model T or thereabouts.
Ehh, well actually, there are some kei vehicles from at least as late as the 90s that were primarily one wheel drive... The hijet with the rear diff lock has a rear axle that basically just powers the rear right tire all the time and then when locked (via vacuum solenoid controlled by a switch on the dash) will connect the left half shaft and spin both rear wheels together... (The more common non-locking rear axles were just plain open diffs though).
You'd also be a little bit remiss only really speaking to clutch style limited slips. Helical or "torsen" being the other major style such as the Eaton Truetrac, Torsen, Quaiffe and other brands that are more popular for off-road and long service life applications. The optionable front lsd in the Ford Raptor for instance is this style. I believe many of the military HMMWVs were as well and I'm sure others over the years.
There is no virtual locker on the AWD. That is 100% known. Where did you get the write up on Diffs? Please tell me you didn't write that.
seenhear
Member
I wrote it. Feel free to point out errors. I can take corrections and criticism well, as long as it's respectful.
I am not heavy into off roading, so if that's your hobby you probably know more than I do. I'm a mech.engr and understand the mechanical designs and physics, but don't know a lot of the products currently out there.
seenhear
Member
Thanks for both your comments. As I said to @outdoors above, this is not my hobby; I understand the design, physics, and basic principles of differentials from an engineer's perspective, but I'm not an expert on all the different products out there that off-road (and drag racing, and drifting) enthusiasts know and love to use. So, I didn't intend my post to be a all-encompassing treatise on differentials.
I mostly wanted to inform those who obviously didn't understand what a differential was, let alone what the different types were. Feel free to add on to the topic and/or correct things I said. Again, thanks for chiming in!Similar threads
