The thing I think most people don't realize is that the axles physically have to push the car (and pull - in the case of the front).
What you should imagine is that the axle(s) is/are like the horse(s) and the rest of the vehicle is the cart. People tend to imagine that the engine is physically pulling the vehicle through the engine mounts, but it's not.
Try to imagine building a toy out of a little car axle mounted under a kid's red wagon. You could sit in the wagon with a cordless drill, reach around underneth and plug it into the axle where the driveshaft is supposed to attach, and you could power the wagon forward by triggering the drill. It might try to twist the drill out of your hand, but you wouldn't feel like you were pushing the wagon. The drill would be like the engine. The axle is is doing the pushing.
When you drive a car, the engine twists the driveshaft, the driveshaft twists the gears in the differential housing in the center of the axle system, and this transmits the twisting force (torque) out laterally into the axle shafts which turn the wheels relative to the axle housing.
As the wheels try to roll forward, there's an equal and opposite force on the axle housing trying to twist it back. It's just like when you use a hand drill. If the bit gets stuck in the work, either the work is going to start to whip around one direction (equivalent to the wheel rolling forward) or else the drill is going to crank out of your hand the opposite direction (equivalent to the opposing torque on the axle housing).
The way that the driveshaft and axle are connected allows for a small amount of relative movement between them, but this needs to be carefully controlled. When the axle housing trys to twist backward, this is called "axle wrap". Too much can start to strain the driveshaft components, specifically the universal joints, and these can fail.
Trucks with leaf spring suspension are more susceptible to axle wrap than linked suspension designs. This is because the leafspring can bend in a way that allows the axle housing to twist in place. It's especially true when lift is added by replacing the spring from under the axle to over the axle - the so called "spring-over-axle" modification. Interestingly, a truck with leaf springs is actually getting "pushed" by force being applied from the axle to the leaf spring and from the leaf spring to the frame. The spring is physically pushing the truck!
The unwanted degrees of flex that are inherent in a leaf spring design suspension can be resolved by using a linked suspension design. "Links" are rigid mechanical struts which attach with pivoting joints and "constrain motion". Our FJ Cruisers use a "4-link and panhard bar" rear suspension design. The engineers reading this will no doubt be quick to point out that the vehicle and the axle are also "links", rendering this actually a 7 link physical system... but they're nerds.
The concept of a 4 link suspension is that there are 2 lower and two upper links which attach to the axle at low points and high points respectively. As the wheels turn FORWARD, the axle housing tries to twist BACKWARD, but this is constrained by the fixation points of the links. As the twisting force is applied, it pushes FORWARD at the bottom on the lower links and it pulls BACKWARD on the upper links. It's a lot like a person standing up on a water ski. The skier leans back, pulling on the rope, pushing forward with their feet on the ski.
In the linked suspension system, these 2 opposing structures constrain against axle wrap. The pressure applied to the lower links is applied forward from the axle up into the frame of the vehicle. This is the pushing force that moves the vehicle forward. The lower links are physically pushing the car.
The beauty of the linked suspension system is that it allows for a lot of movement of the axle up and down (called "ride travel") and in a one-side-up one-side-down twisting motion (called flex) while keeping the driveshaft aligned with the pinion shaft (the part of the gear in the differential housing that the drive shaft physically torques) within specifications.
Our 4 link suspensions, however, have a very poor amount of lateral stabilzation by themselves. The upper and lower links would be perfectly happy to swing side to side. This would be bad, so an additional link is added which goes side to side. That's the "panhard bar".
Links used in this kind of automotive application are typically attached at their ends using a type of spherical bearing joint of which there are several designs. The genius of the spherical rod end is that it can only transmit a push or a pull. This means that the axle and frame, by themselves, can only impose a force perfectly in-line within the link material. Within the allowable movement of the bearings, there is no bending force that gets applied to the link.
However, this link is fairly close to the ground. It's not unlikely that it becomes a contact point in driving over big rocks. It can become a ski. It can even get to the point where the link is physically resting on a rock, while the vehicle's weight is applied to the ends. That will surely bend the stock links! Once you've bent one, you need to replace it. Repairing it by straightening would not be safe because the metal has been altered. Breaking a link is a disaster that can take out other mechanical components as collateral damage.
Replacing these links with something that has enough strength to be a weight bearing surface, and enough material resillience to do this over and over is a GREAT idea if you plan to drive on trails.
I'm sorry if that was too much information or too oversimplified. I was trying to explain this in a way that would be useful to someone with a minimum amount of background.
...my 2 cents ...for free.
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