Can I Use the Differential as a Jack Point

Let me start off by saying that every vehicle is designed differently and you should always refer to your vehicle’s owner’s manual for answers to questions like this. Many times you will find an answer…sometimes you won’t. In regards to this particular question, even when the owner’s manual specifically says not to do it you will find a lot of debate among vehicle enthusiast. The article below is my educated opinion based on many years in the automotive industry as well as considerable research. First of all, some owner’s manuals actually say that it is perfectly acceptable to jack up the rear of the vehicle using the differential or pumpkin. In these cases the answer to the posed question is obviously yes. So what do we do if there is no definitive answer in the owner’s manual, or the owners manual say not to do it. I am not going to tell you its alright to jack up your vehicle using the differential when your manual say not to, but let’s consider a few things to help you make a decision on your own. This will only cover vehicle that have a straight rear axle, not those that have independent rear suspension and utilize constant velocity (CV) axles.

WHAT COULD GO WRONG?

Localized Physical Damage

The first issue I would consider is physical damage to the surface of the differential housing where the floor jack makes contact with it. This might be things such as denting or scratching off the paint. If your floor jack has a rubber pad, or you pad it using a rubber mat or a folded towel this shouldn’t be a problem. The only case that I can see physical damage occurring is if there is any type of cooling fins in the area that your floor jack is going to make contact. In that case the weight of the vehicle could damage the fins regardless of the amount of padding used.

Bending of the axle housings/tubes

This is the debatable issue and really depends on the design of the rear axle. On some vehicles, the differential and axle housings are one solid piece. On others, the axle housings or axle tubes are separate pieces that are pressed into the differential and then welded into place. Either way, it’s clear that the design of the rear axle is meant to support the weight of the vehicle out on the ends of the axle where the leaf springs are mounted. Even with the weight on the outside of the axle there are still bending forces generated at the center of the axle. So how are these bending forces different than those created by using a floor jack to lift the vehicle from under the differential housing? They are very very different. The difference comes from having a longer lever distance between the points at which forces are applied to the axle. In the case of the vehicle resting on the ground, the downward bending force generated at the middle of the axle is created by the weight of the vehicle being applied at the point where the leaf or coil springs are mounted to the axle. The lever that creates this force is the distance between the center line of the tire (where the upward force is being applied) and the point where the springs are mounted to the axle (where the downward force is being applied). This tends to be a fairly short distance in most vehicles and the axle and differential were designed to handle the bending force created by this lever. When you jack up a vehicle by the differential housing you increase the length of the lever which dramatically increases the bending force on the axle. Here’s why. When you raise the vehicle off the ground, you move the upward forces that were being applied by the ground at the two tires to the single jacking point. The downward forces are still being applied at the same point they were before, the spring to axle mounting points. The lever length created by this situation is now the distance between the two force application points which is a longer distance than when the vehicle was resting on the tires. When calculating the force created by a lever system the length of the lever is multiplied by the force being applied to the lever. In this case, the forces are the same, but the length of the lever has increased thereby increasing the bending force as well. The other difference is that the bending force has changed direction and is now trying to bend the axle in an upward arc. So the question is whether or not the axle is designed to handle the increased bending force in the opposite direction. My guess is that this was not part of the design criteria, but that most axles are designed with enough of a safety factor that quickly (and safely) lifting the rear of your vehicle under the differential housing to place it on jack stands isn’t going to do any long term damage. I wouldn’t however recommend leaving it jacked up by the differential for any length of time. Ultimately, you will have to make the decision.[really_simple_share]

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