Jeep Unibody and Ladder Frames What's the Difference?
Unibody and Ladder Frames are probably the two most common designs in vehicle manufacturing today. But there are differences between them that come into play when ordering suspension parts for a vehicle. What can be easy on one type frame can be a nightmare, or almost impossible on another. So what do you need to know? As far as Jeeps go, it's best to start all the way back to 1941. The first Jeeps had a typical ladder frame. For one thing, it was 1940's technology. Most American vehicles built around that era had a ladder frame, including both cars and trucks. Ladder frames are simple to design, build, and are more durable. When you hear people saying the older cars were "tougher and more durable", that's probably because even a car like a '57 Chevy was built on a ladder frame, and the term "built like a truck" was essentially true. The term 'Ladder frame" describes a type of chassis that is commonly two side pieces (called 'rails", like train tracks), with interconnecting frame pieces, bolted (or welded) across ways, tying the two rails together. When you look at the finished frame, it looks like a ladder. Ladder frames are not used in cars much anymore, but full size trucks still use them. Again, because they are stronger for hauling, towing, and payload use. Ladder frames are more versatile too on a truck, since they can accommodate variations on the body that bolts to them.
Jeeps, such as a military Jeep, all the way up to the JK Wrangler, use a ladder frame. There are some advantages to this feature. First of all, you can separate the entire body off the frame. The body mounts to the frame with bolts, washers, and body mounts (also known as a doughnut, puck. etc). Original factory mounts are usually made out of rubber, and maybe 1 inch thick. These cushion the body on the frame, and allow some flex between the two. They can also absorb some vibration that comes up through the suspension and frame, and into the body.
Having a ladder frame means you can add a body lift, which uses thicker doughnuts, say 2 or 3 inches, and will lift the entire body up off the frame. This allows more clearance for bigger tires. But the important thing is that they cost less, and are usually easier than installing a whole lift it.
Now, let's look at the Unibody. This type chassis is normally a combination. The body of the vehicle usually has two sub-frames: one in the front and one in the rear (though not always). There is no frame rail that goes all the way from the front to the rear. The floorboard of the body connects the front and rear parts of the vehicle. Right away you can begin to see how this would not be as strong as a full frame. But most vehicles don't need a robust ladder frame, because they won't be called on to tow or carry a few tons of cargo. Note that many Unibodies of today use a subframe, which is installed from underneath the body. The front suspension and engine attach to the subframe (rather than directly to the vehicle). This is why you sometimes see "frame stiffiners" for performance cars, and even some Jeeps. These connect the front Unibody frame rail to the rear one for added strength, and sometimes, to prevent flexing. Now, since the body is part of the frame, you can't separate the two. You can't put the frame in one corner of your shop, and the body in the other. They are one and the same. With this in mind, you can see how a body lift is out of the question. There are no body doughnuts. Indeed, the body and sub frames are welded together. In the case of a Unibody and sub frame, you must take on the suspension to get the lift you need. This is why there are no body lift kits for Jeeps like a Cherokee XJ, Liberty, or other modern Jeeps. They're Unibody.
If you've got a JK Wrangler, or a CJ / TJ, even a YJ, you're in luck. Those use a Ladder Frame. So a body lift kit is readily available for you.
If you have another newer type Jeep, from a Cherokee, to a Liberty or Compass, you're out of luck. You have a car frame, ie. a Unibody.
This is not the kiss of death though, as any XJ Cherokee owner will tell you. There are plenty of knarly off roading XJs out there, with great lift kits and mods installed, and even with the Unibody, do quite well on the trails and in competition. But when you have a Jeep, it's important to know if your model has a full frame or a Unibody, because that will effect any plans on upgrading your suspension. As an end note, older Jeeps, like an SJ Cherokee / Grand Wagoneer, will have a ladder frame. Remember, those were built when a truck was made to last a long time, and take a lot of abuse. But those days are gone. Will the 2018 Wrangler have a full ladder chassis? Or will it be a Unibody? The case is still out there for that one. We'll have to see. But if they want to keep the vehicle as tough as nails, and easy to modify, chances are, it will be a ladder frame.