dumb question

[BMW X5]

what does a unibody mean when referred to the X5? ?

[jwalsh22]

I believe it is talking about the chassis. There is the unibody chassis, which is, as its name implies, one solid piece. Then you've got one that is assembled from different parts (forgot the name) and is usually done so for larger vehicles where a unibody chassis would cause too much hassle. Correct me if I'm wrong guys...

[BMW X5]

so why do some people say a unibody is bad? is it because if say in an accident one part gets damaged its basically unchangeable?

[obiparis]

Unibody vs. body-on-frame? I maybe wrong.

[LeMansX5]

The new Benz ML are unibody construction believed to be better.

[Kewl X5]

Most traditional pick-up trucks and SUVs were frame-on-frame. However, in terms of ride comfort and handling, having a uni-body is great. However, I wouldn't really go major off-roading in a uni-body car. Each design has its pros and cons.

If I was really going off-road and/or hauling lots of stuff in a pick-up bed, then a frame-on-frame can be more durable.

However, if your AWD SUV is mainly for street use and mild rough road/off-road use, unibody will serve a much better platform for handling and ride. Thus, BMW X5 and Lexus RX300-330 has unibody. The original ML was a frame-on-frame, but most people didn't take their MLs off-roading. However, the on-street handling and ride is abominable when compared with the X5. The new generation ML is built like the X5 (geez, Mercedes have been copying BMWs lately with i-Drive and stalk control for auto tranny, etc.)

[BMW X5]

thanks, i was just curious what the difference was i get it now...

[JCL]

What the unibody has going for it over the body-on-frame construction is torsional stiffness, which helps the engineer obtain better handling. Because the frame (unibody) is stiff, compliance can be designed into the suspension. It will also be tighter, which usually means fewer rattles and creaks.

As Kewl said, the downside (compared to a real truck) is load-carrying ability. I just don't know what a frame-on-frame design is

Another small point: the unibody is not a chassis, it is a frame. The chassis starts with the frame and includes the suspension and other bits.

The unibody is a modification of the monocoque, a design where all body panels are stressed and constitute the body and frame as one. A unibody has a central structure that is all welded, but there are some panels that are then bolted on.

A unibody will generally be better for the passengers in a collision, as the entire frame structure can be designed to absorb the impact. The downside is that since it has been deformed in a collision, it is more expensive to fix. Not impossible, with the right equipment, just more expensive.

While each design has merits, the unibody is more suited to a performance oriented road vehicle, and the body-on-frame construction is more suited to an off-road vehicle or a truck.

Incidentally, having the unibody design is one of the primary reasons why the trailer hitch is so expensive for the X5. It is easy to make a hitch for a vehicle with a separate frame, it is harder to make one (correctly) for a unibody design, particulary when you are dealing with the 6000 lb tow limits of the X5.