Rob Gray :: ontheroad :: wothahellizat :: wot1 :: diaries :: issue-010


24 Apr 1999

So what does all this mean. Below is a scale drawing showing a cross-section of the truck chassis, wheels, fuel tanks and lower part of the body. No obvious problems here, plenty of clearance between the components.

Now let's apply the ten degrees rotation to the chassis and everything directly attached to it.

Oops, the fuel tanks hit the bottom of the body. The main cross-members are high enough, it's just the extension of the body that's a problem. This was intended to bring the body line a bit lower to hide the workings of the chassis etc. Obviously this won't be possible.

I could move the tanks closer to the front where the body and chassis are tightly connected and there is less movement between them but the spare wheel already occupies that location on one side. I could hang the tanks from the body instead of the chassis but I'm trying to minimise the weight on the body.

The easiest option seems to be to raise the sides of the body in the area over the fuel tanks.

Here we see the original design with the body stepping up just before the wheels.

And here is the modified design with the body stepping before the fuel tanks.

Let's back up a little now. As you can see in the first three drawings at the top of this page the body only appears to be mounted at one point, well it's actually mounted at three points so let's go over the idea behind his.

If you have two flat objects tigthly mounted together and you bend one of those objects what will happen to the other one? It will either bend as well or the mounting points will break. With a motor home body we have the same setup, both the body and the chassis are usually flat objects and they must be connected for obvious reasons. Unfortunately the chassis WILL flex. It's the old "irresistible force and immovable object" scenario, the chassis is the irresistible force and we'd like the body to be the immovable object. Something has to give.

One option is to let the body flex as well and on many body types this is valid, trouble is with a motor home we have cupboards and doors and things that won't take kindly to being twisted.

When I started designing this I got to thinking about my camera's tripod. It struck me that, no matter what terrain it was placed on it was always steady. Think about it, a four legged stool is always rocking whereas a three legged stool will be firm no matter what the floor is like. This is because you can always draw a flat plane though any three points, but not any four points. As is happens, when I started talking to those in the know, I found that this is indeed the method used for off-road vehicles.

Let's look at some drawings. In the first we see our two flat objects connected at four points. Let's say the top object is our motor home body and the bottom one is the truck chassis.

So far so good. Now let's apply some twisting to the chassis, as stated above two things can happen, the mounting points can break...

or the body can bend.

However if the body is only mounted and one point on one end, and that point can pivot, then the chassis can do what it likes without affecting the body.

Note that the chassis is connected, and considered stable, at the front, this is arbitrary on my part. One needs a reference point and the front is as good as the middle or the rear. This reference is where the body will be firmly mounted to the chassis. This brings me to another point.

As mentioned above, the difference in angle between the front and rear of the chassis was 10 degrees. I measured the front and rear because I decided to mount the body firmly at the front end with a pivot at the rear, therefore I had to allow for 10 degrees. It seems valid however to place the firm mounting in the centre with two pivots at each end. In this case you would have two lots of 5 degrees rather than one 10 degree angle to contend with.

So much for the theory but how does one actually implement this three point mounting?