Custom bumper assembly

[simonov]

I am working on a 1993 26 ft Jamboree on the Ford E350 chassis with a 460. There is very little storage with this model, so I am designing a replacement rear bumper that can secure a 10x16 foot shade canopy, about 3¾ cubic feet of firewood and a steel elevated fireplace made from an old washing machine tub.

In the image below, the red and gold elements are steel angle and tubing. The pink boxes are to be made from pine, as is the blue rack for the fireplace:



Because of the relationship between the spare tire, the trailer hitch and the Yakima bike rack (the green thing), there is very little room to work with. Still, I believe I have arrived at a design that will work, and extends out only a little more than 17 inches from the rear of the RV.

My concern is with the weight. The main bracket alone will weigh some 60lbs, while the firewood cage (the tall red structure) will weigh another 25lbs to 30lbs empty. The shade canopy is one of those heavy-duty set-ups with steel tubing and weighs some 70lbs. So I am looking at 160lbs on the bumper alone, plus the fireplace that hangs off the ladder weighs about 30lbs.

The four bolts that secure the bumper to the frame are big and stout, so I'm not worried about that. My concern has more to do with drivability and other rig capacity issues.

Here's another more up-to-date illustration of the design:



Does anyone one have any suggestions of what else I ought to be concerned with here? Anyone ever done a project like this?

[j-d]

Rules of Thumb for Axle Weights:
33% or More of Total Scale Weight on Front Axle, or...
Front Axle Loaded to at least 75% of its Rating...
These can work out to about the same thing
And of course neither axle overloaded. These numbers should be on a label in door frame of the cab.
An E350 under a mid-20-ft coach is very close to its max capacity. I don't want to be the Kill-Joy Nay-Sayer, but I'd think twice about carrying extra weight in or slung under the cabover. They're an awkward structure and subject to stress and strain on the road. More weight just increases the forces and threat of developing leaks.
The good news... If you have a reasonable weight distribution, there are a few things that can be done to improve steering and tracking. The first and easiest is to be sure the front tire pressure is no higher than what the axle weight requires.

[simonov]

j-d wrote:
Have you weighed this coach? Four corner weights are best, but at least the Front and Rear axles? What's the Wheelbase? Tiny is right - IF the coach is already Front Light/Rear Heavy, or the Wheelbase isn't at least 55% of Length, handling will suffer.

I guess that's what I'll have to do. On Monday or Tuesday I'll take it out to get weighed with the tanks half filled. It's already a bit squirrelly, so it will be interesting to see what kind of leeway I have.

I understood there would be ramifications with more weight back there, which is why I was looking for some specifics. I didn't think the equivalent of a 200lb man laying on the rear bed was going to render the rig undrivable, but I'll see what the numbers say.

j-d wrote:
Are you making a new "bumper" from the Frame out, or adding to an existing square-thinwall-tubing bumper? We've had cases where a spare tire, mounted to the bumper, has torn the bumper at the bracket welds.

It's a replacement bumper that bolts to the frame using the four original bumper bolts. You can see that in the first illustration above.

I have to take this gear with us somehow. The alternative I have considered is bolting a couple of 2in steel angles to the bottom of the cabover loft on each side of the cab and using them to hold gear just in front of the loft, over the cab. But it would be difficult to haul the gear down and put it back up for every trip. And it still doesn't help with the firewood.

[downtheroad]

I'd be re-thinking this. Loaded I think you are going to create under-steer and will be wandering all over the road. Also, unless this is all attached to the frame AND reinforced, you may dump the whole load on the ground.

[j-d]

Have you weighed this coach? Four corner weights are best, but at least the Front and Rear axles? What's the Wheelbase? Tiny is right - IF the coach is already Front Light/Rear Heavy, or the Wheelbase isn't at least 55% of Length, handling will suffer.
Are you making a new "bumper" from the Frame out, or adding to an existing square-thinwall-tubing bumper? We've had cases where a spare tire, mounted to the bumper, has torn the bumper at the bracket welds.

[Tinyandthegang]

It will put a lot of weight behind the rear axle which will remove weight from the front axle and probably overload the rear axle.