Regarding frame extensions and wheel base adjustments, Rick Jay covered it well. I am just getting into the nitty-gritty.
The Ford E350 and E450 straight from Ford, have the same length frame from the rear axle to the rear of the frame. The difference is the gauge of the frame, the E350 frame is 0.228" thick, the E450 is 0.248" thick.
RV manufactures almost always add frame extensions behind the rear axle.
Frame extensions provide undercarriage space for waste tanks, outdoor storage, and other items, for example a generator.
RV manufactures with their variety of floor plans, they often increase and have been known to decrease a stock wheel base. Stock wheel bases are 138", 158", and 176".
Ford provides special components for wheel base modification. Components impacted from a wheel base adjustment are as follows.
- length of drive shaft or additional drive shaft
- exhaust pipe
- fuel lines
- brake lines
- parking brake cable
- wiring harness
- the frame itself
Some items are replaced while others are added in-line.
Specifically about the frame itself, the original frame is cut, extra frame is welded in-line for an extended wheel base, then the impacted area is reinforced with additional steel plating.
Wheel bases are adjusted to accommodate the requirements of the floor plan. One example is, you can't have a toilet too close over the rear axle, nor opposite the rear axle from the waste tank. The wheel base is also adjusted to better distribute the load of the house and contents to all 6 tires.
Any modification to the frame (wheel base or rear extension) has to follow the modification spec Q-18 provided by Ford. If you search the web, you can find those specs.
When shopping for a motor home, if you want an unmodified Ford chassis, then you want a stock Ford wheel base of 138", 158", or 176". The E350 was offered in all 3, the E450 in the longer 2. By chance, our 2007 E350 rig has a stock 158" wheel base. I have some comfort in knowing our Ford E350 has all standard components and uncut frame.
Maybe last year in 2018, or this year in 2019, Ford discontinued the E350 chassis because of it's limited production. The E450 comes now in all 3 wheel bases.
