My SMI Air Force One is supposed to arrive any day now. This will be my first attempt at working under the coach and the air will have to be bled down completely. So, I need some clearance for my big belly! I did a search for some plans but didn't really find what I was looking for so I started from scratch.
I created these ramps to give myself 7.5 of extra room at the axle. I used some 1x12s I had laying around. Id have used 1x10s if I had enough to save some weight
these buggers are about 60# each! Still, Im not planning on carrying them when we travel so its just a matter of storing them in the shop.
My dimensions for length started with my tires contact area. At the rear they have about 14 of total contact area (close enough for these purposes). I wanted to have plenty of room so I made the top tier 24. Then, to make the ramp gradual enough, I increased each lower tier by 6. So, for each ramp I cut boards in the following lengths: 24, 30, 36, 42, and 48.
To accommodate moving I drilled four holes in the 36 board and tied a piece of line into each one. The larger holes are 1-5/8, a size that was chosen to accommodate the knotted line. (Quick tip: when drilling with hole saws, drill from one side until the pilot bit penetrates the bottom, then turn the piece over and drill from the other side. Makes for a cleaner hole and it's much easier to pull the plug out of the saw.) The smaller holes are drilled perpendicular to the larger ones and are 5/8 to easily accommodate the 1/2" line. Pull the line through one and tie an overhand knot; push the knot into the hole. The pull the other end through and tie it off at your desired length.
I just guessed at the position of the larger holes, but it worked out. From the ramp end I measured 12 for the first hole and 20 for the second. If you scale the length of the ramp up or down you could easily adjust the measurements accordingly. Ive moved the ramps a few times and this dimension seems to work pretty well for me.
Finally, I used some 2-1/2 deck screws I had on hand to put them all together.
I could have trimmed the corners off of each piece to give a more ramp like approach, but its not really necessary. The tires climb the ramps nicely and, thankfully, my belly now fits under the coach with room to spare.
Hope this is helpful for someone.