Today I've been thinking about, drawing, and designing the arms and door-opening-friendly nature of it.
Doors come from 1200 from the front to 1200 from the back. IE, the middle 2400 should be open at full height to accommodate most cars.
This places the pivot points @ 1/4 and 3/4 as mentioned above.
Tyres are in the range 600 to 700mm diameter and should nearly touch (< 100mm clearance) at mid height/max closeness with new full tread and max/above average pressure.
SS-7 bags are 68.58mm fully compressed and 304.8mm fully extended giving a range of 236.22mm of travel IF linear. But we're not. We need a travel range of (tyre diameter minus rail height) at the hub centre. If the rails are 50mm and the tyre is 650mm then we need 600mm of vertical travel to fully clear low doors and jack way up to clear obstacles in the real world.
Maximum load for an SS-7 is more than I wrote above. 250 PSI @ 7" diameter = 3.5 * 3.5 * PI * 250 = 9621.12750162 lb = 4364.075 kg absolute max. If we assume 3000kg total max load and derate 50% then we must be able to carry 1500kg per corner * lever ratio. To achieve our 600 travel that's roughly 600 / 236.22 = 2.54 ratio. 1500 * 2.54 = 3810kg with lots of margin which is under our theoretical max.
Smaller tyres would give a shorter travel and more options, but lower mean ride height. Given the length, and overhangs, more height isn't a bad thing, especially while negotiating tricky non-flat driveways. Looking at trailers outside, they sit at 350mm or so normally.
So carrying on with the assumptions: 100mm clearance + 650 tyre = 350 axle centre spacing from deck centre. 1200 pivot point gives arm length of 850, divided by 2.54 = 334.65 mm to the centre of the bag. 3.5 inches from centre to edge means an additional 88.9mm, bringing the total length to 423.55 mm which should look quite reasonable IMO.
What I learned from this, though, is that to design the arms, I need the bags and wheels/tyres in-hand for final measurements. I also do NOT want to cut all of this stuff by hand, so I'll have to sit down and CAD all of the parts up ready for laser/waterjet/plasma cutting.
My current design for this unit consists of not four, but 6 airbags! Two new ones are for:
- Swinging arm to allow lowering the deck fully flat without straining the vehicle's tow ball assembly. See this pic for the idea source: http://stuff.fredcooke.com/clever.3.air.bag.trailer.jpg
- An air-bag based front stand/jack for supporting it while disconnected and at full or ride height.
This means the shipments will likely be two sets of three bags as one set of 6 would not clear customs without taxation paid to a corrupt government.
Another thing: My truck is 4800 long, so if I want to give this thing optional sides AND get that truck inside with the doors closed sealed and safe, then the deck must be at least 4800 long, preferably more. To this end I will likely make the lip fully steel, and the front box longer/wider such that the alloy part is whatever 2 sheets are, but such that there is ample overall length with walls/doors attached to get them shut with the truck inside.
EDIT: Tow ball base height on the four tow-barred vehicles outside are 300, 300, 350, 350, so yes, towing at about 300-350 is ideal, jacking up way higher is fine, and lowering fully down also fine with the front-end feature.
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