Quote:
Originally Posted by SuperHatch
I misused the word "All", but there is still a great deal of load on the tailgate that would otherwise not be there.
You have the force of the weight of the ATV. For arguments sake, we'll say the ATV weighs 800lbs with you on it (600lb ATV, 200lb rider) and the CG is such that the weight is evenly split on the F/R axles (400lb each). The force B is 400lb, loaded approximately 2/3 the way up the ramp. We can also assume the ramp angle C is 30 degrees. The rear axle load B then is distributed between the resultant force D ("mother earth") and force E (the tailgate or that product). We can break force B into it's components, and when doing so we find out that the force perpendicular to the ramp is ~350lb. [ 400*cos(30) ]. Since that force is 2/3 the way up the ramp, we need to force balance the loaded member. There is a component of force D and E supporting this weight. According to Newton, the sum of the moments on any loaded member equals zero around any given point. If we analyze the moments around the point where the ramp touches the ground we have a moment of 400lb x 2L/3 and another equal and opposite moment of xlb x 3L/3. 267L = xL, the L's cancel, and we have a force of 267lb on the gate perpendicular to the ramp. The component of this force acting downwards is 308lb ( 267/cos(30) ).
Considering the rear axle load assumption of 400lb on the ATV, I would say that 308lb on the gate is a very good deal of additional stress that could cause cracking.
I made a lot of assumtions in this calculation, but the gist is there... |
One minor thing you missed -- the weight of the ramp itself, otherwise a good explanation. How about force per sq inch exerted on the tailgate at the lips at the top of the ramp? This is the primary concerning force for causing the tailgate to display stress fractures.
I'll be posting actual figures when I do some measurements in the real world using *one* ramp as one-half of the representative weight. I have a Polaris sportman 400 quad and a heavy steel ramp for it so I'll go ahead and measure the height of the top lip of tailgate to ground and then measure the angles out for you. This would only be representative of the actual ramp I use to load my dirtbike (A '04 Honda XR250R at about 250 pounds w/full tank) however.
Update:
7' ramp extended towards a 33.55" height, gives me a 23.54 degree angle. With a 250 pound dirtbike w/front tire on tailgate and rear tire off, the force exerted downwards, at the top of the tailgate this would be 270 pounds, with two ramps this would be a whooping 540 pounds if loading two bikes simulateously. IF loading my polaris quad which weighs in around 500 pounds, this would be 545 pounds distributed across two ramps exerting onto the tailgate or 272.50 pounds on either ramp pushing on the tailgate. The pressure applied on the tailgate is even more significant given the little surface area of the bolt that presses down on the tailgate (think of it as a finger pressing down on a ledge and you have the lip design of my loading ramp). These figures would be compounded with the extra weight of the ramp resting on the tailgate's lip. You get the good idea of the pressures involved in causing our tailgates to crack at the seams.
Sanosuke!