Having hauled around the 4 man OEM steel ramps for years, originally issued for the M172A1, would strongly recommend going aluminum. As NDT points out the lifting load is on your back, but even more interesting is the relief to your friends. They show up on time for loading, and aren't volunteering to go get coffee when it's time to set up the steel monsters. One man can position the AL versions!
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Original steel ramps- plenty strong and plenty heavy
Aluminum version, my new best friends
Castle Bravo had suggested the Discount outfit he just mentioned and I talked with the guy there, giving him specs. for the weight of the load, length/width etc. and he came up with a recommendation. With some pride he then mentioned that they were making ramps for the military and even for loading tanks! I then told him that was what I was loading and he got quiet for a moment and then said he would re-work the loading for additional webbing in the structure. What arrived is an amazing piece of work- excellent welding and finish, and has held up beautifully over last couple of years.
I much prefer the completely removable ramp approach, leaving the rear of the trailer open for dock loading and other adventures. With the AL ramps, as light as they are, it's no problem to stow them anywhere along or under a load as needed, and remember you can then move them around to multiple trailers or bridging needs.