I did some simple hand calculations using 30,000 psi as an allowable yield stress. Since we don't really know what the material is, I assumed a non-structural 1010 steel. Also, we cannot be assured the vehicle will be centered on the ramps so I placed all the wheel weight on one of the 1" x 2" x .1046" wall tube members. The maximum stress occurs when the wheel is half way up the ramp. The yield stress level of 30,000 psi is reached at a wheel weight of approximately 500 lbs. That would mean the front of the vehicle could weight a maximum 1000 lbs, (500 lbs per wheel).
This is not to say the ramps will fail if the front of the vehicle weights more than 1000 lbs but rather to say the ramps are better suited for lighter vehicles.
To your question of reinforcing the ramps by placing a bridge under the tube on each side: If a 6" bridge is used and we assume a wheel weight of 1500 lbs, the tensile load in the supporting tie bar would be approximately 4,560 lbs. A 1" x 3/16" thick tie strap could be used with three bridges; one in the middle of the 6' tube (6" long) and one centered on each side of center, i.e. 18" from each end, (3" long).
I would very much recommend that you not reinforce these ramps since 4560 lbs is a high load to join with thin wall tubing. It would be best to start with ramps better suited for full sized vehicles.
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