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Discussion Starter #1 (Edited)
With the brackets which attach it to the cross bars, it weighs thirty-six pounds. The floor is a 48 inch by 48 inch sheet of 1/2 inch treated plywood. The sills around the edge are treated 2X2's. Except for the ring bolts, all hardware is galvanized.

The ring bolts are 5/16 inch with a safe working load of 90 pounds. I have a galvanized 5/6 inch bolt at each corner of the platform. All bolts including the ring bolts have a galvanized flat washer top and bottom, then a glavanized lock washer, and then a galvanized nut.

It does affect gas mileage. I don't know how much due to the fact that I had mostly headwinds and not enough miles to get a good reading. The two tanks averaged 25.6 mpg, my lowest so far. But, there were some healthy headwinds.

If I were to do it again, I would get the 4X8 foot sheet of plywood cut in in a 50 inch long piece and a 46 inch long piece instead of getting it cut in half to make two 48 inch by 48 inch pieces. I would make the long dimension go fore and aft. I would take the eight foot 2X2's and have them cut also in 50 inch and 46 inch pieces instead of getting them cut in half into two 48 inch lengths. I would then use the two 50 inch pieces along the sides of the plywood floor and trim the 46 inch pieces to fit at the front between the side pieces.
 

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Discussion Starter #2 (Edited)
Attaching the platform to the car

It's attached with two independent sets of attachments. The clamps on the cross bars or the lines to the side rails are probably sufficient without the other. But, I like to be sure.

The ines hold the platform both sideways and fore and aft. The lines have four turns, so there are eight lines holding each corner. AFter tying the square knots, I wove the ends through the pieces that went around the ring bolt and side rail, leaving the bitter end of the lines facing aft. This used airflow to help keep tham in place and also made it easy to check them for security by just looking to see that the ends were all pointing aft. You can get an idea of what I'm talking about by looking closely at the close up photo of the left rear corner. I drove for two hundred miles with this configuration with no problem.

I wanted to get some squared off U-bolts to go below the cross bars and up through the edge of the platform. I wasn't able to find them, so I made the the wooden brackets from some more of the treated 2X2. The spacers on each side of the cross bar had to be 7/8 inch tall. I ripped some of the 2X2 length-wise to get a piece that had a cross-section of 7/8 inch by 1 1/2 inch.

On one side of the brackets I have a 5/16 inch ring bolt and the other side has a 3/8 inch galvanized bolt. I used a 3/8 inch galvanized bolt because Lowe's did not have the needed length in 5/16 inch.

To drill the holes down through the top sill, the floor, the 7/8 inch spacer, and the 2X2 pice at the bottom of the bracket, I used a four inch clamp at each end of the 2X2 piece at the bottom of the bracket. I clamped everthing vertically from the top sill to the bottom ppiece and then drilled the holes down through all four layers at the same time. Since I was using a hand held electric drill, drilling thorough all four lawyers in one pass ensure that the holes in each layer would line up. After drilling one hole, I would insert the bolt for that hole and lightly tighten the nut on it to ensure that things stayed lined up.

I have about 1/16-1/8 inch play fore and aft on the brackets.
 

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Discussion Starter #3 (Edited)
More pictures may be coming.

Before removing the platform, I marked things to make it easier to reinstall it. I put arrows pointing forward on each side of the platform. I numbered the bracket spacers L1, L2, L3, and L4 from front to back. I numbered the pieces below the cross bars L1 and L2. I did the same on the right side, R1, R2, R3, etc.

After spending about three hours making the four wooden brackets, I found the "square U-bolts" in a leaf spring attach kit at a trailer parts place. Oh well.
 

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Looks like you put some time into making the Platform, turned out real nice. I did the opposite and took off the side rails and put them in the garage this past Saturday. Nice Work, looks very functional.
 
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