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The carriage bolt going through the 2x4 cross member and down 6 inches into the 2x8 stabilizes the whole sled. Another key is to cut out pieces in the 2x8 so the 2x4 lays into the 2x8. It should be smooth across the top of the sled. You don't want the 2x4 resting on top of the 2x8's. These sleds are easy to put together and take apart too. You just have to take a mallet and hammer the cross member up and off of the 2x8 skis.
So it sounds like your not screwing the cross members to the risers, just putting carriage bolts through the cross members and into holes drilled into the risers, so you can take it apart each time? I like that idea for portability, but does that give the stability needed.