Leif,
Following your post I thought I should look at my Benbo today. When I stripped it down I found that a lot of paint had flaked off but had not fallen out. I think these flakes acted as a slippery surface. Picture attched. I cleaned the paint off and added a penny washer on each side of the bolt. Some improvement when the legs are splayed out, but still not good enough. The design requires the spike section to be the main contact area with the ground. Beyond a certain point the legs will want to carry on splaying out, and there is nothing in the design of the leg top sections to prevent it.
I have had this tripod for years and never thought of the simple solution before today. For other readers of the thread I have attached a picture of a foot. The hole is moulded into the hard rubber end and is obviously, ( I can say that now
) for a spike.This really only applies to use on damp/soft surfaces, not much use into chalk or flint. For harder surfaces swinging spiky plates would probably be the way to go, or possibly even a length of thin rope tying all thee legs together.
I have attached a pic showing the Benbo in use, effectively nailed to the deck with garden plant supports. This really does work. You can even slacken the main bolt to move the arm up or down as the legs are not going anywhere. The file size for this pic is still quite large despite being massively compressed to get it here.
Don