I was very confident about using the factory tie-down cleats for just about any load. They appear strong and lock in place solidly when tightened down. Then when I had LINE-X installed and I saw how easy the tracks were to remove (only a few small sheet metal screws holding each track to the bed wall) and I was not so confident. Especially when the self-tapping sheet metal screws tend to lose some grip upon removal and reinstallation, or upon frequent heavy usage. I also thought it was odd that they didn't have lock washers. I guess you could drill a few holes and add some more bolts to reinforce the rails but I don't plan to haul anything that requires extreme tie down force anyway so I figured I wouldn't bother. Seeing this thread made me do some research on tie down forces and limits. Here is what I found:
The following info is from
another similar thread for the Tacoma which was reportedly confirmed by Toyota via email:
Mini Tie Downs (attached to bedrail) = 110 pounds each
Bed Cleats or Tie Down Cleats (attached to bedrail) = 200 pounds each
D Rings (mounted to lower bed wall) = 440 pounds each
I assume the above info is also true for the Tundra since the rails, cleats, and rings are the same size and type on both trucks. This means that each factory cleat is good for 200 lbs. of force and each ring is good for 440 pounds, thus you have a total tie-down capacity of 2,560 lbs. of force if you used all 8 tie-down points simultaneously. This sounds like more than enough muscle for all hauling jobs unless you carry a lot of weight with a high center of gravity and you hit the turns hard and fast.
This made me curious about the actual force which would be applied to the tie downs when hauling various objects so I did some more checking. It turns out that the actual tie down force required to secure a load is based on many factors including the acceleration / deceleration force measured in Gs, the weight of the load, the amount of friction between the load and the vehicle, the material used for the tie down, the placement of the tie downs, center of gravity, angle of the tie downs, etc.
It gets pretty complicated and there is actually a very detailed standard
published here with all the rules and recommendations. I also found a simple
calculator here but it is based on tie downs made of chain. Basically you need to be aware of the working load limit (WLL) rating for your tie down and you need to follow the guidelines for your specific type of load.
If someone has found a better calculator for common pickup bed loads then please share. My guess is that Toyota and other vehicle manufacturers probably wouldn't put tie down points in the bed if they weren't capable of securing the maximum loads to be hauled by the truck, so to be safe you should use all the available tie downs including the rings and use high quality straps or ratcheting ties.