Safety towing
The crossing doesn't weaken the chain. Tighter angles increases the force depending on what the vectors of that force are.
Sorry, but that is incorrect.
As a rigging supervisor and certified rigging inspector, I feel I have the knowledge to at least point ya'll in the right direction.
While "Catching the bar" is a good reason, that is not the purpose of the chains. The purpose of the chains is to ensure the towed vehicle does not break loose from the towing vehicle. If your tow bar should break/become disconnected for any reason, without chains, the towed vehicle is now a rolling missle. The chains provide some degree of control over said vehicle until it can be safely stopped.
Now the rigging part.
The highest rated lifting angle on rigging (whether it be wire sling, nylon strap or chain) is 90 degrees. Lowering the angle lowers your capacity. If your tow bar should break, you will have "shock load" on your chains. This is a directional pull much greater than the weight being lifted (or towed). If rigging is in a less than optimal configuration, your rigging will fail. Take for instace heavy lift crane operations. Often times you will see a "spreader bar" in use. This is used to reduce the angle of the rigging above the load in order to ensure angles as close to 90 as possible.
Crossing your chains reduces the capacity of your chains. It's simple physics (if there is such a thing).
Here is a quote from "Hoisting and Rigging safety":
Number of Legs and Angle with the Horizontal – The smaller the angle between the sling legs and the horizontal, the greater the stress on the individual sling legs. This increased stress effectively decreases the weight that can be safely lifted with any given sling size. Large (heavy) loads can be safely moved by keeping this angle as large as possible and, when necessary, distributing the weight of the load among more sling legs.
I can send you the actual numbers (angle of rigging=loss of capacity) if any of you like. Feel free to PM me.