On the fly bridle calculations are the bane of many riggers existences. There are many methods, including but not limited to formulas with Greek letters that you need a degree in applied mathematics to truly understand, the Pythagorean Theorem that we all learned in high school, and then there is the trusty cheat sheet that you and your peers have made through trial and error in your home venues. I am offering up an alternative. It is a method that I have dubbed The Universal Cheat Sheet. Using this method, you can calculate your bridle legs quickly and accurately in the majority of venues.
There are two circumstances where this method does not work that should be noted at the onset. If your anchor points (most often I-beams) are at different elevations, or if your anchors are low and have a wide span, this method will not work. With different elevations, the method just simply does not have a mechanism for taking the differing elevations into account. On low ceilings with wide spans between the beams, your hook height is going to be too low. It has been my experience that most ceilings 60’ or taller with spans of 20’ or smaller this method works for a hook height of about 43’. If the ceiling is taller or the bay is smaller (or more often both), the hook height possible becomes taller. For example if you had a 60’ ceiling with 10’ bays your possible hook height becomes about 50’. Or if your Ceiling is only 40’ and your bays are 20’, your possible hook height is about 23’… which is probably too low.
Now that we’ve discussed the limitations, let’s talk about how we use the Universal Cheat Sheet. First you establish how wide the distance between the anchors are. That distance is going to be the length of one of your short bridle leg (indicated as X in the drawings). The short leg is always attached to the anchor point closest to the motor. You then figure out how far between the beams the motor lands (in the form of a fraction) and multiply the length of the known leg (X) by the number on the cheat sheet that corresponds with the fraction you established to get your long leg.
For example, let’s pretend we have 12.5’ bays. If your motor is 1/3 of the way between the bays you would multiple 12.5’ by 1.155 and find out that your long leg is 13’ 11”. In the same bay you have a motor that is 1/8 of the way across the bay. You would multiple 12.5” by 1.335 and find out your long leg is 16’ 8”. At this point all you need to do is translate these numbers into lengths of steel rope, shackles, deck chain, etc. and then decide how long of a stinger to add to get your desired hook height.