Let’s start with a little introductory physics here, about the most basic stabilizer arm you could make. All you have to do is put a spring from one corner of the arm to the other diagonal corner, and it will carry weight, and absorb shock. However, if you boom the arm up and down, the spring’s length changes significantly, and because of Hooke’s Law, that means that the force the spring puts out will change. So booming down requires much more force pushing down, and booming up requires you to pull upwards with a lot of force. This isn’t ideal, as nobody wants to have to work hard just to boom (not to mention that it could disturb the shot), so different designs have been made over the years to help alleviate this issue.
Early Steadicam arms, from the Model 1 through the model 3A, used 3 springs connected with cables. 2 of those springs ran along the bones of the arm, and one that went diagonally across the center. The tension of the arm was adjusted by screws at either end of the springs that pre-tensioned the springs. The force curve of these arms changed dramatically across their weight capacity – as you lightened the load the arm would carry, the arm required proportionally more force to boom. However, near the top of their weight range, they had a relatively gentle force curve, and required only a moderate amount of force to boom.
The term “iso-elastic” was invented by the folks at Steadicam to define a specific characteristic of a new design of arm – that the force curve of the arm was much more flat than previous designs of arms. The first arm to be termed iso-elastic (and result in the creation of the term) was the Master series arm, which didn’t have any controls for varying the force required to boom, but was always set to be “quite iso-elastic”.
The Master series arm was quite different in construction, in comparison to the 3A. It had 3 springs connected with cables, but they were arranged differently inside the arm, and rather than adjusting the lifting range of the arm by pre-tensioning the springs at their end, you adjusted the lifting range of the arm by changing where the end of the spring connected in the arm.
After the Master series arm, Steadicam continued to develop more arms that used the moving of the spring attachment points to adjust the arm’s tension. Notable examples include the Flyer, Pilot, Zephyr, and Aero arms. All of these arms use a spring across a shallow diagonal angle, and a vertical (ish) screw that adjusts the placement of one end of the spring to adjust the load capacity of the arm. This results in a relatively “iso-elastic” arm performance, with a force curve that is certainly usable.
The G series arms took this a step further. The G series arms use a spring across a shallow diagonal angle, with a vertical adjustment screw that changes the position of one of the spring’s ends to adjust the load capacity, but it also adds a second control called “ride”. This control adjusts a series of cams and linkages inside the arm that change the spring’s end as the arm booms. The result is that you can dial in the amount of “iso-elasticity” you’d like, or in more accurate terms, you can make the arm’s force curve flatter or steeper. The mechanism that does this is both quite clever and quite complex, and still covered under patent. There are additional tricks in the G70’s arm design that also play a role, like the use of a second spring hidden inside the main spring, but the real magic of the arm is in its “Ride” mechanism.
The PRO arm uses a very different mechanism, due to its use of compression springs, and is a whole different animal in terms of performance. It does not have any adjustment for the “ride” or “iso-elastic” setting, but is set at a quite pleasingly low amount of force for booming.