JUMP ROPE DESIGN - THE BASIC PHYSICS

This page and the following will be of primary interest to true jump rope enthusiasts and coaches, and perhaps not the casual rope jumper.

 

On the last page, I discussed the general strengths and weaknesses of the major different kinds of jump ropes on the market.  On this page, I will look more closely at the basic physics that affect how a jump rope handles in general.  Don't Worry!  No formidable math formulas here.

 Some people might disagree with the generalizations listed below, but I would encourage them to consider that often times we prefer a particular rope because we are simply accustomed to it and not because it is necessarily the best rope for the situation. For example, from a physics stand point, speed ropes are not the best choice for most freestyle Double Dutch for reasons that will be discussed below. Nonetheless, it took several years for me to convince coaches and jumpers of that fact because they were used to using speed ropes for everything.

It might surprise you to know that the math formula to figure out one of the most basic questions of rope jumping - how much energy is needed to turn a particular rope if you just consider its weight - was too hard to figure out even for physicists that I approached (I'm sure one of them would have tried harder if there was a potential Nobel prize involved!). The problem is not as easy as it seems because the weight of the rope is distributed along its entire length, it's a "U" shape, etc.

Eventually, I did find an engineer who was able to put together the mathematical formula but the math is beyond me even though I had a year and a half of calculus. Nevertheless, I've been assured by physicists and my friend the engineer, that the following discussion approximates how different qualities affect jump rope performance.

Weight

The centrifugal force that is created when turning the rope increases directly in relation to its weight. Thus, a 2 ounce rope requires half the work as does a 4 ounce rope.  As you might guess the lightest type of ropes, called speed ropes, are the choice for speed and endurance events since competitors don't want to fatigue the arm and shoulder muscles sooner than necessary. Using current materials and designs, there seems to be a limit to how light you can make a rope, however, and have it still handle well - the lightest ropes weigh about 2 oz for a 9 ft. rope.  (Some rope jumpers have tinkered with a stiffer material to make them even lighter.)

A light rope is not the best choice for other situations. For example, a light rope has less momentum than a heavier rope and will "wobble" more in the wind, near misses, or against a rough floor. Therefore, a freestyle jumper using a light rope will tend to miss more frequently with skills that don't require tremendous speed if he or she uses a 2 oz. speed rope as opposed to a 3.5 oz beaded rope. (I predict that one day, ropes will be sold for competition according to their weight just like baseball bats, e.g. it's windy for the outdoor show today, let's use the 3 oz. speed ropes.)

Light ropes are also at a disadvantage for certain kinds of jumping like freestyle Double Dutch and "The Wheel" where the rope is moving relatively slowly. A slow moving light rope doesn't hold its shape well (need more centrifugal force here) and doesn't act predictably (need more momentum). Another advantage to a heavier rope is that it pulls on your arms more and gives you stronger feedback as to where it is positioned and how it is behaving allowing you to make more accurate adjustments.

Former "Thump Jumper" team members doing a difficult "Wheel" trick.
Note that the speed ropes are not holding their shape well since this type of
jumping is generally done at slower turning speeds.

Weight Distribution

If you disregard the ends where the handles are, most ropes have their weight distributed more or less evenly along their entire length. The one exception I am aware of is some of the older rhythmic gymnastics ropes that more of their weight at the center. For skills where one or both ends are release, one must also consider the weight of the rope ends or handles. With a "rope toss" (see photo to the right), for example, a rope with light handles is released with less spin than a rope with heavier handles . Although I have found advantages to heavier handles with certain skills, rope with light handles, or even no handles are more versatile in general with skills where one or both ends are released during its execution.

Length

The general rule for measuring a single rope is to stand on the middle with one or two feet and the ends of the rope should reach the armpit level. Often, however, jumpers will deviate from this rule. For example, they will often use a shorter rope for speed events for good reason. A shorter rope is less work to turn because it generates less centrifugal force (because it's shorter and lighter) and has less aerodynamic drag. The same factors might be considered for Double Dutch speed jumping although rules might specify how long the ropes should be for the event. The downside to shorter ropes is the increased miss rate, especially if Arm Action skills like Crosses (see below) are done.

Demonstrating a challenging Front-Inside Leg Cross where a slightly longer rope is desirable.

Aerodynamic Drag

The resistance a rope encounters as it moves through the air is called "aerodynamic drag" or just "drag" for our purposes here. As you might guess, the greater the drag on the rope, the more you must work to turn the rope. In fact, this is the principle force that you are fighting to keep the rope turning, and it's probably more important than centrifugal force for contributing to the work of turning a rope. In case you don't think drag is a significant factor, realize that the middle of the rope can be traveling over 65 miles per hour for an average height adult male speed jumper!  [A little math here (sorry!): length of rope from handles to middle of arc (about 3 feet) x 2π (about 6.28) x 300 turns/minute x 60 minutes/hour ÷ 5280 feet/mile.]

A number of variables affect drag. First, drag increases with speed by the square. That means if you increase the turning speed by two (say from 100 to 200 turns a minute) the drag increases by four. Increase the speed by three times (to 300 turns a minute) and drag increases by nine! Therefore, rope speed is probably limited by drag more than any other factor.

Drag is also affected by the thickness of the rope. As with speed, drag increase with thickness by the square, so a thin rope is definitely an advantage for speed jumpers. Shape also affects drag, but almost all ropes are essentially round in cross section, so any discussion about the advantages of different rope cross-sectional shapes would be hypothetical at this point.

Finally, the roughness of the rope's surface greatly affects the drag. Ropes with a smooth surface, like speed ropes have a lower drag than woven ropes that have a rougher surface.  Therefore, woven ropes require more work to turn and are also more affected by the wind outside. High aerodynamic drag ropes are not always at a disadvantage, however. Drag is not a major factor at slower turning speeds, so woven ropes work well for Double Dutch freestyle. Rhythmic gymnasts, who do many difficult tosses in their routines also take advantage of the longer "hang time" their woven ropes have when tossed high into the air.

I will look at how friction at the handle, rope handle length, flexibility, and elasticity affect jump rope performance and end with a summary on what kind of rope you should have for which situation next . .