Analysis of a Riser
It is the intent of this paper to describe and define the history and design considerations relating to development of the common sport main risers. The development of the 3 Ring Release, which is a subject for another paper, is not included.
Risers are the interface used on parachute assemblies to connect the links holding the lines of the canopy to the harness of the store, or load. The end which attaches to the harness is quick releasable. As are most of the sub components used in the sport today, risers find their origin in the military. Early sport risers were indeed military surplus. They were made of one strand of type 13 webbing doubled over a Capewell main release lug and secured with a confluence wrap. The ends were doubled back and sewn. All stitching was done with a class 7 machine using 5 or 6 cord.
The first modification to this basic assembly was the addition of steering guide rings. Round parachute steering lines were not loaded; to the contrary, they required slack so as not to tear off the guide ring during deployment.
With the advent of the square parachute the riser had to take on a more demanding role within the main system. The steering guide rings now must be load bearing as they would be exposed to about one fourth the opening shock load. We later learned this could go as high as 45 percent of the opening load. Additionally, the provisions at the ring now include a deployment brake release and storage sub system. There are any number of devices developed to accomplish this task; in as much as they are neither critical to the function or strength, they won’t be discussed here.
The “Trim Tab” or front riser leg length adjustment device was developed and patented by Para-Flite and is still found on some risers. This device was developed primarily for use in the development of trim angles for gliding canopies, however found employment on many sport risers. This devise is generally an after market installation.
The square parachute also brought about a reduction in number of lines going to the riser link. In 1975, we received the first batch of Strato-Stars to hit the market. The canopies came on eight inch long, Type 8 risers, which were too short to cover the distance from their attachment point in front of the shoulder, down into main container of our SST’s. The lines were permanently attached to the risers via a solid metal ring. We obviously needed to change the risers. At Jump Shack, we had been building our own Type 8 risers for some time. To solve the problem, I bought a batch of “detachable links” in a hardware store, to replace the solid rings, which we subsequently cut off and discarded along with the eight inch risers. The migration to the lighter webbing, and the reduction in the number of connector lines facilitated the use of what we now call “French” links. This type of link was originally used to attach pieces of chain. It was the first time this type of hardware was so utilized. The experiment was a great success. The “French” or Rapide link was eventually selected to replace the military (“L” bar, “U” bar – 1 ¾” type of ) link .
The next change to the riser was the implementation of the 3-Ring riser release. This implementation brought on the piercing of the web, and the installation of a grommet. The first grommets were #5’s or about one half inch diameter. The ring set is what we know today as the large set. The questioning of the strength reduction because of the hole was considered at that time and dismissed. However the .25 inch grommet was introduced and adopted by Jump Shack because it reduced the hole size and therefore didn’t reduce the web strength as much as the #5 grommet.
The sewing of the riser with “E” Thread using a zigzag stitch was introduced by Jump Shack with the release of the SST.
The mini 3-Ring set was developed with the location of a fourth, smaller ring. The old middle ring became the base ring, and was first used on the Rapid Transit harness/container system. This reduction in size gave designers the capability to utilize a narrower webbing. Type 17 webbing was considered and tested, but not incorporated by manufacturers because it was not available in “shuttle” weave. Some needle weave risers were made by riggers and a number failed. Not until type 17 shuttle weave became available did type 17 risers appear on rigs from the factories.
This riser, type 17 w/mini rings, was used successfully in the sport until micro (spectra) line, tube stows and zero “P” hit the sport almost simultaneously. Line strip, which had been with us for some time, now became more noticeable – and more commonplace. Canopy manufacturers had been improving their designs to where the canopies would now stay together, during a line strip opening, and not dissipate the opening forces before they got to the risers. The riser, which had for years undergone a strength and size reduction, now was failing.
These failures came in a number of types. The most common was the failure just above the middle ring of the set ( bottom ring of the riser), just where the main ring and the middle ring meet, when assembled. There was also a rear leg failure at the grommet. The third failure point which I don’t believe ever occurred on risers manufactured by a certificated manufacturer, occurred where the small ring mounting was pulled off during deployment.
The first failure had more to do with the 3 ring than it did the webbing and was defined as a cutting or guillotining of the webbing by the base and middle ring as they came together during opening. Risers with the edge exposed to this interface were cut during very high loads. Risers with the edge folded under were not damaged as the fold choked the cutting action. This type of failure was also encountered on type 8 risers though not in as high a frequency. The problem went away for two reasons. The first was better control of the rolled edge during manufacture, and the second was education of the field as to line strip, and how to prevent it.
The second failure was only found on risers without confluence wraps above the grommet. Without the confluence wrap the rear leg of the riser was loaded directly on the hole used for the grommet giving the rear leg a 30% reduction in strength of webbing or about 1700 lb. When the confluence wrap is correctly used the rear leg has strength of material. The actual strength of the riser at the grommet hole when a confluence is used is 4500 lb. on a dynamometer test. This test incorporated two thickness’ of type 17 at 2500 lb. and two thickness’ of 1″ square weave at 1000 lb. and a one third deduction for the hole yielding some 4600 lb. theoretical strength. This configuration represents a commonly used design.
In as much as the third ring mounting failure has not occurred on product from certificated manufacturers, and that is a low frequency situation — I think it is safe to assume that the industry knows how to build this component, and we won’t go into it.
A note about compatibility of the mini ring set and type 17 webbing. The strength of the type 17 webbing is 2500 lb. The capability of the mini ring set is about 3600 lb. The load distribution of a square parachute is about 50/50 front to rear. However, it can be as high as 90/10 left to right. Normal opening loads can and do reach 1,000 lb. Aberrant loads can reach as high as 3000 lb. Assuming a 3000 lb. load at 90% on one side and 50/50 to the legs we can compute a leg load of 1350 lb. . Therefore a web with 2500 lb. leg strength is acceptable. The leg strength on type 8 risers would of course be higher, but the overall strength of a type 8 riser with mini rings is no greater than a type 17 riser, as it is limited by the ring set.
In summary, the development of the common, type 17, sport riser has come to an acceptable product which the jumping public can depend on. The further field incorporation of developments to improve the strength of the riser with external grommets is unnecessary, and it involves unknowns. A failure has been documented where the top or small ring locked in the hole of the grommet in the external tab, as the ring rotated or tumbled past the grommet. This happened dynamically and has been documented on video. It was also repeatable. This failure however, could probably be avoided through thoughtful design consideration, and proper location of the tab. There might be a way for the external grommet tab to be used to develop an additional mechanical advantage which would help in a spin up or high load scenario.
Additionally, the reversing of the ring to minimize wear or for whatever reason, involves the reduction of the release range of the 3 ring. In a bag lock or high speed malfunction you would not be pulled into an upright position. This leaves the risers still in contact with your body with no room for the ring to tumble. Depending on ring to harness location and harness to body fit, the release might not occur until you attain a body position greater than 90 degrees. It might even require you to push on the backs of the risers to provide clearance enough for the rings to tumble.
It is not my intent to stifle progress or development with this essay. On the contrary, it is my intent to educate so that progress will proceed without repeating history. It is also my intent to educate the buying public by informing them, and the people who make “bootleg” risers, as to the pitfalls of not knowing history. I hope this paper is of help to all of them.
John B. Sherman