How to Size a Backplate

by David Chamberlin
Photos by Sami Laine and Clinton Bauder

The backplate and wings setup is one of the defining pieces of DIR equipment. This configuration embodies many of the properties that DIR divers demand from their equipment. It is streamlined, durable, easily maintained, adaptable, minimalist, robust, and highly configurable, to name a few. Ironically, one of its greatest features - its ability to be configured for any body size and shape - leads to problems for many new DIR divers. The harness setup truly deserves the one-size-fits-all moniker. Unfortunately most dive store employees are not well versed in the proper sizing and configuring of a backplate harness. The result is that divers are left with ill-fitting equipment that can cause discomfort or even pose dangers to the diver.
By far the most effective way to ensure a proper fit is to have a knowledgeable professional provide direct, personal assistance. With the success and proliferation of the GUE DIR Fundamentals class, this option is becoming more accessible to a large number of divers. The GUE instructors work through proper sizing and configuration of equipment with each student individually, assuring a proper setup, as well as helping the students understand the process. If you are not able to attend a DIR Fundamentals class, or would like to get a head start, the following are some pointers on achieving a proper fit and correct configuration for your harness setup.
It is very important to perform all of the sizing and positioning while wearing the full exposure suit: full undergarments and drysuit, full wetsuit, etc. A thick exposure suit can have a dramatic impact on the sizing of the harness.
The best place to start is with the arm straps. Here we are trying to balance two constraints.
Figure 1
The arm straps need to be loose enough that the harness is comfortable and that the rig can be easily doffed/donned both in-water and out-of-water. The straps also need to be tight enough so that the tanks do not slide around the back and stage bottles clipped off to the chest d-ring stay close to the chest.
Start by loosening the arm straps to the point where there is a lot of slack and put the harness on. Position the plate in approximately the correct position on the back. As a starting point, place the top of the plate a little below the first pronounced vertebrae at the base of the neck.
Figure 2
With the plate in this position, you should be able to touch the top of the plate (Figure 1). If you can accomplish this, your chances of reaching your valves when your rig is fully assembled are good. With the plate in that position, tighten up the arm straps somewhat snug. The straps should not be tight - you should be able to get at least 2 or three fingers underneath the strap (Figure 2). You want it loose enough so that you can slide the straps off the shoulders (Figure 3). This should also enable you to pull the rig over your head when doffing/donning in the water. Again, you need to avoid making it too loose.
Figure 3
With excessive slack in the arm straps, the plate will ride low on your back, making it nearly impossible to reach your valves. In addition, the shoulder straps will hang loose on your body when in the horizontal position, which will be exacerbated by stage bottles clipped to the chest d-ring, causing the stage bottles to hang in the slip stream, inducing increased drag.
Once the shoulder straps are correct, next work on the waist strap. The waist strap adds much of the stability in the harness configuration.
Figure 4
Whereas the arm straps are somewhat loose to add comfort and mobility, the waist strap is fairly snug to keep the rig in place.
As much as possible you want the waist strap to just come straight around and not "sag" too much (Figure 4). However you don't want it to restrict breathing from your diaphragm, so if it is restricting breathing or you have a somewhat pronounced belly, you may need to let it go down just a bit. The waist strap should be snug, but not so snug it restricts movement or breathing. When adjusting the waist strap, make sure to set the buckle so that it is off to your right side. With the buckle in this position, the crotch strap is unable to accidentally open the buckle. The waist strap buckle can also be used to hold the light canister in place. However if the rig needs to be removed in-water, it is suggested that an additional buckle be slid (not threaded!) on the strap to hold the light canister. This will prevent the canister from sliding off the waist strap when the waist strap buckle is released.
The final piece of the harness that needs to be sized is the crotch strap. The crotch strap's two primary functions are: keep the rig from riding up (primarily at the surface), and for use with a tow-behind scooter.
Figure 5
If a failure occurs while scootering wherein a diver needs to be towed, the towed diver holds on to the crotch strap of the towing diver. Therefore the crotch strap needs to be loose enough that a diver can comfortably grab on to it, yet it also needs to be snug enough so that the rig does not ride up.
Start by buckling the waist strap as you normally would, but do not run it through the crotch strap. Now pull the crotch strap up to the waist band. Set the length so that the top of the crotch strap reaches the top of the waist strap (Figure 5). If you want to give it even an extra 1/2 inch or so, that should be fine. Many people make this too short, which then pulls the plate down and restricts the ability to reach the valves.
As a final check of the sizing, with the plate on, the crotch strap on, waist strap tightened, you should be able to reach back and touch the top of the plate (Figure 1). If you can accomplish this, then likely the sizing is reasonably close. The true test will be in the water. Don't be afraid to readjust the lengths based on in-water experience.
Once you have the sizing about right, you will want to get the d-rings into the proper position. Incorrectly positioned d-rings can cause a variety of problems, from inability to effectively clip/unclip gear, to increased drag caused by stage bottles, and even inability to dump gas from the BC inflator hose. The harness should have at most 5 d-rings: 2 shoulder d-rings, one d-ring on the left hip, one d-ring on the rear of the crotch strap, one d-ring on the front of the crotch strap.
The shoulder d-rings are where most of the gear is clipped off. These d-rings should be positioned relatively high on the arm straps for a variety of reasons.
Figure 6
One of the primary reasons is that stage bottles need to be held close to the chest. If the d-rings are too low the stage bottles will hang away from the diver out of the slipstream causing drag, banging the reef, etc. D-rings that are too low will also cause problems with positioning of the backup lights, especially on shorter-torso people, as there will not be enough length to fit the light snugly. An often-overlooked aspect is the ability to dump gas from the wing. Since the bungee cord that holds down the inflator hose runs under the triglide for the d-ring, if the left side d-ring is too low, it will restrict the diver's ability to raise the inflator hose to dump gas.
To position the shoulder d-rings, extend your arms straight out to the side, parallel to the ground. Bend at the elbow and bring your thumb straight to the harness strap.
Figure 7
Where you hit the strap is a reasonably good starting point for placing them (Figure 6). It will likely be almost inline with your clavicle, possibly a shade lower. First check to see how easy it is to clip/unclip gear there and how natural that position feels. Next, with your right arm, reach across your body to (or almost to) your left shoulder. Make sure the d-ring doesn't pinch between your arm and chest (Figure 7). Do the same with your left. If it pinches, it is too low and needs to be moved up. These d-rings generally should be as high as is comfortable/reasonable to clip off one-handed on the same side (i.e., one-handed clip off of gear on right d-ring with right hand Figure 8).
Figure 8
You should not rely on reaching across with your opposite arm to clip gear off, as that arm may be occupied doing something else.
The waist d-ring is used for clipping off the SPG and for stage bottles. A common mistake with the waist d-ring is to move it too far back (towards the plate). Several problems are introduced by having a waist d-ring too far back. It becomes much more difficult to clip/unclip gear, especially if the diver is using a "wreck mounted" argon bottle. In addition, since most of the bottles used as stage/decompression bottles are slightly buoyant, if the d-ring is too far back the tail of the bottle will ride high enough o be out of the slipstream. Of course, having the d-ring too far forward introduces another set of problems.
Figure 9
Stage bottles and the SPG will hang too far forward, out of the slipstream, causing drag and increasing entanglement hazards.
To position the waist d-ring, run your left thumb straight down your left side until it hits the waist strap. This should be a good starting point. It should be right about where you can feel your hip bone (Figure 9). This one may need further adjustments based on in-water testing. Practice clipping/unclipping the SPG and if you wear stage bottles, have your buddy check the position of the tanks.
The rear crotch d-ring is generally used for storing gear that is not used very often, such as reels or lift bags. This d-ring needs to be low enough that ou can clip/unclip gear there (and not be blocked by the bottom of the tanks), yet high enough that the gear does not hang down too far. The rear crotch d-ring is considered an optional d-ring. If you have no gear that needs to be clipped in this position, it can be removed.
Figure 10
To position the rear crotch d-ring, you generally want it about one hand-width down from the bottom of the plate (Figure 10). If it is too much lower than that it will be more difficult to reach, and the gear will likely be hanging too low between the legs, interfering with fin kicks. If it is too much higher the bottom of the tanks will often block access, and those with drysuits may find flexibility is reduced enough that reaching the d-ring will be very difficult.
The front crotch d-ring is only used with a tow-behind scooter. Gear should never be clipped to this d-ring, as it will hang in the slipstream and pose significant entanglement hazards. This d-ring is often sewn in, so there is generally little choice in placement.
A properly configured backplate harness is an amazing piece of gear that can greatly increase a diver's comfort and safety in the water. By following these directions, one should be able to come relatively close to achieving this. However the ultimate test is how it performs in the water. Find a good buddy, go for dive and have fun!