Backmount Double Stability Issues

[divingx]

@tbone1004
Thanks for the advice. Really appreciate it! I thought I was the only one had the stability issues.

I used a 6cuft argon bottle for drysuit inflation. I found the bottle interfered with my waist D ring. Should I move the D ring forward?

 

[divingx]

@macado
I don't have much problem with my trim. Maybe because I have have legs. I used to think short person should use shorter tank. What tanks do you use? AL80s are not so practical here cuz that requires so much weight.

 

[tbone1004]

@QAQTAT inflation bottles usually get attached to the left side of the backplate, shouldn't be interfering with a waist d-ring at all. If it does then move it to wherever comfortable

 

[macado]

@macado
I don't have much problem with my trim. Maybe because I have have legs. I used to think short person should use shorter tank. What tanks do you use? AL80s are not so practical here cuz that requires so much weight.

I'm using mostly LP85s now but I also have a set of PST HP120s. The PST HP120s are pretty long for someone my size but I trim out well in them. I own bunch of sets of HP100s and I still use them but I need about ~8-9lbs in a tail weight to fix my head heaviness with them. I fill my own stuff so I can "cave fill" LP85s and get an appropriate amount of gas.

My legs are pretty short so it's hard for me to get more weight down. I can only move my legs back so much to counterbalance and that's with heavy jets. I've got a big barrel chest, beer belly and I struggle with shorter sized doubles honestly.

Just my experience, may not be your issue but I've found people of shorter stature 5'3" - 5'7" have similar issues.

 

[EireDiver606]

Hi OP, doubles aren’t technical, they’re simple enough and I don’t think they take a lot of time to get used to, for me it was around 20 dives to feel pretty comfortable in them. It is a big misconception that they are hard to dive and are only for “advanced” diving, if you’re squared away, doubles will not be a problem. That said, your harness and where the tanks sit on the twinning bands will need to be set up correctly for that to happen, not to mention good training. With good training and correctly set up doubles, mastering them will only take patience and a lot of dives to get really dialed in until it becomes apart of you rather than a heavy unit that can pivot your entire body. I definitely do not claim to have mastered doubles.

You felt like they were pulling you down because a they are very negative and b you were probably overweighted. They are negative for sure especially at the start of the dive and if you have air in them.

Your best bet is GUE fundamentals, or at least a doubles primer.

Edit: Specific equipment considerations:
Make sure the twinning bands are sitting as high as possible on the tank before the curve near the valve.
Wing shape should be a donut not horseshoe.
Shoulders need to be looser (easily fit hand between you and harness) and crotch strap tight.

 

[Jack Hammer]

May I ask what tanks do you use?

For OC mostly Double hp130s/Lp104s or sometimes hp100s.

 

[tbone1004]

oh fwiw
I'm 6'4", 260lbs. I primarily dive double HP120's and LP121's from PST but have lots of hours in double 130's, HP100's, and LP72's. I don't really notice any appreciable difference between any of them when I'm diving in terms of pitch/roll problems but that's after a lot of hours so it's all dealt with subconsciously.

 

[EireDiver606]

The next solution is avoiding negative doubles.

Negative doubles are not to be avoided in cold water. You need the weight to offset the buoyancy of thick undergarments and a puffy drysuit. For warm water you want lighter tanks like AL80s.

My tank(s) trying to roll me annoyed the **** out of me when I tried single LP85s or steel 72s

If you’ve set up your kit correctly and have good stable trim, rolling shouldn’t be an issue.

Yeah putting the weight in the plate does not help much, as it is all still behind you

I understand what you mean and agree with it in terms of hypothetical physics but you are coming from a perfect hypothetical physics world. In reality, with diving, you don’t want to put lead on your front as you want a clean front where you’re operating and be able to reach pockets easily. No clutter on your front like weights that you can put on your back. Counteract it with stable trim and a wide surface area - bringing arms out.

Thanks! I'll try this tomorrow. Do you think stages might help a little bit since they are kind of 'hanging below me'?

No, stages should be less negative and more buoyant like an al80 to have a balanced rig.

 

[MichaelMc]

Doubles will ALWAYS be unstable when you are rolled all the way over on your side and there is nothing you can do to really fix it.

tbone, Will this be the case with double AL80s on a cutout SS plate, so minimal fixed mass in back, with all the needed lead on the front of the body? Or if that makes the diver too belly heavy, then the lead distributed more to the sides to align with the divers center? Not wanting the AL80's lead or not wanting lead in front are separate issues.

I'm currently diving with 20 lb. of lead. I have not dove double AL80s on a cutout plate. But that's a lot of lead available to shift my center of gravity to get me trimmed out front/back and head/toe if that is what I want to do.

 

[tbone1004]

tbone, Will this be the case with double AL80s on a cutout SS plate, so minimal fixed mass in back, with all the needed lead on the front of the body? Or if that makes the diver too belly heavy, then the lead distributed more to the sides to align with the divers center? Not wanting the AL80's lead or not wanting lead in front are separate issues.

I'm currently diving with 20 lb. of lead. I have not dove double AL80s on a cutout plate. But that's a lot of lead available to shift my center of gravity to get me trimmed out front/back and head/toe.

Always is not an absolute, if you do a lot of work to get the balance between ballast on a belt in front of you equal to that on your back, there will be a small window during the dive where you can roll on your side and the tanks will be comparably negatively buoyant to your body. Practically though it's not going to happen.
If you chose to dive double AL80's in a drysuit with a lightweight plate and had a ton of lead on a vest or belt, then you would have all of the ballast on your front and when you tried to roll on your side, the lead would just pull you face down again.
The wing also plays into this where a properly sized wing will have gas on both sides of the tank when you roll over and that will naturally want to level itself out. It doesn't really care if you are face up or face down, but it will want to be level.

Practically speaking though, normal divers will have the rigs much heavier than they are and for that, the natural tendency when you roll on your side is to turtle over. I'm sure guys like @kensuf can back me up, but after you spend enough hours in doubles, even double 104's while cave diving with light undergarments, your body learns what it needs to look like to be able to stay on your side for extended periods of time. It's not something I like to make a habit of, but it's not something I'd consider a problem either.