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Rubber band strength

 


hopnpopper0429  (C 36648)

Oct 24, 2011, 2:18 PM
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Rubber band strength Can't Post

I'm an experienced skydiver but new to packing. Yesterday I replaced a broken rubber band with a new one from the bag I recently bought and it seemed stronger than I'm used to. A single stow on the new rubber band seemed to be as strong as a double stow on the old one on the other side of the bug. However, I still went ahead and did a double stow on the new one cause I heard somewhere that the number of stows for each rubber band need to be the same for all.

Did I do this correctly or could I have a bag lock due to the new rubber band being stronger than the old ones?


dragon2  (D 101989)

Oct 24, 2011, 2:27 PM
Post #2 of 21 (9083 views)
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Re: [hopnpopper0429] Rubber band strength [In reply to] Can't Post

By "strong", do you mean tight? Ie, the newer rubberbands are smaller than your old ones?

What type of lines do you have on your main canopy? Dacron, microline, vectran or HMA?
If you have dacron lines you use bigger bands than for the other linetypes. Better get bands that fit your lines, so you don't have to double stow at all.
All bands on your bag should be the same type and size, although some people use a different (tighter) type for the locking stows.


sundevil777  (D License)

Oct 24, 2011, 2:32 PM
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Re: [hopnpopper0429] Rubber band strength [In reply to] Can't Post

The strength of rubber bands largely depends on whether they start to rip right away when they get stretched. Sometimes they will do that when you're just installing them on the d-bag. It is for this reason that I use tube stows and would rather accept the risk of a bag lock over the risk of canopy dump. Many threads exist on the subject. Bag lock can still happen (and has happened) even when std rubber bands are used.

I suggest that the objective is not to be sure to single or double stow all the lines uniformly, but that the force it takes to unstow the lines be consistent. How that is achieved does not matter, IMO.

Your mileage may, and in fact will vary.


(This post was edited by sundevil777 on Oct 24, 2011, 2:33 PM)


JohnRich  (D License)

Oct 24, 2011, 2:38 PM
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Re: [hopnpopper0429] Rubber band strength [In reply to] Can't Post

You're very unlikely to have a bag lock because of one rubber band being different.

That being said, stow bands are intended to have a certain breaking strength for this very reason. A rubber band should not be so strong that it prevents the canopy from coming out of the bag. If something is wrong with deployment, the stow band should break to allow deployment to proceed.

Every once in a while you see someone using some type of stow bands that are designed not to break, with the idea of saving money from broken stow bands, and saving packing time from not having to replace them. That's a bad idea. They break for a reason - a reason that may save your life. You just have to accept occasional broken stow bands as normal.

And all the rubber bands should be the same strength. However, a double wrap on one type of band does not necessarily equal a double wrap on another type. It's the strength that should be consistent, not the number of wraps. If one band is stronger than the others, a single wrap on that strong band might equal a double wrap on the weaker bands.

And whether you use one or two wraps can depend upon what part of the lines you're stowing. Because of the cascades which branch one line into more lines, the upper line groups are thicker than the lower line groups. So, depending upon the diameter of your bands, you might need a double wrap on the skinny lower lines to hold them firmly, and only a single wrap on the fatter upper lines.

Get an opinion from one or more of your local riggers and/or instructors.

Disclaimer: I'm not a rigger; personal opinion only.


monkycndo  (D License)

Oct 24, 2011, 2:48 PM
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Re: [hopnpopper0429] Rubber band strength [In reply to] Can't Post

Unless you are using the black milspec bands, there can be a bit of variation between manufacturers. If you have that much difference between the old bands on the d-bag and the new batch of bands you just bought, change all of the old ones. It most likely won't really make much of a difference having some variation in the bands, but this will eliminate that concern.


sundevil777  (D License)

Oct 24, 2011, 3:01 PM
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Re: [JohnRich] Rubber band strength [In reply to] Can't Post

In reply to:
Every once in a while you see someone using some type of stow bands that are designed not to break, with the idea of saving money from broken stow bands, and saving packing time from not having to replace them. That's a bad idea. They break for a reason - a reason that may save your life. You just have to accept occasional broken stow bands as normal.

A lot of people use bands that aren't designed to break, and I don't think it is right to label that philosophy as a bad idea without acknowledging the merit. Having to "accept the occasional broken stow band as normal" means to me that you should also accept that sometimes your locking stows will break before line stretch 'gets to them'. That can have worse consequences (canopy dump) than a bag lock.

I used to have (can't find right now) a pic showing how much the locking stows get stretched as the bag is accelerated off your back. At exactly the wrong time for locking stows to break is when they are under the most stress.

The guy with the big beard that invented a bunch of stuff we use had recommended tube stows for the locking stows in the owner's manual of his rigs (at least it used to be that way). The safety benefit of difficult to break locking stows should not be dismissed.


gowlerk  (C 3196)

Oct 24, 2011, 4:43 PM
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Re: [monkycndo] Rubber band strength [In reply to] Can't Post

I've only seen those black bands a few times. I'd love to know where to get them. Anyone?

Ken


JohnSherman  (D 2105)

Oct 24, 2011, 6:15 PM
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Re: [hopnpopper0429] Rubber band strength [In reply to] Can't Post

Reading "Rubber Bands Break For A Reason" @ http://www.jumpshack.com/...ds&SortBy=DATE_D might shed some light on the subject.

There is no way you are going to cause a bag lock with a Mil Spec Stow band. The breaking strength of this band is 40 pounds. They seem to maintain that level of strength until they all of a sudden break.

If the stows are divided into equal parts, by the bands, so as to balance the mass of the line bights, there is no need to double wrap.

A designer knows that the breaking strength of a rubber band is directly linked to the drag ability of the pilot chute


chuckakers  (D 10855)

Oct 24, 2011, 6:28 PM
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Re: [hopnpopper0429] Rubber band strength [In reply to] Can't Post

In reply to:
I'm an experienced skydiver but new to packing.

6 years and nearly 600 jumps and you're new to packing. I give up, folks.


hopnpopper0429  (C 36648)

Oct 24, 2011, 6:59 PM
Post #10 of 21 (8945 views)
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Re: [chuckakers] Rubber band strength [In reply to] Can't Post

In reply to:
In reply to:
I'm an experienced skydiver but new to packing.

6 years and nearly 600 jumps and you're new to packing. I give up, folks.

Yes, been paying packers up until this season.


Thanks everyone for your input. It looks like I'll change the double wrap of the tight rubber band to a single wrap, just to eliminate the concern as one of you said. Then I'll replace the old rubber bands soon after. But chances are the next time I'll be at the DZ will be for a repack, so I'll ask my rigger.


chuckakers  (D 10855)

Oct 24, 2011, 7:06 PM
Post #11 of 21 (8942 views)
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Re: [hopnpopper0429] Rubber band strength [In reply to] Can't Post

In reply to:
In reply to:
In reply to:
I'm an experienced skydiver but new to packing.

6 years and nearly 600 jumps and you're new to packing. I give up, folks.

Yes, been paying packers up until this season.

Not surprised that you don't see the irony.


sundevil777  (D License)

Oct 24, 2011, 7:23 PM
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Re: [JohnSherman] Rubber band strength [In reply to] Can't Post

Would you trust just two rubber bands to hold a reserve bag closed? Apparently not, as your "conventional" freebag uses a safety stow like others do. Your speed bag doesn't rely on just two wimpy rubber bands to hold it closed. I don't like having the main d-bag held closed with so little strength either.


sundevil777  (D License)

Oct 24, 2011, 7:42 PM
Post #13 of 21 (8924 views)
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Re: [sundevil777] Rubber band strength [In reply to] Can't Post

Here's the pic I was thinking of. I can't remember where I saw it, but it shows how the canopy is slumped in the bag, stretching the bands severely. Maybe it is a tandem, maybe it was staged to make the bands stretch out, but I don't like the idea of two rubber bands that are hard to install and use without them getting pre-broken/cracked being the only things preventing canopy dump.
Attachments: locking stow stretch.jpg (86.1 KB)


sundevil777  (D License)

Oct 24, 2011, 7:45 PM
Post #14 of 21 (8916 views)
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Re: [JohnSherman] Rubber band strength [In reply to] Can't Post

From your article:

Quote:
...the Type 17 riser problem that was traced to a non-Mil. Spec Stow band.

Please expand on this, thanks.


JohnSherman  (D 2105)

Oct 25, 2011, 2:21 PM
Post #15 of 21 (8799 views)
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Re: [sundevil777] Rubber band strength [In reply to] Can't Post

Quote:
Would you trust just two rubber bands to hold a reserve bag closed? Apparently not, as your "conventional" freebag uses a safety stow like others do.
Not true! We don't use the Safety Stow on our Free bag and haven't since 2003. Our Free Bag is also a "Speed Bag" see http://www.jumpshack.com/...ag&SortBy=DATE_D

Quote:
a pic showing how much the locking stows get stretched as the bag is accelerated off your back. At exactly the wrong time for locking stows to break is when they are under the most stress.
I have a copy of that pic on my bulliten board. It shows the power of "Bag Extraction" and the forces encountered. If some of the guys messing around with stowless lines would digest that pic they would move on to something else. It clearly demonstrates the need for the "Speed Bag" BTW; that is on a Vector Tandem with a collapsed drogue. (lower lift forces)

Quote:
...the Type 17 riser problem that was traced to a non-Mil. Spec Stow band.
------------------------------------------------------------------------------
Please expand on this, thanks.
The problem was line dump or line strip or bag dump or ILS or whatever. They were using tube stows with very small bights. Stows located on the edge of the bag. They were getting openings in the 2500 pound range.


(This post was edited by JohnSherman on Oct 25, 2011, 2:23 PM)


sundevil777  (D License)

Oct 25, 2011, 3:06 PM
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Re: [JohnSherman] Rubber band strength [In reply to] Can't Post

My understanding was that a racer could be ordered with what I termed a "conventional" reserve d-bag instead of your speedbag. Anyway, before 2003 you had "conventional" reserve d-bags, so did they use a safety stow?

Knowing how regular bands split and break so easily, I don't think many would be comfortable with the reserve bag being held closed by just 2 of them, so why should we rely on that for the main bag? Your speedbag is of course one way of fixing this issue.


JohnSherman  (D 2105)

Oct 25, 2011, 6:26 PM
Post #17 of 21 (8751 views)
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Re: [sundevil777] Rubber band strength [In reply to] Can't Post

Quote:
Anyway, before 2003 you had "conventional" reserve d-bags, so did they use a safety stow?
Yes, they did. I had developed the "Speed Bag" about 5 years earlier which was a spin off of the "Balanced line stow bag". All an evolution which began about the time of the introduction of the Safety stow. I didn't like the safety stow and I began testing. I had to ride the tide until I had proven my case. I told my people in 2003 that I thought 5 years of constant use of the "Speed Bag" on mains without any malfunctions meant we should consider it for our reserves. They had all ready done it.
The USAFA reduced their malfunction rate from 14 per year to 1in 3 years using home made speed bag copies.


riggerrob  (D 14840)

Oct 26, 2011, 10:47 AM
Post #18 of 21 (8680 views)
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Re: [sundevil777] Rubber band strength [In reply to] Can't Post

"
In reply to:
... Knowing how regular bands split and break so easily, I don't think many would be comfortable with the reserve bag being held closed by just 2 of them, so why should we rely on that for the main bag? Your speedbag is of course one way of fixing this issue.
"

.......................................................................

Speed-bags have 8 or a dozen locking stows.
Speed-bags make way more sense than the next dozen methods for closing tandem main d-bags. The biggest advantage is using stock MIL SPEC rubber bands as the highest-wear component.

All the other tandem manufacturers have had problems with simple MIL SPEC rubber bands, so have been forced to develop a variety of solutions.

For example: Parachutes de France uses extra locking stows, that vaguely resemble Speed-Bags.

Every solution has its good and bad points. For example, Strong's Anti-Line-Slump d-bag adds and extra flap, closed by three bites of line through bungee cords. The problem with bungee cords is that if the bungees are tight enough to control the lines (soft openings keeping TIs and customers happy), packers whine that they are too tight to pack.
OTOH if the bungees are loose enough to be easy to pack, TIs complain about hard openings.

Similarly, Vector/Relative Workshop/UPT use double-width rubber bands on civilian tandems, but triple-width rubber bands on military tandems. Heaven forbid if you mix those different sizes of rubber bands!!!!


riggerrob  (D 14840)

Oct 26, 2011, 10:48 AM
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Re: [sundevil777] Rubber band strength [In reply to] Can't Post

Vector Tandem before they switched to double-width rubber bands.


JohnSherman  (D 2105)

Oct 26, 2011, 2:59 PM
Post #20 of 21 (8616 views)
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Re: [riggerrob] Rubber band strength [In reply to] Can't Post

Oh, BTW: The Racer Tandem does not require wide rubber bands. We only need the 3/8 wide standard because of the speed bag and the balanced stows.
John


mjosparky  (D 5476)

Oct 27, 2011, 11:34 AM
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Re: [hopnpopper0429] Rubber band strength [In reply to] Can't Post

This should cover everything you want to know about "stow bands".

Sparky
Attachments: Mil-Spec. rubber bands.pdf (184 KB)



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