Brake line twists shortening brake lines?

[pchapman 280]

There are various reasons discussed why brake lines shouldn't be left to get too twisted. Just how true or important the different factors are, is not generally clear. Opinions vary, and little actual data is available to the average jumper.

Issues that are suggested include:

- greater wear along edges of the line from slider grommets
- greater shrinkage of Spectra line due to increased friction and thus heating by the slider grommets moving along twisted line
- possible loss of strength from twisting (but the filaments are already twisted in creating a woven line)
- possible increased chance of tension knots due to the tendency of a twisted line that is not under much tension to tangle up
- shortening of the brake lines due to the twists (affecting openings, especially if one side is twisted more than the other)

Various threads can be found debating the issues, by searching for "twisted brake lines" for example.

I was feeling a little skeptical about the idea that brake lines get significantly shortened by twisting, and it was easy enough to actually collect some data.

While it is only one set of simple tests, the results suggest that brake line twists DO NOT tend to shorten the brake lines all that much.

Still, it might have some effect on openings, whether symmetrically applied on both brakes, or asymmetrically between a twisted and untwisted brake.

It can be debated how much twisting is seen in practice and what the effect on openings would be.

The tests:

I twisted up a few lines that happened to be 68" to 80" in length, comparable to lower steering lines from eye to cascade for quite small canopies. (E.g., lower steering lines on a Stiletto 97 are 78" long; an FX88 ~79" long.)

All were measured when under 20 lbs of load.
(Under more load, such as 30 lbs, the difference between twisted and untwisted length tended to be a little less.)

A 1000lb Spectra brake line lost about 1/2" in length when given 20 full twists. (0.7% of its original length)

A much thicker 600 lb Dacron line lost more in the same situation, about 7/8". (1.2%)

A Vectran suspension line, thinner than a typical brake line, demonstrated how a thinner line will be less affected as one might suspect. It lost only 1/4" (or 0.3%).

Twenty full twists seemed like a reasonable simulation of a badly twisted brake line. One tends to notice 180 degrees of twist as as a distinct twist in a line. Those lines with 20 full twists had a half twist slightly more than once per two inches.


The testing was hardly exhaustive but I did want to provide some data where I hadn't seen any before.

[format 1]

Is there a suggestion?
... like: "never mind the break line/s twists"?

no significant difference?

What goes around, comes later.

[mdrejhon 8]

Add reason to the "reasons" list:
Another reason to untwist lines is because when they get twisted enough (over 5 twists), they start slowly making packing more inconvenient. Interfering with the cascades while packing, making it harder to visually verify the line groups while packing. When I pick up the lines to pack and smooth the lines until I put the canopy over my shoulder, the twists also tend to get pushed to the top of the canopy, exacerbating this, too.

[councilman24 37]

Are defining a twist as 180 or 360 degrees? If 180 I might suggest 20 twists are not enough. And I've pointed twisted lines out to jumpers who never heard of it or realized it was happening, probably with more the 40 twists.

A lot of people can't figure out how the twists get there. One way.... When I land I almost always let go of the left toggle, hold on to my right toggle and turn to my right ducking under my right riser to face the canopy. This also turns the canopy into the ground when windy. This it the habit I got into to both pick up my parachute and collapse it in higher winds. Most of the time now I either set my brakes in the LZ, or if there is traffic and I need to leave at the packing area before I take my rig off. Little opportunity to twist or untwist unless I make it.

By doing this I put a 180 degree twist in the right brake line only on almost every jump. 20 jumps would equal 10 full 360 degree twists. I untwist my lines enough that only ever get 10 or 15 twists in it. But almost exclusively the right line.

1/2" difference I'd suggest would be outside of tolerance for most canopy brake settings. Certainly 7/8" difference is for any canopy.

I'm old for my age.
Terry Urban
D-8631
FAA DPRE

[pchapman 280]

To clarify, I tested the lines with 20 twists of 360 degrees.

However when we look at lines we tend to say, "there are two twists in that line" if we see two 180 degree rotations.

[AggieDave 6]

Quote

Another reason to untwist lines is because when they get twisted enough (over 5 twists), they start slowly making packing more inconvenient.

Twisted brake lines can also cause malfunctions. On dacron lines I have noticed that when the brake lines get twisted, the cascade fingertrap turns and flattens to a horizontal position. I have had at least one tandem ride with a twisted control line as a contributing factor. The flattened fingertrap makes it easier for the C/D cascade to catch it and lock in place with the slider.

I'm hesitant to say for sure that it would happen on a sport rig, since its been a long time since I consistently jumped a sport rig with dacron lines. Maybe another jumper can chime in here.

--"When I die, may I be surrounded by scattered chrome and burning gasoline."