Change in rope length vs rope twist

[ski6jones]

I measured my rope today with various numbers of twists and found a maximum change of just over 1 inch with 50 twists.

 

I measured the 28 off loop with 0 to 50 twists incrementing by 10 (red) and 50 to 0 decrementing by 10 (green). The rope is about a year old. I tried to reduce measurement error by;

 

Measuring under constant tension, about 45 pounds,

Using constant tesnsion on the measuring tape by pulling it to the same length on a fixed object before making the measurement.

Divisions on the tape were 0.002 m.

 

It would appear that even with 50 twists, which seems like a lot, the change in length just exceeds 1 inch. The differences are from the initial measurement so there appears to be some hysterisis when unwinding the rope. Below 30 twists the difference is negligible.

 

 

 

 

[Drago]

What I thought

[mwetskier]

yeah, but slalom has been described as ' a game of inches '. plus which jamie beauchesne once said that the path needed to run 41 off is no wider than a garden hose. so theres that.

[Zman]

Any thoughts how springy or soft a rope might feel if excessively twisted?

[Chef23]

@mwetskier if you are running 38 and shorter the inches make a difference. At 32 and longer not two inches doesn't make a difference.

 

How much stiffer a twisted rope might feel to me is a bigger issue. I can see where a twisted rope would feel stiffer which is a bad thing for your hands and elbows.

[mwetskier]

@Chef23 -if i was running 38 and shorter i wouldn't care if the rope was shorter by an inch or two because the only time i'd be running 38 or shorter is in my dreams.

 

[ski6jones]

Below about 30 twists and the change is a few millimeters. When twisting the rope up to 50 it really started to resist being twisted. I don't think the rope would naturally twist that much, which I think also means the change in length is negligible for the number of twists the rope will take before the force required to add more twists becomes too great.

 

I took out about 10 twists from my rope that were there from the last time I skied before starting to measure.

 

On the stiffness of the rope I would think twists would make it feel less stiff. Another project for someone.

[Than_Bogan]

Cool data! I think you slightly buried the lede, though. The take-home is that any sane number of twists has negligible effect on rope length.

 

Pretty fascinating that the rope just barely lengthens from the cycle of twisting and untwisting. (Right?)

[ski6jones]

Fair point @Than_Bogan, edited original post. And yes the rope lengthened a couple mm over the cycle of twist/untwist.

[teammalibu]

So I've been untwisting my rope all this time for nothing! Guess I'll keep doing it! @ski6jones thanks for the info! And great skiing this weekend congrats on your PB at quickset, the new windshield is on order!

[BraceMaker]

@Than_Bogan or all the loops shifted during tensioning and are just loosened.

 

I think youd have to measure twist untwist. Ski it let it dry. Measure.

 

 

[ConPexEr]

@mwetskier you don't run 38? what do you run?

[mwetskier]

@ConPexEr -i mostly run from ex-wives and similar dangerous encounters.

 

[JackQ]

I don't think the difference of a twisted rope will make a difference in my skiing, but it still drives me mad and I will continue to unwind it. My questions, is when I fall and picked up the rope is always (or all most every time) twisted counterclockwise. Seem that it should be random.

[dave2ball]

A twisted rope will affect your skiing. Not only does the rope get shorter but the twisting does not allow the fibers to stretch as designed to.

 

[BobF]

@jackq I think it depends on the handle. Some handles tend to propeller one way or the other based on how they are strung. I've found that since I started using a head protector on my handle, no more rope twist.

 

btw, I also can't help but unwind if needed while waiting for a start!

[dave2ball]

I really wouldn’t worry which way the rope twists.

[thager]

Unless you are south of the equator then they spin the other way! B)

[dave2ball]

@thager you are correct.

[liquid d]

It's just like an MLB batter stepping out of the box, adjusting the gloves, and getting back in the box. It keeps ya loose, and focusing---take every inch you can get!

[6balls]

Totally subjective but a twisted rope feels "springy" to me. Don't like it.

[DaveD]

"Untwisting the rope" is code for taking a pee break on our lake.

[Zman]

@DaveD Or, need to get the soap out of my boots. Same code.

[DW]

I would expect a twisted rope to stretch and contract more than a non stretched rope. As it shortens, a load on it will cause it to stretch back closer to untwisted condition. A straight rope has less ability to stretch than a twisted one.

[Triplett]

I always thought of it as a stretch/springy-ness issue rather than length. Not entirely sure how we could quantify that.

 

 

[DW]

I was pondering the how to - perhaps attach one end to a fixed object, use a (fish) scale on the other and measure load v distance and overall length all relative to # or twists.

 

Might make a great kids school project.

[dave2ball]

@DW the fish scale is a great thought. But when you ski a person can load the rope 600 lbs pls. If you have a fish scale for that and can get a measurable accurate number you are the man.

[DW]

So, I may need to reconsider my previous comment. I untwisted my main line and took a set and surprisingly the rope seemed to have a bit more stretch. Could be other factors but it sure did seem to be a bit more forgiving and stretchy. Just adds to my thought that it would make a great experiment. Of course like everything else in this sport, could be simply a mental thing.

 

@dave2ball : the fish scale was more of a reference, I would think a 0-500 # scale would be wanted to get enough pull and accuracy. You could do it crudely with a few people of different mass and off the back of a tow hitch, simply measure distance v person leaning back at a prescribed angle, may not get enough accuracy that way. You may just get a subjective response using people leaning back in a fixed setting.

[The_MS]

We just flat out say we are warming up the lake when we pee. It is always good to add warm water to lake

[BraceMaker]

Get a cable and a pulley with a hook.

 

0-500 lb scale. Scale and cable connected to post with pulley and hook forming a V to the ski line. Now scale measures one half of the tension in the system. 0-1000 lbs. Multiply reading by 2.