The Ultimate Poly Pre-Stretch Experiment!

[travlerajm]

Hypothesis:
This post was inspired by an interesting recent thread questioning whether the tension stability of poly strings can be improved by pre-stretching.

To truly test this idea, I decided to try an experiment with a TRUE pre-stretch of poly. That is, a true pre-stretch requires holding the tensile stress for a long time to allow the string to creep under the the tension. A true pre-stretch cannot be accomplished in a few seconds.

In short, I pre-stretched a poly at 70 lbs and held it under tension for more than an hour.
Here are the details of Part I of this experiment:

String:
Prince Tournament Poly 16g.
I chose this string because it’s my favorite string to use as a slippery poly cross when it’s under appropriate tension. It has a glassy smooth dent-resistant surface (harder and smoother than any other string I’ve encountered). It’s downside is that it has the poor tension stability of a typical poly string.

The setup:
I cut off a little bit more than 20 ft of string. To anchor one end of the string, I created a lasso at one end to loop around the end of my kitchen bar countertop.

I set up my Gamma X2 dropweight stringer on the carpet 20 feet away, and placed the other end of the string in the jaws of the tensioner.

Before pulling tension, I pulled the string enough to straighten it (but still with plenty of slack), and with a sharpie pen marked off a 10-inch gauge length for the string in the relaxed condition.

To adequately anchor the stringer to the floor, I first attempted combinations of luggage and furniture, but nothing in my condo seemed to be heavy enough and conveniently shaped to suit the job. I decided that for this first experiment, I’d need to stand on the base of the stringer in order to anchor the stringer securely enough – so that’s what I did.

The process:
I set the weight at 70 lbs. Pulling tension with the dropweight was extremely difficult, because raising the dropweight 90 degrees did not release much of the tension (normally, 90 degrees translates to a significant percentage of the total length under tension, but that is not the case with the full 20 feet under tension.)

I found that I could muster enough torque with my hand to advance the ratchet through one click (10 degrees of rotation, or about 4mm length extension) with each raise of the dropweight (my hand is quite sore though).

After each added click ( raising the dropweight 10 degrees above horizontal), I would wait a few minutes for the dropweight to creep back to horizontal.

As I continued to repeat this cycle, the creep rate got slower and slower, so that it was taking more than 10 minutes for the string to creep another 4mm.

Something else interesting started to happen too. After a while, I started to notice that the initial equilibrium position of the drop weight (after adding a click) was getting higher (because the creep rate was no longer fast enough to balance the tension increase).

Eventually, the equilibrium position got high enough that there was no longer an equilibrium position (without forcing the dropweight down to horizontal, which I was afraid to do, for fear of exceeding the string’s ultimate tensile strength). If I tried to take my hand off the dropweight while it was 20 degrees from horizontal, the tension in the string would slowly pull the dropweight back up. A few times in a row, I patiently held the drop weight steady until the creep was enough to reach a quasi-equilibrium again. And the final time, holding it in place for 10 minutes was not long enough, so I risked gently forcing it almost to horizontal, which worked.

I performed these cycles for about 90 minutes total time under 70 lbs.

Initial observations:
After the stretching process, I asked my wife to take a measurement of the distance between the gauge markers while it was still under tension (I couldn’t do it myself, since I needed to keep my body weight on top of the stringer). It measured 10.5”.

Then I released the tension and measured the new relaxed gauge length: 10.14”.

This means that the string creeped by about 1.4% during the process. For a typical racquet, that’s about 2.5” in total string length.

This also means that the string can still stretch about 3.5% elastically under 70 lbs of tension. This can be used to calculate a tensile elastic modulus – I’ll do that later.

I found it interesting that after the stretching process, there was no hint of coil memory left in the string. Rather, it had a new memory: it wanted to be a 20-ft-long straight rod.

More to follow…

[travlerajm]

Part II:

Strung up my siliconed & shortened diablo mid.

I re-used the 18g kevlar mains that were already in the frame that had been mostly broken in (about 3 hours) but been in there for a year, and then restrung the crosses using the pre-stretched tournament poly. I used 45 lbs in both mains and crosses (I tightened up the mains a bit without re-weaving to get back up to 45 lbs reference, since kevlar is easy to cut and tie on an extension).

Bouncing the ball on the strings in the living room, feels crisp with short dwell time like a fresh bed of kevlar/poly normally does at that tension. But seems a little more lively than normal, as if it has better energy return.

Will test later against the wall...

[corners]

Great experiment Trav. Look forward to hearing how it plays and holds tension.

[travlerajm]

I worry that I will really like the pre-stretched stringbed a lot, and then there's no going back.

I'll need to go to the hardware store to get some bricks and sandbags to anchor my stringer to permit an hour+ pre-stretch every time I want to re-string my racquet. Or maybe I'll just stick with the ZX crosses.

I just did a quick calulation based on the creep and elastic deformation:

My 70-lb, hour+ pre-stretch (to 1.4% creep) is equivalent to letting a stringbed strung at 45 lb reference tension relax all the way down to 18 lbs. Should make a big difference in tension stability.

[corners]

Originally Posted by travlerajm

I worry that I will really like the pre-stretched stringbed a lot, and then there's no going back.

LOL

I'll need to go to the hardware store to get some bricks and sandbags to anchor my stringer to permit an hour+ pre-stretch every time I want to re-string my racquet. Or maybe I'll just stick with the ZX crosses.

Have you considered something like bench clamps or a bench vise?

I just did a quick calulation based on the creep and elastic deformation:

My 70-lb, hour+ pre-stretch (to 1.4% creep) is equivalent to letting a stringbed strung at 45 lb reference tension relax all the way down to 18 lbs. Should make a big difference in tension stability.

That's about how much tension Tournament Poly 16g lost during TWU's pre-stretch protocol. After being so stretched and then re-tensioned to 62 pounds the string lost 17 pounds, compared to 25 during the pre-strech. It looks like Tournament Poly is pretty prone to creep and stress relaxation, but the difference between 25 and 17 pounds of total loss is pretty significant. I think you can expect it to hold tension much better after the application of your (very arduous) pre-stretch protocol.

[travlerajm]

Originally Posted by corners

LOL



Have you considered something like bench clamps or a bench vise?



That's about how much tension Tournament Poly 16g lost during TWU's pre-stretch protocol. After being so stretched and then re-tensioned to 62 pounds the string lost 17 pounds, compared to 25 during the pre-strech. It looks like Tournament Poly is pretty prone to creep and stress relaxation, but the difference between 25 and 17 pounds of total loss is pretty significant. I think you can expect it to hold tension much better after the application of your (very arduous) pre-stretch protocol.

Actually, I suspect that my pre-stretch procedure will have much larger effects than the TWU pre-stretch procedue.

The behavior of the string while stringing was much different than when I string it without pre-stretching. Without a pre-stretch, the dropweight will creep about 10 degrees/minute with every drop during the stringing process (roughly from my memory). With the pre-stretch, there was zero noticeable creep. It was as if it was a completely different string.

Given this observation, I'm guessing that I've probably captured at least 90% of the creep/relaxation curve. In contrast, the TWU protocol only captured about 30% [(25-17)/25] of the creep/relaxation curve.

Anyway, the on court results are what matter. We shall see...

[travlerajm]

Tried the pre-stretched bed against the wall tonight.

It played amazing. Terrific bite and rpm. Short dwell time, but soft muted yet full-bodied feel. Nice control.

The 45 lbs was slightly softer than my usual preference, but still great all-around and rebound angle not too high. It seems to have more energy return and pop than kevlar/poly usually does, without sacrificing accuracy.

Unfortunately, the poly snapped after 40 minutes of wall hitting (equivalent to about 1.5h singles). It snapped on the outside of frame between 3rd and 4th string from the tip when I caught a serve off-center near the frame. So tentative conclusion is that the pre-stretching process can make the string more fragile and sensitive to stress concentrations.

Also: I couldn't resist making a few purchases today in preparation for building a home pre-stretching device (a wire-frame shelf unit that can take 500 lbs per shelf, a 100-lb set of disk weights, and a set of pulleys).

[pvaudio]

Goodness, that sounds like some sort of Inquisition equipment you're building

[Power Player]

It's much better to string on a constant pull machine with poly. simulates prestretching, but does not weaken the string as much as actual prestretching does.

[corners]

Deleted post

[travlerajm]

Ok. I officially set up my home pre-stretching device in my lliving room this morning:

Components:
1. A "3-Step Project Stool" from Home Depot as a frame (which is basically a 5-foot-tall step ladder with steel tube frame).

2. Four 3-inch diameter clothes line pulleys.

3. 3/16" diameter Neon twine.

4. A 20-lb nylon sand-filled disk weight.

-----

I'm doing a pre-stetch on 16g Prince Tournament Poly again.

Step 1: I attached 2 pulleys to the top crossbar of the frame, and 2 pulleys to the crossbar at the base. I used 2 small loops of twine (each triple-knotted to avoid knot slip) to attach each pulley to the bar (I needed two loops to orient the pulleys correctly).

Step 2: I looped a "safety net" of twine through the disk weight and around the top crossbar to catch the weight before it crashes to the floor in case the string were to break.

Step 3: I anchored one end of the 20-foot segment of string to a loop of twine wrapped around the top cross bar.

Step 4: I threaded the string down, up, down, up through the 4 pulleys.

Step 5: I placed the disk weight on top of a 3-foot tall bar stool (to give me some slack while I attached it). I looped some twine through the hole in the disk weight, and then tied the other end of the string to the twine.

Step 6: I marked off a 10" gauge length with a marker while the string was still slack.

Step 6: I made sure that the string was properly seated in all 4 pulleys, and carefully removed the bar stool and gently lowered the disk weight to tension the string.

Observations so far:

I have the weight hanging about 2 feet off the ground. I proabably have about 19 feet under tension. It looks like the 20 lbs of tension is too low -- I can't notice any creep happening.

Next step: I'm going down to my car to grab more disk weights. Will add another 20 lbs and see how it works.

[corners]

Good ol fashion industry and ingenuity. Tournament Poly is a good one for this as it loses a lot of tension (according to the lab tests) but has that surface hardness and lubricity that we look for. If you can get it to hold tension better with the pre-stretching you'll have a winner.

[travlerajm]

Update:

I attached another 20-lb disk weight to the first one. So now 40 lbs of constant tension.

This seems to be working, the weights dropped about an inch due to to slow viscous creep in the first 15 minutes after the initial stretch. I noted the height of the center of the outer weight was about 18.75" off the floor when it started. Now it's at 17.75".

I plan to let it hang for at least 24h, maybe even a few days - however long it takes for me to stop noticing the movement. I have about a foot of clearance between the ground and the weights.

[travlerajm]

Update: 17.5".

[travlerajm]

17-1/8". (I might as will use this thread as my lab book).

[travlerajm]

17.0. slowing down.

[corners]

Originally Posted by travlerajm

17-1/8". (I might as will use this thread as my lab book).

Open lab book, good idea.

[travlerajm]

16.75".................

[corners]

40# seems to be working nicely.

[travlerajm]

Yes - still at 16.75".

I expect it might extend another inch by tomorrow.