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Thread: Why +2lbs On The Cross?
01-01-2012, 12:48 PM #324
Thank you for your answers. Two more questions:
1- The crank machines have to be calibrated frequently?
2- How often they break?
01-01-2012, 06:39 PM #325
... The string grip in the crank machines could be adjusted? I am asking, because I am think in buying a pros pro crank machine, but I don't know the efficiency of the grip.
01-02-2012, 01:41 PM #326
calibration frequency is pending upon the care you give to the machine and usage. there isn't a certain rule, but I normally do a check (verification of the reading) once a month or sooner (like every 2 weeks).
the machine normally lasts LONG time (10+ years), with proper care and storage location.
for the string grip. assume you are talking about the two parallel plates that hold the string. If so, yes, they can be adjusted for different string diameter. often time, you don't have to do anything about them, unless they are slipping or out of alignment. again, this is something that normally doesn't go bad at all.
02-02-2012, 09:01 AM #327
Hi everyone, I have read most of this thread but not all, so forgive me if this question has come up already...
I am not a stringer, so I wondered, when a player gives you a racket and asks for it strung at say 24lbs, do you string at 24x26, or 22x24? Or 23x25?
02-02-2012, 10:51 AM #328
02-02-2012, 12:09 PM #329
I wonder why YY has not suggested this +2 Lbs on the cross anymore in its annual catalogue
Instead YY suggests to use same string tension for main and cross strings.
05-25-2012, 08:36 PM #330
This 1lb higher tension on the crosses issue has been bothering me, so I also did some calculations (I'm an engineer).
I agree with the first part of your calcs (the mains are forced by the crosses to stretch about 0,6 mm over their length), HOWEVER, I think your next steps are incorrect.
A 0.25% extra strain (strain = change in length/original length) doesn't give 0.25% extra tension.
Actually Tension is proportional to Strain x Young's Modulus (YM is effectively the "spring constant" of the string material. I went though the theoretical calculations but the problem is finding reliable values for Young's Modulus for the string materials (they range from 6 Gpa to 28 GPa...too broad a range to be useful!). So...I did some experiments:
What I found was that adding 5lb (20lb to 25lb) of tension on a 480mm length of BG-65 stretches it 4,5mm. Since we can assume the BG-65 is elastic in this tension range, we would see 2,25mm stretch over a 240mm length (the length of a long main) with +5lb...so, calculating 5 x 0,6/2,25 = 1.3lb would stretch a main by 0,6mm (the length we agree the mains get stretched).
The conclusion from this analysis is that the crosses should be strung 1.3lbs higher tension than the mains to end up with the same tension on the long mains as the crosses.
By the way, reversing out the YM gives a value around 13 GPa.
05-25-2012, 10:57 PM #331
did you take into account string density in your calculation?
the main strings are closer together and the crosses are further apart. the effective tension for the mains are thus higher.
05-26-2012, 06:00 AM #332
05-26-2012, 06:53 AM #333
05-26-2012, 08:44 AM #334
While not super-productive concerning the ongoing discussion, I tried different tensions on my BS09s, trying to find a satisfying tension for the BG66UM where it doesn't feel dead after 3 sessions but I can still smash harder than a small girl
Anyhow, during that, I varied the main/cross string tensions as well, as I was wasting string anyway....and had interesting results. The stringjob yielding the best head shape was a 12x12,4kg one [26.4/27.28lbs]. The 29x30 one was better in playability, but too hard for my taste, and the most recent one resulted in the best feeling of all - a 27x29lbs one. Even though the head shape is very slightly off, it just feels more alive than any other previous tension.
I would try and guess that it's due to the cross strings being slightly tighter than the mains, giving a bit more feedback than a more homogeneous stringjob while retaining most of the repulsion of the softer mains. Personally, I'm very surprised by this, as I was pretty sure a string job with the perfect head shape would yield the best results, and I still have no real 'scientific' explanation for it.
Still, fact is, the 27x29 feels more alive (harder too) than a 27x27, more repulsive and slightly more direct than a 28x28, and more powerful than a 29x29 (while not as hard and direct).
Stringing was done on a Signum Pro machine modded with a WISE tension head. Same pattern and equipment for every one of those stringjobs, of course.
05-26-2012, 09:31 AM #335
If you're a top-downer like me, keeping the "corners" of the iso-frame is the tricky bit. I've found that 27.5/30 with 26->30 from the top cross does OK in this regard. IMO the crosses stretch the mains, so the mains need extra - end of story.
A TD will probably never keep the frame shape, but my width/length ratio is good and I'd rather have stress end up at the bottom of the frame than the top.
05-26-2012, 10:15 AM #336
Yep, top-down with lowered tension at the beginning. For others I wouldn't add quite as much tension as on mine, as it DOES deform the frame, but enough to even out the difference created by lengthening the mains and pulling at worse angles than on the mains (0.4-0.6kg usually).
05-26-2012, 11:11 AM #337
I don't use the WISE prestretch, but I'm considering using it to see if more of the cross friction is pulled out - I'm wondering how much of the target tension is lost to that friction (even with ECP and the "massage" technique).
05-26-2012, 01:50 PM #338
Hard to put a number on it, but I find that with 10% pre-stretch I need to add very little on the crosses to get a pretty homogeneous stringjob. Without the pre-stretch, I would need to 'massage' the strings (taking much time, which I don't really want to spend^^) and probably still end up short in comparison...
05-26-2012, 03:35 PM #339
If the mains are done first and tied off, the crosses will stretch the mains and cause the mains tension to increase. This is basic mechanics of materials: Stress (which is Tension/String Cross Section Area) = Young's Modulus x Strain. So for a homogenous string bed, the mains should be tensioned LESS than the crosses.
05-26-2012, 03:45 PM #340
Yep, that's what everyone does....and I'm studying to be an engineer as well, so no need to explain the basics of mechanics to me I'm just surprised a rather big difference in tension lead to such a great result, as I would've expected it to feel slightly 'off' with the crosses being tighter (even taking into account that the mains have tightened by pulling the crosses).
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