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View Full Version : Could Somebody 'Splain this... Frame Deformation & String Tension


ChicagoJack
10-26-2005, 09:51 PM
I was just flipping through the USRSA certification study guide for 2005 and was wondering if somebody could offer a little further elaboration on the following passage. I'm not taking the test, I don't even string my own frames, just indulging my inner techno-geek self.

[..]

Quote: "Why then, are shorter cross strings looser than longer main strings? Other factors come into play here, mainly the friction of the crosses being pulled through the mains. It's not uncommon for a racquet to have main strings with 55 lbs. of tension and crosses with 40 lbs. Matching main and cross string tensions would require increasing tension on the crosses to the point of deforming and probably damaging the frame. Also, the resulting stringbed stiffness would certainly be too high for most players. So, when that same customer shows up with a Stringmeter demanding to know why the crosses are so much looser than the mains, you can confidently explain why."
--USRSA Supplemental Certification Study Guide 2005-2006

[..]

What I am not understanding is how a lower tension in the Xs relative to the Ms would somehow prevent frame distortion. I would have guessed you need even tension to prevent frame deformation. I've experimented with slight dif tensions and never gave such things a second thought. But here is mention of ratio spreads as high as 55/40 being not only common place, but possibly necessary.

-Jack

Steve Huff
10-26-2005, 10:09 PM
They're not saying to string the crosses at 40. They're saying the ACTUAL tension could be 40 as it has lost some tension due to the friction of being weaved around the main strings.

Midlife crisis
10-27-2005, 09:00 AM
What I am not understanding is how a lower tension in the Xs relative to the Ms would somehow prevent frame distortion. I would have guessed you need even tension to prevent frame deformation. I've experimented with slight dif tensions and never gave such things a second thought. But here is mention of ratio spreads as high as 55/40 being not only common place, but possibly necessary.

What is probably the case is that there are more crosses than mains in most rackets, and the combination of the hoop stress created by all of the crosses is then more equal (and more equally distributed around the head) as well.

Man, reading that, it does not seem clear what I'm saying. How about this: in a 16M X 20C pattern, if the 16 mains were at 55 pounds, their combined tension on the racket head is 1760 pounds. For the 20 crosses at 40 pounds, their combined tension is 1600 pounds. This is a variation of 10%. If the crosses were at 55 lbs tension, their combined force would be 2200 pounds, for a difference of 25% between mains and crosses. The forces would be concentrated at the points where the hoop has the greatest curvature, usually at the top corners of the head, where the racket also can get whacked on the ground pretty good.

ChicagoJack
10-27-2005, 09:22 AM
They're not saying to string the crosses at 40. They're saying the ACTUAL tension could be 40 as it has lost some tension due to the friction of being weaved around the main strings.

Follow up Question,

IF
... The friction caused by pulling the Xs through the Ms is enough to lower the actual tension by 15 lbs,

THEN
... Strictly speaking, the meaning of a players request to have their strings at a particular tension, really is not a request describing the actual desired end result/condition of the string bed itself after stringing is completed, but describes only the force used by the machine to pull the string?

I am aware that tension loss is happening all the time, I use a tensionmeter to measure this fact of life in all my frames. I guess at some point this is a linguistic question, players see tension as an end result, stringers see tension as the pulling force used by the machine?

-Jack

LoveThisGame
10-27-2005, 09:33 AM
As Steve said there is friction with the main strings when pulling the cross string, the cross string not "liking" the up and down of the weave. It is also a shorter length span for the crosses, which helps a bit in the tension, although the friction is probably a more significant factor.

The point made by USRSA was that to get equal tension on crosses and mains would require higher cross tensioning which could result in frame deformation. Instead, tension is set for crosses in order to make the stringbed effective and comfortable. In the process, frame distortion happens not to be a factor. IOW, the object is not necessarily to prevent frame distortion, but to deliver a desirable stringbed.