Most commonly:
- What tensions for mains and crosses? Any differentials?
- Which string for mains?
- Which for crosses?
- What gauge?
- Prestretch?
Personally, I do not enjoy hybrids. (I have even tried tri-brids and quad-brids!) No matter what I pair together - kevlar, syn gut, multifilament, natural gut, soft poly, stiff poly, fishing line, steel wire or grass trimmer line, it was extremely difficult to balance them well.
Inevitably, one string would dominate over the other. Usually, the feel and playing characteristics of the mains prevail. Once in a while, the crosses.
I am far from solving this puzzle, but I believe it has something to do with how the strings behave at different tensions.
When pulled to 55 lbs, the amount of total stretch observed in the following strings were:
- natural gut 6~7%
- synthetic gut 9~10%
- multifilament 10~12%
- poly 3~4%
- kevlar 1~2%
The numbers were in a range due to different batches and brands.
These same strings, when pulled an extra 10 lbs, bringing them to 65 lbs, yielded an incremental stretch of:
- natural gut 0.4~0.7%
- synthetic gut 0.8~1.1%
- multifilament 0.5~1.2%
- poly 0.1~0.3%
- kevlar 0%
The conventional wisdom is to string the softer string a couple pounds tighter. Using poly/syn gut at 53/55 lbs for example, would see them stretch about 3.5% and 9.5% respectively when strung.
However, during ball impact, the amount of additional poly stretch available is less than half that of the syn gut. Even though the strings are inter-weaved together, hard impacts bring the poly into deformation territories.
In other words, poly gets stretched beyond its elastic range and loses tension. Over time, the feel of the syn gut increases. (see "Impact Loss" in table below)
And that's only assuming one plays with the racket immediately off the stringing machine.
(from Tennis Warehouse University String Performance Database link)
When allowed to rest from being strung at 51 lbs each, the "Actual Pre-Impact Tension" has already dropped to 30/39 lbs! This differential would only get wider with every ball hit.
Having established this, then how about we string poly higher instead? Would that help to compensate for its larger pre-impact tension loss compared to syn gut?
Probably yes. But only for a while before impact tension loss brings down the tension again. How fast this happens depends on how tightly strung it initially was and how hard one hits the ball.
Another often overlooked factor is string gauge while pairing. Many choose the same gauge for hybrids, typically 17g or about 1.25mm.
Using the same example of poly/syn gut earlier, from my own measurements, the 1.25/1.25 mm combination would thin out to about 1.20/1.13 mm.
Below pic shows a 1.40mm syn gut became 1.25mm strung.
An easy way to calculate this diameter change would be to use the percentage stretch as a proxy. A 1.25mm string with 10% stretch, usually becomes 1.13mm (0.90 x 1.25) strung.
Simple engineering dictates, the thicker the material, the stronger the support. This could be another factor why the poly mains dominate when paired with syn gut in the same gauge. Well, but many people like it, so it's their choice.
Perhaps after reading this, one could start deducing why Federer is touted to be using 1.30mm gut mains and 1.25mm poly crosses:
0.93 x 1.30 = 1.209 mm
0.96 x 1.25 = 1.200 mm
The same gauge combination of 16G gut and 16L poly for Serena Williams as well? (link) And Sharapova? (link) And Nishikori? (link)
That said, my personal preference is still a full bed of the same string. Primarily for its ease of adjustment and consistency after play. If I were to cut strings out after every hour of play, then it might change.
After all, according to the Luxilon String Analysis at the 2013 US Open (link, refer pg 6-10), more than 70% of the competitors used the SAME STRING in both mains and crosses.
Special thanks to Irvin for highlighting that report. (link)
