I've built up a couple of the spg 720's and 780's lately, and I've now got myself quite confused. In examining the two blanks, it appears
that they have the same taper, but the 780 is just 6" longer than the 720.
It seems the general conscensous is the 720 makes a good 5/6, and the 780 makes more of a 3/4. My question is, if these two blanks do have the same taper, how
is it that the 720 is supposedly a more powerful rod?
I'm fairly green in rod building yet, but my understanding is that when you shorten a blank by removing material from the butt, this will make the rod
slower (as a result of moving the butt of the rod closer to where the rod's natural curve is, and secondly, if anything, it will decrease the power of the
blank.
If the above is true, then in the case of the 720 and 780, we are saying that reducing length (in the case of the 720 when compared to the 780) is actually
increasing the power of the rod? Am I interpreting this all wrong? Or perhaps they actually have different tapers?
Any clarification on this would be much appreciated.
AK
photo by Loudog99 |
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SPG 720/780 Taper Question
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 Nijinski.theunofficialbl... |
SPG 720/780 Taper Question |
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Sport
Posts: 15 (05/13/09 12:59:29) |
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 pearow |
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Master Guide
Posts: 1600 (05/13/09 14:49:55) |
well, I was with you on that until i was corrected by more knowlegeable folks on this board. Shortening the rod actually makes the rod faster. Like i said I
thought like you, but it does. I was wrong. I'm not an engineer but you can take a 720 and test it on the Cents scale; cut off 6 inches and test it again
and it'l take more pennies to bend it 1/3 its length. Makes me scratch my noggin, but it does. I have found that on the 720 and 780 blanks the only
consistency is inconsistency. Mountainshark has dealt a lot with these blanks and can help in this regard but i have found the blanks differ in as much as 3
line weights. I made a fellor a rod out of a 720 and i thought it might be a 5 weight; he uses a 3 weight line on it ; says it really is a dog with a 5 but is
a nice rod with a 3; so who knows. I have not miked the tapers but i also believe the 780 to be the softer of the two blanks; I recently built a rod out of a
781 blank, adding 6 inches to the butt making it a 7 footer; actually 6'11". It is a nice bass bug rod; lots of beef in the butt and a nice quick tip
I havent cast any of the new bass rods by sage, redington, ross, etc. but its in that category, and lots cheaper-p-
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jgestar |
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Posts: 3659 (05/13/09 17:52:06) ezOP |
Think of it this way. If the 720 and 780 have the same basic taper, but the 780 is 6 inches longer, you have added 6 extra inches of fiberglass to the 780.
Fiberglass bends. The first 72 inches of the 780 rod blank will bend at least as much as the 720 PLUS you will have the additional bend that can occur in the
extra 6 inches. That results in a more flexible rod.
Tom |
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gypsy |
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Master Guide
Posts: 468 (05/13/09 18:08:43) |
Think of two wooden dowels of the same diameter but one longer than the other. The longer one is more flexible than the other one.
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Nijinski.theunofficialbl... |
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Sport
Posts: 16 (05/13/09 20:28:02) |
But with adding extra material at an increasing taper, aren't you adding material that will flex less than the thinner part of the taper, giving you a
blank that does not bend as far down in proportion to the overall length of the rod? Also, what started this for me, is I can't understand how adding
material at an increasing taper will decrease the power of the blank (i.e. 720 rating at 5/6 and 780 rating at
3/4).
The wooden dowels is a good example, but they aren't tapered, so it's not quite the same is it? I guess most of my confusion goes back to data from CCS test, which I understand is not the most effective tool to measure the power and action of fiberglass, but it seems that the properties affected in relation to removing material should remain constant regardless of the blank material. This is where I wish I was an engineer. I guess sometimes it's easier to not try to think things through, and just find a line that feels good for each blank, and leave it at that. |
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jgestar |
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Posts: 3660 (05/13/09 21:00:33) ezOP |
Yes, the taper increases and that 6" portion of the rod will flex "less" than any other 6" portion of the rod. But flexing less is still
flexing. So the overall flex of the rod increases - you are adding the total flex of the 720 rod to the flex of the additional 6" portion. That small
amount of deflection of the 6" portion, when leveraged through the remaining 72" of rod blank is a big movement at the tip.
Since you asked about the engineering, rod deflection is roughly =  where W is the weight you use to deflect the rod, l is the length of the rod, E is the modulus of elasticity, and I is the
moment of inertia. For this case, W, E, and I
will all stay basically the same, but l will increase 8%. Because
the deflection is proportional to l cubed, that 8% increase in length
results in 27% increase in deflection. If you want a rod to deflect more - then make it longer, or make it less stiff (the 3EI factor on the bottom). If you want a rod to deflect less, then make it shorter or stiffer.
You are right, it is easiest to just test cast the final rod and see what line works best. As P mentioned, these blanks vary a lot anyway. Tom
Last Edited By: jgestar
05/13/09 21:03:55.
Edited 2 times.
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Nijinski.theunofficialbl... |
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Sport
Posts: 17 (05/14/09 10:23:19) |
Thanks Tom!
Appreciate the engineering side of it...now I'll just have to try to wrap my head around that formula! AK |
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taniwha2 |
780 + 6 | ||
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Master Guide
Posts: 89 (05/14/09 14:16:43) |
I'm with you in the 720 vs 780 and I get the the gist of the formula. But a 780 extended 6" seems faster and much more powerful ....
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pearow |
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Master Guide
Posts: 1602 (05/14/09 15:57:27) |
Thats my favorite configeration also; a 780 plus 6 inches; and a 780 plus 3 inches; both 6 weights or approximate-p-
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