My spider is nearly flat. I've never actually measured, but when laying on a flat surface I'd wager there's maybe 1/32" clearance in center. Does spider arch affect pressure distribution (and thus tone)?
Nice question to open a can of worms...
First, you should probably loose the term "pressure" (as in PSI) and think in terms of "force", specifically "downforce".
The arch in the legs of a spider is the result of gently forming the legs to achieve proper string height in relation to the fretboard while also creating the desired break angle of the strings over the bridge. While a reso's string height is pretty much established at around 3/8", the proper break angle is debatable.
The break angle determines the amount of downward force applied to the bridge by the cumulative string tension and ultimately distributed to the cone by each leg. For example, if the tuned up tension of six strings is 220 pounds and the break angle is 3 degrees, the downforce on the bridge is about 12 pounds. Roughly, the distributed force on each leg would be about 2 pounds. You can play with doing your own calculations here...
one note about making this all work is that the spider needs to be "leveled" in that the spider rests on a flat surface and does not "rock" when any given leg is pressed downward. Using a flat sanding surface is usually the method used to level a spider.
So... yeah. If you have a proper set up on your reso you almost assuredly have some arch in your spider legs. And yes, it most assuredly has some effect.
Arch should be immaterial if I'm understanding you correctly. String height (and break angle) is determined by the height of the inserts rather than any arch in the spider; I generally strive for as much height as I can get under the palm rest under full tension. Unless you've got a spider leg that's way high or low, I'd advise against trying to adjust the leg height, however gentle you might be. If you can't level the feet by sanding, be damn careful when adjusting. Cast aluminum is brittle - ask me how I know.
When you're done sanding the feet against a true flat surface (window glass works if you don't have a flat saw table), put some downpressure on the bridge area and check each leg using a scrap of paper as a feeler gauge. When correct, the paper should encounter the same resistance when moving under each leg.
Hence the can of worms... eh, badger?
Unless you are lucky enough to have a bin of spiders to select from, it's not uncommon to receive a spider that needs to have one or more legs gently coaxed into place. I totally agree that attempting to bend a leg is courting disaster but ( after a costly learning curve) it can be done and in some cases a must. If merely sanding the spider will do the trick, so much the lucky you. My reference is with those spiders where sanding to a level state would remove too much material all around due to a leg being abnormally high.
I'm not suggesting that bending legs to achieve string height and break angle is the preferred way. I should have mentioned that the proper installation of the bridge insert is a given. But there are those cases where, for whatever reason, things don't mesh using common (safe?) set up practices there are remedies.
Yeah... I avoid bending legs on a spider like the plague, but I'm not afraid to do it if the situation warrants. Judging from spiders I have removed/replaced from various builders there are other who remain fearless also.
Edited by - Phaedrus on 11/26/2021 12:00:14
All No. 14's are slightly arched using a jig. partially because, as they come from the foundry, the legs are not in an even plane due to the way the spiders come out of the mold.
Once in the same plane, the tips can be/should be leveled for perfect consistency across the 8 legs.
Due to alloy differences, I'm not personally aware that you can arch an import spider. They're too brittle and the attempt usually snaps a leg.
Place spider on jig. 8 wacks with an appropriate mallet. Ready to level.
So, that's my Beard No. 14 spider recollection.
Edited by - hlpdobro on 11/26/2021 12:05:21
Thanks all. Guess I should have included the caveat that I already understand the importance of spider leveling and string height/break angle. So, yes, badger, you understood me correctly, and since no one else included the degree of arch in and of itself in their can of worms, I'm inclined to accept your answer that it is irrelevant. Everyone else concur?
Try to get a high action under a "modern" coverplate without arching and report back. Bridge inserts are only so tall.
The spider bridge height and bridge insert height contribute to string height above the surface of the top of your guitar. The string height contributes to the volume, tone, and playability of the guitar. Your goal should be to have the guitar sound and play the way YOU think is best for your playing style.
Please, tell me more. How does arch have an effect (other than on string height), and what should be my goal?
Arching is necessary to facilitate the leveling of the tips of the spider legs.
In my experience, arching the spider casting raises the resonant frequency of the spider.
As to what your goal should be…
Only you can answer that.
Edited by - Schoonie on 11/26/2021 20:26:13
This is a great subject! Glad you asked the question and love to hear the answers given thus far from some guys who have really delved deeply into the mysteries of dobro construction and done the math.
I timberframed my house by using bark-on trees with 45 degree bracing. My structural calculations consisted of standing back and saying, "Yep, that looks about right" or not and everything turned out fantastic from a structural and aesthetic standpoint.
My take on an almost flat spider would fall into the "or not" category. If you were in super shape and held your arms flat out and tried to hold your body up with your arms, it would be very difficult. The more you lowered your arms (put more arch into the spider) the easier it would be to hold yourself up. I don't know the specifics, but structurally, the downward forces are very different and absolutely have to affect the sound.
I was going to jump out of this thread because it seemed to be getting a bit contentious, but Kent's post resonated with me... no pun intended.
I'm a hobbyist... a shade tree builder. I started building reso's around 2005 when I entered retirement. Since that beginning I quickly came to realize that the reso is a 'system' which relies on major and minor components to properly function. The goal, as requested by arficus, is to fine tune these components to work together as a well oiled system. As with any finely engineered device, changing something on one component will very likely have a positive or negative effect on the whole.
As a builder the design of the 'system' evolves as things tried pass or fail or new information is presented from other sources. For example, Kent's post above shed light on something I never considered... the arch of a spider affection resonance. I've always looked at other components and methods to achieve the "sweet spot" in the system. Now there is something new to add to the mix. Brilliant!
As for spiders I have always arched the legs if not already done by the manufacturer. As Kent alluded, it's pretty darn difficult to level a flat spider. Now it appears I need to pay attention to the severity of the arch...
Edited by - Phaedrus on 11/27/2021 07:15:35
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