If Eight was Three

Tom Olson · From: Spokane, WA

Does anyone know why a resonator spider bridge has eight legs? I would think that three would be better because if you had three you could be certain that all three would contact the cone fully. Sort of like when you have a three-legged stool versus a four-legged stool. You never have any wobbling problems with a three-legged one. I would think the same theory would apply to the spider bridge.

Sage · From: Boulder, Colorado

Howdy Tom- I've wondered exactly the same thing. What you're talking about makes perfect sense to me. All you'd have to worry about is the pressure adjustment in the middle and not the contact around the outside. (I think I'll make one like that).

mikey · From: New Jersey

John Dopyera had enough patent problems w/National in the 30's...I think a 3 legged bridge would have been a bit too close to a tri-cone...if you want 3 legs, get a tri-cone...Just a thought,
mike

Tom Olson · From: Spokane, WA

Mikey,

You have a good point. It's possible that the 8-legged spider was developed to avoid the tri-cone patent. However, in light of the fact that Dopyera built an instrument that was similar to the tri-cone in so many other ways, it's doubtful that the number of legs on the cone would pose a big problem in the overall scheme of things. In any case, if there was a Tri-cone patent that claimed a three-legged bridge, it's been LONG since expired (they only last for 17 years). I'm wondering if the purpose for the 8 legs has a technical reason. For example, maybe you need that many legs in order to get a fairly full frequency response from the cone. I don't really know -- but that's what I'm hoping to find out here.

Sage,

If you try it, let me know the result -- I'd really like to know. I'd try it, but, at the moment, I don't have a resonator guitar. All you'd have to do is buy a cheap foreign-made spider (Janet Davis sells a Korean-made unit for about $25) and cut off 5 of the legs. I would do it this way: If you assign numbers to the legs sequencially starting with number 1 for the leg which points straight down the neck toward the nut, then I would cut off legs #1, #3, #4, #6, and #7, leaving #'s 2, 5, and 8. This way, you'd have two in front where you'd need more strength to counteract the pull of the strings toward the nut due to the downward slant of the strings behind the bridge.

Scott Camara · From: Connecticut

I believe the reason for more legs on the spider is to distribute the load more. With the strings tuned to pitch, there is tremendous pressure on the cone. Notice that after a cone has been played for a while, there will be indentments where the spider starts to sink...this would be even worse with only 3 legs.

Mike D · From: Phx, Az

There is a fair amount of downward pressure (but not all that much or we wouldn't get so many buzzes in the first place) If three legs were used they may need to be thickened up. I would also speculate that it would reduce output somewhat. Dobro-style instruments work by transfering vibrations through the legs of the spider onto the edges of the cone, less contact area may equal less sound.

If your spider has not been flattened I would recommend looking into having it done, or do it yourself with plate glass and sandpaper.

The real problem as I see it is that if the bridge has only three legs it could no longer be called a "Spider" A Trider maybe?

[This message was edited by Mike D on 07 February 2001 at 03:34 PM.]

Sage · From: Boulder, Colorado

I think that one of the problems with the 3 contacts around the outside is getting enough action out of the top, as was mentioned previously. IMHO cast aluminum is too soft and will "creep" over time, as well as not transmitting all the vibrations it could. I'll make one out of carbon fiber since I'm using that for my other prototype instruments. (don't be looking for it next week though, I'm still finishing up my divorce and won't be back in my shop for at least a month)

[This message was edited by Sage on 07 February 2001 at 12:48 PM.]

Mark van Allen · From: Watkinsville, Ga. USA

This has nothing or little to do with the rest of this thread, but... a while back a guitar amp was made (Washburn I think) with a nifty gizmo inside over the front of the speaker, looked like a Dobro Spider, that held just the element from a Shure Sm-57 mike right over the center of the speaker, behind the grillcloth. It was hardwired to a female XLR jack on the cabinet, just a genius idea, your amp was already miked up ready to go for any gig. I was told they had to quickly discontinue them from threat of lawsuit by the patent holders on the Dobro Spyder.

------------------
Mark van Allen-"Blueground Undergrass" Pedal, Non-Pedal, Lap, and Dobro

Tom Olson · From: Spokane, WA

Mark,

That's some interesting info -- sounds like a great idea they had. I find it difficult to believe, however, that the patent holders of the Dobro spider would have any right to claim such a device as an infringement on their patent. It's two different applications -- no similarity between the two that I can see, save for the spider itself. Besides, the Dobro patent has got to be ancient. Wasn't it patented in the 20's or 30's? Patents only last for about 20 years at the most. Kind of weird -- I'd like to know what really happened.

Mike D · From: Phx, Az

The patents on both National and Dobro's are long expired. Anybody can cast up a Dobro-style spider bridge or cone.
The Trademark names are still in effect though.

Sage · From: Boulder, Colorado

That's a great idea- to have a mic element mounted right in the spider at the focal point of the reso cone- cool.

Tom Olson · From: Spokane, WA

Sage,

I might be wrong, but in order to have the focal point of the cone at the spider, the cone would have to be parabolically shaped (ie, shaped like a parabola). Most cones are purely conical (I guess that's why they're called cones) in shape.

Still, you're right though -- it seems like a cool idea to mount a mic on the spider. But, I wonder if you'd get some vibration interference from the spider itself? I think a lot of people mount the mic/transducer/etc. on the cover plate. That way, there's not much vibration in the mic support.

Sage · From: Boulder, Colorado

That's true, The sound might be pretty bizarre if the element was mounted on the spider- might try it anyway just for fun and see how the sound turns out. The element would have to be light. There are some full range AKG's that might work.

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