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If, on the otherhand, you want 2.4mm on the low E and 1.6mm on the high E, all the time, well, my guess is you are going to be fiddling with it a good bit. Shims and truss rod adjustments are the way its done.
An adjustable bridge like one sees on archtop guitars would be nice, but they weight 25-30 grams minimum, often mroe and that would just kill the sound on Selmer style guitar. If someone cooks up an adjustable 10-12 gram bridge, please let me know. In the mean time, shims are very effective and not hard to use.
http://www.luth.org/images/backissues/al113-116/al113_monteleone.jpg
Maybe a little heavy and I've heard rumors they were fiddly to operate, but some thing along that line might do it. Most of it could be a light hardwood like walnut and only the top part of the center piece where the strings lay would need to hard like ebony. Humm.....
Some guys have an aversion to shimming. Nousche Rosenberg told me he didn't like to shim unless there was no other way, and it sounds Like CVH feels the same way. They have a valid point. But if you need to shim, here are a few things to keep in mind:
1.) Shim material/number can form an acoustic filter:
There's a concept called 'constrained layer dampening' that exists because sound waves lose energy when they are forced to pass through materials of different density. This attenuating filter is a result of mechanical impedance differential. This is the reason that insulating a room cuts down on sound, and why truck-bed-liner reduces vibration and why a person can't speak to someone who is underwater. Essentially, sound waves that travel well through one density of material don't travel well through significantly different densities of material, and a few other things come into play etc. Anyway, guitars are already only ~10%+/- efficient at turning vibration into sound, and one of the reasons is that vibration goes from metal strings into a hardwood bridge, into a softwood top. These transitions steal energy and turn it into heat & whatnot, and in so doing, they filter the energy. Fortunately, we perceive that filtration as a positive thing, as we are used to the sound of a guitar, and part of the character of the sound is this filtration. It's also why bridges of different materials & weights sound different; they form different filters. But no matter what your bridge is made of, if you stack a bunch of shims of different types of wood under it, you can dramatically increase the number of impedance transitions. What does this mean in the real world? Usually it means you lose treble, volume & attack/response. I recently did some work on a Favino for an artist. It was a great guitar, but the bridge had been "lightened" (translation - shaved to within an inch of its life) and had 3 or 4 shims of dissimilar wood densities (poplar, maple, rosewood) under each foot. The bridge didn't have enough mass to function properly and the shims presented a huge dampening filter. I made it a decent bridge and the guitar woke right up. But a rosewood shim under a good rosewood bridge wouldn't be a huge problem.
2.) Unequal numbers of shims under the bridge feet change the bridge fit:
Imagine laying a book on a table. It lays flat. Now imagine laying that book across two equal height drink coasters. The middle of the book is suspended, but the ends of the book lay flat across the coasters, and the bottom of the coasters lay flat on the table and the bottom of the coasters bear equal weight all across their bottoms.
Now, add one drink coaster under one side of the book so that you have two under one side and one under the other. You'll see that the book now lays across the inside of the higher stack of coasters and the outside of the lower side. Instead of the weight of the book bearing evenly across the coasters, it's borne by the inside and outside edges of the coasters. This is exactly what happens when you unequally shim the feet of a bridge. Now, in reality, shims are thin, so it's not a big deal if you put a shim under one side, or two under one side and one under the other, but if you get carried away with unequal shims, not only are you likely to crack your top, but your bridge will no longer bear squarely on the tonebars connecting the 3rd and 4th braces and that will dramatically steal power from the guitar because the bridge will no longer bear on a node. Somewhere I did a video of the power transfer difference between on-node & off-node loading... I think it was in response to someone asking about where to mount a contact mic but the same principal applies to an ill fitting bridge.
Anyway - now you know more than you ever wanted to know about shims.
Bottom line:
- Use all one type of wood - preferably the same type of wood as the bridge
- Don't use a lot more under one side than the other. If you have to do this, then glue the shims to the bottom of the bridge feet and re-fit the bridge.
Ahhh... found the vid. Again, the effect of an ill fitting bridge not bearing on the bridge tonebars is less pronounced, but... a similar thing is happening.
I love it when people explain stuff in my native language
> Again, the effect of an ill fitting bridge not bearing on the bridge tonebars is less pronounced, but... a similar thing is happening.
By tone bars, do you mean the two short braces that run perpendicular to the bridge and placed right under the contact points? The bridge on one of my guitars has unusually long feet, which I think extend beyond the width of those braces. Is this bad, or just fine? If I needed to shim, should I cut pieces as big as the existing feet or smaller ones to overlap with those braces only?
That's what I found too when I was searching for adjustable archtop bridges some time ago.
Looks beautiful but doesn't get a whole lot of praise for friendliness.
In absence of something like the above, I myself prefer two different bridges. Even with Chicago weather one of the two would always work fine. Actually the one with higher action, a touch over 3mm does the job all year long. I change it only when I want to go back to the lower action. That one can end up with string buzz during the winter season though.
It's fine either way as long as you're loading the top through those two short braces. You can overlap those little braces with the bridge feet, and in fact it's probably good to do so, because the downforce on the bridge is ~15lbs+/- and so if you're loading that 15lbs on two chicklet sized feet, you will probably deform the spruce/cedar under the bridge feet. This is a guess, but Selmer eased the ends of the bridge on purpose, and one good reason to do that would be to make it harder for an ill-fitting bridge to bear outside of those two braces - perhaps to improve the sound of a bridge that was a close but not perfect fit, and perhaps also to reduce the possibility of the very outside of the bridge digging in near the inside of the mustache if the bottom of the bridge was over-arched or if the other side of the bridge was over-shimmed.
Looking at my description above, I might have played fast and loose with terminology. I haven't done the math on this sort of thing since the Clinton administration (Bill, that is) so what I'm talking about is the difference & abruptness of mechanical impedance of substrates through which the energy will travel... Differential equations are used to describe dynamic systems and I think I just moshed them together. Again, it's been a while ;-) Oh well. It sounds like you understand the point of it even if I was a bit fuzzy with language. If the bridge bears outside of or inside of these cross-braces (tonebars / supports / whatever a person might call them) and does not bear directly on them, then the bridge is not loading into the bracing, but rather into a leaf-spring straddling a brace, and the directness of loading becomes dependent on the modulus of the soundboard, which is... well - its nothing more than a few millimeters of spruce or cedar... Lol... so... it'll eat some efficiency for sure.