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I haven't built with WRC before but I recently got a set of sinker that's pretty nice and very stiff, great tap. I've seen a lot of comments here about bridges coming up from WRC tops ripping up part of the soundboard as well. A friend of mine wants WRC as a soundboard but if I choose to use that set I'd like to take steps to prevent this from happening. For those of you that use WRC what steps do you take to ensure that the bridge doesn't come up? I thought of maybe adding 2-3 dowels into the bottom of the bridge that would run through the top and into the bridge plate upon gluing. Doesn't seem like different adhesive options would do much good since the wood typically fails more often than other woods.

The dowels are more likely to cause problems than solve them.

Just don't cut into the top layer of fibres when you clear finish from the bridge area and you'll be fine.

+1

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Pat

I don't have yet have any issues with it and I own two guitars that I built 15 and 20 years ago, one a steel string the other a classical, which are still going along just fine with no issues. I have several others out there in the wild that are similar ages and have not heard back about any problems. So I think if you just stick to your best practices, freshly planed or sanded gluing surfaces, perfect joinery, use proper cauls and so on you will be fine.

I really like WRC, especially for Spanish guitars.

When I first started I used a lot of Cedar. Bridges coming off got me to shift to spruce and problems went away. No doubt part of the problem was my technique. Time has taught me to do better fits and procedures. One thing you can do is a wider footprint but try to be careful not to add too much weight. A slopping back section on the bridge helps. Also as Ed says no cutting into lacquer when fitting the bridge. Lots of folks make it work so give it a try.
Tom

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I've done three with WRC over the last decade and have seen no problems. During that time I bet Olson and Ryan have done a gadzillion.

I just use masking tape while French polishing, that shouldn't be a problem as long as it doesn't rip up small fibers of wood I suppose? Maybe heat it with a hair dryer to be sure?

I've made 8 cedar top steel strings to date and no issue with bridges coming loose or off.....nothing special done either. Just don't cut into the fibres when removing finish as Ed said.

Oh, and I've used titebond, HHG and fish glue all with success.

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WRC can make a very fine guitar and I built with it lots of times. But repair guys everywhere if they are very experienced with acoustic guitar repair will tell you that bridges seem to do far more top damage with WRC when they lift and I'll even suggest that they may lift more frequently although I have no empirical data to support my hunch.

Being careful when scoring most certainly will help but when we see WRC bridges lifting and taking a goodly scoop of top wood with them it's not always because of the scoring of the finish. The wood just seems to tear up more than nice spruce.

It's typical when a bridge lifts for the failure to show glue line and some wood with spruce. With WRC much if not most of the time you won't even see the glue line and will see only WRC still stuck to the bottom of the bridge.

WRC sure is pretty though and makes a great finger style guitar. Love that chocolate brown color!!!

WRC has lower peel strength than the spruces. Any sort of stress riser at the back of the bridge can start it peeling up; that's why the caution against cutting into the top when removing the finish. Often enough when it comes up you'll see that you've got 100% wood shear: the glue line didn't fail, the wood did, leaving a layer of cedar on the bottom of the bridge.

The key to making it work is to reduce the stress along the back edge of the bridge, where its highest, and where the problems start. Aside from avoiding scoring the top the other ting you can do is make the bridge wider: deeper along the line of the string pull. This is what Martin did with the 'belly' bridge when they had issues with their bar bridges lifting. Note that it's not so much the footprint area as the distance from the front edge of the bridge to the back that helps. I often use a lower density wood, such as walnut, for bridge on cedar tops. this allows me to make the bridge larger without adding too much weight.

Yes, and keep in mind the physics of the process. On a pin style bridge, it is not the string tension trying to pull the bridge off the top, since the ball ends are anchored beneath the top. String pressure against the saddle is trying to tip the saddle over forward, taking the bridge with it, so the whole system is acting like a lever with the saddle acting as the arm of the lever. So the Martin belly bridge and the Gibson reverse belly bridge are both diminishing the mechanical advantage of that lever. String pressure is also exerting a downward force on the saddle. If the pin hole distance behind the saddle is increased, that also changes the ratio of downward vs forward pressure and decreases this mechanical advantage. So, narrow bridges with tall saddles are the worse case scenario, thus bridge design can minimize the potential problem.

Something to think about.

Grant

I'm working on a WRC top now and intend to make the bridge wider as mentioned here. Grant's post got me thinking more about how I am going to do that. I had planned to just make the bridge wider on the bridge pin side (not a belly bridge), but hadn't really thought much more about it than that. I'm guessing the added surface area would be best spent behind the saddle rather than in front.?. It sounds like moving the pins back slightly would be a benefit also (If I had that right). So the best place to add this width would be between the saddle and the bridge pins.?. Or, am I over thinking this?

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Bryan Bear PMoMC

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Thanks, that's a great explanation. I'll have to give WRC a try. I do love the tap.

Out of curiosity, has anyone had similar experience with redwood? I'm on my second redwood top build and this set seems much nice than my last one. Neither set is close to as stiff as any spruce that I've worked with or handled so it's been in the back of my mind while working on my current Redwood project since I had read about the bridge lifting problems with WRC.

Grant's explanation is excellent so when you have more material behind the saddle moving toward the tail end of the guitar it's not unlike having a longer handle on a wrench to deal with the tipping forward forces. Most of the real counter force to that saddle/bridge tipping forward is the very back edge of the bridge.

Regarding moving the pins back further from the saddle there are some other considerations here as well.

First a proper break angle is important and if you've ever taken a saddle down as low as you can go and watched the break angle disappear you may have also noticed that with too little break angle, such as a Fender style bolt on neck and the G string with no trees breaking over the nut..., the note becomes soft and muddled and eventually won't sound much at all as the vibrating wave of the string can breach the saddle termination point.

With this said the acoustic guitars that we make will need neck resets in the vast majority of cases some day. Some of you may not be all that concerned about the serviceability aspects of your creations but I'm sure that you do want to provide the most value for your clients and/or recipients of your instruments.

The need for a neck reset does not happen over night where it was fine on Tuesday and sucks on Wednesday. It's a gradual and slow process that can take anywhere from a few years to 40 - 50 years until it has to be done. We've seen a small builder guitar need a reset in 5 years and a major manufacturer guitar not need one until it was 50. Along the way as the neck angle is gradually changing over time we still want to enjoy our guitars and have them be capable of being set-up great and playing great too.

The fix over time to keep the action where we want it is to lower the saddle as need be. Keeping in mind that your saddle height will not be static and will be coming down in time and it may be a goodly amount of time before the neck gets reset the pin distance from the saddle should permit the saddle to be lowered and still have a decent break angle. If not neck reset time may have to come quicker and this in my view reduces the value that an instrument provides if it can't be serviced well or needs premature major work because of a design flaw in pin spacing from he saddle.

If you have any beaters that need a new saddle take the saddle down progressively until the break angle is nearly nonexistent and you will see what I mean. Having the pins closer to the saddle, say what Martin does, will permit the saddle to be lowered a great deal before someone has to pay the piper for a neck reset and the associated "whole nine yards" that often comes unfortunately with this.

A related topic is the idea of angling the saddle back. This can redistribute the forces to a degree and may be a very good idea in terms of helping bridges not lift. Problem is again in time that saddle will likely need to come down as the neck angle increases and since it's angled back the break points on the saddle will be coming forward as the saddle decreases in height likely causing intonation problems as the "speaking length" of the string is shortened with reduced saddle height.

Although these things look pretty darn simple there is a LOT of physics with an acoustic guitar and even more interplay between how the forces are distributed. This complexity has kept many of us going with Lutherie and countless doctoral students writing their thoughts about how an acoustic guitar works.

This is my stardard design bridge. 1 3/4" at the widest and 7.5" long. That saddle position is go I ng to get pushed forward as it was in the guitar pic above. This WRC is a baritone build and another reason I'm concerned. Should I look at going even wider?

Sent from my SM-G900V using Tapatalk

I don't do anything different from what I do with spruce. Same (small) bridge, minimal long-grain runout, scrape finish off the full footprint, glue/clamp accurately and quickly (whichever glue you use) etc.etc. I've built a good few of these. It's pretty much my only "standard" model. Never a problem. Same with classicals.

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Grant Goltz wrote:
"Yes, and keep in mind the physics of the process. On a pin style bridge, it is not the string tension trying to pull the bridge off the top, since the ball ends are anchored beneath the top."

I question that. In terms of the way the string vibrates all of the tension has to be acting on the saddle top, pulling the bridge toward the nut. That's the same no matter what happens behind the saddle. The way the strings anchor behind the saddle, whether in pins or a tie block, should not make any difference in the load on the glue line of the bridge as far as I can see. At any rate, having spent a lot of time doing repairs in the dissipated days of my misspent youth, I can't really see much difference in the way bridges start to peel up. Once they start, of course, the ball ends do pull the top up, but I can't remember any systematic difference up to that point. I realize this is an iconoclastic view, but, again, without any data one way or the other it seems just as valid as any.

Hesh wrote:
"First a proper break angle is important and if you've ever taken a saddle down as low as you can go and watched the break angle disappear you may have also noticed that with too little break angle, such as a Fender style bolt on neck and the G string with no trees breaking over the nut..., the note becomes soft and muddled and eventually won't sound much at all as the vibrating wave of the string can breach the saddle termination point."

Here's one where I CAN put up data. Break angle is pretty over rated: you don't really need much for it to work. In a test I did with a Classical guitar I measured the sound output and did listening tests for mechanical 'standard plucks' of the open strings with different break angles, and string heights off the top. When the height stayed the same but the break angle differed from 6 degrees to 25 degrees there was no significant difference in the sound output, and listeners could not reliably distinguish any difference. When the string height off the top was raised from 11mm to 18mm (don't try this at home!) there were differences in the sound and people could hear them. It has been objected that there is a difference if you go above a 45 degree break angle, but I have not seen any real data on that. Absent that I'm going to say that something on the order of 6-10 degrees of break is 'enough', since it suffices to keep the string in contact with the saddle top through the whole vibration cycle. Beyond that no further signal will be transmitted to the top. From what I can see the main result of increasing the break angle over the saddle is to raise the tipping force that's trying to split out the front of the saddle slot. You can get around that by angling the saddle back, but that's another issue.

Great information here and good food for thought. It is much appreciated.

When I look at Trevor's guitar and the way he has set the pins back on the bridge, away from the saddle, it seems apparent to me the ball ends of the strings must add greatly to the 'hold down' force. The are locked at the bridge plate and turn nearly 90 degrees across the bridge to the saddle. The section from the ball end to the turn has to increase in length for the bridge to pull away from the top. The farther the distance from the saddle the more the string has to lengthen to allow a rotation. Of course this is all supposition but it makes sense to me.

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Exactly, the tension is acting at the saddle top, not the saddle as a whole nor the bridge as a whole. And if you look at the cross section of a typical bridge and saddle, that force is causing the saddle to act as the handle of a lever system where the force is applied. That lever is attempting to rotate the bridge forward. That is what causes the common "bulge and belly" behind and in front of the bridge, respectively....the front edge of the bridge is being pushed down and the back edge lifted up, and if the glue joint holds well, this force distorts the top. That force can be strong enough that the saddle slot cracks out rather than the bridge glue joint fail.

This is all simple, basic physics. Shall we next take on gravity

BTW, I totally agree on the break angle.

Grant

Good conversation. I guess the forces don't "know" whether or not a string is tied as in a classic bridge or a ball end mounted to a bridge plate but intuitively i cannot help but think of the ball end set up as having six little clamps holding at least some of the bridge in place to the soundboard and plate.

As to the Martin VS Ugly backwards Gibson belly bridge I'm scratchin my head thinking about the applied forces and how they relate to a bridge ripping off on a cedar top.

Well, make a 1-string mock-up without the "bridge" piece glued to the top. Do 2 versions, one with a steep break angle and one with a shallow. Be sure to have a solidly glued bridge plate piece. Tighten the string and note any movement of the "bridge".

In a real-life situation, the top actually peels off from the bridge rather than the bridge peeling off from the top. Just like peeling off a piece of tape, the flexible half (top, not bridge) is where the failure initiates....it is much easier to peel tape off a piece of wood than to pull a piece of wood off an immobile piece of tape. In that regard, the bridge plate plays a very important role in helping to keep the bridge glue joint intact by stiffening that portion of the top (so it resists initiation of the peel). Actually, a bridge plate with the grain direction the same as the top and extending about 1/2" or so behind the bridge would be idea. Not sure what that would do for tone, though.

Obviously, a tie on bridge is a whole different animal.

Hope this is helpful.

Grant

This is true but the compensation for the bulge in the belly in theory should be a rigid center of the X-brace. This is something that I struggle with while thinning bracing and tapping for that bell tone. I'm always reluctant to thin around the area that the x-brace and bridge overlap. Maybe I'm off base in this thinking but I'd be very interested in hearing the community's thoughts on that subject.

This is a really great conversation and it really makes me appreciate the community that we have here. Merry Christmas everyone and best wishes, hopefully our best work is ahead

The comment I made earlier about dowels through the bridge had me thinking about what you are talking about in the above post for most of the day while working in the shop. For the sake of conversation what about a mortise/tenon between the bridge and bridge plate (matching materials and grain). Sounds like a cool CNC project/experiment build. Can anyone think of a reason not to try it? It won't prove anything but with a sinker WRC top, in theory should give peace of mind if successful. On the other hand it presents a ton of problems/challenges with flush glue joints and very tight tolerances unless the tenon were to come from the bridge and extend through the bridge plate.

Grant Goltz wrote:
"Well, make a 1-string mock-up without the "bridge" piece glued to the top. Do 2 versions, one with a steep break angle and one with a shallow."

I got some interesting numbers off the guitar I used for the string height and break angle experiment. I had three cases:
A) strings 11mm off the top with a 25 degree break angle,
B) strings 11mm off the top with a 6 degree break angle, and
c) strings 18mm off the top with a 25 degree break angle.

One measurement I took was of the bridge rotation under tension. I clamped the body of the guitar to the bench at the rim and used a dial gauge to measure to deflection 50mm in front of and behind the saddle, and also how much the saddle pushed down when the tension was applied. What I found was that a taller saddle gave more rotation, and pushed down further, as you'd expect: the A and B cases were the same in those respects, while C was greater. You can subtract the downward movement of the bridge from the rotation, and find a 'centroid'; the point behind the saddle around which the bridge is rotating (in other words, think of it as two movements; one pushing the top down and the other a rotation of the bridge around some axis; the centroid). It turns out that the location of the centroid seems to depend on the break angle: the A and C cases, with the same break angle, had the same centroid, while the B case had it's centroid further back. It makes sense if you think about it.

Bridge torque does distort the top, of course, and that can change the way it vibrates. Thus, at least in theory, changing the break angle could alter the sound. In practice there was no statistically significant difference in the sound with the two break angles I used so long as the string height off the top remained that same, either in objective measurements or listening tests. The very small changes in top mode pitches simply were not enough to detect.

As for the stress in the glue line: I got some good information on this in a little book entitled: "The New Science of Strong Materials; or Why You Don't Fall Through the Floor". At one point he talked about the stress in a glue line that is loaded in shear, such as the we see with a bridge glued to a top, except that he didn't account for torque. What happens is that there is high stress at the leading and trailing edges of the joint, and it falls to a low and constant level in the middle. The total area under the curve is equal to the load. The level in the middle is determined by the nature of the glue line. If the middle is longer (the leading and trailing edges are further apart) more of the total load is take up there, and there's less to cause the spikes in the leading and trailing edges. Since it's the stress at the edges, and in particular the back edge, of the guitar bridge that fail and cause the thing to come up, the longer the distance between the front and back edges of the bridge the longer it's likely to stay put. From this standpoint it doesn't matter whether the belly is in the front or in the back, and I suppose there's something to be said either way. I just think Martin got it right the first time, and Gibson was simply trying to be different.

For the case under discussion here (remember?), the allowable stress along the back edge of the bridge is lower for cedar than it is for spruce. It occurred to me this morning that it's particularly low because we glue to a radial face: the meddulary rays weaken the bond between cells in that case, reducing the splitting resistance, as everyone knows who's split enough fire wood. Since torque adds to the stress reducing it by lowering the string height off the top should help. However, the basic mechanism is to make the bridge wider; longer along the line of pull of the strings.

Yes, that seems consistent with what I have been trying to say

Grant