Bridge length on WRC OM build

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Doug Ferguson
Gidgee
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Bridge length on WRC OM build

Post by Doug Ferguson » Fri Jan 06, 2023 7:25 am

Hi, this is my first post to the Forum. Last year I built three engleman spruce/black walnut OM's. Builds 3,4&5. I will note that they are loud compared to Yamaha I own with more sustain and base. They were gifted and all are pleased.

This year another three builds only WRC/Mahogany OM's using the same basic design, mold, jigs etc. During the research my concerns have grown regarding western red cedar and its strength. I have shortened the scale to 24.84 to reduce the static load and will join at the 13th fret to approximate the traditional bridge/saddle location. I am using WRC bracing and will run them out to to reverse kerf linings at about 1mm thick to reduce concerns of peel up. All braces will be let in. The main T1 brace will be Mahogany in an effort to remove flexibility and I may A frame the neck block through the T1 and to the X braces. For the X braces I have laminated 1.25mm HOG to each side of the 6 mm WRC core and increased the height to 17.6 mm from 16mm.

I am trying to hold the 94 deg X angle and have the lower bridge wing corners land on the centerline of the X braces. This would require that the bridge would be lengthened from 6" to about 6.375" as the saddle is moving about 7/16" towards the tail block. The bridge, a Martin style will be increased in width to 1.5" and the bridge plate 1.75" with the excess hanging out the back.

Ok now for the questions - Is it common to modify the length and width of the bridge? Is the increased surface area sufficient to reduce the likelihood of bridge pull up? What direction tonally is expected from lengthening the bridge? Am I mis-guided seeing the finger and tone braces having limited structural significance?

Thanks for any input one can offer
Doug

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TallDad71
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Re: Bridge length on WRC OM build

Post by TallDad71 » Fri Jan 06, 2023 7:57 am

At a facile level, the bridge contract area is sufficient for a specific pull strength of the strings. The bracing underneath does not determine what contact area is required.

A larger contact area could lead to more mass on your soundboard as the bridge is larger. The technical term for this is ‘ bad’.

There is discussion that making the bridge connect at the lines of the x brace is good practice but, in my opinion, this is less important than reducing the mass of the bridge.
Alan
Peregrine Guitars

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kiwigeo
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Re: Bridge length on WRC OM build

Post by kiwigeo » Fri Jan 06, 2023 9:58 am

A bridge should have as little mass as possible.....no point wasting string energy thats already in short supply on moving an oversized bridge.
Bridge wings should have X brace legs running under end of same to reduce tendancy for bridge to rotate under string tension.

I'm a great fan of Trevor Gore's bridges that are a sandwich of wood and CF. For a steel string one layer of CF in the bridge and a second layer between bridge patch and underside of the top.

Re your plan to use WRC bracing. I like my bracing light and stiff so moved away fron using WRC for bracing...spruce is the go for me.
Martin

Doug Ferguson
Gidgee
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Re: Bridge length on WRC OM build

Post by Doug Ferguson » Sat Jan 07, 2023 1:34 am

Thanks for the responses,

I have calculated 145 lbs for the short scale string tension and the bridge and bridge plate (wenge) weigh 21.5 and 6.5 grams respectively. The wing ends are 2mm thick and in this case extended by 4.7mm each end compared to Martin bridge. I could save a few grams by changing to a lower density wood. My concern is that the bridge will peel up given the strength of the western red cedar. Currently I do fit the bridge to the arch of the top.

The other option I see is to narrow the X brace angle to return to the traditional 6" length on the Martin guitars but I am apprehensive on going this as it moves away from the fabrication drawings I have and my inexperience in understanding how to influence the tone of a given build. It also does not address the bridge failure aspect I have read about for WRC tops.

I do think I will return to Spruce bracing stock in the future hand split and tested of course. I have not tried to deflection test the WRC/HOG laminated braces but they do seem as stiff as the spruce pieces I have here. I can say that I am interested to see the impact of WRC top & bracing for the finger and tone bars as the entire top should come in 25-30 grams less than a spruce equivalent.

Doug

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Mark McLean
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Re: Bridge length on WRC OM build

Post by Mark McLean » Mon Jan 16, 2023 9:08 pm

You are concerned about the bridge peeling off with a cedar top. There are anecdotes about this - and perhaps cedar sheds bridges a bit more often than spruce does - but it is certainly not the norm. There are a lot of cedar-top guitars out there with bridges that have stayed stuck for decades. If your bridge glue-up is done well the likelihood of a failure is pretty low. And if it happens, glue it again. So, I don't think you need to make a change in plan (or scale length) just for that reason. OTOH the rule about making sure your bridge ends overlap the X-braces is really important. If your 13-fret join means that the bridge is a bit further south relative to the X then you will want a longer bridge for this reason, or a narrower X angle. You don't have to feel that the 94 degree angle in the OM plan is some kind of law. At various times in the history of Martin they have used 94, then 98, then 94 again. Some builders use 90 and some Gibsons are as wide as 103 degrees. Some custom builders vary it according to the characteristics of the individual top. If this one is a bit floppy across the grain close the angle down a bit (more like fan bracing), floppy with the grain open it up (tending more towards ladder bracing). Naturally, a narrower angle strengthens the centre of the soundboard and makes the outer area more mobile. Some say that this favours a brighter tone, and a wide X accentuates bass a bit more.

Regarding brace material - many cedar-topped (or other non-spruce top) guitars still use spruce for the bracing because it is great for weight to stiffness ratio.

It is really interesting that you are making multiple instruments with the same basic design but varying these build elements. It should give you a pretty good feel for what difference those variables make. Please report the results of all experiments here!

Doug Ferguson
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Re: Bridge length on WRC OM build

Post by Doug Ferguson » Thu Feb 09, 2023 2:44 am

Thanks Mark for you input. I have elected to leave both the X brace angle an the length of the bridge the same. The drawing had what might be considered an oversized bridge plate and I modified it such that the leading edge of the bridge will be just behind the plate edge about 1mm. and the back edge passes by 3mm. The back edges of the wings do come over the X braces. What I have found with laminating the main X braces HOG x WRC x HOG is that they a very stiff if I am describing it right. Not at all springy compared to Cedar or spruce even though I tried to calculate dimensions to obtain characteristics of spruce. I have been using the tapered approach to the carving the X braces and have reduced main brace height to 15mm tapering to 5mm and then down to 1mm at the edge for the lower legs to get the top to loosen. I might experiment with laminating main braces in the future but this exercise tells me spruce is the way to go for the main X braces.

FYI on the first box I used reverse kerf HOG lining which I have done in the past. I like being out of the mold for gluing on tops and backs but I can hear the fibers cracking on glue up an/or sanding out the kinks. So I am giving a go at solid linings. Three pieces 2mm of soft maple, so far it bends a treat. Not sure exactly how I will manage the tapper on the back though?

To date all of my weight and density checks are within 3% of each other
Tops trimmed (6mm oversize) with rosettes in - 135 grams.
Uncarved brace weights - X 60 grams, main neck 33 grams, tone and fingers 25 grams
Tops with bracing added and carved - 228 grams
Tap tone G# perhaps a touch more
First box weight 1009 grams

Cheers Doug

Doug Ferguson
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Re: Bridge length on WRC OM build

Post by Doug Ferguson » Wed Aug 16, 2023 12:12 am

At long last I have managed to complete this years three guitars.
The three sisters proved to be more trouble than I imagined, that' sisters for yah!
The details of the guitars are in the conversation string.
I have attached sound profiles using REW software and UMK-1 mike.
Any thoughts on these files would be appreciated.

In my effort to build identically to see how much variation I have in construction technique I note the following
sister 1.1 file I had to sanded out scratches from thumb nail (careful with those cedar tops) so thinner than target.
sister 1.1 file I used solid linings (debate on this is still out) but I think directionally good.
sister 1.1 file has a body depth 1/8 greater than the other 2 at 4.375" (in the past I have made much thinner 3.5 for a punchier sound)

Subjectively they all sound like short scale OM's. The Cedar tops did not in any way make a lower volume guitar or less bass response.
My sound preference was in order 1 followed by 2 followed by three with one being the loosest or warmest of the three.
I note that I did laminate the main cedar X braces with 1.25 mm mahogany on the outside edges. Using guidance from commentary on this site I used young's modulus to mimic the strength and weight of spruce to get a place to start the carve. They were 9 x 18.4 mm and much stiffer than any spruce brace I have started with. while I initially carved/tapered the bracing to specific dimensions, after closing up each box I began to realize that I could not recall the flexibility of the previous tops with any accuracy.

Any thoughts are appreciated.

That's the story for this year
Attachments
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sister 3.2 aug 5 2023.jpg
sister 3.2 aug 5 2023.jpg (142 KiB) Viewed 8962 times
sister 2.2 aug 6 2023.jpg
sister 2.2 aug 6 2023.jpg (187.12 KiB) Viewed 8962 times
sister 1.2 aug 15 2023.jpg
sister 1.2 aug 15 2023.jpg (166.75 KiB) Viewed 8962 times

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kiwigeo
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Re: Bridge length on WRC OM build

Post by kiwigeo » Wed Aug 16, 2023 10:00 am

Nice work Doug and thanks for putting up the data.

I've done a couple of paired builds and they were endless trouble.
Martin

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