Archtop action changes

[Dominic]

Hey, a question for the archtop makers. I had lent my archtop to a friend when I was away. He dropped it back around last week and it was raining and it got a bit wet. Once it had dryed out I noticed the action was quite high. It was perfect when I gave it to him. So my question, what should I expect from an archtop as far as movement goes over time. I am trying to work out what moved, was it the top coming up with the humidity or just normal string tension? It has a pretty hefty neck block but perhaps it has tipped in a bit but it only a few months old so I doubt it. The neck is perfect and straight with the correct relief. I used a solid (unadjustable) bridge so it would be helpful to know which way I can expect the action to move over time.
Cheers
Dom

[Clancy]

When we arrived in Alice my archtop's action was higher.
I knew something had changed the moment I got it out of the case to check it and gave it a strum.
Close inspection revealed a gap had appeared between the base of the heel and the body.
The joint is the 'square dovetail' type shown in bennedetto's book.
It had just spent 5 days in the car being moved from Canberra to Alice.
I don't know if it was due to sitting in constant airconditioning or if the driver (dad) left it in the car in the sun at any time.
Anyway, it now lives in it's case (always has) in a house that has a swampy running & humidifying almost continually.
The heel/body gap is gone and the action has returned to normal.

Prior to this I had never noticed any major change in action in the years since it was built & living in Canberra.

On the other hand, the Kauri/Qld Maple archtop I built went to the Gold Coast hinterland last August.
When I went to Brisvegas for chrissy I took my fretting kit.
Dave (the owner) had called to say the action was now so high he didn't like to play it (I had actually expected a change in action and discussed it with him when he collected the guitar in canberra).
As well as the normal high summer humidity, they had had months of rain leading up to xmas.
Again, this guitar has the solid dovetail neck joint.
I was able to reset the neck/bridge/action but also had to do a bit of fret levelling on the fingerboard extension.

So that's 2 examples, one from canberra to dry air and one from canberra to wet air.
Hope it helps.

[Dominic]

Thanks Craig, so in both your cases the problems was the neck joint?

I've got the bolt on neck and don't have any gap. I did think it could be the neck heel bending relative to the FB surface and extension (which has CF rods all the way through) but I can't push it back down and it seems solid. I didn't mention that the gap between the FB extenstion and the top had closed up in one spot.

So I assume the string tension over the top is pulling the top into a higher arch which raises the action and the pressure on the bridge is not enough to overcome this force. And the very wet conditions is perhaps compounding this. Sound right?

Dom

[Clancy]

Mostly neck joint for the one that came to alice. Probably would have been alright if I'd slackened off the strings before packing it for the trip.

With the one that went to qld, more a problem of the neck and some up movement of the fingerboard extension.
The neck was made from rock maple that had spent 16yrs at Monaroe Timbers so was very conditioned to canberra.
It was a 3 piece neck and although it was all vertically grained, I still shuffelled the directions of the pieces around to try to counter any movement. Still happened though.

[rocket]

Hmmm, Dom, i can't see how the string tension can be pulling the top upward, the string tension can only be pushing the bridge downwards. It is a puzzle, besides atmospheric conditions i have no other ideas as to what is causing the ultra high action.
Cheer,,,, Rod

[Clancy]

I should say that the archtop that came to alice probably had neck movement as well and I just focused on the gap that had appeared at the base of the heel as the most obvious indicator of movement having occurred.
I didn't do anything about it, just left it for a while & it corrected itself.

You braced that top with parrallel braces if I remember right.
I'd be surprised if the whole top structure rose against the compression of the strings, especially so quickly (but then I don't have decades of experience.)
Last I saw of that guitar it didn't have any jam on it and you wern't planning on spraying it for a while. Is it still in the white?

[Mike Thomas]

It is quite possible that the top has a higher transverse arch due to humidity i.e. as the top has expanded, the arching height has increased, lifting the bridge with it. If so, as it dries, the process is reversed, and the action returns to normal.
Violinists who travel regularly between dry and humid locations have different height bridges, lower for humid.

[Dominic]

Thanks guys. Rod, the string tension is mostly trying to fold the top up with only a small part of the force acting down on the bridge. But I don't believe it is enough to do this anyway. And it was very stable for 3 months before that so as Mike suggests, it is probably humidity. But, I am currently making my second archtop and so wanted to determine what forces were acting where so I understand how to build to counter them.
Cheers
Dom

[rocket]

What the?????
" The string tension is mostly trying to fold the top up " " only a small part of the force is acting down on the bridge"
I'm no authority when it comes to building guitars but this sounds silly to me, if the instrument in question was a flat top it might make some sense but, an archtops strings are held at one end by the tuners and at and the other by a tailpiece that in turn is anchored to the tail block, idealy the strings pass over the bridge at a break angle of around 14 degrees and the resulting downward force both holds the bridge in place and transmits the strings vibrations through it to the soundboard and suitably fitted braces beneath will, along with the built in strength of the arched top bear [ All ] the downward load delivered by the strings combined tension.
I may be completely wrong, and pardon me if i am, but given the above senario i cannot see how the string tension could possibly "fold" the top up! I'm sorry!
I need a coffee,,,, Cheers
Rod.

[Mike Thomas]

Surely the string tension, which would be exerting a compressive force on the top, is much, much greater than the downward pressure of the strings on the bridge, and through the bridge onto the top, caused by the break angle. I think Bob Benedetto gave a very convincing demonstration of this on one of his videos, if I remember correctly.

[Dominic]

It makes sense to me. With the strings attached at the nut and the tail, the tension on the strings is trying to bring these two together. Either by folding up the neck or buckling the top upward. The small portion of the force acting down on the bridge is nothing compared to the force acting between the nut and tail. This is high school physics.
Anyway, as i said i don't think this is what is happening but it is easy to imagine that with a very light top the neck and head blocks will want to tip inward and the only way the top can go is up. But I still suspect humidity is to blame. The action is coming back but is still a little high.
Dom