Bass bar springing?

[Janito]

Just imagine a bar set for a flat top, then curl the ends of the top down over a non-plane ribset [imagine the violin sitting bridge-up, horizontally], which pulls the ends of the bar down, which pulls the middle of the bar down because the bar is a bar, and stiff, also (the bulk of the top being held UP by the corners and the ribs) and you can see that positive spring can quickly become negative as the middle of the bar is pulled downwards by the ends, and the bar, rather than pushing the top up in the middle, pulls it downwards, in the same direction as the bridge is pushing.

Thanks for the extra info.

Couldn't one also argue that the spring actually helps to contour the top plate to a rib garland that has a longitudinal taper from the top corner blocks to the neck block?

If this is the case, there could be a problem when a bar is fitted with spring whilst the top plate is held in a rigid frame.

[Darren Molnar]

Thanks Stradofear

[stradofear]

Thanks for the extra info.

Couldn't one also argue that the spring actually helps to contour the top plate to a rib garland that has a longitudinal taper from the top corner blocks to the neck block?

If this is the case, there could be a problem when a bar is fitted with spring whilst the top plate is held in a rigid frame.

That would be a logical question to debate, yes.

[Melvin Goldsmith]

I seem to remember an early BVMA article where the author talked about prestressing the bar before fitting. ie the bar blank was first bent and laminated to another piece of wood to keep it bent. It was then fitted peceisely to the plate and once the lamination was cut away the stored stress forced in the wood sprung the bar.

[Janito]

Just imagine a bar set for a flat top, then curl the ends of the top down over a non-plane ribset [imagine the violin sitting bridge-up, horizontally], which pulls the ends of the bar down, which pulls the middle of the bar down because the bar is a bar, and stiff, also (the bulk of the top being held UP by the corners and the ribs) and you can see that positive spring can quickly become negative as the middle of the bar is pulled downwards by the ends, and the bar, rather than pushing the top up in the middle, pulls it downwards, in the same direction as the bridge is pushing.

Whilst I remain on-topic, let me turn this observation on it's head and suggest that this negative 'pull-down' effect is exactly what was intended!

The negative pull at the middle of the bar would be anticipating the depressive effect the bridge-strings unit has on the top plate when in playing condition.

Perhaps these folks were trying to accelerate the stabilisation of the top plate by introducing the spring.

[polkat]

Having read all the responses, I'm left with the feeling of..."duh?!" Sort of leaves the question of tension or no tension still up in the air as a sort of personal choice.

In his first reply to this post, Anders said, "Fitting bars with tension is mentioned by Bagatella (1782) and Count Cozio (b. 1755 - d. 1840." Is there any evidence that makers from the classic golden period used tension in their bars? Bars were generally shorter then, and if they introduced tension it must have been a more difficult process.

[Andres Sender]

I believe there's more in Cozio's notes that says something to the effect that it was rumored that springing was standard practice in Cremona. I think Michael Darnton mentions it in an old thread. But this is something like 4th-hand so you should try to get your hands on the Cozio book and see if it's in there.

Springing a Strad-sized bar wouldn't be any more difficult than springing a modern one.

[Johnmasters]

The nylon wing nut is tightened to cause the hinged end holder to touch the top. Half a turn more buckles the center of the top down about 4mm. Then I fit the bar to the curve. The gig is a handy holder for working. No need to have some kind of fancy aluminum plate. I use carbon paper, not chalk. Xylene removes any slippery stuff.

By the way, it works fine. No bumps on the outside.

Here is a lower resolution picture, does that help?

[captainhook]

John,

Could you cut that picture size in half so I can enjoy it too? I can see only the top half with no wing nut in sight. It looks like a very interesting jig. Thanks

Lyle

[Johnmasters]

In case the other did not work.......

[Oded Kishony]

Occasionally I need to place all my bass bar clamps along one side. I then support the clamps by putting wedges under their ends so that no twist is inadvertently forced on the plate by the weight of the clamps hanging in the air.

~OK

[stradofear]

I've worked on violins where if you'd done it John's way, unsupported with all the clamps on one side, you'd have run the risk of chasing half of a couple hundred thousand bucks of violiin top across the floor, probably in smaller pieces than just the half that would break off initially. As the boss of that shop once said to me about something else: "you can use that on your own violins, but I'd better not see you using it on any of mine!"

I have a set of cello bar clamps with supporting screws in the middle of the clamps to take the pressure like your wedges, Oded. I used to use wine corks before that with cellos, where the clamps are large and often heavy.

[Oded Kishony]

Not that I necessarily do this, but a really well fitted bar shouldn't need more than 4-5 clamps max.

~OK

[Johnmasters]

I've worked on violins where if you'd done it John's way, unsupported with all the clamps on one side, you'd have run the risk of chasing half of a couple hundred thousand bucks of violiin top across the floor, probably in smaller pieces than just the half that would break off initially. As the boss of that shop once said to me about something else: "you can use that on your own violins, but I'd better not see you using it on any of mine!"

I have a set of cello bar clamps with supporting screws in the middle of the clamps to take the pressure like your wedges, Oded. I used to use wine corks before that with cellos, where the clamps are large and often heavy.

That is not a criticism of course. I do this with new tops. Also many rebuilds. I don't think a customer of mine has ever asked for a new bar. I work for the hoi poloi. The top is supported well at the bar as you can see from the jig. I also use wedges for cellos where I have to use some larger and heavier iron clampls.

But I am sure you can see the justification. It makes more sense for a new violin. If it is a sensitive question to do or not to do, I think it makes more sense to let the top find its own weaknesses. But perhaps it does not make not differnece a difference in any case.

Not that I necessarily do this, but a really well fitted bar shouldn't need more than 4-5 clamps max.

~OK

Oded, I just happen to have a lot of clamps. Why be stingy. Or are you and Michael REALLY trying to pull my chain ?? The bar fits very well. I use carbon paper, not chalk. I can see carbon paper much more easily. Perhaps there is a bit of torsion to worry about. I use the extra clamps in part because the piece is slightly springy when I push to bar to the paper. Remember, I don't use a gig that took me a day to make. In the end, the results are to my satisfaction. Any built-in torsion would be completely neglibible.

[Oded Kishony]

Just yanking your chain a bit There's ALWAYS someone who does it better, faster cheaper.

Oded

[Janito]

Perhaps these folks were trying to accelerate the stabilisation of the top plate by introducing the spring.

From my rocking armchair 'empiricism', this might look something like:

Sprung bar = 1 week for sound to settle

Unsprung bar = 3 weeks for sound to settle

--------------

So the spring could:

(i) help shape the top to the step between the upper corner blocks and neck block

(ii) make the assembled instrument reach a stable sound more rapidly

(iiii) many others...

[stradofear]

I also like to use a lot of clamps--I use nine. Not because I need them to fit the bar, but I like to see an even glue line, and infinite clamps is the way to do that. If you put another clamp between the clamps on your perfectly-fit bar, Oded, I bet you will get some squeeze out, which indicates the space between the clamps doesn't get the same clamping pressure as that under the clamps (an obvious point, I guess.) The alternative would be to glue in a very large bar (or a bar with temporary a steel bar spine, if we want to go to extremes) in a top that's in a mold, and I know people who do that.

[Oded Kishony]

I don't trim the upper part of the bar so there is plenty of stiffness to spread the pressure. Also if there is a bit of spring and you start clamping at the middle adding more clamps won't cause more squeeze out.

Anyway, these are theoretical ideals, I often use more than 5 clamps.

One other point is that I bevel the edges of the bar slightly so that clamp pressure is always centered.

Oded

[stradofear]

It's not the bar I'm worried about--it's the 2.5mm thin, flexible top underneath it. Anything short of a fitted, stiff caul is not much better than nothing (that's why some people use a plaster cast).

[Bill Yacey]

I'm not an advocate of springing the bar. I prefer to put my instruments under zero tension until strung up. When I taper the ribs, I taper from the bottom block to the neck block so the plate still has a flat, plane surface for gluing. My belief is even if you spring the bar, after a number of years under string tension the wood fibres comply and relax to the point that there isn't any tension left between the top and the bar. If you are worried about the top sinking under string tension, perhaps the bar has been over shaped past the point of sufficient strength.