Philosophical question about chair design

[John Seybold]

I'm working on some prototypes for a set of bar stools, and I've run into a practical problem that is leading to a philosophical question. Take a look at the latest iteration pictured here:
354081

I like the overall design (with a few smaller modifications), but there is a wood movement problem: the seat will expand and contract across its width, stressing the joints on the crest rail and front stretcher. I'm not too worried about the front, since the stretcher is about 18" below the seat and the legs are fairly slim. The structure can flex a little, I think, without too much trouble. The crest rail is a bigger problem. There is no rear stretcher, so the rear legs can flex in and out if needed, but the crest rail is stout, and only 8" away from the seat. Eventually, in the fullness of time, either the seat might crack or break the joints on the crest rail.

Now, I know there are different design solutions to this problem (like putting an apron under the seat and letting it float above). But here's my philosophical question: is it OK to take a chance that the chair won't tear itself apart for a long time? I mean, eventually it will be destroyed by fire or neglect or accident (or, most likely, by somebody in the future who finds it ugly or unstylish and throws it out). I usually like to think that anything I build could in principle last forever, if made correctly. But can I go ahead and just hope it lasts long enough?

Thanks for any thoughts,
John

[Yonak Hawkins]

John, this is an interesting question. Are you actually seeing stress on the joints or are you just concerned about it ? I really can't see that it could be a problem. First of all, the seat is, likely, less than 18" wide and the movement over that span can only be a small fraction of an inch. Second, at both the front and rear, there is no real stress as both are restricted on only one side of the fulcrum leaving the other side to move freely. If you actually are seeing stress you could put single pins in the back through the rail.

[Don Orr]

How is the seat attached to the rear uprights? It appears to be notched into the uprights. Is there a way you can attach the seat to the uprights so there can be some lateral movement of the seat (expansion/contraction) without the seat separating from the uprights? Something like a sliding dovetail where the seat has the tail and the uprights have a socket. Just a wild guess as I am not a chair maker or designer by any stretch of the imagination.
In reality I think your design is just fine the way it is.

[rudy de haas]

You are the only one you should listen to on this.

Personally I am neither a perfectionist nor good at woodworking so the question doesn't come up, but I know people to whom this issue is important and, in general, see a stronger argument for getting it right (i.e. for the person meeting his own view of what "right" means in this context) than for pragmatism. Basically, and only in most cases, if the person making the thing isn't happy with it, then it isn't good enough - even if it will outlast its first owner by a considerable margin.

[pat warner]

Yes, there should be concern.
In 18" W, you may get a 3/16" - 5/16"/year in width.
If there is some slop where the seat meets the back legs,
I would not sweat.

[Yonak Hawkins]

The thing is, even if there is slop in the joint with the legs, the legs will move with the movement of the seat material. I'm not understanding what would keep it from doing so.

[Prashun Patel]

I made a couple of these style, low-back stools and the backs have held up fine.

I'll admit it if this is naive: but I can't imagine leaving slop in the rear legs. The most stressed joint I can think of is the seat-to-rear-leg connection on a chair. I'd sooner run the grain on the seat side-to-side or run the crest rail vertically then compromise the seat to leg joint.

[John Seybold]

Thanks, everybody - I haven't seen any problems, but then again, I just glued up this prototype a couple of days ago. I've been toying with the idea of soaking the seat with water to make it expand and see if I can get it to fail. But as we know, it's the more insidious repetitive expansion and contraction, over and over, that can really break a joint.

I'm sort of working with Yonak's theory (which I like), that leaving the rear stretcher off allows the rear legs to move in and out by flexing as the seat grows and shrinks. The rear joint has no slop in it (at least, none that I intended!), and I agree with Prashun that it needs to be rock solid there or risk a catastrophic failure. It's the same joint Nakashima used on his cantilevered chairs, and those are still standing decades later. So that joint, if made properly, should be fine. Actually, Nakashima has the same issue of wood movement in his chairs, since the crest rail runs cross-grain, but the backs are much higher allowing more room for the legs to flex slightly. I'm also encouraged by Prashun's experience with his stools.

I'm probably over-worrying here. Thanks for the insights.

[John Seybold]

I guess it would also be possible to run the grain in the crest rail vertically - there's probably enough wood in there to make it strong enough, and it doesn't have to take a lot of load across the grain. I'd have to use a floating tenon to attach it to the legs, instead of a sawn tenon. I think I'll prototype that and see how strong it is. The people at Moser must think it's OK: http://www.thosmoser.com/product/2262/auburn_stool/

[Mel Fulks]

I don't see anything about whether they are for your own use ,or a commission. If they are for your use I'm sure they are fine. It's your friends I'm worried about. If the wood is kiln dried ,I think the estimate on movement is too much.Might not move a whole 1/16th.

[John Seybold]

They're for my own use, Mel. I'm not too worried about catastrophic failure! The disaster, if/when it arrives, should come in slow motion...

Also, it will be air-dried lumber, so maybe more movement there.

[Wayne Lomman]

John, I think you would be better off going with vertical grain in the top splat. I would also use 2 or 3 loose tenons rather that a single large one in each end. Good design. Cheers

[John Seybold]

Thanks Wayne - I'll give it a shot.

[Patrick Walsh]

On a seperate note, that is a sharp stool!

[Art Mann]

I suggest you take a look at commercial chair construction. I have seen chairs built with similar techniques that have been in service for many, many years and they haven't self destructed yet. If Ikea can do it, then why can't you? I went on the shrinkulator website and got an estimate of the dimensional changes of a slab of cherry about the size of your seat (18 inches) assuming every piece of wood is oriented in exactly the worst way. The number came back 0.15 inches over a humidity range of 6 to 9 percent. That is much more than you will ever see in an environmentally controlled house. Even if it does change dimensions, wood is compliant and can absorb significant dimensional changes before self destructing, especially if it has somewhere to move to. I know dimensional stability is all the rage in woodworking forums these days and sometimes it is for good reason. But, too many people get too hysterical about wood movement without ever doing any actual calculations or analyzing the situation rationally.

[John Seybold]

Thanks, Patrick!

[John Seybold]

Wow, thanks, Art - I had never heard of the shrinkulator. I've always just guessed at how much movement there *might* be. I tried the tool myself for walnut, and got about the same results. I live in Colorado, where the climate is quite stable (always dry). My air-dried lumber for this project ranges from 7-10% moisture content. So I'm guessing that the movement will be quite small, and certainly within the ability of the structure/material to compensate, especially once everything is coated with finish.

I appreciate the help.
John