Chair Rockers

For all those other associated crafts.

Moderators: jrccaim, Bob_Fleet, gavin, Robin Fawcett, HughSpencer

Chair Rockers

Postby Donald Todd » Fri Sep 25, 2009 10:53 pm

I'm picking up the thread from "Chair Making", but not the earlier part.

I am starting to make a child's rocking chair and selected a bole with a natural bend in it. This bend is not enough for the rockers. I could finish shaping them and steam bend them to suite, or I can let the pieces dry out and then either cut or steam bend them to shape. any suggestions as to the best strategy? Is there a standard profile for the bend of the rockers? I know they have to flatten out toward the back, otherwise the chair will roll over backwards! I've just sat in one like that.
Attachments
Rockers 1.jpg
Roughed out Rocker billets
Rockers 1.jpg (76.43 KiB) Viewed 14713 times
User avatar
Donald Todd
Regular
 
Posts: 504
Joined: Thu Apr 28, 2005 10:49 am
Location: Kilspindie, Carse of Gowrie, Scotland (halfway between Perth and Dundee)

Re: Chair Rockers

Postby RichardLaw » Fri Sep 25, 2009 11:50 pm

If you're going to steam them, you'll need to do it while they're green I believe.
User avatar
RichardLaw
Regular
 
Posts: 570
Joined: Tue May 13, 2008 10:17 am
Location: Skipton North Yorkshire

Re: Chair Rockers

Postby Donald Todd » Sat Sep 26, 2009 7:12 am

No, you can steam dry wood, it just needs longer in the steamer. I thinks it's about 50% longer. It's the heat that makes the wood malleable; luthiers use dry heat to bend the sides of guitars.
User avatar
Donald Todd
Regular
 
Posts: 504
Joined: Thu Apr 28, 2005 10:49 am
Location: Kilspindie, Carse of Gowrie, Scotland (halfway between Perth and Dundee)

Re: Chair Rockers

Postby RichardLaw » Sat Sep 26, 2009 9:23 am

OK, just me again. Maybe I was thinking about the steaming drying back splats ready for mortising into the back legs.
User avatar
RichardLaw
Regular
 
Posts: 570
Joined: Tue May 13, 2008 10:17 am
Location: Skipton North Yorkshire

Re: Chair Rockers

Postby Donald Todd » Sat Sep 26, 2009 5:39 pm

It's certainly better to steam while the wood's still green. I found that (Ash) Windsor combs that had a bit of "wind", retained it after steam bending. With these rockers I feel I need to see the natural movement due to drying, before I do anything. I suspect that in the 19th Century they steam bent large numbers and paired them up afterwards. I don't have that luxury. I'm making enough rungs to be able to abandon the rockers if it doesn't work. This is Gean by the way.

What kind of drill bits are you using?

I'm intending to fit the legs into the rockers, probably secured with a cross pin, though fox wedging is a possibility. That however would compromise future securing by other means. Best look at an old chair for the curve of the rocker. I saw a comparatively new one a few days ago which had rockers that were part of a circle: when you leaned back you just kept going! The back of the curve should flatten out. I have a pair (natural curve) that are too small for any of my chairs but I can sit a chair on them to see how things work. At a certain point leaning back, the front legs will start to lift off the rockers; so they have to be fixed down.
User avatar
Donald Todd
Regular
 
Posts: 504
Joined: Thu Apr 28, 2005 10:49 am
Location: Kilspindie, Carse of Gowrie, Scotland (halfway between Perth and Dundee)

Re: Chair Rockers

Postby steve tomlin » Thu Jun 03, 2010 7:12 pm

i'm interested in this and keen to keep the thread going so Robin, Donald did you steam or saw your rockers, what did you use as a pattern? I''ve just got hold of a copy of The Chairmaker's Workshop by Drew Langsner who suggests steaming a piece of 2"x2" quarter-sawn oak for rockers over a former which is a part of a circle 42" radius. He lets it dry then bandsaws it into the 2 rockers. I'm guessing the springback flattens out the curve a little.
Can we have an update on your chairs please?

Matt J, more details about Jim's process would be great.
User avatar
steve tomlin
Regular
 
Posts: 255
Joined: Mon Jun 23, 2008 5:06 pm
Location: Cumbria

Re: Chair Rockers

Postby Brian Williamson » Fri Jun 04, 2010 5:45 pm

A rocker is the one chair that I've always wanted to make (haven't made any variety yet), so despite limited knowledge I shall chip in anyway.

The only reference book I have for this is Jack Hill's. He says that the rockers for his chair should be cut to a 43" radius - no suggestion of a flattening.

My take on it is this. Cut your rockers to a short radius and you get a chair with a fast 'action' - needs a firm push and comes back briskly. Cut your rockers to a longer radius and you'll get a slower action - needs less of a push and comes back more gently. Compare the action of polelathes with long and short poles!

The fact that the rockers are cut to a single radius has nothing to do with it's stability. This will depend (and I'm fishing deep in to old memories of boatbuilding here) on the relationship between the centre of gravity of the combined chair/person (I would guess this to be somewhere around your stomach and maybe 28" or so off the ground) and the point around which the chair is rotating (rocking), which is the centre point of the circle from which the arc of the rockers was drawn and is (according to jack Hill), 43" above them.

When you're sitting at rest in your chair the centre of gravity will automatically find it's way directly below the centre of rotation and all will be stable. If you rock the chair gently you will always return to rest where you started. As you rock harder, your centre of gravity approaches the imaginary line drawn between the centre of rotation and the end of the (back) of your rocker. Cross this line and POW! Collapse of stout party and much indignity.

A number of things will cause a rocker to capsize. Push too hard and you'll go over backwards regardless of how well the chair was designed. Make your rockers too short at the back, and you bring that imaginary line in closer to your centre of gravity, with possible dire consequences. Set the chair up wrong, so that your centre of gravity is shifted backwards and again, the distance between it and the imaginary line closes up with dire....... Giving the rockers a really tight curve brings the centre of rotation down towards the centre of gravity and, yet again, moves that imaginary line in closer........ Oh the pitfalls in building rocking chairs! I've heard tell that bean bags are very comfortable.

I'll see if i can draw a little diagram which shows this more clearly than my words.

Might the best shape for a rocker be part of an elipse? Slightly steeper at the front and flattening out ever so slightly (but never being flat) towards the rear?

Does any of that sound the least bit plausible to anyone?

Brian.
http://www.westcountrycoppice.co.uk

undergreenwood.wordpress.com

'Measure twice and cut once'
User avatar
Brian Williamson
Regular
 
Posts: 344
Joined: Wed Dec 30, 2009 11:33 am
Location: Stroud, Glos..

Re: Chair Rockers

Postby Brian Williamson » Fri Jun 04, 2010 6:39 pm

OK. Lots of apologies. Started talking before I started thinking there.

Forget all about the centre of rotation. The length of radius that you cut the rockers to won't affect its stability, though it will affect its action.

No, the crucial thing (as I soon discovered when I started playing with bits of paper and compasses) is the relationship between the centre of gravity of the chair/person unit and the (back) end of the rockers. I hope that the attached diagram will make it clear.

chairs.JPG
chairs.JPG (72.67 KiB) Viewed 14319 times


(I wish I could see it as I write). The centre of rotation (Cr) is now irrelevant. As long as the centre of gravity (Cg) is between the ends of the rockers you will be stable. Push back too far, have too short rockers or your centre of gravity too far back and, whoops!, over you go. With all the same loss of dignity as before.

So, although I stand to be corrected, i still maintain thatcutting the rockers to the arc of a circle doesn't make the chair unstable. Other things will do that for you.

Over to someone who knows more than I.

Brian.
http://www.westcountrycoppice.co.uk

undergreenwood.wordpress.com

'Measure twice and cut once'
User avatar
Brian Williamson
Regular
 
Posts: 344
Joined: Wed Dec 30, 2009 11:33 am
Location: Stroud, Glos..

Re: Chair Rockers

Postby Brian Williamson » Fri Jun 04, 2010 6:43 pm

Sorry about the diagram Still haven't got my sizes sorted. Hope this views in one.

chairs2.JPG
chairs2.JPG (3.01 KiB) Viewed 14319 times
http://www.westcountrycoppice.co.uk

undergreenwood.wordpress.com

'Measure twice and cut once'
User avatar
Brian Williamson
Regular
 
Posts: 344
Joined: Wed Dec 30, 2009 11:33 am
Location: Stroud, Glos..

Re: Chair Rockers

Postby Donald Todd » Mon Jun 07, 2010 7:06 pm

My post 26 Sept.
Donald Todd wrote: I saw a comparatively new one a few days ago which had rockers that were part of a circle: when you leaned back you just kept going! The back of the curve should flatten out. I have a pair (natural curve) that are too small for any of my chairs but I can sit a chair on them to see how things work. At a certain point leaning back, the front legs will start to lift off the rockers; so they have to be fixed down.

If you make the rockers to a circle, the centre of rotation will move horizontally as you lean back; the centre of gravity moves back faster. The centre of gravity is that of the chair plus the sitter, whose torso will be the major part of the mass. Once the centre of gravity goes behind the back ends of the rockers the chair will tip over.
I've been ill so I haven't finished this chair. I did not steam the rockers at all while green.
Look at a commercial or antique chair to get a better appreciation.
User avatar
Donald Todd
Regular
 
Posts: 504
Joined: Thu Apr 28, 2005 10:49 am
Location: Kilspindie, Carse of Gowrie, Scotland (halfway between Perth and Dundee)

Re: Chair Rockers

Postby Brian Williamson » Wed Jun 09, 2010 6:59 am

I think that we're getting the language sorted, even if we haven't reached agreement on the conclusion!

I think the most useful scenario I can come up with is that of a barrel.

Put an empty barrel on the ground and give it a gentle push and it rolls. This is because it has no inherent stability. The centre of gravity and the centre of rotation are in exactly the same place, move sidewys together, and there is no 'righting moment' It is only friction and air resistance that will stop it.

Now put a weight in the bottom of the barrel. The centre of rotation stays exactly the same, but the centre of gravity moves downwards. As soon as you push it, the centre of rotation moves sideways in the same horizontal plane, but the centre of gravity moves upwards and more slowly in the same direction as the centre of rotation. You get a separation between the centre of gravity and the centre of rotation, a righting moment develops, and the barrel will try and return to where it came from.

Now substitute your barrel for a rocking chair with circular rockers which share the same centre of rotation as the barrel. And sit on the rocking chair. Assuming that it was a large barrel (radius 43"), the combined centre of gravity will be somewhere above the chair seat but below the centre of rotation. Push back gently. Just as with the barrel, the chair rocks back (rolls along it's circumference), the centre of rotation move horizontally backwards, the centre of gravity moves upwards and backwards more slowly, a righting moment develops, and the chair returns to its starting point.

Because the chairs rockers are only a part of the circumference of the circle, if you push too hard you can tip the chair over the ends of the rockers before the righting moment can bring it back. Circular rockers aren't the problem with tipping chairs. That will lie with the rockers being too short at the back; the chair being poorly designed so that the centre of gravity is set too far back. Or that you are pushing too hard!

Brian.
http://www.westcountrycoppice.co.uk

undergreenwood.wordpress.com

'Measure twice and cut once'
User avatar
Brian Williamson
Regular
 
Posts: 344
Joined: Wed Dec 30, 2009 11:33 am
Location: Stroud, Glos..

Re: Chair Rockers

Postby Donald Todd » Wed Jun 09, 2010 10:05 am

Donald Todd wrote: If you make the rockers to a circle, the centre of rotation will move horizontally as you lean back; the centre of gravity moves back faster. The centre of gravity is that of the chair plus the sitter, whose torso will be the major part of the mass.

Brian,
This is WRONG the centre of gravity moves forward relative to the centre of rotation. This is why the chair returns to the starting position. I think the key is how close the two centres are. You can't do much with the centre of gravity, which rotates about the centre of rotation, but increasing the radius of the rocker will separate the centres. I originally estimated the same as you (43") for the radius but increasing that toward the back acts as a brake and affects the rate of recovery.
User avatar
Donald Todd
Regular
 
Posts: 504
Joined: Thu Apr 28, 2005 10:49 am
Location: Kilspindie, Carse of Gowrie, Scotland (halfway between Perth and Dundee)

Re: Chair Rockers

Postby Matt Jarvis » Thu Jun 10, 2010 6:57 pm

It's all getting a bit technical for me with this centre of gravity stuff!
Jack Hill's rocker is superbly balanced so I would say 43" is right for the curve on a rocker. Whether that would work on all chairs though I cannot say but I suspect so.
Jim steams his rockers, Jacks instructions say saw them. I can't see any problem with either method. The key is to make the chair feel safe. I have sat in some rockers that feel really scary. The chair should gently rock you, returning to a position that feels comfortable.
'Making Classic Country Chairs' by David Bryant shows a picture of different shaker rocker patterns. They vary massively on curve and length. His shaker rocking chair looks really unstable but of course you can never tell until you sit on it! Image
Matt Jarvis
Regular
 
Posts: 37
Joined: Sat Feb 19, 2005 6:03 pm
Location: Warwickshire

Re: Chair Rockers

Postby Donald Todd » Thu Jun 10, 2010 9:44 pm

(F) and (G) look like Jack's (I don't have the book to compare) and similar to Mike Abbott's in "Living Wood"; that is: not a truly circular arc.
(H) and (I) look scary !
User avatar
Donald Todd
Regular
 
Posts: 504
Joined: Thu Apr 28, 2005 10:49 am
Location: Kilspindie, Carse of Gowrie, Scotland (halfway between Perth and Dundee)

Re: Chair Rockers

Postby Donald Todd » Mon Oct 25, 2010 9:42 am

I finally got these chairs finished. One was supposed to go on the rockers, but I'm a bit stumped how to fit them. I don't want to cut the legs to saddle the rockers. I would prefer a method that leaves the rockers as an option: say socketing the legs into the rockers and securing with pins. Any idea how it was originally done?
Attachments
Cherry Chairs.jpg
Cherry Chairs.jpg (108.91 KiB) Viewed 14095 times
User avatar
Donald Todd
Regular
 
Posts: 504
Joined: Thu Apr 28, 2005 10:49 am
Location: Kilspindie, Carse of Gowrie, Scotland (halfway between Perth and Dundee)

Next

Return to Greenwood crafts

Who is online

Users browsing this forum: No registered users and 6 guests