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samwsamw (Electrical) (OP)
15 Aug 09 3:58
There's a slow motion discussion on Wikipedia about how a glider works versus a rocking chair.  See:

Are gliders deliberately designed to move like a rocking chair or is the design intent to make the motion different?


NomLaser (Mechanical)
15 Aug 09 8:25
Guess you would have to ask the original designer if he intended it to behave the same as a rocking chair.  If not then he succeeded, otherwise he failed.  The motion is completely different.

The question isn't something that anyone other than the designer can really answer.  It's like asking if a flat head screwdriver was intended to open a can of paint.
Twoballcane (Mechanical)
15 Aug 09 8:39
Well I can attest to this question.  I have a glider at home (got it for my wife when we had your first child) and the difference is the wear and tear on the floor and comfort.  Since all of the motion on the glider is on ball bearings, there is no harmful interface to the floor.  Also, I do believe that the seat stays more horizontal than the rocker which makes the glider more comfortable.

"If you avoid failure, you also avoid success."  

samwsamw (Electrical) (OP)
15 Aug 09 20:58
Thanks for the responses.   I agree the motion itself of the glider is different than that of a rocking chair and more horizontal.  Is yours closer to a "parallelogram" 4-bar linkage or a "double-rocker"?  Do you agree a parallelogram 4-bar linkage would keep the person horizontal at all times?

IceStationZebra (Mechanical)
16 Aug 09 0:12
"Do you agree a parallelogram 4-bar linkage would keep the person horizontal at all times?"

Yes, only if:
1) all the bars are the same length
2) all the lower pivots are on a plane which is parallel to the floor
3) all the upper pivots are on a plane which is parallel to the floor

IRstuff (Aerospace)
16 Aug 09 2:16
Only requires that opposite sides be the same length, I think.

My glider has a bottom link that's about 3 inches shorter than the top link.  The side links are only about 1/3 as long as the top link


FAQ731-376: Forum Policies

samwsamw (Electrical) (OP)
16 Aug 09 8:47
Thanks for confirming that the glider isn't actually a parallelogram.  So can we conclude that glider designers want that and want some angular rotation of the occupant?  Thus the various references that state gliders simply move back and forth are incorrect?

e.g.  See:

    "A glider is an updated version that's designed to slide forward and backward rather than rock in an arc".

See also:

     "Unlike swings and rocking chairs, gliders have no arc to their movement; instead, they move on a flat plane."


williedawg (Mechanical)
16 Aug 09 11:04
Whoever wrote that short definition on the glider doesn't understand geometry.

Yes, it is an updated version of a rocking chair, but as Twoballcane said, it's much easier on the floor and has a less-pronounced "rock". And the motion does describe an arc or some single line curve; depending on the following....

The basic layout of the linkages is a trapazoid; however, the lengths of the "opposite" side links and upper/lower members as well as the mounting points can be varied to change the path of the seat.

On a typical glider, if you made a small X on the side-board, then watched it through the forward-rearward motion of the seat, you'll see it describe an arc or curve.
It would not be an ellipse or some other closed-end curve because the forward displacement is the same as the rearward.

Here's a Patent that shows a basic layout.

On the first page, there are many more Patents which can be viewed on Google's patent site.

williedawg (Mechanical)
16 Aug 09 11:39
meant to add..
a parallelogram will describe a straight or level forward-rearward motion; however it will change a fixed point on the moving member in the vertical distance, thus describing an arc.
IRstuff (Aerospace)
16 Aug 09 14:03
A parallelogram will undulate, but will not rock, it can't.  So long as the moving horizontal member remains horizontal, even if there's a change in the vertical position, it's not rocking.  It's doing a wave undulation.  Rocking requires an angular change.  If you draw out the vector between the CG and the center of the moving horizontal member, you'll see that it's always parallel to the floor normal vector, so there is no "rocking."

That said, there is probably no consensus in the actual particulars of the gliders on the market.  There will be some where strict parallelograms are used, and there will be others where a rhombus is used.


FAQ731-376: Forum Policies

samwsamw (Electrical) (OP)
16 Aug 09 21:26
Thanks for confirming that a parallelogram can't rock.  Thanks for confirming that real-world gliders aren't parallelograms. Would anyone like to speculate as to the engineering considerations on the choice of parallelogram or not?  Is it purely what customers want or are there technical reasons?

IRstuff (Aerospace)
17 Aug 09 0:01
I don't think your question makes sense.  Customers do not care how it's done, just that there's no rocker leg that can squash toes or animals' tails.  Beyond that, noise, smoothness, etc. are considerations.  

Given the lack of intimate knowledge in an audience of engineers, why would you think there were any customer influence?


FAQ731-376: Forum Policies

samwsamw (Electrical) (OP)
17 Aug 09 0:44
OK thanks.  I think you've answered my question:  there's no engineering reason for gliders using either parallelogram or more complex 4-bar linkages.  So a reasonable conclusion is that the design of the 4-bar linkage is to make the glider rock in a way most pleasing to the occupant.

TheTick (Mechanical)
17 Aug 09 9:34
My 1-year-old son can tell the difference.  The rocker puts him to sleep.  The glider makes him cranky.
hydtools (Mechanical)
17 Aug 09 19:02
Analytically your cg while rocking moves about in a straight line assuming your cg is near the center of curvature of the rockers.  The center or curvature moves parallel to the surface on which the rocker radius rolls, especially if the rocker curve is a constant radius arc.  Think of a wheel rolling on a surface, the wheel axis moves parallel to the surface.  The rocking motion is due to the fixed frame of the chair moving through the angle of rotation; like the spokes of a wheel fixed to the rim.

The glider can do whatever you want it to do, within limits, as you select the arrangement of the four links.


samwsamw (Electrical) (OP)
17 Aug 09 20:27
Agreed, the glider can do whatever you want it to.  So the question is how glider designers chose.  Is there some engineering/structural reason or is the motion of the glider designed to somehow please the occupant more than a rocking chair.  Given no compelling engineering reason for the motion design, we can only conclude the designers think the actual motion best pleases the occupant.


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