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I just built this storage rack for my basement out of 2x4s and plywood. It is relatively stable but does have some front to back wobble. More than I would like. Is there a way to increase the stability of the unit? I've read something about adding diagonal braces but am unsure how exactly that would work. Any help would be much appreciated.

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Most things we build are rectangular, and rectangular structures are prone to racking, which is illustrated in this drawing of a deck, but the same problem affects your rectangular shelves:

Racking

As you see a structure can collapse due to racking if the joints flex without tearing apart.

There are various ways to counter racking. Other shapes, like triangles and trapezoids, are not prone to racking. In the picture, by adding knee braces, you attach the rectangle to triangles, which are not prone to racking.

Another way is to make joints that are not as flexible, it looks like the vertical boards in the front of your shelf are attached in a way that won't flex and allow your shelf to rack left to right.

Another way to prevent racking is to attach a sheet over the rectangle. For example, if you attached plywood over the sides of your shelf unit, there's no way it could rack front to back. The sheathing doesn't have to cover the whole side. You could add gussets - just a triangle of plywood in the four corners - and make the shelf nice and solid.

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    Thanks for the detailed response! So just cutting some plywood triangles and screwing them over the four corners on each side would prevent the racking? – agentbanks217 Feb 15 at 13:30
  • Yes but I am not sure from your picture but it doesn't look like the base is in the same plane as the posts, so you may not be able to screw them right on. If that's the case you could just make four 6" strips of plywood to span from front post to back post, and attach one at the bottom and on at the top on each side. – batsplatsterson Feb 15 at 13:43
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    Just to make sure I understand, you mean like this? imgur.com/a/UF5LAqt – agentbanks217 Feb 15 at 13:54
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    @agentbanks217 - exactly. The wider they are the stronger the bracing effect. Thickness won't matter much at all. – batsplatsterson Feb 15 at 14:05
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I would consider

  • Joinery
  • Bracing
  • Tethering

You probably can't retrospectively improve the joinery but it is worth noting that some methods of fixing pieces of wood together result in a more rigid joint. A well-made and glued mortise and tenon is one of the most rigid joints, more so than say a single screw.

Adding diagonal bracing can make a very large improvement, This can take the form of struts, tension-cables or sheets.

Simply bolting the top rear rail to the wall behind it (perhaps through a shim or solid spacer) will in many cases stop any forward and backward wobbling. If the shelves are free-standing in the middle of a room, your options are much more limited - but in a basement maybe a tall post secured to both shelving and to a joist might be feasible.

  • Thanks for the reply! Where would the diagonal bracing go? – agentbanks217 Feb 15 at 13:21
  • @agentbanks217: Ideally, both back corners to opposite front corners. Though one is often enough. An alternative is across the corners of the sides. – RedGrittyBrick Feb 15 at 14:46

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