Does MDF flex?

[ParadigmDawg]

So I built a center channel shelf using 3/4 MDF and it seems to be bowed a lot now that the speaker is on it.

It is 33'' long x 20" deep. I didn't think MDF flexed but I know nothing about wood. I don't want to brace the bottom, what is a good wood to use for this?

Speaker is around 45lbs.

 

[Rickster71]

Any wood that's 3/4" will flex and bow over time.
You'll have to brace the back, by screwing a piece to the back edge, forming an "L"

 

[j_garcia]

Yes it flexes. 33 FOOT long shelf will definitely bend with a 45lb speaker in the center of it Bracing would take care of it. A 1" or 2" lip across the back where you probably won't see it, would keep it from sagging.

 

[ParadigmDawg]

Any wood that's 3/4" will flex and bow over time.
You'll have to brace the back, by screwing a piece to the back edge, forming an "L"

My kitchen cabinet shelves(oak) have more weight than 45lbs on them and they don't flex at all.

Explain the brace better to me.

 

[ParadigmDawg]

Yes it flexes. 33 FOOT long shelf will definitely bend with a 45lb speaker in the center of it Bracing would take care of it. A 1" or 2" lip across the back where you probably won't see it, would keep it from sagging.

Sorry...it is 34 inches long.

 

[gmichael]

Sorry...it is 34 inches long.

I have a lot of shelves made of 3/4" MDF and none of them have bowed. Some have a lot of weight on them too. Bracing is about every 3 feet for me.
Do you have a lot of humidity where you are?

 

[GlocksRock]

Most sub cabinets are built from mdf, and if they didn't flex, they wouldn't require bracing. Just use some L brackets ever few feet and you should be ok.

 

[j_garcia]

I wouldn't expect 3/4" MDF to bow either, but in this case it apparently did. I used 5/8" to build a sub before and that thing was solid as a tank. The baffle with the two drivers was 1" thick though.

 

[Rickster71]

My kitchen cabinet shelves(oak) have more weight than 45lbs on them and they don't flex at all.

Explain the brace better to me.

It's like John said, screw a 2" lip to the rear edge. Do that with the weight off of it, and make sure it's flat before screwing it together.

It depends on the wood and the amount of weight.
I took out a support column in my basement and ran a quadruple 2" x12s" as a girder. I even bolted them together.
It took about 12 years, but it sagged an 1/8"

 

[Rickster71]

I put a bunch of cabinets in a foyer off our kitchen. My wife keeps big pans and her cook books on a shelf and it bowed.
I flipped it over and moved the heavy books to the sides, instead of the center of the shelf. They're 36" wide.

 

[j_garcia]

The lip resists flexing in the vertical direction because of its orientation and because there is more structure to it in that direction (on end). The shelf piece flexes because that is its weakest direction, flat and horizontal. Combinging them into an L creats a sort of I-beam effect.

 

[Rickster71]

My kitchen cabinet shelves(oak) have more weight than 45lbs on them and they don't flex at all.

MDF is garbage.
It works well for speakers because it's density is consistent and lacks the knots that some of the inner layers of Plywood has.

Oak is at the other end of the wood spectrum; it's very hard and strong. It can bow if subjected to the right conditions.

 

[j_garcia]

MDF is garbage.
It works well for speakers because it's density is consistent and lacks the knots that some of the inner layers of Plywood has.

Oak is at the other end of the wood spectrum; it's very hard and strong. It can bow if subjected to the right conditions.

MDF is great for speakers for another reason: inexpensive.

How is the shelf supported or attached? Edges, back, underneath? If you put the shelf supports about 2/3 of the way on either end, it will help keep it from bending in the center also.

 

[sawzalot]

I agree with the added two inch rip on the back, just make it the same length as the shelf and keep the top edge flush with the top of the shelf and it would be not so noticeable from the front,or place it under the rear edge of the shelf kinda like a pinrail and attach with a few fine finish screws, also if possible don't make the shelf "too" much longer than the center so the load bearing would be closer to the sides thus avoiding the laws of gravity.

You could always use a rip of Birch, Oak and or Maple ply wood you would just have to wrap the edges with a pre-made trim stock to hide the edge grains showing the ply layers.

 

[j_garcia]

45lbs supported at edges:

(have a video, but doesn't want to run).

With a 2" lip at the back (doesn't matter if it is up or down):

This is also an extreme case with all the force at the center of the top face and support only at the ends, but you get the idea.

 

[Rickster71]

45lbs supported at edges:

This is also an extreme case with all the force at the center of the top face and support only at the ends, but you get the idea.

Very nice John!
It should work nicely with the center's four feet being closer to the edges.

 

[ParadigmDawg]

It is just held my 4 pins at the edges. There is no back or front of the cabinet unless you count the 1/8" masonite that hides the wires but I can't screw a brace into that.

Here's some pictures. Don't judge the paint job, I'm not done with it yet and the first shot didn't turn out as dark as I wanted.

I would like to use oak or something but I don't understand how to do the edge that Sawz referred to.

The dog always walks in front of the camera.

 

[j_garcia]

You need a 1-2" strip of scrap or whatever that is the same width as the shelf. You can screw through the top or bottom of the shelf into the edge of that strip so it supports the shelf, and that would be at the back side so you don't really see it from the front.

 

[ParadigmDawg]

You need a 1-2" strip of scrap or whatever that is the same width as the shelf. You can screw through the top or bottom of the shelf into the edge of that strip so it supports the shelf, and that would be at the back side so you don't really see it from the front.

I don't quite get what you are saying. Just add a strip of the same stuff to the back length of the shelf? That doesn't change the 4 pins for support. Does it just strengthen the shelf?

 

[ParadigmDawg]

Oh...and here is one more pic for Sawz....

I wish it was more gross but I have perfect feet.