Hold the foam???

Discussion in 'HO Scale Model Trains' started by Doug, Oct 18, 2006.

  1. jcoop1

    jcoop1 Member

    My plan is to support every 10-12 inches, have the bench work done with 12" spacing. Might go up to 3/8 ply or what ever I have laying around.

    I have used the foam and I know it works. I am not too concerned with the weight factor. I want to cut down on the road noise. May even do the WS foam again

  2. ocalicreek

    ocalicreek Member

    hang on a second...

    I've long wondered about sagging plywood...have you experienced this first hand, or seen it on another layout?

    What exactly would cause the ply to warp? Moisture? Train weight?

    Also, the foam and ply combo won't sag but laminating the soundboard and ply might?

    Sorry to sound like I'm jumping on your case here, I'm really not. It's just that I have always heard stories or rumors of warping plywood (usually 1/2") and have never seen any hard evidence or heard any first-hand accounts of this actually happening. I'd love to be proven wrong here just so the truth can come out.

    It's kinda like that homasote swelling/shrinking myth.
    (see http://www.homasote.com/strength/index.html)

  3. pgandw

    pgandw Active Member

    Don't worry about jumping on my case. I like to question information put out as "fact" as well, especially when I'm a little supicious for some reason.

    I've worked with plywood in various structures - shelves, model railroads, and boats with thicknesses ranging from 1/4" to 1.5" (2 3/4" pieces laminated together).

    Horizontally mounted 1/4" plywood without sufficient closely spaced supports will sag between supports. It will sag more with any kind of weight being put on it, whether by leaning or stuff being put on it. In my experience, once it sags in this application it will not return to flat without being taken up and re-installed. Finally, cheap grades of 1/4" plywood are more prone to warping than 1/2" or thicker plywood due to only 3 plys instead of 5 or more.

    I have successfully used 1/2" plywood supported every 16-18" with joists of 1x3 or 1x4 on several layouts. Some had bare plywood, some had Homasote glued to the plywood. My findings concur with Westcott's book on benchwork.

    The real problem is sagging of as little as 1/50 of an inch (0.020") is enough of a vertical irregularity to allow model flanges to climb the track under the right conditions. Giving ourselves a safety factor, I allow no more than 0.010" sag under load.

    But you really should run sagging experiments for yourself. Get a couple of bricks, place them under a 3" wide, 3ft long strip of your chosen plywood. See how far you can move them apart before you get a 0.010" sag with at least a 50 lb load centered between the bricks (or simply push down hard to simulate the load). The load is to simulate an accidental lean on your part, which will happen sooner or later. I personally believe a 100lb load would be more representational of what could happen. And if you want to be able to walk on your layout, go with 300 lb load.

    To avoid sagging with 1/4" thick plywood by itself, I'm willing to wager you are going to need the supports closer than 12".

    The sound absorbing board I saw at Home Depot had very little rigidity of its own. The same is true of the Homasote strips I saw. Both were easily bent into curves. I would not count on either to add much strength to my 1/4" plywood unless installed as a filler in a sandwich configuration with 2 layers of plywood. This is how a door gets its incredible rigidity from 1/8" plywood skins. Cardboard filler and wood strips hold the plywood skins over an inch apart. This gives the door most of the bending resistance of an equivalent thickness of solid plywood at a fraction of the weight.

    Extruded foam (not beadboard!) sheets or strips of 1" thickness or more have a surprising amount of bending resistance. Check it yourself. So a lamination of foam and 1/4" plywood will combine the bending resistance of both together.

    my thoughts, your choices
  4. ocalicreek

    ocalicreek Member


    Thanks for the thorough reply! Finally, some data to work with, not just 'well, I heard it from so-and-so'. I may take you up on the testing and share the results with the group when I reach a point closer to construction on my own layout.

    My little timesaver layout is 1/4" ply (that I found in an alley) screwed to risers lifting it about 5" above 1"x3" framing. The overall size of the frame is 14"x5' and cross members are spaced about every 12-14" or so. Subroadbed is yardsticks (can't beat free!) and all track is handlaid code 70. So far no major warping or sagging has happened (except for a minor twist in the framing that I carelessly built into it...has nothing to do with the ply...in fact, the ply may have actually reversed that somewhat when I mounted it).

    SO, as I mentioned, I'll eventually get around to some testing and see what happens. I've done some research on foam/ply sandwiches (google stressed skin panel) and have thought about the inherent strength involved there. But I wonder if any or how much strength is lost when/if the integrity of the whole is damaged by cutting this sandwich into strips or jigsawing it in a cookie cutter fashion? OR, if the ply is cut first, then foam is applied over that (for instance, on a grade).

  5. Russ Bellinis

    Russ Bellinis Active Member

    Regarding the resistance to bending of foam, my other hobby is cars in the 12 inch to the foot scale (by the way that hobby makes model railroading cheap by comparison!) I was over at a friends house who has a welder and we were building a fire grate for my daughter from 1/2 inch rebar. We are both into the G.M. f-body cars, Firebirds & Camaros. One of the classic things to do to the cars since the first one was built is to weld in subframe connectors to stiffen the unit body so that the only moving parts in the suspension are the suspension, not chasis flex. Well, he mentioned to me that he has seen some of the latest ideas on sub frame connectors for the cars, especially useful for restorers who want the chasis stiffness on a stock appearing car is to drill a small hole in a reinforcing tube on the floor of the car, and fill it with foam from spray cans like you find at Home Depot. Believe it or not a tube filled with foam is as torsionally stiff as a steel subframe connector! Back in the 1970s G.M. built an experimental 2 seater sports car using Vega components. The car, if produced, would have been comparable to a Mazda Miata. It wasn't put into production, but the really significant thing about it was it had no steel in the frame! It was a unit body made up of an inner and an outer tub of fiberglass. The 2 peices were laminated together, and then filled with foam. The suspension mounts were molded into the fiberglass. The resulting chasis was so stiff that if one wheel was jacked up, as soon as the suspension travel was taken up, 3 wheels would come off the floor. That body would not bend without breaking!

    Because of it's stiffness, I would laminate a small piece of foam to a piece of plywood. Perhaps both pieces being 3 feet long by 3 inches wide or so. Then see if it will bend. I'm not sure you can do a cookie cutter layout with a plywood /foam sandwich because I'm not sure the foam will bend enough.
  6. Jim Krause

    Jim Krause Active Member

    There are some really rigid foam products available, Hetrofoam and Hexafoam that can be machined in their more dense versions. I have used them when I was building models for a living. They are of course pretty pricey for the average MR hobbyist. I remember one project where we added a dummy fairing to a 747 engine pylon. It took two days of chisel and hammer work to remove it after the test.
    On the subject of supporting the pink or blue insulation foam, I would suggest 16 to 18 inches between supports.
  7. JAyers

    JAyers Member

    Oh, plywood will warp under it's own weight, over a span of four feet to be sure. I store pieces out in my shop and if I don't have them supported across the whole width, or stored completely vertical, they'll sag, especially the 3/4" stuff. Sagging of sheet goods, even solid wood, under its own weight and load over spans is a well known entity. Just ask any cabinet maker that makes book cases. Many of the wood working magazines publish tables of span, deflection and material. It's no mystery that this happens.
  8. gottaBreal

    gottaBreal Member

    Can you use Foam and still handlay track? Thats what I cant seem to understand how you can get away with useing Foam and still be able to have the spikes GRAB into the foam.
  9. ChadYelland

    ChadYelland Member

    Hi guys, i plan to use 1/2 inch MDF board supported every 32inch or 16 if nessisary,(using mdf for better smoother glueing surface) with 1.5 or 2 inch pink foam over the entire surface, i did something simular but on 3/8 ply wood and knelt or stood on it many times with minimum defection. I liked the WScenes inclines will use some again. Also have heard of solid foam being noisyier so i might try the soft foam roadbed from WS i belive also i think its black,,,
  10. Russ Bellinis

    Russ Bellinis Active Member

    I don't think the spikes will grab into foam. I suspect most modelers using foam for subroadbed will use flex track rather than handlay. If hand laying, the choice is to either glue down the ties and glue rail to the ties, or glue down homobed (roadbed made of homosote) to give you something to spike into.
  11. ChadYelland

    ChadYelland Member

    I have heard mention of this Homosote,,, what is it? i have never seen any in canada and no lumber yard workers seem to know either.
  12. Russ Bellinis

    Russ Bellinis Active Member

    Homosote is a pressed paper product. It comes in 4 x 8 sheets like plywood, but is actually pressed paper. It holds spikes very well, so is popuolar with those who handlay track. It is also sold in strips by a model railroad company and then it is called Homobed. I don't know where you would get it in Canada, but I think Walthers sells Homobed.
  13. MasonJar

    MasonJar It's not rocket surgery

    But if you install and laminate the layers in the "bent" configuration (grades or whatever), there is very little chance that they will deform.

  14. pgandw

    pgandw Active Member


    I don't have answers for the second part. I've only done one layout with 1.5" foam and 1/4" plywood (plywood on top). It was 46" x 60" with no support other than an outside frame of 1x3. Because I glued the foam into the frame on a sloping 3D basement floor, the whole assembly was not quite true in the vertical dimension. The twist in the foam caused the plywood to take on the same twist when it was glued on. That built-in twist never went away during the whole 4 year life of the layout (O27 and O track so the irregularities didn't matter much) despite being laid on flat floor with a heavy Christmas tree on top and occasionally being walked on. Between seasons it was stored vertically against the wall.

    I know that in boat construction, various foams and wood products (plywood, balsa) are used as sandwich filler between 2 fiberglass skins to increase bending resistance. Voids in and/or failure of the filler are major structural defects that have to be carefully watched for because that's how the load is transferred from one skin to the other. But boats are subjected to stresses many times what our layouts are. From boat construction, I have found the ultimate lightweight girder for model railroad or other similar uses (in my opinion) is a box girder with 1x3 sides, 1/4" plywood top and bottom, and 2" foam interior, all glued together. That box girder will easily span 10ft or more with no sagging.

    The best recommendations I have seen for foam use in layout construction, and that make the most sense to me, is to use 2 or more layers of foam. The lower layer is structural, sized accordingly, and should not be compromised except for wiring holes and the like. The second and subsequent layers are provided for removal as desired for below track scenery.

    In your Timesaver situation, I would submit that the yardstick roadbed adds significantly to the rigidity of the plywood, and as a unit will probably not sag over the distances listed, especially if not heavily leaned on. Also, widening the plywood beyond the width of the subroadbed up to the length-wise span distance does add a very small amount of additional rigidity (maybe some civil/structural engineer can provide the equation).

    While I may want to minimize weight in my layouts, rigidity is more important to me. I want the grades and elevations I build into my track to stay there as long as I am using the layout. So I may use heavier and thicker materials than are absolutely necessary to ensure my goal. Others will accept and get by with less.

    my thoughts, your choices
  15. ocalicreek

    ocalicreek Member

    Fred - thanks again for good dialogue on a subject that has at other times earned me the ire of others. Benchwork can be somewhat of a sacred cow, I guess.

    I agree, others will accept and get by with less. It's good that we have a forum for enriching the discussion of these techniques in order to increase our enjoyment in building and operating model trains.

    As for foam, up here in the Pacific NW lumber is so cheap that I'll probably go with a homasote/ply combo. I have seen foam at HD, but it was in 2'x8' sheets (not 4'x8', or thinner to fit between framing...hmmm) of varying thicknesses and per square foot more costly than the lumber. But, as was mentioned before, this can be a pricey hobby and perhaps the extra cost is worth the flixibility and strength benefits.

  16. Russ Bellinis

    Russ Bellinis Active Member

    I think a lot depends on what sort of layout you are building. For modular modeling where the modules are ocnsidered portable, weight may be a very important consideration. On a home layout that is to but built and never moved, weight is probably way down on the list of considerations.

Share This Page