Adding frame strength

[Jim Boemler]

I'm looking at ways to make a super-strong 914 chassis. In other areas (like aircraft) they'll use a sandwich of two layers of light metal around a foam core. The core contributes significantly to the strength by simply keeping the metal from buckling.

What I'm considering is filling a 914's frame voids with polyurethane foam (a 2-part liquid that reacts and solidifies very rapidly). This would minimize the flexing of the sheet metal frame pieces, and by coincidence would keep water from getting access to the metal (it's a tight-adhering closed-cell foam).

Question is, am I re-inventing the wheel? Has this already been done? With what results? If not, anybody got any thoughts on the value of this technique?

jim

[Dennis Marer]

I have the same question myself - but had a slightly different approach.

What if a few layers of carbon-fiber composite were laminated to the metal? This would improve rigidity at a low weight cost. Two layers would also make a water-tight seal.

The foam would help reduce bending in situations where it would be compressed. The carbon fiber could prevent bending over long sections if it can be securely bonded to the existing metal, stiffening them.

Some modern race cars eliminate the metal frame completely...

[Jim Boemler]

Interesting idea! Along the same vein, I chatted with Bill Frey at Restoration Designs today. Turns out he makes a metal piece that's just like the outer longitudinal, except it's made just a bit bigger. The idea is that you weld it OVER the existing piece for extra strength. It's stamped exactly like the original, so after it's in place it looks just like the original, but thicker.

This costs some weight compared to carbon of course, but is much easier to do. He's sending me info, but as I recall he said cost was about $300 for both sides.

BTW, Bill's piece is longer than just the longitudinal, and covers about four of the existing frame pieces.

[Dennis Marer]

Let us know the details when they arrive - this sounds like an acceptable solution if you can live with the extra weight. Thanks!

[Bha57]

Hi Jim,
Used this on the 914, V8. All voids in the body have been filled with foam. This has been in place since the body was reworked and has been no trouble for almost ten years. Go for it.

[halbl]

No experience but like the idea. I have a friend who is sold on the idea and will try it on his restoration. Last spring I found a product called Frothpak at an industrial convention. It is two part with 2 nozels for application.I tried putting a free sample in a sealed, completely full, jar of water to determine if it would absorb water. Didn't, so I located the mfgr, and got a local distributor to order in a small (3 cubic feet ) supply. Would also like to have more input on this subject. Can the trailing arms be filled? What parts of the chassis? Do holes have to be drilled for air to escape when filling? Should the heater ducts be filled or foamed around?

[Mark Killar]

In regard to using carbon fiber on the outside of sheetmetal for strength...I don't have any experience with that application in particular but have been involved in the building and repairing fiberglass/kevlar kayaks where strength,stiffness and weight are usually one of the biggest considerations. Carbon fibers are considered to add little to stiffness and strength when applied to the outside of a lamination. It does make a BIG difference when incorporated WITHIN the lamination. It would seem to me that your looking at a similar situation in this application. Foam cores are also used in boat building and add rigidity with light weight with excellent results (again, within the lamination). Using foam in this case sounds like an interesting application. One concern would be with the accumulation of moisture between the foam and the bare sheet metal. Anyone that lives in areas where rust is a problem can find a good example of what I'm talking about if you look at some old VW Rabbits. You'll notice the 'A'posts at the bottom of the windshields are usually rotted out because VW used foam in that area to reduce wind noise within the 'A'posts. Also, old Beetles have a similar problem in the 'C'posts where foam was used. I don't think the foam itself would hold water but moisture was trapped in those areas.

[Paul]

I may have some more info on foaming stuff. I have an old boat making catalog I was looking through and I noticed that it had a space filler/floatation foam stuff. It expands and says it resists absorbtion of water and is resistant to gas oil and such.it hardens to 40lbs per square in compression strength and weighs 2 lbs. per cu. ft.

If you called and asked maybe these fellas could give you some answers to Mark's question concerning water trapping between the foam and metal.

I found this stuff in a clarkcraft catalog, the company is located in NY and is supposed to be a very reliable boat company. their number is 716 873 2640
I am posting a pic of the add at:
http://members.aol.com/madd001/float.jpg

-Paul