Improving Chassis Stiffness by Foam Injection

[scrimshaw]

Hi
I am trying to wade through some hype on a product called foam-seal which some people are saying increases chassis stiffness by a huge amount. (i've heard claims of up too 300% in some cases!!)

The product is a 2 part polyurethane foam which is injected into the rails, expands, fills all the voids then hardens. Even the conservative estimate was I believe 30% more stiffness for the lowest density foam (2lbs/ft^3).
I understand the principles behind a foam core in a sandwich laminate and I suppose it works the same way, but I have a few reservations.

First I would think the foam must adhere completely to all sides of the walls for it to be any benefit and I am wondering how the foam can do this in metal that has been enclosed for possibly years and is not prepped?
Second if used in say 2"x6" rect. tube steel with a wall thickness of 0.125" and a length of 10 feet can any usefull stiffness increase be expected assuming good adhesion on all sides?

Thanks for any help you can offer.

 

[MacGyverS2000]

I gave up my mechanical ways many moons ago, but I can't see how a low-density foam can add any appreciable stiffness to a frame... vibration damping, yes, but not appreciable stiffness. And 300%?! I call BS.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[Fabrico]

Maybe they meant fill in everything outside the tubes.

 

[MechanicalChef]

A syntactic polyuretane foam could potentiall boost stiffness. Worth the hassle? Probably not.

 

[patprimmer]

It would depend a lot on the structure.

If the frame was aluminium that was microns thick, and therefore exceptionally flimsy, then the foam would have a significant increase, however if the frame was heavy wall chrome molly tubing, I doubt the increase would be measurable.

Polyurethanes are very good adhesives and stick well to most metals. Some generate significant pressure as the foam expands on cure. This pressure increases stiffness just like air in tyre, and like air in the tyre does not need a bond to get some (but not optimum) effect.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[dicer]

It would create expansive stress on the structure.
Have you ever seen what happens to a couple of vaccum packed magazines? They become as stiff as a board. The foam deal maybe like doing that in reverse.

 

[bvanhiel]

Most of the stiffness from a steel-foam-steel sandwich comes from the increased section modulus because the foam "spaces out" the steel. In a tube the steel is already "spaced out", and adding the foam will not change the section of the steel. I would expect no appreciable increase in stiffness.

-b

 

[bvanhiel]

Let me add that it may delay buckling on thin wall tubes, so perhaps that is the "stiffness" increase they are referring to.

-b

 

[Peter7307]

scrimshaw,
Ford Australia used a foam filling in one of their production cars to help get through crash tests not long ago.

Perhaps an email will help give you some clarification on the points you era talking about.

Apart from reading about it in a magazine I have no other knowledge of the "how to" and the results.

Pete.

 

[patprimmer]

The prevention of buckling is the main contributing factor. This is only relevant to very thin wall sections.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[scrimshaw]

Thanks for all the replies and and for making me see it will not work as it does in a composite laminate. As for the expansion property there is no way really to accurately predict it unless I calculate the inside volume and inject a measured amount to get the required force. Not very practical.

So this process will do nothing except maybe some noise reduction.

Thanks again for the help.

 

[Compositepro]

The statement that it will not work as it does in composite laminates is not correct. Foam core will work exactly in the same way in both cases.

Two large flat sheets of any material will be greatly stiffened by a foam core. But there are other ways of stiffening such as ribs, curvature, corregation, etc. A tube is already stiffened by its shape and adding foam will have minimal effect in mast cases.

Sheet metal can be stamped to shape very economically and therefore that is the most common method of forming stiff structures.

 

[scrimshaw]

"The statement that it will not work as it does in composite laminates is not correct. Foam core will work exactly in the same way in both cases."
compositepro

I was talking about the specific case which I mentioned in my original question i.e. rect. tube steel 2"x6"x10' with 1/8th wall. and as you mentioned later in your post a tube will not really benefit.

 

[izzmus]

Thing is, cars are not made of 2"x6"x.125" tube (that's more like a truck's framerail!), they're spotwelded together from thin stampings.

I believe Toyota and Nissan have been using the process for a while now in their luxury cars.

 

[GregLocock]

I've seen it used on several cars, for several reasons. Given the tendency for cost cutting I can only assume that it had some benefits... but quite what they were is a bit unknown.

If your spotwelding was amazingly bad it might help to improve the integrity of the spotwelds.

It will certainly reduce panting of the skins of box sections. This could increase the overall stiffness of the car.

It will prevent transmission of noise along the tube.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[MacGyverS2000]

It will prevent transmission of noise along the tube.

I would imagine this was the main reason...

Dan - Owner

http://www.Hi-TecDesigns.com

 

[GregLocock]

Yes, but we use foam blockers to do that, much cheaper and less likely to explode in the paint ovens.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.