Gonna try this: Urethane foam for targa chassis - advice?

[tekdeus]

Well after much research, I'm going to do this, and report my application progress and results. I'm looking at either the 8 or 16 lb foam from this site: http://www.shopmaninc.com/foam.html

Check out the descriptions of the these densities:
"8lb: This 8LB density foam is extremely hard and rigid, like that of a soft wood. You fingernail can penetrate its skin, however it cannot be dented by hand.
Parallel Compressive Strength: 250 psi
Tensile Strength: 225 psi
Shear Strength: 130 psi
Flexural Strength: 350 psi

16lb: This 16LB density foam is essentially as hard as a rock, you would need a hammer in order to make any dents in this product."
Parallel Compressive Strength: 580 psi
Tensile Strength: 450 psi
Shear Strength: 230 psi
Flexural Strength: 750 psi

I've calculated approx 2.5 cubic feet to fill the rocker panels, and the rear under-seat rail. My goal is to eliminate targa shimmy and the need for an unsightly targa brace.

From what I've gathered there is a risk of squeaks developing over time so I will take the time to clean/hone/scuff the insides of the panels for maximum adhesion. In addition, I'm going to use foam 7 times more rigid than the 2lb foam others have in the past.

Talking with Randy, he was wondering if the 16lb foam might be TOO rigid, and put stress on other parts of the chassis. I was thinking that a more rigid foam is less likely to compress and lose adhesion with the metal and cause squeaking. What do you guys think?

This is all theory at this time, and I just want to put my ideas out there since this is a major irreversable step and I want to make sure the outcome is ideal.

By the way, the 8lb foam would add 20lbs of weight, and the 16lb foam would be 40lbs.

Assuming that the rocker panels flex and twist, do you think the foam would experience more than 250psi (pounds per square inch) under maximum load? say hard cornering on really stiff suspension with the targa off?

What would you do?

 

[mkIIIman089]

If I were going to do it, the weight gain alone by using the 16lb product would definitely shy me away from it.

That being said I would never do it, but then again, I would never put a snake oil additive in my car either.

 

[Doward]

What would I do? Put my Supra on a 4 post lift, and design some appropriate perimeter style subframe connectors.

You could try it, and let us know how it does. I've wondered about it - but I've also wondered that the foam would eventually break down

 

[Enraged]

lexus uses foam in their chassis, so i doubt you have to worry too much about foam breakdown.

 

[Junior]

Assuming that the rocker panels flex and twist, do you think the foam would experience more than 250psi (pounds per square inch) under maximum load? say hard cornering on really stiff suspension with the targa off?

first off, psi isn't the measurement of load, the measurement you're looking for is ft*lbs, or nm for us metric thinking folks.

secondly, the measurement for chassis flex is knm/mm(kilonewton meters/millimeter) meaning if you apply x number of kilonewton meters of force, the chassis will flex y millimeters, it's generally a quadratic function, linearly more force will 'cause a squared function of flex.

thirdly, resistance to chassis flex is primarily a function of design, not materials, buttressed aluminum will be stronger than unbuttressed titanium, or unobtainium for that matter.

fourthly, to give you a ballpark figure, a rough idea of what amount of chassis flex would be, it'll generally be in the range of 100-200knm/mm or in imperial terms 1875 thousand pound feet/inch to 3750 thousand pound feet per inch

the imperial ones wont be accurate tho, because you're into the "yield" part of the stress curve by the time the chassis flexes that much (atleast I would think so, I don't have anything to back that up, but an inch of lateral flex is alot...) but it gives you a rough idea of what you're working with here. So if you think that an extra strength of 250psi applied indirectly against the chassis is gonna make a measurable difference... have at it I guess. Let us know how it works out, but I wont be trying it anytime soon.

Enraged: lexus use foam in their chassis as a sound dampener, and vibration dampener too, to keep the cabin quiet and seperate from road/tire vibrations.

 

[tekdeus]

I could be wrong but I think the 250psi compressive strength suggests that it would take 250psi of pressure before it buckles or compressess. It does not put any preload on surfaces as it hardens(areas with relief holes).

A reducion of 25%-50% of the flex in the rocker panels might eliminate 100% of the targa shimmy, which in my car is more of a fast vibration/wiggle twisting between the front and rear of the car.

 

[supra90turbo]

lexus uses foam in their chassis, so i doubt you have to worry too much about foam breakdown.

Not all foams are created equal.

3M Automix "Pillar Foam" != Root Beer Foam

the 3M shiz: http://products3.3m.com/catalog/us/...us_aad_3_0/command_AbcPageHandler/output_html

more info for yo brain: a LOT covered in here, as well as links to a few other discussions on MKIIITech.com and supraforums.com: http://www.supramania.com/forums/showthread.php?t=21218&highlight=foam+chassis+injection

 

[Junior]

yea they covered densities, but not strengths, not other than "hard as wood" or "hard as rock" I'd like to see something abit more quantitative before I'd start spraying the stuff. Like I say tho, please, somebody do it and get us the results, if you prove me wrong, we all benefit (including me).

 

[aye mate]

Wasn't Simann going to do this? I remember a thread about that. It might have been pre-hack days though. Anyone else remember?

 

[tekdeus]

I posted the strengths in the first post: Parallel Compressive Strength, Tensile Strength, Shear Strength, and Flexural Strength.

yea they covered densities, but not strengths, not other than "hard as wood" or "hard as rock" I'd like to see something abit more quantitative before I'd start spraying the stuff. Like I say tho, please, somebody do it and get us the results, if you prove me wrong, we all benefit (including me).

 

[Junior]

errr, sorry, you're right you did, I was looking for them in the other linked post and didn't see any.

can you give me a rough idea how much surface area on each axis we're talking about here? on the horizontal axis (parallel to the floor) and verticle? if so I aughta be able to give you a rough idea how much strength you'll have thru that section. That's not to say that it wont just flex the same amount an inch up the road from that section tho, but it'd be good for everyone if we had some solid numbers.

 

[Nick M]

So where exactly does Toyota use foam for bracing on their Lexus' line?

 

[tekdeus]

Side rocker panels are approx 4ft long x 6" x 6". The rear floor brace is 4.5ft wide and approx 3" x 6".

errr, sorry, you're right you did, I was looking for them in the other linked post and didn't see any.

can you give me a rough idea how much surface area on each axis we're talking about here? on the horizontal axis (parallel to the floor) and verticle? if so I aughta be able to give you a rough idea how much strength you'll have thru that section. That's not to say that it wont just flex the same amount an inch up the road from that section tho, but it'd be good for everyone if we had some solid numbers.

 

[Facime]

I would be willing to try it on a car I didnt care much about, but my fear would be...What if I dont like what it does or in essence ruins the car in some way. I mean, once you put that stuff inside the frame its never coming out (especially the 16lb stuff). If it did develop squeaks over time its would be something you just have to live with.

That being said, I hope you do give it a shot and report on the process and findings. That would be very valuable information

 

[Junior]

72000lbs support for the sides supports, with the 8lbs, 167000 or so with the 16lbs.

for the center you're looking at about 40.5k psi with the 8lbs, and 94k or so with the 16lbs stuff.

strikes me that for the side walls, it's really valid, but for the floor brace, you'd be better off with a piece of 2" square tube steel welded in, and plates on the outside to distribute the load. then buzz the floorpan back up ontop of it. I guess that's beyond most people tho. But for the sides it looks to be a pretty valid way of getting rid of some flex.

 

[tekdeus]

Thanks for crunching the numbers Junior. Do you figure the 8lb foam would be strong enough? That would weigh about as much as a targa brace and if it worked, it would be fantastic.

By the way I am running a floor bar brace that supports the points you are talking about (I think) and I did notice a slight reduction in shimmy with it. Perhaps I should just leave that in and not bother foaming the rear floor brace?

I did a google search for things like "chassis foam squeaks", etc, and could not find any other mention of the foam squeaking, other than IJ's story about a rusty datsun 510 many years ago.

72000lbs support for the sides supports, with the 8lbs, 167000 or so with the 16lbs.

for the center you're looking at about 40.5k psi with the 8lbs, and 94k or so with the 16lbs stuff.

strikes me that for the side walls, it's really valid, but for the floor brace, you'd be better off with a piece of 2" square tube steel welded in, and plates on the outside to distribute the load. then buzz the floorpan back up ontop of it. I guess that's beyond most people tho. But for the sides it looks to be a pretty valid way of getting rid of some flex.

 

[Junior]

hmmmm, I think that 8lbs would be ok, but thing is that once it's in, it's in, there's no upgrading it later. so with that in mind I'd probably go with the 16lb. I guess it depends entirely on whether it's about weight or appearance.