Yeah, unusual question here. I noticed our k-members are 2 pieces of welded steel. Can I spray in expanding foam to fill the gap? I'm looking for reduced road noise and engine vibration. Will this help or am I smokin crack?

put down the crack pipe.

the k-member has nothing to do with road noise, adding foam will do nothing.

engine vibes could be a tuning issue, or a dirty engine, missfire, ect. Road noise can be bad bushings in the suspension, strut mounts, steering linkage ect.

My engine runs fine, but my thinking was that it would absorb some of the noise. Kinda like adding dynamat to the firewall. I'm looking for "better than new" level of quietness, but yeah, this probably isn't the way to go.
Thanks tho.

people sometimes put expanding foam in the frame rails of their cars to give extra rigidity, but usually all that happens is it makes a big mess and deteriorates after a few weeks and ends up adding a bunch of useless weight.
i wouldnt do it, but its your car!
check your motor mounts

NVH is the main reason the OE's have gone to things like cast accesory brackets, cast oil pans and 360* bellhousings, magnesium alloy subframes (99+ W body) and other techniques.

NVH is all about reducing vibrations and "tuning" the remaining vibrations to frequencies that we dont notice.

For example, the cast aluminum brackets uset exclusively on modern production cars have a lower natural frequency than a piece of stamped steel.

Since the K member is such an open piece filling it would be hard, look at the rocker boxes and frame horns first. They'll make a much more significant difference, even if you dont use the structural foam specificaly for cars. The difference is the structural stuff can add significant stregnth and rigidity to the chassis while non structural foam simply dampens the resonance of the sheet metal.

Make sure whatever you use is closed cell foam, open cell foam WILL invite rust.


Look at what the OE's are doing that you can copy. Weather stripping that sits nearly flush along the sides and front of the hood to keep wind noise down, make some foam inserts for the rear of the fenders to keep wind from whipping through at speed.

I'm planning to use 4th gen inner door weather stripping on my car as I rebuild the interior for NVH reasons. It'll run from just under the window at the front to the back. Just trim the exccess and modify the stock sill cover. Double and tripple seals are common on modern cars and I've seen real differences on friends cars.

If NVH is the real concern dont use poly bushings or heim joints in the suspension and steer clear of big rims and low-pro tires, consider road noise when picking tires. Tread patterns make a big difference.

A thick cover over the cargo area is a good idea, I notice a difference in my Mazda when the rear seats are down, the larger the open volume of the cabin the "din-ier" the car will tend to be.

Get the "wedges" used in verts and T top cars if yours dosent have them and set the doors for a slight interference fit.

Oh and make sure the spare stays tight Thats the main source of noise in my car and it took me forever to realise it wasnt the famously flimsy chassis.

There are several types of foam specialy designed for automotive use.

^Now THAT's what I'm talkin' about.

I didn't think household expanding foam was the best, but I didn't think it would hurt. Good to know. I'll look non structural automotive foam and look for more places to fill. Not looking to be a Buick, just quieter than now. Thanks.

why not just use the structual stuff? make the car stronger and quieter at the same time. stronger chassis means less squeeks etc. SFC made a big difference. and i would use something like lizard skin (they have great spray on products for noise and temperature) or dyna matt etc.

would a tubular kmember be quieter than the stocker?

would it be hard to make your own Kmember? using some rollbar tubing and 3/16" plate. maybe use the stock kmember as a template. hmmmm

you know, ive thought about that, makig a k-member, but i wouldnt trust my ability on something so crucial... what would happen if the mounting point for the a-arm failed... ya know?

sorry, i wasnt aware that there was foam specifically designed for automotive use.
a guy on another forum used great stuff in his frame of his caprice and the consensus was that it helped for about 300 miles but then broke up to the point where it just added weight and made noise

I would only fill it after I had the thing electroplated or zinc dipped. The foam will retain water and it will accelerate rusting, check out old panteras ...the frames rust like nothing else.