How To – Cheap DIY Chassis Stiffening Foam
By Richard Gantry, Photography by Joey Leh
The stiffer a chassis is, the less it will flex when forces are applied to it, i.e. through the suspension. This means that forces from the road surface will be contained and absorbed by the suspension rather than being allowed to beat up the body shell. As the chassis is stiffened, ride quality will improve and stiffer springs can be used before the chassis throws a fit. Hell, even OEMs use the methods outlined here on such cars as the Acura TL and the Mazda RX-8 R3.
Our thought process behind this urethane foam method is – when it comes to making your car stiffer and stronger, sometimes you just don’t want a full roll cage. Tying together the chassis with a custom welded cage would increase chassis stiffness by a large amount but if your car sees more street time than track time, cracking your skull open on a metal tube doesn’t exactly sound like a great Friday night.
This leaves you with the options of stitch welding, bolt-on chassis bracing bars, a simple four-point roll bar or urethane foam. Bolt-on braces and bars often have little effect or gain on chassis stiffness and they often cost a pretty penny. Doing something for dirt-cheap in your own garage sounds pretty tempting, huh?
Keep in mind though that the foam you want to put into your car isn’t the cheap spray insulation that you can buy at your local Home Depot. Many of those foams have a density in the range of 0.5 lb per cubic foot. We were after the 2 lb per cubic foot injectable kits for use in upper A/B/C pillars (made by Handi-Foam) and the 8 lb per cubic foot stuff for the rockers and frame rails.
Foams in the 8 lb per cubic foot density are commonly used in marine applications and we’ve never come across one that was an easy to use two part injectable kit. You’ll have to mix and pour these tougher foam types. Our expanding urethane foam pour kit was sourced from US Composites and came in two unassuming metal cans. The liquid in the two cans is mixed in a 1:1 ratio and has a claimed 45-second pour time. In actual use, the pour time (the time you have to pour it into your car before it bubbles over and turns into a solid) seems to be closer to half that.
We mixed up about 20-ounces at a time and a paint mixer/power drill combo came in handy as we zapped the mixture for 15-seconds before beginning the pour into our rocker panels. Any longer and the foam would begin to harden in the funnel as we poured, clogging any more of the mixture from entering our tester Nissan chassis. If you want to avoid ruining and wasting any foam, funnels or supplies, you’ll have to be quick with your hands.
The pourable urethane foam expands to eight times its liquid volume, meaning the 16 lb net weight size kit is more than enough to handle most any car’s rocker panels/side rails. Buy more than one kit if you intend to fill up a subframe or tubular control arm. Handling the foam itself isn’t difficult at all but proper prep is a must. The foam is pretty much impossible to clean off once hardened and hardens very quickly.
Before mixing and pouring began, we covered our 1989 240SX’s interior completely with a plastic drop cloth, placed newspapers underneath the car (foam will drip out of the rails on to the floor), wore disposable work clothes (no tuxedos here) and pre-cut bits of duct tape were placed next to every orifice that we pouring into. This was so that the holes could be covered after pouring and the expanding foam could be captured inside, instead of bubbling out. We also made sure to tape up the backside of the lower seat belt holes in the rockers, so that the foam didn’t fill up the threads for the mounting bolts.
The injectable 2 lb per cubic foot foam was by far much easier to use. A kid could literally figure out how to use the kit. Not recommended though as any kid on Earth would probably try to eat some of this stuff. Nasty. The Handi-Foam kit comes pre-assembled with two cans in an injector setup. You pop open the two cans, release the nozzle and you’re ready to go. Stick the injector tip into you’re A/B/C pillar holes and let the stuff loose. Be sure to use the duct tape trick again to seal the foam inside as it hardens.
Although we don’t have any exact chassis stiffness increase percentages, a simple garage measurement yielded a stiffer chassis with a 1/4-inch front and 1/8-inch rear increase in frame to ground height as the car was jacked up at the front left, with a floor jack saddle height of 10.75-inches. Road testing the car also provided a noticeable difference, with less creaks going up driveways and a noticeably smoother ride over broken pavement. For one afternoon’s work in the garage, and less than $80, why wouldn’t you try it?
Source
US Composites
www.uscomposites.com
Handi-Foam
www.fomo.com
| 8 lb/cu. ft urethane foam | Parallel Compressive Strength: 250 psi |
| Free Rise Density: 8.0 lbs cubic ft. | Tensile Strength: 225 psi |
| Expansion Rate: Approx. 8x Liquid Volume | Shear Strength: 130 psi |
| Buoyancy (flotation): 54 LBs per Cubic Ft. | Flexural Strength: 350 psi |
