Project S13 Nissan 240SX – Part 2 – Chassis Stiffening For Cheap
By Joey Leh, Photography by the author
When we last left off with Project Nissan 240SX, we just started getting acquainted with our second-hand project car. The base platform for cheap speed was already in place when we started – SR20DET swap, 198whp, new brakes, and an adjustable suspension.
Pouring the urethane foam into the chassis
Shaking loose nineteen years worth of age was as simple as new Energy Suspension urethane bushings, brake pads, tires, and a comprehensive fluid and water pump swap. Almost.
Project 240SX’s fastback chassis is the softest of all 240SXs. And 240SXs are not stiff to begin with anyways. The back hatch glass, which might as well be made out of cast iron, leaves a massive and heavy non-structural void in the rear. Thus, the fastback is substantially less rigid than even the coupe model S13-chassis 240SX. We desperately needed to address chassis stiffness.
One thing was for sure, we didn’t want a full roll cage in the car. Tying together the chassis with a custom welded cage would increase chassis stiffness by a large amount but this car sees more street time than track time and cracking our skulls open on a metal tube doesn’t exactly sound like a great Friday night.
This left us with the options of stitch welding, bolt-on chassis bracing bars, a 4-point roll bar and urethane foam. We may try out all these methods eventually, there’s no such thing as a car that’s too stiff, but the urethane foam is one of the most intriguing and so we started there first.
We’ve seen expanding urethane foam in automotive applications before. It’s used in such OEM applications as the Acura TL and the Mazda RX-8 R3. The stiffer a chassis is, the less it will flex against forces applied to it, i.e. suspension movement. 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 even stiffer springs can be used before the chassis is upset again. Foam filling a chassis is a cheap and light method with which to dramatically increase chassis stiffness.
There are concerns though – the foam can trap moisture and rust your car from the inside out. Here in Southern California we don’t see a lot of rain, or any snow whatsoever, so we took the risk. Those in colder climates that are planning on keeping their cars for a very long time will have to weigh these negatives.
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 the upper pillars (made by Handi-Foam) and the 8 lb per cubic foot stuff for the rockers. 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.
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.
Our filled rocker after hardening
We mixed up about 20-ounces at a time and a paint mixer and power drill came in handy for us as we zapped the mixture for a mere 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.
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 the entire 240SX chassis. Handling the foam itself isn’t difficult at all, but proper prep is a must. The foam is nearly impossible to clean off once hardened and sets very quickly.
Before mixing and pouring began, Project 240SX’s interior was completely covered in a drop cloth, newspapers were placed underneath, disposable work clothes were worn and bits of duct tape were placed next to every orifice that we would be pouring into. This was done so that the expanding foam would 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.
Although we don’t have any exact 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 (less chassis sag) as the car was jacked up at the front left, with a floor jack saddle height of 10.75-inches.
Driving the car is also a revelation, with less creaks going up driveways and a noticeably smoother ride over broken pavement. Project 240SX may well see extra bracing and bars soon enough but for one afternoon’s worth of work and less than $100, we’re not complaining.
<<< Continue to Project S13 Nissan 240SX – Part 1 – Buying An S13 >>>
<<< Continue to Project S13 Nissan 240SX – Part 3 – Unsideways Track Suspension >>>
Sources
K&N Engineering
www.knfilters.com
US Composites
(561) 588-1001
www.uscomposites.com
Mavrik Motorsports
(714) 523-1896
www.mavrikmotorsports.com
Gotta try this on my e30. Hopefully it doesn’t rust from the inside out because of snow.