How to form your own custom foam seat insert

By J.G. Pasterjak
Dec 4, 2022 | DIY, seat, Foam, Mold | Posted in Shop Work | From the Oct. 2015 issue | Never miss an article

[Editor's Note: This article originally appeared in the October 2015 issue of Grassroots Motorsports.]

Driving a race car is difficult enough, what with the noise and the g-loads and the throngs of onlookers cheering your every move. But if you’re being thrown around the cockpit like a rag doll, lap times will suffer, throngs will not cheer, and your trophy shelf will be a barren reminder of all that could have been.

We’re here to help. 

We all know that a proper driving position is key to best performance behind the wheel, but maintaining it in the face of physics can be daunting. This is especially true in formula cars and sports racers, where there usually isn’t a “seat” as we know it–just a hole that is roughly human-shaped.

The easy solution to the problem is expanding polyurethane foam. Common in the marine industry, this foam is the product of a two-part chemical reaction. When those chemicals are combined, the liquid transforms into an expanding cloud of goo, eventually hardening to the consistency of Styrofoam. It’s an easy and cost-effective way to create a seat mold that properly locks the driver in place for high-g fun.

No silver cloud comes without raindrops, though, so here’s our default warning: Polyurethane foam and fire are not the best combination. It’s not highly flammable, but it will melt, and the resulting sludge is a hot, sticky mess that can further complicate the already unpleasant environment of a burning race car. 

So if you’re concerned about fire, we recommend covering your foam seat mold with Nomex cloth to increase its fire resistance. We used common headliner material in our autocross-only Formula 500, but then we typically autocross in shorts and a T-shirt, so our seat fabric is certainly not the weakest link in our fire safety chain.

A self-extinguishing foam option is available from BCSI, the popular roll bar padding manufacturer. This SFI-approved kit uses EIS W50, an energy-absorbing foam that, like our low-buck urethane, starts off as a liquid.

For our project, we used a foam with a 2-pound density from Fiberglass Coatings, Inc., of Fort Lauderdale, Florida. They sell all sorts of molding chemicals for hobby and industry applications, and they deal with customers both directly and via Amazon.

The “2-pound” in our foam’s description refers to the weight of one cubic foot of set foam. This is the lightest of the expanding foams, the one commonly used for seat molds, filling gaps in boat hulls, and simply amazing your friends with science.

Step 1: 
Pouring a mold is a job best suited to a team of three. If you’ve done a few of these projects, you can get by with two people, but three is the sweet spot. 

One is, of course, the driver for whom the mold is being poured. His job is to sit still. Our advice to sitters: Pee before you do this. The second person is responsible for mixing and pouring, and the third holds open the containment bags for the pour. 

Start by covering everything nearby in thin plastic sheeting. Be sure to fill or tape off any “negative” space inside the car where the expanding foam could get trapped. 

Step 2:
Next, position the bags that will receive the foam. Depending on the areas you need to fill, these may go under, behind or around the driver. Use large, heavy-wall bags, like contractor bags.

Step 3:
Insert driver. The driver’s job is to maintain his intended driving position while the foam expands around his body. 

Depending on the configuration of the car, this is easier said than done. For some cars, it may mean that the driver has to support himself in an uncomfortable position for extended periods while the foam expands and hardens. Figure this out before you mix the foam, and rig supports accordingly. 

Remember, the driver’s position needs to be as close as possible to how he’ll actually sit behind the wheel. Having both arms straight out to the sides is not how someone drives. Keep this in mind when rigging your driver supports.

Step 4:
Mix your first pour of foam. Some inserts will take only a single pour. Larger ones may take several to get properly fitted. We felt most comfortable working with 10 to 12 ounces of material at a time. 

The chemicals combine in a 1:1 ratio and begin reacting almost immediately. Mix thoroughly for about 20 seconds and then pour quickly, as expansion will dramatically accelerate very soon.

Step 5:
Your first pour needs to get all the way to the bottom of the bag, so the driver may need to lean out of the way. The driver should be wearing his driving clothes, since the foam will expand to precisely match the contours of the body.

Step 6:
Now, the driver needs to sit as still as possible and wait. The foam is pliable as it expands, so he can sort of squish it into position. But once it sets, it’s done. 

If you don’t like where it is when it sets, you can always pour some more material, but the basic lesson is for the driver to be very conscious of his movement during the process. 

Remember also that the mixture is undergoing a powerful chemical reaction, and during that process it gets hot–not painfully hot, but hot enough that it can make the sitter a little uncomfortable and sweaty. Tell him to get used to it, and have a cold beverage nearby.

Step 7:
After 20 to 40 minutes, depending on ambient temperature, the driver can hop out and cool off. If you taped up the gaps correctly, your mold should lift out with little effort. 

After about an hour, the foam is set enough to trim. The bag should come off easily–it won’t stick to the foam–and the foam can be trimmed with a large bread knife or reciprocating saw. An electric knife works great as well. 

Step 8:
When you’re satisfied with the shape and fit of the mold, you can upholster it to your liking. As we noted before, we recommend fireproof covering for potentially fiery situations. In our case, we simply used headliner material and spray adhesive. And there you go: one custom-fitted seat insert, ready for the autocross course. 

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View comments on the GRM forums
triumph7 Reader
8/7/20 9:11 p.m.

I've done a couple, here's the latest in the Zink 19.

triumph7 Reader
8/7/20 9:11 p.m.

classicJackets (FS)
classicJackets (FS) Dork
10/2/21 6:22 a.m.

Older article and maybe a dumb question, how do you then mount the foam seat to the car?

Junghole SuperDork
10/2/21 10:27 a.m.

They left a step out: clean that foam crap out of the drivers hair after it has set up and he's been in it. 

triumph7 HalfDork
10/2/21 7:18 p.m.
classicJackets (FS) said:

Older article and maybe a dumb question, how do you then mount the foam seat to the car?

Mount?  When the foam expands it not only conforms to the driver, it conforms to the car as well.  In fact, you have to be careful that you can still remove it, usually with cardboard to keep the foam out of where it shouldn't be.  That's part of the reason why mine is a two piece seat.

triumph7 HalfDork
10/2/21 7:22 p.m.
Junghole said:

They left a step out: clean that foam crap out of the drivers hair after it has set up and he's been in it. 

Not a problem if you use a large enough trash bag and do multiple pours, build up the bottom first then the back, maybe using a second bag for the back.

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