There are a huge variety of foams, as well as foam manufacturing and fabrication processes. There are so many different uses for foam, each having specific requirements. This site makes no attempt to explain the science behind foam (there is a lot). The goal here is to provide a simple, very basic understanding of some key characteristics of foam so that you can make your seat more comfortable. That's pretty much it.
Most foam used in motorcycle seats is open-cell (polyurethene) foam or closed-cell (polyethelene) foam. The fourth bullet below describes the basic differences between open- and closed-cell foams. If you want more detailed info on foam characteristics, search online (e.g., American Chemical Council, Foam Factory have some good explanations). And at the bottom of this page are a bunch of videos related to making foam that you might find interesting.
- Cell structure. There are two basic types of cell structures—open-cell and closed-cell. Open-cell foam is composed of tiny cells of foam that are not completely closed. Since these tiny cells are broken, air fills the little open spaces. As you might expect, this typically gives open cell foams a softer feeling. It also allows it to absorb water. Closed-cell foam, on the other hand, is comprised of tiny foam cells that are "closed" and not broken. These closed cells are filled with a gas that helps the foam rise and expand. The pics show the difference, with the closed-cell foam having its cell walls intact.
- Density or weight. In general, the denser the foam, the heavier it is and the longer the foam will last and keep its shape. Density is considered to be the most important indicator of foam quality. Density is not related to the firmness of the foam (see below). That is, you can have a dense foam that's very soft and will feel the same for years; or you can use light, very firm foam that breaks down quickly. Foam density/weight is measured in "pounds per cubic foot." Picture a cubic foot of foam as a square box, 12" long and 12" wide and 12" high, filled with foam. The weight of that box of foam is measured as pounds per cubic foot. The denser the foam, the more that box will weigh. Thus, when you see foam that has a density of 1.8 lbs, it means that a 12" long by 12" wide by 12" thick piece of that foam will tip the scales at 1.8 lbs.
- Firmness (ILD). Remember that density is not related to the firmness or softness of foam. The Indentation Load Deflection (ILD) is a rating that describes the firmness (or softness) of a piece of foam. Basically, the ILD is calculated by measuring the pounds of force it takes to compress the thickness of a 4-inch piece of foam by 25%. Thus, the lower the number, the softer the foam; and the higher the number, the firmer the foam.
- Support factor. Essentially, support factor is another measure of a foam's ability to support weight. You don't want soft, cushy foam bottoming out on the seat pan. Good supporting foam also more effectively distributes the weight of the rider. The support factor is also known as the "compression modulus." The support factor for a piece of foam typically ranges from 1.8 to 3.0. The higher the number, the greater the foam's ability to provide support.
- Total vertical motion (TVM). TVM, also known as "ride," and is how far down a person sinks into the foam when they sit on a seat. A seat with an insufficient support factor results in too much ride, causing you to sit too deeply into the seat. This causes a rider to slouch more, and makes weight shifting on twisties more difficult. Determining the optimum amount of TVM or ride for your seat is influenced by the type of foams used, type of riding, the construction of the existing seat pan, handlebar height, and the existing seat angles of the bike.
Where to buy different types of foam? Here are a few ideas:
- Try your local auto or marine upholsterer. See if they have extra pieces of foam they'd be willing to sell to you. Depending on the type of work they do, they should have a variety of foams.
- If you have a local foam manufacturer or distributor, that's even better. For example, I used to live in Richmond, VA and Foam To Size is a local distributor and a great source of almost any type of foam (and tools) that you would use. If you live in Central Virginia, give them a visit.
- If you need to get creative, you can find useful foams in exercise mats, flotation devices, and other things you can find at Walmart, sporting good stores, and other retailers. The foam that I've found at big-box sewing and craft stores, however, is typically way too soft for use on motorcycle seats. It might be worth checking out though.
- I've had good success on the Internet with FoamByMail.com. Though I haven't used them, FoamOrder.com seems like they stock a good variety of foam. And, of course, McMaster-Carr (look in the "Raw Materials" section) seems to have just about everything but endangered species.
- Lastly, check out the Google Ads on this website. They're inserted by Google, are context-specific, and can turn up some good sources.
These sources may require you to buy larger pieces of foam than you need for your specific job. If so, consider going in with your friends on the purchase.
Generally speaking, you'll want to use a firmer, more supportive foam as a base layer, and a softer type of foam as a top layer. Shape the base layer to give you the desired cradling and seat angles. Use the top layer to hide minor imperfections in the base layer foam and to give you some soft cushioning.
Gel and memory foam are also pictured below since they are often mentioned as alternatives to giving seats a softer feel. Although there are different perspectives on this, memory foam is probably not as functional and cost-effective as good quality, open-cell foam for use in motorcycle seats.
Firmer Foams Used Primarily As A Base Layer
Rebond Foam. Open cell foam that is made from shredded pieces of foam, which are then glued together, forming one solid piece of foam. Rebond foam is very dense and provides a solid, firm support base. Note how little the 10 lb dumbell depresses the rebond in the picture below. Rebond foam is typically about 7 lbs density with an ILD of at about 70 lbs.
Closed-cell Foam. This foam is smooth, high support, light-weight, and does not absorb water. Types of closed cell foams that work for seat modifications include closed-cell polyethylene and minicell. As with the open-cell rebond, you can see the 10 lb dumbell is only making a slight depression in the picture below.
Neoprene Foam. Neoprene is a closed-cell, sponge rubber foam that also can be put to a variety of uses, from base-layer padding to thinner smoothing foam. Neoprene is a very versatile, firm, flexible, and durable foam. It provides outstanding thermal and moisture insulation, and is resistant to ozone, sunlight, oxidation, many petroleum derivatives and chemicals.
Softer Foam Primarily Used As Top Layer
Polyurethane Cushioning Foam. Open cell foam that is flexible and comes in a variety of different densities and firmness. This is softer (though still relatively firm) foam that is best used as a top, softening layer over a firm foundation foam, such as rebond. Thinner pieces of it (e.g., 1/2" thick) are also good for smoothing the top of the seat surface to make sure that it's nice and smooth once the cover is reinstalled. High quality polyurethane foam will be at least 2 lbs density with an ILD of at least 40 lbs.