Foam involves knowledge of complex chemistry and engineering. There are a huge variety of foams, and foam manufacturing and fabrication processes, for different customer uses. This site makes no attempt to explain the science behind foam. The goal here is to provide a simple, very basic understanding of some key characteristics of foam so that you can more effectively improve the comfort of your seat.
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. The differences between polyurethane and polyethylene foams essentially lies in their chemical properties and how they're manufactured. Both these types of foam are used as cushioning for a wide variety of consumer and commercial products, including furniture, carpet cushion, transportation applications, bedding, packaging, and other uses.
- 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 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 greater its pounds per cubic foot. So, foam that is said to have a density of 1.8 lbs means that a 12" long by 12" wide by 12" thick piece 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 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. Good support from foam means that the foam sufficiently holds up the weight of a person. It doesn't "bottom out" or compress to a point where the rider's buttocks is actually against the seat pan. A 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.
- 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. These tiny cells are broken and air fills the "open" space inside the material. This gives the foam a softer feeling (than closed-cell foam). It also allows it to absorb water. Closed-cell foam, on the other hand, is comprised of tiny foam cells that are "closed" (i.e., not broken) and packed together. These cells are filled with a gas that helps the foam rise and expand. The pictures below show this difference. Note the cells on the right in the closed-cell example have their cell walls intact (at least that is what it's supposed to show).
- Total vertican 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 will results in too much ride, causing the rider's buttocks to sit too deeply into the seat. This forces the rider into an excessive slouch position, and makes weight shifting on twisties 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.