| 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. The base layer can be shaped to give you the desired cradling and seat angles. The top layer will hide minor imperfections in the base layer foam and provide some soft cushioning. The firmer, base layer foams shown below are rebond and closed-cell foams. The softer, top layer foam pictured below is what's called an open-cell, polyurethane foam. These two types of foams will work well for pretty much any type of seat modification.
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.
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FIRMER FOAMS USED PRIMARILY AS A BASE LAYER
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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. Purchase
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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, the 10 lb dumbell is only making a slight depression in the picture below.
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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.
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SOFTER FOAM PRIMARILY USED AS A TOP LAYER
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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.
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SOME OTHER FOAMS
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Gel. This is also called viscoelastic polymer gel. Being "viscoelastic" means that it has both viscous and elastic properties. Gel pad material is designed to absorb vibration and shock, and distribute weight more evenly across the points of body contact. Like memory foam (also a viscoelastic material), its firmness is affected by temperature. Gel is typically used in the medical industry (e.g., wheelchair seat pads, shoe inserts), though it has become popular as a cushioning insert in motorcycle seats as well.
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Memory foam. Memory foam is an open cell foam, often referred to as viscoelastic polyurethane foam. It tends to be firmer in cooler temperatures and softer in warmer environments. Memory foam is often used in sleep products (mattresses, pillows, etc.). My research suggests that memory foam is not as functional in motorcycle seats, given that it can bottom out under the pressure of a rider, and is sensitive to changes in temperature and humidity.
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