If you are making a motorbike seat from scratch, here are a few pointers to keep in mind. If you intend to upholster and cover a custom bike seat, using some of these ideas will make it much easier and cheaper for us to trim and finish...
More often than not, most people who are making a "special" come to us with what is basically a flat piece of metal or fibreglass, cut to the shape they require and often with the rear part bent up a bit to provide some kind of a bum stop. They then ask us to make the foam and upholster it. This is fine if money is no object, or if the overall style and design of the bike warrants a flat base and that is definitely the shape that is wanted, however it's not the ideal base for a decent looking seat, as it will cause problems when foamed and covered, and takes longer and costs more to do. We can take a flat base, and come up with what is hopefully a nice result, but it can take longer and you may end up paying us more than if it had a little more planning.
A flat metal or fibreglass seat base suffers from the following issues:
1. There is no easy way to fix the seat cover to the base. Most modern seats have the cover stapled on, which is obviously not possible with a metal or fibreglass base. Glue on it's own is not ideal, as it will peel off with time, no matter how good the glue. In particular with this flat type of base, the cover will go down the side of the foam, and then turn a right angle to go under the base. This makes a particularly poor basis for fixing with glue, as the cover only goes round a right angle and every time you sit on the seat it will peel it a bit from the edge. A glued fixing is much stronger if you can fold the cover back on itself as it goes round the base, ie there needs to be some kind of a rim around the base..
2. The edge of the metal base will show by pushing through the seat cover, because it is harder than the foam above it. The flat edge of the base will also tend to cut through the seat cover if anything hits it from the side, even fairly gently. A plastic trim can be put on the edge of the base before the cover is put on, this will help but not eliminate this problem. Also, If the edge of the seat pan is further out than the frame rails of the bike, it may also cut into the rider's leg in the event of an accident. A rim round the base will solve this problem.
3. The normal way to fix the cover to a metal or fibreglass base is to use pop-rivets. BMW used this method on their bikes from the '80s, which had a metal, and sometimes a sort of fibreglass base. They also used thin strips of metal or plastic underneath the rivets, to further clamp the seat cover material. This method works, but it is time consuming. When putting on a seat cover using staples, you normally put the first ones in and adjust the cover to get it square etc, particularly if it is has a pattern on it, and then take the first few staples out and replace them as they are unlikely to be in exactly the right place. This is difficult to do with rivets, as you are unlikely to get the first few in exactly the right place first time, and they are more difficult to remove than staples.
4. If the base is flat, these rivets will be probably be visible from the side or back of the bike, and look bad.
These problems can be solved by making the seat base out of plastic or wood, so staples can be used, and if possible by putting some kind of a downturned rim around the base. Obviously if plastic or wood can be used, then the cover can be fixed using staples, which is quicker and easier. However it is still advisory to put a rim around the base, to hide the staples/rivets. If you look at any manufacturer's motorcycle seat base, either metal or plastic, it has a down turned rim around the edge. This allows the seat cover to be pulled back on itself before it is fixed on, which is much stronger however it is fixed, and helps stop glue from peeling. It also hides any fixings from view, and means that if the seat is hit from the side, the base will not cut through the seat cover, nor the rider's leg. Metal bases from older bikes normally had this rim, this allowed the cover to be clipped on in some way from below. There was then some kind of a trim fixed on to cover the clips. Alternatively the rim sometimes turned back on itself again, so these clips were out of sight. So if you are starting absolutely from scratch, where at all possible you should be aiming to produce a base with a rim round it, whatever the material used.
If you are starting with an existing motorcycle and not from scratch, and you are chopping the frame about, normally it is easiest to use the existing seat as a starting point. The existing seat base will fit the uncut parts of the frame exactly, and also the back of the original petrol tank if that is to be used, so the seat base can just be cut and altered to match the change in the bike frame. This normally saves a lot of time, and has excellent results.
Metal bases can easily be cut and welded, and it is actually just as easy to do this with plastic bases. Most are made of polypropylene (PP). This is easily cut and can often be bent into a new position or shape. We have a plastics' welder and PP filler rods here if you don't have access to them.