How to reshape a motorcycle seat
Using a Kawasaki Ninja 250 as a practical example
Posted July 12, 2004
See the story if you want to know about my process. This page is intended to be as informative as possible without all the distracting backstory.
If for whatever reason you've decided your motorcycle seat needs to be reshaped, but you can't afford to send it off to one of the custom seatmakers, then you're in the right place. I'll attempt to describe a process you can use to make a seat shaped just for you, which is comfortable and looks good into the bargain.
In my example, I was adding height and moving the seating position back, but these procedures work well for any additive change you'd like to make. Note that I don't cover making a seat shorter at all in this page, you'll have to find that information somewhere else.
Disclaimer on the pictures: the pictures that illustrate this article are from my attempt at reshaping a seat, which included a lot of learning curve (from which you will be benefiting shortly). Please look at the pictures as being interesting, without necessarily illustrating the correct way to do things.
Tools and Materials
You'll need the following materials (about $70 total, excluding the seat):
You'll need the following tools:
Note that the electric carving knife should be one you're willing to completely donate to the project. It will probably get goo on it that will preclude its ever being used for turkey again.
The process of customizing your seat shape falls into three broad categories: preparing the seat, shaping it, and covering it. I'll deal with each of these categories in turn.
Preparing the Seat
Using the screwdriver, remove the staples from the seat. Remove and discard the old cover (it'll never fit on the new seat, unless you're only removing foam).
Lay down your foam on top of the seat without any glue, to get an idea how much of it you'll really need. Mark off the size with a marking pen. This will be your cutting guide once the foam is away from the seat. Mark the lines with generous clearance, since it'll be cleaner and easier to cut away foam than add it. Make a note of whether the original seat foam is adhered in any way to the seat pan. If it's not, you probably want to pull it off and use the foam adhesive to stick it to the pan.
Using the electric knife, cut along the lines you just marked. No need for perfection at this point, since you're going to be doing a lot of trimming once the foam's on the seat. Note that the knife "likes" to cut at a certain speed that may well be too slow for you. This foam is very dense, and it's possible to overload a cheap carving knife if you're not careful. Use patience and cut at the knife's pace.
Note: DO NOT spray foam adhesive near your motorcycle! If any gets on your bike, chances are you'll never ever get it off.
Working somewhere that can afford to get messy (ie, lay down a dropcloth if necessary), spray down the top of the seat with foam adhesive. Only spray the area where you're planning on building up the seat. There's no need to mask off the "no spray" areas unless you're really worried about your ability to aim a spray can. Let the adhesive sit for at least 30 seconds before attempting to stick anything to it. Read the directions on your adhesive to get actual curing times.
Spray down the foam you just cut and let it sit for 30 seconds minimum as well. Once both the seat and the new foam have cured for their allotted time, carefully lay the new foam onto the seat. Bow the foam into a U shape, so only a little bit contacts the seat at a time. This will allow you to work it on carefully. Try to avoid trapping any air bubbles, and make sure it sticks everywhere it's supposed to. Apply firm pressure over the whole area to ensure it's sticking tightly.
Cutting the Shape
You know how old tractors have those big, butt-cheek shaped seats, stamped out of steel? That's the shape you're aiming for. You want something that will put even pressure everywhere your body contacts the seat.
First, (making sure the glue is dry) put the seat on the bike, and sit down on it where you want the seating position to be. It may be helpful to have an assistant for this, but use the felt marker to mark on the foam where the back of your butt is. You can use that as a guide for where to cut down the foam.
Take the seat back off the bike, and carefully mark a centerline down the length of the seat. Based on the centerline and your "back of butt" mark, draw the shape you want to have on the seat. Make sure the shape you've drawn is even by using the ruler, and measuring from the centerline. You'll use these marks to cut the shape with the knife.
If your proposed seat shape is wider than the stock seat was, remember to leave support for it. You can't just leave a thin flap of foam as an outrigger and expect to get any support from it. Build everything up so it's solidly supported. It's also helpful to have foam overlying the bottom edge of the seat, so that when you're done, you don't have to make up the missed area.
Grab the electric carving knife, and cut out your seat shape. You don't have to get it perfect, but aim for the "not cutting out enough" side of perfection. Use the knife to carve out the big swaths of foam and get the shape generally right. Don't worry right now about getting the shape perfect.
Once the seat is in roughly the right shape, put it back on your motorcycle, and give it a test sit. It won't feel right, but make sure you're on the right track. Note that most people can't tell what a seat feels like when riding until they're actually riding. Unless you know you're one of the exceptions, it's worth taking the seat out for a ride every time you make a change, and this is one of those times.
Once you know you're on the right track, grab your grinding disc, and "prepare" it for seat foam duty: locate a rock, or piece of concrete or something hard that you don't care about scuffing up. Grind the wheel against it until the wheel is no longer sharp to the touch. If the grinding disc is sharp, it will probably take off foam faster than you want it to. The advice I received on grinding discs was to have three of them, each 40 grit: the first was just barely ground against the concrete, just enough to make it slightly less than sharp. The next was ground for a minute or two, so it was still raised and sandy feeling, but definitely rounded. The third was ground down so it was just sort of nubbly with almost no sharpness at all. For myself, I ended up just going with the second disc, and it worked fairly well. (The reason for the three were to have one to take out foam fast, one to take out foam slowly, and one to smooth the surface.)
Using the appropriately-desharpened grinding disc, grind down the seat until it's in the right shape. The right shape will be a surprisingly deep concave shape (but remember to test, your butt and my butt are different shapes!), with the sides sloping up to form a bowl in the back and sides. The front will slope down where your legs come out, so you can get your feet on the ground comfortably. You'll discover in this process why you want either compressed air or a good broom handy -- those bits of foam get everywhere!
Grind and test. Remember to suit up and actually ride around the block on the seat to locate any problems. Just sitting on the seat is not the same as riding on it, for the vast majority of riders. Also remember to do your test rides in the same riding gear you're likely to be wearing when you "actually" ride the bike. Whatever riding pants you have will make a difference in how the seat fits. Repeat the grinding and riding process until you have a shape you like (this part took me over a month).
This process is extremely forgiving of mistakes. If you grind too far, grab one of the big scraps you cut off with the knife, spray down the foam and the seat with adhesive, and lay up some more foam to work with. You can do this as many times as you need to, up to the limit of your patience, your budget, or your time.
The only downside that I found to this kind of mistake fixing is that when grinding through the glue layer, it would gum up the grinding wheel. Based on this, it's best to shape the seat in one pass if at all possible.
If you end up with a shape that's just completely not working for you, it might be best to lay on a fresh new layer of foam and start over. I had to do this twice before I had a shape I liked.
Covering the Seat
The final part of the process is covering the seat. Get your headliner material and vinyl out. Throw the vinyl into the dryer to get it warmed up (warm vinyl is easier to stretch).
Lay the headliner material over your built-up seat, and trim it so you've got a margin of at least 3" all the way around (this makes it easier to work with, but you can get by with less if you have to). You'll be applying the headliner with the fabric side up.
Spray down the butt of the seat with adhesive. Spray down the matching section of the headliner with adhesive. Once they've cured, carefully stick them together. Make sure the headliner is lined up enough that you'll have coverage all the way around on the seat. Carefully press the headliner onto the seat, making sure you don't have any creases or air pockets. Working out from the butt-section of the seat, spray down the seat and headliner with adhesive, always leaving curing time, and work the headliner onto the seat so it's smooth and crease-free.
Once the headliner is fully adhered to the seat, trim it with scissors so that it doesn't wrap over any of the edges of the seat pan.
Next, prepare the vinyl. Lay your vinyl over the seat, and once again, ensure that you have at least 3" (7 cm) overlap on all edges. With vinyl, I recommend having more than 3", probably more like 5" (12 cm). This gives you something to grab onto when stretching the vinyl into place. Trim off gross excess fabric.
I recommend using adhesive to stick down the vinyl wherever it will go over a concave (inward curve) shape. The reason for this is that, as you stretch it, the vinyl will naturally pull taut over any concavities, making the covering job look amateurish (just because we're amateurs doesn't mean we have to do amateur-quality work). Use the same method with the vinyl that you used with the headliner -- spray down the areas you want to stick one at a time, and work them carefully to avoid wrinkles, sags and air pockets.
Note that you should only glue down the concave sections. The vinyl will look better if it's loose over the convex sections, and can stretch naturally.
Once the vinyl is glued on, it's time to start stretching and stapling. I recommend tacking down the edges at four points: front, rear, and each side in the middle, first. This allows you to ensure that each quarter of the fabric will be stretched evenly. Pull the vinyl over the edge of the seat pan, and staple it in place just like the original seat cover was stapled in. You'll want the staples to be closely spaced, so that no one staple is holding too much tension.
Voila, the Finished Product
Put your seat on your bike and have a celebratory ride around the block. If you've done it all right, you should have a seat that addresses whatever problems caused you to read this in the first place. In my case, my seating position is further back and higher up, both of which make the seat-to-peg distance much more bearable on this small bike.
Created by Ian Johnston. Questions? Please mail me at reaper at obairlann dot net.