This instructable outlines the steps required to build an inexpensive wooden recumbent trike seat from 1/8" plywood. Any thin sheet of wood could be used but I found 3-ply birch plywood bent quite well. I purchased the plywood from my local Rockler Woodworking store. The finished seat is comfortable and very sturdy. I am 6' 1" and this seat fits me very well. If you are much taller you might want to make the seat a little longer.
If you want to see how I build the bamboo trike frame, check out my instructable:
Step 1: Making a Template
The first step is to make a template of the curved seat. I downloaded a PDF printout of this curved seat at http://ia700209.us.archive.org/9/items/RibSeat/Rib.PDF. I've included a copy of the PDF below. The seat outline prints on four sheets of paper and can then be taped together on your wood. I used a 1/4" plywood scrap for making the template. Transfer the outline to the wood and cut it out on your bandsaw or with a jigsaw. Make the cut just a bit bigger than the outline and sand it to the line using a sanding drum on a drill press (pic 2 & 3).
In the first photo, you can see the final shape of the template after cutting and sanding. Take your time making the template since it will be used to make all the other pieces on the router table with a flush trim bit.
Step 2: Using the Template With a Flush Trim Router Bit
To use the template to make an exact copy of the curved seat, you'll need a flush trim router bit. Because these pieces are narrow, it is easiest to use a router table, however, it can also be accomplished with a hand-held router. Take your template and draw the outline on a piece of wood. In this case, I'm using 3/4" birch plywood. Cut the shape out with a bandsaw or jigsaw about 1/16-1/8" outside the line. The end result is a rough cut piece slightly bigger than the template which will be trimmed smooth using a router. Double sided carpet tape is sufficient to hold the two pieces of wood together for trimming.
A flush trim router bit has a guide bearing which is exactly even with the cutline of the knives (pic 2). The height of the bit is set so that the bearing follows the edge of the template as the blades trim the other piece of wood. The end result is an exact copy of your template. When the trimming is complete, the 2 pieces of wood are simply pried apart and the carpet tape removed (pic 3).
Step 3: Building the Cradle
Make 2 copies of the template in 3/4" plywood and stick them together with carpet tape. Drill the four 1/2" holes in both pieces at the same time so they are in precisely the same location on each piece. Pry the pieces apart after the holes are drilled and round over the convex outer edge using a 3/8" round over bit. Cut four 7 1/4" long x 1/2" dowels. After sanding all the pieces, glue and assemble the cradle.
Step 4: The Seat Press
A press is needed to hold the wood in place while it dries. The concave surface of the original template is used to make the first bottom piece using 6" wide plywood. After making the first piece for the base, it is used as the template for trimming the remaining 3 pieces so they are all exactly the same.
In a similar fashion, the original template is used to cut one of the upper pieces. The only difference with the 4 upper pieces of the press is that they are 1/2" smaller than the original template. The reason for this is that the seat will be 1/2" thick and making the upper pieces smaller accommodates this thickness. So after trimming the first piece to match the original template, a line is marked 1/2" inside the edge and the 1/2" is trimmed off. The trimmed upper piece is then used as the template for the remaining 3 pieces of plywood.
The 2 halves of the press are glued together with 2" thick blocks between the plywood pieces. This makes the total width of the press 9" which is sufficient for a 10" wide seat.
Step 5: The "Steamer"
To get the plywood to bend we'll need steam which means we need a steamer box. Since I have an old kitchen stove in my woodshop I simply drilled out the top of a kettle and installed a hose end in the lid. After adding another hose end to the wooden box, I used a washing machine hose to connect the kettle and the box. This setup gives you tight fittings and is easy to disassemble and store.
The box was made out of plywood scraps and is approximately 6"x12"x48" to accommodate the large plywood pieces needed for the seat. Before putting the box together I drilled a series of holes in the 6" sides for metal rods so the steam would be able to circulate completely around the pieces. The box is open on the end furthest away from where the steam enters the box. Pieces to be steamed are placed in the box through this opening and the opening is covered by a towel during steaming.
With everything assembled, water is brought to a boil and steam is allowed to build up in the box. When steam is rolling out of the box, 2 pieces of 1/8" plywood are placed inside the box and the end covered with a towel. The plywood remains in the steamer for an hour. My experience was more than an hour caused the plywood layers to separate and an hour was long enough to obtain good flexibility.
Step 6: Bending the Tops
In figure 1 you can see the top pieces are 34" long, 10" wide at the bottom and 9" wide at the top. Allowing for a little error on all 4 sides, the pieces to be bent are cut to 36" x 11 1/4" x 1/8". After bending, the piece will be trimmed to it's actual size with 4" radius curves on the bottom end and 3" radius curves on the head end.
I was able to steam two pieces at a time in my steam box and after steaming for an hour they were removed and clamped in the press (pic 2 & 3). Each time I put pieces in the press I allowed them to dry 24 hours before unclamping. So with 4 pieces, I let the first 2 pieces dry before steaming 2 more pieces and adding them the next evening. Since each piece you add will be a bit smaller than the one underneath it, don't clamp the pieces individually. Simply remove the clamps and add the next pieces on top of the stack. I was impressed with how much force it took to clamp the press down even after steaming the wood. Even at an 1/8" thickness, the 3 ply wanted to remain flat especially in the curves of the bend. Takes some strong clamps and ample elbow grease to lock it all down.
Step 7: Gluing the Seat Together
After allowing the plywood to dry in the press for 24 hours, it's time to glue them into a single slab. Nothing fancy just slather the glue, stack up the pieces and put them back in the press to dry for 24 hours. Interestingly, it took all four pieces to really hold the shape of the seat. 2 or 3 pieces isn't enough to prevent the seat from springing out when removed from the press, but 4 pieces seems to be the required amount to hold the shape. Make sure you let the wood glue dry a full 24 hours before unclamping as there is a lot of kinetic energy in the wood.
In pics 2 & 3 you can see the cradle and seat top match each other nicely.
Step 8: Trim the Seat to Size
Draw the outline for the seat on brown craft paper and glue it to the plywood using spray adhesive. Using a bandsaw or jigsaw, trim the seat a bit wide of the line and then sand it even with a drum sander (pic 2). Sand the seat top and the cradle to at least 150 grit sandpaper if you are planning to paint and 220 for stain or varnish. The paper comes off very easily with a random orbital sander.
Before you upholstery the seat, you need to determine how you will attach it to your bike/trike. Although I don't have pictures of this step, I simply drilled holes for carriage bolts in the seat top. I locked the carriage bolts into the wood with a lock washer and nut on the back side of the seat. Depending on your bike/trike, you'll have to determine where the appropriate place is for the carriage bolts for your particular brackets. With the carriage bolts in place, you can upholster over the top of the heads and never know they are there. This is a much better approach over the long haul than putting screws into the back of the plywood seat.
Step 9: Putting on the Finishing Touches
After sanding and before screwing the cradle and seat together, prime and paint the dowels as they are difficult to spray paint under the seat. If you are painting with a brush then you don't necessarily need to do this. Alternatively, you could also have simply painted the dowels before cutting them for assembly.
Screw the cradle and seat top together using 1 1/2" or 2" #8 wood screws. Since you are screwing into a relatively long thin piece (cradle plywood), make sure you drill deep pilot holes to prevent splitting the plywood and wax the screws. Apply wood glue to the flat surface of the cradle ribs on the edge adjoining the seat and insert screws. I used 5 screws on each side or 10 screws total.
With the seat assembled, prime the surface and apply a few coats of paint. I used spray paint which made it easy to cover. Sand in between coats with 220 paper to get a smooth finish.
Step 10: Upholstering the Seat
If you know how to upholster, give it a shot. My local upholstery shop did it for me for $40 including the material. For that price it really wasn't worth me doing the upholstery even though this is a pretty simply job. Check around and you might find a similar price.
The most comfortable seat for most people is a 1" layer of closed foam which is very firm. A cheap source is the foam camping mats at Walmart for placing under your sleeping bag. A roll the size of a sleeping bag is around $5. The camping foam is 1/2" thick so you'll want to double it up. Use spray adhesive to adhere the foam pieces to the seat and also to each other. Don't make the mistake of using 2" green foam because it compresses to about 1/8" thick when you set on it. Not comfortable.
After that you're on your own, however it's pretty simple (in theory). Pull the vinyl tight and staple it to the back of the seat. The trick is keeping everything even and snug across the entire seat. Honestly, if you can build the seat, this part should be a snap.....unless you can find someone to do it fast and cheap;) With the upholstery finished, bolt your seat down and go for a spin.
Thanks for taking the time to read this instructable. I look forward to hearing your suggestions on how this seat could be even better! Happy trails!
st_indigo made it!