Seat building part III

So the next step in building the seat was to mark out the centerline. I had marked a rough centerline on the mold, but I wasn't too sure about it. So I pulled the set glass form off the mold and put it on the sling seat form and sat in it. I then had my lovely wife mark my spine location directly on the glass. I then put it back on the mold and used the string trick to draw the line.

From that line I marked out the edges of the ribs by marking out lines parallel to the centerline. I think the ribs are something like 6cm from center but I can't remember and I'm not running downstairs to measure right now. One of the things about my design is that the seat stay mounts have to go on the ribs rather than between them. To accommodate this I am going to leave flat sections on the ribs at those spots and the bulk them up with several layers of glass.

To create the ribs I ended up using blocks of pink foam cut and spray-mounted to the back of the seat. I started by ripping strips that were 2" wide by 1.5" tall. I then applied them by mitering them by hand and mounting them 1 x 1.




The next step in shaping them was to use a compass to scribe a line on both sides 1" high. I then used a hacksaw blade to trim the height to this line. I then shaped the foam as best I could using a surform plane and sand paper to a nice round cross section with tapered ends. The spray mount didn't work real well for this BTW, the blocks kept falling off and generally being a pain. If I do this again, I'm going to use epoxy or possibly just spray foam.





I then mixed up some fairing compound using glass bubbles and epoxy and used that to glue down any particularly recalcitrant blocks, fillet the join to the seat, and fill any gaps between blocks. This also didn't work as well as I'd have liked. It was difficult to put it where I wanted it and get it smooth. In the end its OK but not great. Also something I'd do differently next time.





I came back the next day when the filler set up and tried to smooth out the nasty bits using the surform, files, sandpaper, and various dremel attachments. In the end I found the wire wheel on the dremel to give the best control, but it was a pain and ate up the foam if I wasn't really careful. I did mess up a few spots, so I used spray foam to back fill them and then cut and smoothed it out. This worked reasonably well. The spray foam isn't totally consistent throughout, so there are some bubbly bits, but it's good enough to keep the glass in place. Finally a technique that shows some promise!




While the spray foam was setting, prior to trimming it, I cut up some fiberglass. I cut 4.5" wide strips to cover the ribs from a lighter glass (5oz I think). I also cut them on a 45deg bias to allow it to drape better. I cut a total of 6 strips, but I was only able to apply one layer tonight because I ran out of epoxy. Darn. I ordered some this morning expecting this to be a problem, but I won't have it till Thurs or Friday, so I'm stuck till then. Still, things are progressing. I discovered that the bottom of my epoxy container contains a lot of semi-hard gunk. I'm a little worried that the stuff might not be 100% good, so it's probably good that I'm getting new stuff. It seems to have hardened ok so I'm keeping my fingers crossed and we'll see how it goes.

Seat building part II

Once the plaster set up for about 24 hours on the seat, I scribed a piece of plywood with the shape and made a set of plywood ribs/base. These both stabilize the plaster and give me an easy way to set it on a bench to work on it. To attach them I used two layers of burlap strips soaked in plaster, and just mushed them along the joint.





After that I took the whole assembly downstairs (much to the relief of my sainted wife) and flipped it onto the workbench. I added 3 or 4 (I can't remember, I think 3) more layers of burlap and plaster onto the back of the seat. I am using the mold as a male mold and so I need to account for some amount of seat pad thickness. 6 layers of burlap plus the finishing coats gives me about 1/2" of thickness. I figure this is a reasonable number once you factor in foam compression on the pad. In any case it should get me close enough it won't matter.

After the plaster set up overnight, I went back and started adding layers of drywall compound. I was both smoothing and evening the seat out. There were some odd low-spots and a slight twist to the whole thing which I attempted to correct with successive coats of the compound. The drywall compound is MUCH easier to work with than the plaster for smoothing coats. It stays nice and easy to work for a long time and lets you work at a relaxed pace. Unlike the plaster which is rather frantic. It's actually a very enjoyable process. I was in the throes of a nasty sinus infection while doing most of the drywall compound and I found it quite therapeutic to head down there and sculpt for an hour or so. The other nice thing about the DC over plaster is how easily it sands. I was able to get a very smooth finish very quickly. The drawback to DC is how slowly it sets. The plaster sets up in about an hour. The DC overnight. So it took several days to finish this step.







The last step before applying the glass was to mark a centerline. This was rather harder than I thought it would be. I didn't have the foresight to mark my spine location while the mold was still wet and I was actually sitting in the thing. So instead I just used a piece of string weighted at the ends and laid over the mold and moved it around until it visually looked like it was centered. It's not perfect, but it should be fine.

To apply the glass I started by measuring and cutting out pieces that were about 38"x13". The finished seat length will be somewhere around 35" and the width about 10", so this gives me a bit of wiggle room. I then covered the mold with plastic wrap. In retrospect I should have waxed it and used it directly since the plastic wrap slides around a bunch and is a bit of a pain, but that would have taken a lot longer. For the first layer of glass I weighed it and found it was about 100g. So I mixed 95g of resin with 25g of hardener (100:27 mix for my particular epoxy). I then laid out the glass and carefully applied the resin by pouring it on and brushing with a disposable brush. I applied each successive layer (total of 4) by applying it directly over the previous layer while wet, pressing it in by hand, and brushing more resin on to soak. In all I mixed a total of about 310g of resin and applied about 385g of glass. That's actually a little less resin than I'd ordinarily expect, but it looks good so I'm not concerned about it.







The next step is to build the stiffening ribs on the back of the seat. I'm still figuring out how to do this, so stay tuned.

Making a new seat

So I've decided that I need to replace my seat. The shape just isn't in agreement with my anatomy. I think I need a somewhat more laid back chest position with a more gradual transition into the shoulder area. This will mean more work holding my head up, but I prefer that.

I decided to build the seat from fiberglass. I carefully weighed the options between carbon fiber, glass, and wood, and decided on glass for the simple reason that I already have about 3 yards of the stuff in the basement and thus won't need to order anything.

My general approach is to make a simple wooden frame and stretch fabric over it, then make a plaster mold of me sitting in the fabric seat. This should recreate a comfortable shape for me while allowing a lot of control over the position.

I started by using an adjustable seat mock up I made some time ago to decide on an approximate shape. I then pulled this shape off it by scribing a piece of plywood, and made a simple form from plywood to hold tensioned fabric. I added more lumbar curve to the form from what I had on the mock up because I expected that to collapse under weight more than other places on the seat. I also scribed a line about an inch smaller than the line I took from the hard seat to account for fabric stretch.

I covered the seat with several layers of scrap fabric until I got the right amount of strength and stiffness. Most of the fabric is either muslin or some other lightweight non-stretchy cotton fabric. A heavier fabric like canvas would be preferable and would require fewer layers, but this is all stuff I had around the house so it didn't cost me anything.

I had originally intended to saturate this fabric directly with plaster, but a test showed this wouldn't work well. So instead I covered it with plastic and saturated a few layers of burlap. Much furious mixing and spreading ensued, but with the help of my lovely wife, I was able to get everything in place, another layer of plastic over the plaster, and sit in it before the plaster set up too much. I hung out and watched some TV for an hour while sitting in my seat. And then carefully pulled myself up by using a sling I had pre-rigged from the ceiling (to avoid cracking the plaster) .

Tonight I'll add some stiffening ribs to the plaster and then tomorrow pull it from the "seat". After that I'll add some more layers of burlap to build up the thickness to account for a seat pad, and also give it more stability. Then pull the glass mold from it.