Of course, in my brilliance, I forgot that I didn't have an office chair that I could cannibalize.
So first I had to find one. I went to a local used business furniture store and asked if they had any damaged chairs. The best ones have a square base to attach to the bottom cushion. The one I wound up getting was fairly close to that ideal.
While I was there, I bought another chair cheap and got a free loveseat to boot. A huge coup for someone who otherwise has trouble talking to people.
Admittedly, it helps to live in a large metro area where lots of businesses need and unload furniture. But you may be able to use a castoff from friends or family whose cushions have flattened out or become otherwise unusable.
You might just now think to yourself, "Dude, with all that junque in your garage, why do you NEED a rolling chair?" But as an instructabilista, you already know the answer...which may go something like, "If I only worried about what I need, I'd live in a cardboard box with an inflatable doll (from American Yakuza, an under-appreciated film with Viggo Mortensen.)"
Step 1: The Mating Ritual -- Choose Your Material
A perfect match between the chair base and the racing seat has absolutely no chance of occurring, or if it did, would have wasted a piece of luck better spent on winning a lottery.
So assume you'll need to make some kind of adapter.
What to use? The average Joe or Jane has three choices --
1) Wood -- easy to cut and drill but you'll need at least 3/8" (10mm) thick plywood or maybe 1x2 boards of something not too soft. Keep in mind the grain of the wood in relation to your adapter design.
2) Aluminum (Aluminium) -- almost as easy to cut and drill as wood but you'll likely need 1/4" (6.5mm) thickness minimum.
3) STEEL -- takes a bit more work to cut and drill but even 1/8" (3mm) thickness will work in most cases. 3/16" thick steel will handle just about anything.
Note that most car frames are made with 1/4" (6mm) mild steel, and they support static loads of over 2000 lbs (900 kg) no problemo.
Other materials -- plastics, composites, or exotic metals such as titanium -- if you have these, you probably don't need to read this instructable.
Step 2: The Mating Ritual -- Line Up the Pieces
In the end, two pieces of 12" x 2" x 1/4" (30cm x 5cm x 6.5mm) mild steel served as suitable pieces. A nice, big, square plate (about the size of a text book) would seem like a better solution, but in this case would not do much more because --
1) The base has plenty of strength where it attaches to the chair. It will not spread under load until maybe 300 lbs (140kg.)
2) The seat holes are basically nuts embedded in plastic so any failure will most likely manifest itself by pulling them straight out and not from the side.
Step 3: Mark the Pieces
Then I drew lines from the center of each of the three seat holes until they intersected the main line. I will drill through these intersections.
Did the same thing -- making a mirror image -- for the other side.
Step 4: Drill the Holes
1) Select your drill bit to be the same size or a little (not much) bigger than the bolt that will go through the hole.
2) A drill press is best, but a hand drill will do in a pinch.
3) Use clamps or otherwise secure the piece. One whack on the hand from a spinning piece will provide sufficient incentive to follow this rule.
4) WEAR EYE PROTECTION. Gloves too. Metal shavings hurt.
5) Lubricate the drill bit. Cutting oil is best but even canned lube works.
6) Clean up your shavings when you finish.
The more carefully you mark (scribing works best) and the more accurately you drill your piece the better your final result will turn out. But you might have to elongate a drill hole to make it match up with the seat. Here in the Southern USA, we refer to hole elongation as "hogging".
Step 5: Drill the Holes -- Hogging
I used a marker (the green on the right side of the hole) to help me remember which direction I need to hog.
Use a file to remove the metal that clings to the holes -- it makes for a safer-to-handle piece.
Repeat for the other side.
Step 6: Attach Plates to Seat and Mark for Base Holes
Set the seat on the base and get the most even distribution of mass possible -- both side-to-side and front-to-back. Err towards the front if like me you don't want to measure with ruler or tape.
Mark the holes for the base, remove the plates for drilling, re-attach for checking and hogging if needed.
Step 7: Final Assembly
If you used bare steel, consider slapping a coat of paint on it for a few years' added protection.
My chair also needed the pivot pin knocked back into place and secured with a c-clip but the artistic part was finished.
Once you assemble your piece, sit back, pop a cold one, and sip or chug to a job well done.