jjflyer, Your plan is a great one, but I also suggest that you also buy an already built Avid so you have one to fly now. If you wait to build your C model, you will miss a few great years of flying. Even a complete one without an engine will get you in the air in just a few months. In any case, welcome aboard and good luck!
The FAA departure stall maneuver is designed to help train us in this scenario, to recognize the signs, fix the problem and fly the aircraft out of the danger area. The FAA learned the hard way, after dozens of stall accidents, that the "normal" way to use the controls ("stick back, houses smaller....") is simply wrong. I have bought a copy of "Stick and Rudder" for each of my students back in the day, the author really describes this all so well.
I used a double design, by having a second shorter spring bent to the same angle, but about 2" shorter than the main spring, and placed on top of the first. I clamped the two with a small sandwich to hold them in place, works fine. see the shot below
Simplicity is essential. Rotax Rick says to use a big plastic tub, lots of pillows, and ratchet tie-downs. I did this to send my 582, and he used it to send back the 670, and it was perfect. He sends and receives dozens of engines this way. I used Fedex Ground, cost about $250 each way across the country (Utah to Florida), with declared value of $5000 Zero bother, too. Here is Rick's discussion: https://rotaxrick.wordpress.com/shipping/
The Rotax Rick 670 is a viable alternative, and a drop in. I uses the 582 mount and fits like a glove, produces 91 HP and starts and runs great. It costs about $3750 plus your old 582 for a zero time 670. The whole story: http://avidfoxflyers.com/index.php?/topic/5734-new-rotax-670-install/
109jb, I think your math looks great and we agree! I think an easy way to check the poly disks is to just put the diagonal strut together, and put 2400 lbs of tension on it to see the deflection. A simple rig with a hydraulic cylinder and a strong mount should do the trick. Or you could pick up a VW beetle with it!
I did some trigonometry on the gear dimensions and angles in your sketch, which seems to agree with the basic dimensions of my Highwing llc (Fitt) gear, shown below. The force on the diagonal rod with the disks on it is about 2400 lbs of stretch on the rod during a reasonably hard landing (3 g's, about 8 feet per second sink rate). That means the poly pucks will see a compression of about 2400 lbs during that event. For my springs on the Highwing gear, that compresses the steel by about 1.5", and at 3.8 G's, maybe 12 feet per second, it compresses the spring completely. It might be great to see what the compression on the poly pucks is at that load, 2400 lbs, just as a comparison. the durometer doesn't measure spring rate at all, it is a measure of how the material resists being dented by a sharp pressure. My guess is the stuff is very stiff and would work well as a stiff spring.
Looks like great gear! Here is my measurement of the Lowell Fitt (Highwing LLC) gear that I bought, installed and use on my Avid Mark IV. Very similar geometry. The gear is fantastic, tracks straight on rollout, smooth touchdowns. My calculations say the gear is good for about 12 ft/sec landing sink speed, much more than Part 23.