Physics question regarding a spring?

Question

This question is for the physics gurus, or just anyone who understand general physics better than I.
Here is a link to the diagram I will be refering to
http://i1.pinkbike.com/photo/1311/pbpic1311021.jpg
The 50 lbs to compress bit is very flexible, basically im looking for a range between 25-50lbs, no more than or less than that range however. This is a project I am having to delay because of not having the correct spring - and I know one exists! I just dont feel like quessing and checking without having a starting point.

physical limitations for the spring:
-Must fit in the 1" chamber, preferably a sixteenth or so smaller for squirm room.
-must fit around a 7/16" rod
-Compressed length must be around 1"
-Extended length can be w/e is neccesary to meet the requirements.

Sorry for the detailed post, as you can tell I pretty much know what i'm doing, just havent made it all the way through general physics yet and my mechanical skills are ahead of my physics skills

edit: Spring material is preferably steel to keep cost down. If steel cannot meet the demands of this situation, other metals are acceptable.

Answer 1

About all I can tell you is that the spring wil have a stiffness of ~100 lb/inch and its length will be about 1.5 inches plus its fully compressed length. The pressure from the piston is 130*pi*d^2/4 = 102 lb. If the spring compresses 0.5 inch with 50 lb more added, then it has already compressed 1 inch with only the piston force. (I'm ignoring the small air pressure change as the piston moves that last half inch.) So you want a spring with a stiffness of ~100 lb/inch, with the length I mentioned.
If you settle on 25 lb for the half-inch compression, the spring has stiffness = 50 lb/inch and a full length of 2.5 inches plus compressed length.
I think you may end up at a hardware store trying out all the compression springs you can find. You'll probably end up with spring steel, since beryllium and spring bronze are somewhat exotic, and toxic beryllium requires special handling.