With what we have learned since I introduced this thread, I thought I would put all the ideas together in a drawing, and add one thing I thought about a week or so ago.... Instead of having the thimble extend down over the OD of the larger part of the poppet, I wondered about shortening it and fitting a spring over the outside, instead of having the spring inside the balance chamber where it increased the volume.... Note that this has NOT yet been tried, but it should work just fine.... I also incorporated the idea of a shorter balance chamber, with enough depth to safely allow the poppet to travel 1/2 the throat diameter.... In addition, I shortened the stem, since it is no longer needed to create a spring seat on the front of the poppet, and drilled a hole in front of the stem, slightly larger than the vent diameter, to keep the resistance to the airflow filling the balance chamber, and the chamber volume itself, to a minimum.... This stepped hole helps avoid too little room between the OD of the stem and the bottom of the O-ring groove.... Here is the drawing with all those changes.... It is "generic" in that the proportions are basically correct, but no dimensions are shown....I feel that the range of balance chamber diameters that will work OK are from 60% to 72% of the diameter of the outer sealing edge of the poppet.... With all the changes we have made, you might even get to 75%.... I would still use 71% as my goal, which means that the cracking force of the valve is reduced by half.... The back of the poppet is undercut slightly (about 5 deg.) to insure a narrow seal, right at the outside edge.... Here is what the cracking force is, as a percentage of what it would be without the balancing chamber, relative to the diameter of the balance chamber as a percentage of the seat diameter....60% balance chamber diameter reduces the cracking force by 36%66% balance chamber diameter reduces the dracking force by 44%71% balance chamber diameter reduces the cracking force by 50%75% balance chamber diameter reduces the cracking force by 56%Here are some examples for the dimensions you might like to try for a valve for a full power .257 cal. bullet shooter.... (could also be used for smaller PCPs)….Throat ID 0.312"Stem OD 0.125" (using 5-40 threads on the forward 1/4" to provide shear strength)Vent Hole ID 0.052" (hole forward of stem 0.062" ID)Exhaust Port ID 0.257"Poppet OD at Valve Seat 0.375"Balance Chamber ID 0.250"Balance Chamber Length (ahead of poppet) 0.170"Poppet Travel Before O-ring Bumper Contact 0.156"Stem Protrusion Behind Valve Body 0.150"The length of the larger part of the poppet would be about 0.25"The length of the smaller part of the poppet would be about 0.50"The front OD of the thimble is 0.250" and threaded inside 8-32The overall length of the thimble would be about 1.00"The center of the perforated mounting wheel is drilled 0.187" and counterbored 0.281" for the front of the thimbleThis assures a loose fit between the thimble, thimble screw, and the supporting wheel to allow some radial movement for self-alignmentThe thimble is secured to the perforated wheel with an 8-32 screw cut to length to give minimum end float when tightenedThe OD of the thimble, the poppet at the spring seat, and the spring would be about 0.42" minimum (matching the spring OD)A good spring would be the Century #71332, which is 1"LOA, 0.42" OD, 0.038" wire, 0.344" ID, 11 lb/in, and 0.26" length at coil bindNearly identical is the McMaster Carr # 9657K307, same specs except the wire is 0.039", 0.342" ID, and 0.29" length at coil bindThe ID of the valve would be about 0.625" minimum, and larger is better, to provide double the throat area past the poppet, thimble and springYou would want 8 holes of 0.156" min. ID in the front mounting wheel, to provide double the throat area into the valveThat means the mounting wheel needs to be 0.75" OD or larger, it is retained with a circlipThat circlip groove OD in turn dictates the minimum OD of the valve body, in the case of a 0.75" front wheel, the valve must be at least 0.875" ODThe overall length of the valve would be about 2.75"I hope this will give you a basic "road map" to follow if you want to try a balanced valve....Bob
A bit different approach, poppets I can make - stems I need to buy as spares:balanced-valve by abbababbaccc, on FlickrAfter making that hole through the poppet I can say that it would be easier to file a flat on the stem.