Hi All, What is the best way to mount a shock cord to the motor tube, only having almost a half inch of centering ring? I was thinking that there is not enough ring there to support an eye bolt and want the masses opinion before I continue.
You could epoxy strap to the motor tube and feed it through a slot in the forward centering ring. Maybe you could wrap the strap and motor tube tightly with some nylon thread before the epoxy sets for extra strength.
You could put an eye screw (with threaded end, nuts on both sides) or use the Estes method, or a steel or kevlar line just short of the edge of the tube (zipper protection) with the loop, and glued under the fore ring.
I had the same dilemma when I wanted to put a 75mm motor-mount into a 4" airframe. What I did was use tubular kevlar. I drilled a couple of 1/8" holes on opposite sides of the most forward CR. Next, I took ten feet of 1/8" tubular kevlar, folded it over in the middle, and pushed it through one of the holes. (Note that the CR is bonded to the motor-mount tube by this time...) Pull enough of the cord through the hole to form a loop large enough to pass over the end of the motor tube, and then pull it all back to tighten the loop. The free ends now get pushed through the opposite hole, looped around the motor tube, and tied ... either to each other, or looped again around the tube and tied back onto itself. Pull the whole thing tight, and liberally apply epoxy to the points where the kevlar and the motor tube come into contact. This will make the whole assembly quite strong.
John I've had the same problem when putting a 4 inch mount in a 5.5 inch tube. I did use eye bolts....2 rather than one (and stainless, of course:-). I first bent the bolts, some (to make sure I could had enough room to get a quick link between the eye and the tube). I then mounted them 180 degrees apart, with the bend facing in. This gives me 2 mounting points, with 2 kevlar straps going to the shock cord. I used 2 not so much for the strength, but to try to get the rocket to land "level." I'm sure one would be more than enough for all but the VERY heaviest rockets. Just make sure you use lot's of epoxy on the motor tube to ring joint:-).