If the "down tube" is the tube which connects the fork mounting tube to the lower portion of the frame then the length is 12 inches. The most important measurement is the fork angle which is 65 degrees. To disassemble the shocks, remove the 2 quarter inch bolts, spray WD-40 in all the openings on the forks including the grip section. Twist the shock both directions to make sure it is free. Then pull the lower section of the shock out of the tube. What you should have is the lower section of shock, the compression spring and the upper stop section all attached together. If the shock is stuck and you pull too hard on the shock, you will disconnect it from the spring or the stop.