Driveshaft Clearance Calculator

Take six tape measurements off the car, labeled A through F below and on the drawing. The calculator tells you how much room the driveshaft has over the #1 bar sitting still and when the rear suspension extends on the launch.

Measurements

Car on a level floor at ride height. All heights are from the floor straight up. Lengths D and E are measured level, not along the driveshaft.

in
in
in
in
in
in
in
3.0 in

Result

Driveshaft at ride height Driveshaft with rear extended #1 bar Side view, front of car on the left, drawn to scale. Letters match the boxes above.

Clearance at the #1 bar

Condition Driveshaft center over bar Bottom of driveshaft Gap to top of bar Status

Gap is straight down from the bottom of the driveshaft tube to the top of the #1 bar. Anything under your minimum shows Tight. Below zero means it hits.

How to take the measurements

  1. Set the car up. Level floor, ride height, driver weight in the seat if you can. Every height below is floor to the point, straight up.
  2. A. Floor to the center of the front U-joint cross. With a slip yoke on the trans, push the yoke all the way in, pull it back out about 3/4 to 1 inch, then measure to the U-joint center.
  3. B. Floor to the center of the pinion U-joint cross.
  4. C. Floor to the top of the #1 bar tube, right where the driveshaft passes over it.
  5. D. Center of the front U-joint to center of the pinion U-joint. Hold the tape level, don't run it along the shaft.
  6. E. Center of the pinion U-joint forward to the center of the #1 bar tube. Tape level.
  7. F. Outside diameter of the driveshaft tube you have or are thinking about.
  8. Rear travel. How far the rear end extends from ride height when the car launches. If you have shock travel numbers or video, use those. Set it to 0 to check ride height only.

What the calculator does

  • Draws a straight line from the front U-joint center to the pinion U-joint center. That is the driveshaft centerline.
  • Finds how high that line is directly above the #1 bar, subtracts half the tube diameter to get the bottom of the tube, and subtracts the bar height to get the gap.
  • Runs the same math again with the pinion dropped by the rear travel amount, because that is when the driveshaft is closest to the bar.
  • With a slip yoke on the trans, the driveshaft stays the same length and the yoke slides out as the rear drops. With the slip in the driveshaft, both U-joints stay put. Either way the difference is a few hundredths.
  • Driveshaft loops, weld beads, and shaft whip under power are not included. Leave yourself margin.