How to film it
- Switch the phone to slow-motion (120 or 240 frames per second). Note which one; the tool needs it.
- Stand to the side, far enough back that the foul line and the pins are both in frame, or at least the foul line and the arrows. Keep the phone still; prop it on the ball return if you can.
- For rev rate, wrap one strip of white tape around the ball through the finger holes. One bright line is all you need to count.
- Film a normal shot. Save the clip without trimming effects or speed ramps.
- Open it here, pause, and step frame by frame to the moments you want to mark.
How the maths works
Speed is distance divided by time. The time comes from counting frames: a clip recorded at 240 frames per second has 240 frames in each real second, so if the ball crosses the foul line at frame 310 and reaches the head pin at frame 870, it travelled the lane in 560 ÷ 240 = 2.33 seconds. The distance is the one measurement in bowling you can rely on without a tape measure: the USBC Equipment Specifications Manual requires 60 feet, plus or minus half an inch, from the lane side of the foul line to the centre of the No. 1 pin spot. 60 feet in 2.33 seconds is 25.7 feet per second, which is 17.5 mph.
If your clip only shows the first part of the lane, you can use the arrows instead. The same manual allows the targets to sit anywhere from 12 to 16 feet beyond the foul line, so “15 feet” is a typical figure, not an exact one; speed measured that way carries a few percent of uncertainty, and it is the speed off your hand rather than the average down the lane, which is lower because the ball slows as it rolls.
Rev rate is the same idea with turns instead of feet: count how many times the tape comes round between two frames, divide by the elapsed real time, and scale to a minute. Three full turns in 0.5 seconds is 360 rpm. Count over as long a span as the clip allows, because a quarter-turn miscount over a short span becomes a large error in rpm.
Automatic tracking (beta)
Instead of marking frames yourself, press Find both marks automatically, tap the foul line where the ball crosses it, tap the bottom of the head pin where it stands on the lane, then step to a frame where the ball is clear of your hand and tap the ball. The tool learns the ball's colour, follows it from frame to frame in your browser, and works out the moments it passes the foul line and the head pin, including between frames.
At the foul line the ball is usually still beside your hand, so the tool often estimates those last few frames from the ball's path just after release, and says so when it has. In our tests on computer-generated clips with a known speed, the result was within about 1% filmed side-on, and within about 1.3% filmed from behind the bowler, where the far end of the lane is only a few pixels tall. Real footage varies more, so treat it as a fast first pass: it works best with a steady camera, a ball whose colour stands out from the lane, and nothing crossing in front of it. When it can't see the ball reach one end it tells you, and you mark that end by hand. Always step to the two frames it found to check them; a mark you set by hand replaces the automatic one.
What the numbers mean
Treat your first readings as a baseline rather than a verdict: lanes, cameras and marking choices all shift them a little, so film three shots and look at the spread. The useful questions are whether your speed and revs suit each other and your ball, which is what the rev rate and ball motion pages cover, and whether they repeat from shot to shot, which is what release and footwork work is for.
Sources: USBC, Equipment Specifications and Certifications Manual (last updated 03/26): lane length 60 feet ± ½ inch from the foul line to the centre of the No. 1 pin spot; targets (arrows) 12 to 16 feet beyond the foul line.