Ducati has a new method of expressing belt tension. The old method relied upon springs that could drift out of calibration without warning. The new method expresses tension as the frequency (in Hz) at which a section of belt will vibrate at when plucked like a guitar string. The more tension a belt has, the higher the frequency it will resonate. This method has more accuracy and is much more repeatable, especially from instrument to instrument, because it doesn't rely upon springs. A frequency meter doesn't need to be calibrated because it is simply measuring the number of oscillations per unit of time. As long as it has an accurate clock, it will remain accurate.

I'm not the most cautious person when it comes to belt tension as is evidenced by the fact that I've been setting the tension using the guestimate method since I bought my first Ducati. And I'm not shy about running my belts 18,000 miles and well over the two years Ducati recommends. I haven't exactly been losing sleep over this but I am pleased to have discovered a new way to verify proper tension without forking over a lot of money for expensive tools that you may only use a few times. And, you will be pleased that it's every bit as accurate as the horrifically expensive Mathesis tester with optical probe and more accurate and reliable than the expensive spring tension gauge that needs to be periodically calibrated.

The graph above is a visual representation of a .wav file I recorded using the built-in microphone of my camera. I simply plucked the belt like a guitar string while holding the camera body against the adjustable metal cam pulley. The vibrations of the belt are picked up very cleanly through the pulley. I used some excellent audio editing software called "Goldwave" to analyze the recording. First, I selected the portion of the sound file I was interested in by left-clicking to delineate the left border and right-clicking to delineate the right border. The result is shown above. Then it is a simple matter to count the number of complete waves and multiply by the fraction of a second that you choose to sample. As you can see at the bottom of the screenshot, Goldwave displays the length of your selection as 0.171 (of a second). Taking the reciprocal of this number and multiplying by the total number of cycles (waves) selected to give you the frequency in Hz. We have 25 cycles displayed in the example above. 1/0.171*25=146.2 Hz. That would be about right for a Ducati ST3 but I've been told the specification for the ST4s is 110Hz +-5Hz or 105Hz-115Hz, which means this belt has been adjusted too tightly. Of course that assumes the measurement was taken while the engine was at or near TDC of the compression stroke as Ducati specifies.

Here is a series of sounds recorded at 20 degree intervals of crank rotation to illustrate how belt tension changes with engine position. I plucked the belt twice at each engine rotation.
Click here to listen to .wav file

Nobody wants to hear the sound of an engine munching itself to pieces but this is exactly what happens when a belt shreds to pieces from being too tight or jumps a few teeth due to being too loose. And since belt tension affects cam timing to some degree, one tight belt and one loose belt can result in an engine that does not run smoothly at certain RPM's. Now that I have an accurate and repeatable way to measure belt tension, it was an easy matter to measure the tension at 10-degree intervals of crank rotation while leaving the tension pulley in the same position for the entire graph. Because a four-stroke needs two complete revolutions of the crank to complete one rotation of the valve train, the resulting chart has 720 degrees of crank rotation represented. To illustrate the importance of the valve train position on belt tension, I set the belt within specs (112Hz) while the engine was at TDC of the EXHAUST stroke to see if the belt tension would still be within specification when the engine was rotated such that the valves were not applying pressure to the belt. The black line and lighter grey stripe across the center of the chart represent the specified tension and the acceptable range of deviation when setting the tension with the valves at rest. The peaks and valleys are the result of the closer springs trying to rotate the cam pulleys and thus changing the belt tension depending upon how the cams are positioned. I imagine this effect is more pronounced on engines such as the ST4s that have steeper cam profiles.

If you look to the far left and right sides of the chart you will see the flatter area around 145Hz-150Hz where the valves are at rest. That is way out of spec even though the belt was adjusted within spec at TDC of the EXHAUST stroke. Imagine how much worse the situation would be if a sloppy mechanic happened to adjust the belt tension with the engine resting at 420 ATDC (point 42 on the chart)!

To get consistent results it is necessary to measure the belt tension when the closer springs are not adding (or subtracting) tension to the belt. This is true no matter what method is used for specifying/measuring tension. Some really smart people believe they are smarter than the engineers who designed and tested these magnificent engines and think it's OK to take time-saving shortcuts because they know better. Now that I've done the measurements I'm can say with certainty that Ducati doesn't train their mechanics to position the engine with the valves at rest before the belt tensioning procedure simply because they are psychotic engineers who make up time-wasting rules as they go along. Any mechanic worth his fees will take the extra minute to turn each cylinder to TDC (compression) before they adjust belt tension. Some of the vibration resulting from having one belt tighter than the other will be tuned out when the throttle bodies are adjusted but the engine will still be out of balance at other RPM's. Modern belts may be extremely durable but they still fail from time to time. And I would be willing to bet the vast majority of cam belt failures are due, in whole or in part, to improper tension by a mechanic who didn't take the time to do the job right.