HERE'S THE ANSWER TO THE CONTEST:

well, it's time to reveal the result, before excitement leaves place to boredom.

the "special feature" of the prototypes was that the Pantah was born with a Morse "silent" chain primary drive. Never before (and never after) had a primary chain drive been used on Ducati bikes, as far as I know. The idea was to make the engine less noisy than with gear primary tranmission (for rider's comfort but also to pass the ever more stringent noise regulations). The first prototype was ready to run on the dyno bench by summer on 1976 (yes: 1976! and it had been designed to accept water cooling: just imagine what a time warp jump Ducati could have been able to do, had they had more resources!)

All went well as long as the engine was running on the bench, but once the motor was put on a real bike (the frame was hand-built by Franco Farnč then refined by road tests and finally drawn by Fabio Taglioni, in a reverse fashion which was popular in Ducati at the time) the "silent" Morse chain proved all but silent. A few "what to do now?" meetings of the staff then the painful decision: Mengoli (the engineer in charge of the design of the Pantah's bottom end) had to re-draw the whole bottom end to accept a helical gear primary transmission. Only that this time, tolearances had to be much more precise than allowed in previous motors. The result was a long delay of the scheduled timing and the fact that the release of the Pantah took place only at the end on 1979 was in great part due to this "mistake".

But what is the external "telltale" feature, and in which photo can it be seen?

With a chain primary drive, the crankshaft rotates in the opposite direction of a gear primary setup. As a consequence, also the camshafts will rotate in the opposite way so the tensioner pulley (which is always on the "slack" side of the belt) will be on the opposite side. This is clearly seen in Photo no. 7 where the belt running to the front head is partly visible. See a comparison of old and new Pantah motors on this page.

 

Is it not at all clear? Want a better explaination?

Ok, now think of what happens when power is transmitted with a chain or with a couple of gears. Let's start from the end, the rear wheel. Looking at the bike from the right side, the drive wheel must turn CLOCKWISE in order for the bike to be propelled ahead. There's generally a chain between the drive wheel and the engine sprocket. A CHAIN DOES NOT REVERSE MOTION BETWEEN SHAFTS so also the engine sprocket will turn clockwise, seen from the right (it doesn't matter if on the Pantah the sprocket is actually on the left. Imagine that the engine is transparent).

Therefore, also the gearbox driven shaft turns clockwise. The gearbox driving shaft is connected to the driven shaft by a couple of gears. GEARS REVERSE MOTION BETWEEN SHAFTS therefore the gear driving shaft will turn ANTICLOCKWISE. the clutch, which is co-axial and solid with the gear driving shaft, will also turn anticlockwise. You noticed that I included a few drawings to illustrate these concepts. Sorry: no java animation but mere good old hand sketches.

Now comes the interesting part: if the crankshaft is connected to the clutch drum via a couple of gears, then the crankshaft must turn CLOCKWISE, but if the crankshaft is connected to the clutch drum via a chain, then the crankshaft must turn ANTICLOCKWISE. therefore, the crankshaft will rotate the opposite way in the two setups. When Mengoli re-designed the engine, he also reversed the rotation of the crankshaft (and therefore of everything that was connected to it: flywheel, starter sprag clutch...)

OK, but how do I see it from the outside? Well, the overhead camshafts are driven by two belts that take their motion from a central pulley that sits between the cylinders. Also the belts, like chains, do not reverse the sense of rotation of parallel shafts, so the camshafts will rotate in the same direction as the central pulley. But how does the central pulley get its motion? It is connected to the crankshaft via a couple of gears, therefore its rotation, and hence the camshafts' rotation, is reversed from that of the crankshaft.

Starting to get the picture? Let's summarize: a Pantah engine with gear primary drive has the crankshaft that rotates CLOCKWISE (always imagine it seen from the right side) and the camshafts that rotate ANTICLOCKWISE. The Pantah with chain primary drive has the crankshaft that rotates ANTICLOCKWISE and the camshafts that rotate CLOCKWISE.

The belts need a tensioner pulley which is always placed on the "loose" return branch (just like the final transmission chains on off-road bikes, that often have a tensioner pulley on their loose side - the lower - and never on the tensioned side - the upper). So the "tensioned" branch of the belts will run straight from the central pulley to the pulley at the end of each camshaft, while the loose side will be angled to the inside by the tensioner pulley. If the camhsafts rotate anticlockwise, the loose side of the belts will be the lower one for the front horizontal cylinder and the rear one for the rear, vertical cylinder. You can see this very well on the "blow up" of the production Pantah engine that serves as a background to these pages.

but if the camshafts rotate CLOCKWISE as in a Pantah with chain primary drive, then the loose side of the belts will be the UPPER one for the front cylinder and the front one for the rear cylinder, therefore the belt tensioner pulleys will be in the opposite position in respect to the production engine! Now have a good look at pic no. 7: take note of the path of the belt along the front cylinder. The upper side of the belt runs angled, while the lower side runs straight from the central pulley to the camhsaft pulley. Clearly the tensioner pulley is along the upper side of the front belt and this means that the belts runs clockwise (i.e. the upper side goes forward while the lower goes backward and "pulls" the camhsaft pulley). Hence the camhsaft rotates clockwise; the central pulley driving gear, as well and the crankshaft, must therefore rotate ANTICLOCKWISE, which is overwhelming proof that the primary drive is via a chain!

Look up this page for a comparison of pic no.7 with a view of the final Pantah's engine. Incidentally, anyone who has experience of changing belts on a Pantah (or any other modern 2 valve Ducati) has certaily noticed that the tensioners' retaining bolts are hidden by the belts and a bit of "belt wrenching" is needed in order to have access to these bolts. This is probably due to the fact that the cylinders were not re-designed after the "great primary drive shift" and the tensioner bolts were made to be behind a belt that followed a completely different path!

 

And what about the bevels?

Now, you certainly remember that I hinted to bevel gears a few paragraphs ago. What about them? Well, anyone familiar with bevel twins knows that they have an extra pair of gears at the sprocket end of the driven gear shaft so that the sprocket is aligned with the driving shaft and rotates the same way. This way, the 5th gear is actually a "no gear" and the driven shaft rotates completely idle. It was a common device in the past, to save a few Joules of energy dissipated in friction.

But if the sprocket rotates in the same direction as the driving gear shaft, this means that also this shaft must rotate CLOCKWISE (seen from the right) and the crankshaft, which is connected to the clutch drum via a pair of gears, rotates ANTICLOCKWISE.

ciao! Luca

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