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Author Topic: FAQ: What is desmo? Why are Ducati valves unique?  (Read 27733 times)
Crashin' mofo
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« on: June 15, 2009, 09:04:18 AM »

Desmodromic is the valve actuation system** found in Ducatis.  Everyone talks about Ducati "valves" for two reasons:  it's a unique system, and it requires more frequent valve checks than many bikes.  But what is desmo?

Engine basics.  Here's a good explanation and animation of how an internal combusion engine works (cylinders, valves, pistons, etc) in general:  http://www.howstuffworks.com/engine1.htm

Desmodromic basics.  How is desmo different than the engine described above?  The short version is that desmo has a second rocker arm (see below) to close the valves instead of using springs or some other system.   Here's an animation of how the desmo system works:  CLICK FOR VIDEO

More detail.  For a more detailed explanation about desmo, I've taken the liberty of quoting below a post from wooodyracing.  His post explains how valves work generally and how they work on a Ducati with a desmo system.  Note that woodyracing is talking about a 4 valve bike.  A 2 valve Ducati (like an aircooled monster) will have only 1 exhaust valve and 1 intake valve and only one camshaft in each head.

Valves are what let the air/fuel into the cylinder and exhaust gasses out into the exhaust.  Your bike has 4 valves in each cylinder head (vertical and horizontal), 2 exhaust valves and 2 intake valves.  The belts turn 2 camshafts in each head, one camshaft for the intake valves, one for the exhaust valves (in each head obviously).  These camshafts have lobes that move rocker arms which push the valves down into the cylinder (open) and on Ducatis a 2nd rocker arm driven off of a different cam lobe pulls the valve back up (closed).  Ducati is unique in having the camshaft close the valves, almost every automotive/motorcycle engine in the world uses springs to close the valves.  Keep in mind all this movement is happening VERY VERY FAST.  The gap between the rocker arms and the cam lobes when the valves are closed is critical to keeping the engine running properly.  To set this gap there are shims of different sizes positioned on the valve, the valve adjustment process is basically measuring the gap and swapping the shims as needed to achieve the appropriate gap.  Obviously to do this you have to get to each cylinder head which means removing a LOT of stuff.  As I mentioned Ducatis are different in that they have a rocker arm closing the valve as well, which means there is a shim for opening each valve and a shim for closing each valve.  Check out this animation of a Ducati valve movement including the camshaft and rocker arm.  This animation is from a 2-valve engine so it is a bit different than yours, it only has one camshaft in each head.  


Why use Desmo?.  One reason for using desmo is that it is a signature Ducati feature.  Another reason is that it allows more precise valve actuation a system that relies on springs.  When the clearances and cam timing are set properly, you can control much more precisely when and how quickly the valves open and shut and how long they stay open.  At low RPMs, this precision is not that important.  However, at higher RPMs like you find in racing, this precision is critical to ensure that the engine is operating at maximum potential.

** Saying that Ducati "valves" are unique is not really accurate.  It's the whole valve actuation system rather than just the valves.  Desmo has also been used on engines other than Ducatis.  Wikipedia has more info if you're interested:  http://en.wikipedia.org/wiki/Desmodromic
« Last Edit: November 18, 2010, 04:19:33 PM by Spidey » Logged

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