Ducati desmodromic redline thoughts

[Denver_FZ6]

I have a friend interested in a Ducati Monster 696... can't talk him into an FZ6... says the Duc just "fits" him better... whatever!

Anyway, was looking up info on the Ducati valve system. It's an interesting design and is supposed to overcome valve float at high RPM. My question is, then why is the doesn't the motor rev to 14k like the FZ6? Anyone know?

Found a nice animation here ==> Ducatidesmo.com

Matthew

 

[wolfc70]

The main reason Desmodromic valves were used was that 40 years ago metallurgy was not that great. And having cams and rocker arms open and close the valves eliminated the possibility of valve float. Valve springs back then could not keep up with the ever increasing redlines. That and the dominant bikes of the era were British parallel twins (Norton, Triumph, BSA) and rarely went above 7000 rpm, while the 90 degree Ducati with better primary and secondary balancing were approaching 9000 rpm. (racing bikes, street bikes were much lower)

Now days when valves can keep up with 14,000 rpm, desmo seems obsolete. Ducati keeps them around for heritage, kind of like HD uses a single throw crank. It is maintenance intensive design and even the new Ducs need valve adjustments every 10,000 miles. Along with the timing belts. Desmo valves are painfully arcane to adjust and require lots of patience. Parts are not that expensive but the labor involved is shocking. Expect $600 every 10,000 miles.

The reason for the lower redline than the FZ6 is due to piston speed. A 600cc four with tiny 150 cc cylinders and a 44.5mm stroke can rev to 14,000 rpm and have relatively normal piston speeds of 4088 feet per minute, a 696 vtwin with 348 cc cylinders and a 57.2 mm stroke have much more mass to move up and down and attains a piston speed of 4150 fpm at 11,000 rpm (redline, I think). So you can have two different engines and have totally different redlines, and both attain similar piston speeds. Formula: Stroke x 2 x rpm answer is in MM, use a calculator to get to feet.

 

[Hellgate]

I have a friend interested in a Ducati Monster 696... can't talk him into an FZ6... says the Duc just "fits" him better... whatever!

Anyway, was looking up info on the Ducati valve system. It's an interesting design and is supposed to overcome valve float at high RPM. My question is, then why is the doesn't the motor rev to 14k like the FZ6? Anyone know?

Found a nice animation here ==> Ducatidesmo.com

Matthew

Great question, maybe Fred would jump in on this one.

Your are right, the demsmo has a "return" fork on the valve train that closes the valve, where has pretty much everyone else in the world uses the valve spring to do the job.

The design does prevent float in the way we would get with a spring. Float is where the engine is turning so fast that the spring can't close fast enough to keep up with the cam so it just kinda sits there open. When you install performance cams most often you need to install firmer springs too.

Redline is more than just valve float. Also of it has to do with the ability of intake and exhaust systems to move the fuel/air and burned exhaust gases out of the cyclinder. Each engine is unique and can only go so far until changes must be made, port shape, valve size/shape, fuel delivery ability, etc.

If you go past redline the power will drop off on the engine because it can't flow any more air in and out of the engine. So the MFG will back off from that point and determine the correct redline for that engine.

On a side note our engine does not redline at 14,000, but at 13,700. Our tach is a bit optomistic, like our speedo. The 14K is indicated to sell a bike that has that number. :thumbup:

 

[Denver_FZ6]

Desmo valves are painfully arcane to adjust and require lots of patience. Parts are not that expensive but the labor involved is shocking. Expect $600 every 10,000 miles.

Oh yeah... he's mentioned you can pre-pay the first maintenance when purchasing bike and get it at a reduced cost, although even the reduced price is still outrageous to me! Will be his first bike too!

Great bit of Ducati technical history there Wolfman!

 

[wolfc70]

Oh yeah... he's mentioned you can pre-pay the first maintenance when purchasing bike and get it at a reduced cost, although even the reduced price is still outrageous to me! Will be his first bike too!

Great bit of Ducati technical history there Wolfman!

Yeah, I'm too young for that history, but since my dad and his brother and friends had bikes, I hear all of the stories, and their lust for Italian machinery. My dad had a Ducati Mach 1 and several British twins, and a Laverda Jota, all before I was born. I remember him getting picked on for his BMW, a real bike has only a kick starter dontcha know. I heard how a fraged connecting rod on a BSA will make you late for an interview. When you finally get to the job interview, in oil soaked clothes, the boss hires you anyway because the same thing happened to him a few weeks earlier. The list goes on.

I still want a Ducati, but the maintenance costs are always going to be a hinderance. No matter how much money I have, I always seem to want a bike I can afford to own, but not maintain. I would not want a Duc as a first bike, kind of pricey, tell him to buy a Ninja 250, master it, then get his dream bike.

 

[Fred]

Great question, maybe Fred would jump in on this one.

Yes, I will.

Desmodromic is based on the latin "Demondromedary" which roughly means,"Working on this (mechanical assembly) is less fun than taming a satanically possessed camel."

It is believed that desmodromic valves were designed by Nazis as a way of torturing victims, as a valve adjustment was more more painful than the usual drilled teeth or severed fingers. This is incorrect. Ducati is an Italian company, so the Gestapo are the ones who were responsible.

These days Ducati uses desmodromic valves because regular valve springs, after having been used on every other internal combustion engine ever produced, have proven to be reliable.

So, how does the desmodromic valve system work? The system consists of a total of six camshafts per valve. Each shaft has 1/3 of a cam lobe on it, which is engaged to its valve for a portion of the 720 degree engine cycle. Rather than the valve opening and closing once per cycle, it rapidly oscillates due to the six cam lobes alternately opening and closing it. This rapid vibration of the valve creates a pulsed shockwave in the intake, which stalls the incoming air charge at the correct time, effectively closing the valve. The same thing happens on the exhaust, except that the valve is inverted to account for the opposite direction of the exhaust flow. Also the exhaust valve contains magnetic monopoles (North polarity, unless you have a sub 500cc model). These are to trap any escaping thetans in the engine.

Timing of the system is handled by a Saberhagen Proccessing System. The radioactive decay of a piece of plutonium (mounted just below the rider's seat) determines how often the Ducati will win races, attract women, or break down on the side of the road.

To service a desmodromic valve system, first perform the Ritual of Nyarlhotep. Eat the congealed blood of a dung beetle every night for a month prior to the service. On the full moon, arrange your Whitworth sockets and left handed moebius wrench in a pentacle. Repeat the chant,"Ish oodaga tayam aha" two dozen times while walking around the bike in a widdershins direction. Finally call the dealership and book an appointment. If you've performed the ritual correctly, they'll be able to get you on the schedule sometime in the next two months.

Hope this helps.

 

[wolfc70]

These days Ducati uses desmodromic valves because regular valve springs, after having been used on every other internal combustion engine ever produced, have proven to be reliable.


To service a desmodromic valve system, first perform the Ritual of Nyarlhotep. Eat the congealed blood of a dung beetle every night for a month prior to the service. On the full moon, arrange your Whitworth sockets and left handed moebius wrench in a pentacle. Repeat the chant,"Ish oodaga tayam aha" two dozen times while walking around the bike in a widdershins direction. Finally call the dealership and book an appointment. If you've performed the ritual correctly, they'll be able to get you on the schedule sometime in the next two months.

Hope this helps.

Priceless, absolutely priceless!:rockon:

 

[Hellgate]

Priceless, absolutely priceless!:rockon:

Fred is always good for an interesting take on a situation. :thumbup: