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View Full Version : And I thought I was a little obsessed with Desmodromics



A.K. Boomer
04-05-2009, 02:17 PM
This guys unreal, a ton of info...


http://members.chello.nl/~wgj.jansen/


(I don't know why the hell he put a spring in the animated unit on the home page!)

bruto
04-05-2009, 03:59 PM
This guys unreal, a ton of info...


http://members.chello.nl/~wgj.jansen/


(I don't know why the hell he put a spring in the animated unit on the home page!)My old Ducati Desmo 350 had a hairpin spring, needed because otherwise with valves adjusted for proper clearance they would not close when cranking.

edit to add, you can see it in the second 125 GP triplenocker illustration, probably better described as a clothespin spring on the lower rocker.

clutch
04-05-2009, 05:14 PM
I read the topic, thought it was related to Ducati. No other comments. Not senile yet. ;)

Clutch

rogee2
04-05-2009, 07:57 PM
My old Ducati Desmo 350 had a hairpin spring, needed because otherwise with valves adjusted for proper clearance they would not close when cranking.

edit to add, you can see it in the second 125 GP triplenocker illustration, probably better described as a clothespin spring on the lower rocker.


Not wishing to contradict anyone but a desmo duc will run very well without valve springs, the valves close mechanically to a point and then cylinder pressure does the rest. For a street machine this might not be a desirable set up but for a vintage racer it works just fine, and it makes bump starting a snap.

bruto
04-05-2009, 09:00 PM
Not wishing to contradict anyone but a desmo duc will run very well without valve springs, the valves close mechanically to a point and then cylinder pressure does the rest. For a street machine this might not be a desirable set up but for a vintage racer it works just fine, and it makes bump starting a snap.No doubt it would run fine without springs, but I am under the impression that better starting was the purpose of them. They were pretty light.

Spin Doctor
04-05-2009, 09:20 PM
Wouldn't a sleeve valve of been simpler. Not as cool, just simpler

rogee2
04-06-2009, 12:25 AM
No doubt it would run fine without springs, but I am under the impression that better starting was the purpose of them. They were pretty light.


You will note I was referring to a vintage racer, most racers removed the starting gears and kick starter lever. When you are push starting (bump starting) having low initial cylinder pressure is advantages in letting you get the flywheel rotating. Any one interested in a 450 desmo head with new red & white cam and rocker arms, I have one that I could be traded out of.

A.K. Boomer
04-06-2009, 08:35 AM
There are a plethora of different reasons for some desmo's adding an "auxiliary spring" These are generally just a very small percentage of travel springs and are not to be confused with typical valve springing, Reasons vary on design and manufacturer with everything from keeping the valvetrain quiet due to being able to run "zero clearance" between the opening and closing cams -- A type of "pre-load" so to speak,
To actually buying the closing cam a little extra time and keeping the "abruptness" of returning the valve back to a minimum -- so much so that it can salvage rocker arm destruction at high RPM's --- Just take a look at the radical profile of the closing cams lobe and it will become evident as to just how fast things need to happen --- Its basically the mirror image opposite of the opening cams lobe - BUT - The opening cam has a huge advantage in the fact that the entire rocker from lobe to valve is strictly a compressional unit --- Generally (but depending on design) this is not so with the return rocker as its usually a forked end --- What this means is the rocker can catch havoc in both directions as at certain RPM's the valve its returning can try and out-accelerate the very mechanism that got it going, these are of course the first rockers to disintegrate and did in many of early desmo designs as the opening rockers actually had an easier life than that of a typical "spring valve" engine as they were just opening a valve in typical fashion without the added burden of compressing a spring...

Adding a small auxiliary spring can also make valve adjustments on a desmo much more forgiving as it can add a pre-load exit in the springs compression - Instead of the interference and therefore the inevitable destruction of parts between the opening and closing rockers.

Furthermore --- in some applications - calibrated return springs can be incorporated into a desmo and designed to "float" at very high RPM's -- If utilized on the intake valves return rocker this can be ideal for two things -- one; It lightens the load on the rocker on the largest most heavy valve (therefore the one most likely to fail its rocker) -- two; It can allow for a pre-determined and totally controlled valve float on the engines most dominant H.P. producing valve (the intake) and create a well needed longer duration period for said valve (@ high RPM's)--- and unlike a typical "sprung" engine still have a guarantee of returning the valve back in time so that the piston clears --- The added bennies of variable duration --- At least on the intake side...

I will also conclude that both You and Bruto have covered a couple others depending on manufacturer --- one thing for certain is Duke hands down has the most successful desmo on the planet.

A.K. Boomer
04-06-2009, 08:58 AM
Wouldn't a sleeve valve of been simpler. Not as cool, just simpler


I built my 4 stroke port system almost 25 years ago, unlimited RPM and zero reciprocating drag and flow characteristics that are off the charts and extremely simplex -- except when it comes to sealing --- remember when I stated that there's a reason why the piston engine has survived all these years?

there's also a reason why valves have too -- they are the perfect sealing devises - BUT- with direct injection gas and no longer the worry about a little combustion getting past the intake once in awhile and lighting off the next fuel injector mix prematurely --- who knows, It will at least be interesting to see the attempts, If done up right the benefits would be unreal.

Spin Doctor
04-06-2009, 11:09 AM
In terms of sealing, efficency and cost IMO the conventional poppet valve engine wins hands down in almost every IC application. Desmos are neat but for street engines IMO they are technological overkill. The sleeve valve in the era of emissions is I think a dead end. True they have better volumetric efficency than a poppet valve engine but there are few applications today where they would really shine. I think one of the few being the world of large deisels