Desmonto (sp?) engines

[Biff]

Have any of you lot heard of a Desmonto (spelling?) bike engine? Apparently they were/are used in some race spec Ducatis. If I remember correctly they work on the basis that there's a cam to open the valves and another set of cams to close valves, doing away with the need for valve springs. Am I correct? Am I over simplifying things?

As much info as possible please troops.

Thanks.

[speedpro]

Desmodronic is the word. Should find it on the web. Or just desmo - common abbreviation.

[Paul in NZ]

Desmo valve gear has been around for a long time. I believe that Moto Guzzi, Norton, AJS and a whole bunch of other people played with it decades before Ducati.

For the most part, I believe the pioneers of this system were driven to it because of the unreliability of early valve springs. It was a common failure on racing bikes in the early years and effectively limited the number of rpms they could sustain. Remember these were often 500cc singles with 2 valve designs (large heavy, difficult to control valves).

A combination of better materials, lubrication and better exhaust port cooling in the 50's meant they did not need to go down that route as coil springs became better and better and replaced the old hair spring valves. This allowed max rpm to rise before the limit of valve float and eventual destruction took over.

Honda played with a very interesting DOHC / torsion bar sprung semi desmo design on the 450cc twin in the 60's.

One plus of the system is a lot lighter loading on the cams and valve gear allowing (with better control of valve float) higher rpm I think.

Essentially, I guess it's a bit of a vanity though. People like Honda and Suzuki can make very reliable and powerful V twins without the expensive desmo valve gear.

I believe most desmo engines have a very light valve spring to assist at low speed.

Paul N

[Kiwi]

Desmodromic, what does it mean?

The term "desmodromic" arise from the two Greek words: "desmos" (controlled or linked), and "dromos" (course or track). It is used to describe an old but still innovative valve train system. (1910, Arnott from England realized the first desmodromic timing system). The system is also known as a system that provides "positive valve actuation" or in other words, both strokes are "controlled.

The desmodromic valve system uses one lobe and rocker to open the valve, and another, complementary lobe and rocker to close it – there is no spring.

Only Ducati and the German car manufacturer Mercedes Benz used the desmodromic successful. Mercedes with the legendary race car W 196 in the year 1954.

And, of course Ducati, 1956 in the 125 ccm GP Bike. Which won the Swedish GP first time out. And 1968 in the Mark 3 D

Kiwi

[Marmoot]

(to top it up a bit)

...and that guarantees accurate valve timing in all rev range, even at very high rpm, since there is no potential of valve float (valve not closing properly due to spring cannot close valve quick enough at high rpm).

That also gives Ducati a distinct engine noise.

Apparently need more maintenance than normal valve, though?

[Sniper]

Sounds like Vtec in a Honda but obviously worked out and utalised better

[Kiwi]

more maintenance than normal valve????
Not really, on the fuel injected models, you have to check the valves after 10 000 km, it's a little tricky thats all - for a good mechanic - no prob

Kiwi

[Blakamin]

mmmmm... desmo.....

[Biff]

Thanks for the info' guys. Here's what I've managed to find out in addition to the info' you've kindly posted:

Race spec Desmodronic engines are notoriously fiddly to set up and maintain. This is in addition to the fact that they appear to be very expensive (Although you can apparently convert a GXR1000 to a Desmo engine for around $8000) and require regular, specialist servicing in order to ensure that the cams responsible for opening and closing the valves are in sync. Otherwise all manner of nasty things can take place. Apparently the Ducati engines used in races demand a total rebuild every couple of hundred K's, while Suzuki's and Honda's that were based upon Desmo' engines lasted for a couple of thousand K's. I'm not sure why this is, I'm just repeating what I've just read.

As Paul pointed out, many Desmo race engines also have springs in order to assist the engine at slow RPMs. In addition these low compression springs also appear to be used in order to assist with 'chatter' and to ensure that the valves seals are tight at very high engine RPM's.

It is apparently not recommended to simply convert and engine to a Desmo, without fitting lightweight piston rods, preferably to Titanium ($3000 each!!! Thankfully most Desmo engines are twins). In addition, it's also recommended that the piston heads are changed for lighter weight variants.

I'd better point out that all of the fluff I've just written appears to apply only to race spec' bikes, and not road going variants.

Thanks again for the info' guys, and please keep it coming.

[**R1**]

Have any of you lot heard of a Desmonto (spelling?) bike engine? Apparently they were/are used in some race spec Ducatis. If I remember correctly they work on the basis that there's a cam to open the valves and another set of cams to close valves, doing away with the need for valve springs. Am I correct? Am I over simplifying things?

As much info as possible please troops.

Thanks.

WHY???

you told me ducs were for homo's.........J/k blakamin

[pete376403]

Honda played with a very interesting DOHC / torsion bar sprung semi desmo design on the 450cc twin in the 60's.Paul N

Que? Yes the CB450 used torsion bar valve springs but apart from that was just a plain ol' DOHC. And when you think about it, every bike uses torsion bar valve springs, it's just that most of them are wound in a spiral. But the spring travel still comes from the bar/wire twisting around its centreline.

[Paul in NZ]

you told me ducs were for homo's.......

Hey! That's RICH Homos son....

[Biff]

WHY???

you told me ducs were for homo's.........J/k blakamin

No mate, what I actually said was that all bike riders, including Ducati and HD riders were Homo Sapiens. You on the other hand are from another planet!

[**R1**]

Hey! That's RICH Homos son....

They would have 2b to pay for servicing every 10,000k's

[**R1**]

No mate, what I actually said was that all bike riders, including Ducati and HD riders were Homo Sapiens. You on the other hand are from another planet!

Yeah thats all fine .....but y do u want to no about desmo??