Sportbike World banner

1 - 20 of 23 Posts

·
Registered
Joined
·
8,798 Posts
desmo is short fot desmoquattro, which is a technology ducati employs in their engines. Its a type of valve train. I'll try to explain.

The "classic valve train is the pushrod style. A series of rods ride on the cam shaft which is located in the "V" of the engines. The rods transfer the energy to the top, but in the opposite direction, so its reversed by a lever called the rocker arm. The rocker arm opens the valve. A spring closes the valve. This is the standart chevy engine. The problem is that all these parts in the valve train weigh quiet abit and thus give you a low red line.
Also there is the concept of valve float. At a high enough RPM the valve open very fast. So fast, that the spring cant close it fast enough, leaving an unsealed combustion chamber when the plug fires. So the higher RPM you want to run, the stiffer spring you need, the more power you are wasting to compress this spring.
Enter the overhead cam. Put the cam over the valve and let the valve itself ride on the cam (ok with a lifter, but lets forget about those), get rid of pushrods and rocker arms, so mass is lighter, but there is still the spring problem.
Enter the desmo. Use 2 rocker arms. one opens the valve, the other one closes. No springs. The theory goes, that there is no valve float, no power wasted on compressing springs. Sounds great no?
so how come ducati's put out less power, and have lower red lines than their jap counterparts? well, becouse all those crasy rocker arms added mass to the valve train.
Also now you got not one but two rocket arms to shim up, and the shims counteracting each other. Thats 2 rocker arms per valve*4 valves per cylinder * 2 cylinders. 16 rocker arms. So maintanance is quiet abit more intensive. valves must be shimmed every 6k miles.

Duc owners for the most part swear by this concept. And I gotta admit its pretty cool looking, and if you take the plugs out of the engine, you can spin the crank shaft with your fingers. Which is just awesome!
 

·
Registered
Joined
·
8,798 Posts
ZX6R1033 said:
!




( I said something, then I realized how stupid it sounded, to I deleted it. )
You know how many times I wished I could do that in real life?
 

·
Registered
Joined
·
9 Posts
Vash said:
Duc owners for the most part swear by this concept. And I gotta admit its pretty cool looking, and if you take the plugs out of the engine, you can spin the crank shaft with your fingers. Which is just awesome!
Here is where you are right, you can turn the cams by hand, once you take off the belts and make sure the piston is out of the way. What you are talking about is internal energy losses. The less energy the engine needs to use internally the more energy it puts out per power pulse. That is why desmo engines put out more horsepower than any other twin of comparable displacement. The rest of your comments were generally on except for a few points.

The whole valve float thing is from early development of the system By the way Desmo stands for desmodromic, revering to the valve system. Desmoquatro, means a Four valve desmo engine, Desmoduo is the Two valve engines found in the Monster and SS series. Desmotre, yes three valve, found in the new ST3.

http://www.ducati.com/bikes/techcafe.jhtml?artID=5&detail=article&part=technical.

Valve float really hasn't been an issue in modern engines for quite a while and is not the main reason for Ducati using the system. The 999 puts out 140 rear wheel horsepower from two cylinders. The reason the twins don't rev like a I-4 had to do with weight. When you have a piston/rod assembly that weights twice what a smaller unit weights in a I-4 you just cant change direction as fast. Then you start to run into mass problems to make the thing as strong. That's why the 999R has full titanium pistons and rods and rev's as high as the smaller base 749. So twins are tuned to put out power at a lower RPM but over a broader range and some what higher torque. Read usable power vice peak power, this is a tuning decision to take advantage of the twins nature. Ducati has been improving the design of the bikes and the desmo system. All are engine discripters. Yes twins generaly need more displacement to make as much horsepower as a I-4. The diffrence is that the twin generaly makes good power over a broader range with out the top end rush of the I-4 which makes most of it's power at the top of the rev range, for sporting engines. Ducati does have a 4 cylinder engine that they use in MOTOGP only, but it's a L-4 engine. The street going version is going to be a very limited production model offered to 999R owners. $60,000 a copy is estimated.

Valves need to be checked every 6200 miles and after the first major service, the valves tend to settle and don't really need much in the way of shims but you'll have one or two that needs to be done most every time. Belts have to be replaced every 12K miles. that is really the only additional maintenance required over any other sport bike. Some good news is the new valve retention system just introduced in the 749R and 999R only need to be checked every 12K miles. Duc owners are hoping against hope that the system will work it's way down to the more obtainable bikes.

Ducati is winning again in World Superbike, and is about the only make that is that is giving Suzuki a run for it’s money in AMA Superbike. Not bad for what a lot of people claim to be an outdated design. The compition is great and it's making Ducati improve the bikes which is great for the rest of us when that tech works it's way down to the street.
 

·
Registered
Joined
·
8,798 Posts
In responce to BCebrian

Holy factory propaganda!
First off, the 999 makes 119hp at the rear wheel not 140. (Spare me the aftermarket game, they can all be modded). Second the RC51 puts out more power (also not by much). So while the duc is a sweet bike its not the most powerfull twin.
Then we get to the whole "usable power" VS "Peak power"
Twin power

Inline power

So power at the top end isnt usefull? Or is the duc's twin not tuned for high end power?
You can see the last graph has 999 on it. And inlines are making more midrange than the ducati!
Point is that ducati tunes their engines to spin as fast as the possibly can. between the fact that its a twin, and the added valve train mass, they cant spin as fast as the inlines, and therefor make less power. But it has nothing to do with being tuned that way for more "usable power" they are tuned for as much peak power as possible.
 

·
Registered
Joined
·
9 Posts
The energy losses in a Desmo system is lower than in a conventional spring and keeper system is fact, not my opinion. The added mass of the extra rocker arm is negligible compaired to the resisttance of conventional valve springs. You can look it up anywhere you want.

I apologize I was referring to the “R” model when I stated that HP but forgot to add the designator. The 119 HP number is correct for the '03/04 base 999, the '05 999 got last years 999S deep sump motor and puts out approx 123 HP at the rear, the 999S is now using the '04 R motor good to about 127, and the R is good stock to about 140. It also depends greatly on what dyno you're using use a different dyno get slightly different numbers. The comparative numbers are what is important, good you posted them as you can see the 999R puts out a lot more HP than a RC-51 so yes Ducati does have the most powerful twin. Of which I was referring.

Twins spin slower due to the reciprocating mass of the pistons and rods, not the valve train. I-4' don't need as much displacement for an equivalent power output because they can spin faster due to the lighter pistons/rods. RPM=HP, the key is piston speed, the slower the piston speed the faster the engine can rev, basically short stroke, high RPM =more power. Again engineering fact. Yes, the companies that produce twins want them to spin faster, Ducati’s answer is the new Terrastratta (narrow head short stroke) engine that can rev higher than the old desmoquattro engines. But they still can’t match the I-4’s lower piston speed. So Twins are tuned to put out power at the lower RPM, Your own post show that. I never said top end power wasn't useful, but a wider power band is more useful to the average street rider. It’s a different story on the track where you spend most of your time at high RPM.

There is more to useable power than pure HP, the thing you not addressing is Torque, twins are torque monsters and that torque hits lower down in the rev range and holds longer in a twin. The RC-51 has the same rep for loads of torque and it's justified, it is one, so is the 999 series. Yes the I-4 liter bikes out there produce more power and some more torque. Never said they didn't, but it all comes on higher in the rev range
From your own website source
 

·
Registered
Joined
·
8,798 Posts
BCebrian said:
The energy losses in a Desmo system is lower than in a conventional spring and keeper system is fact, not my opinion. The added mass of the extra rocker arm is negligible compaired to the resisttance of conventional valve springs. You can look it up anywhere you want.
You are absolutely right on energy losses. No argument. But the additional rotating mass isnt negligable. While it has little effect on energy losses, its yet enother eccentric spinning part, adding vibration, and robbing from the redline. And as we both agree Revs=HP.


I apologize I was referring to the “R” model when I stated that HP but forgot to add the designator. The 119 HP number is correct for the '03/04 base 999, the '05 999 got last years 999S deep sump motor and puts out approx 123 HP at the rear, the 999S is now using the '04 R motor good to about 127, and the R is good stock to about 140. It also depends greatly on what dyno you're using use a different dyno get slightly different numbers. The comparative numbers are what is important, good you posted them as you can see the 999R puts out a lot more HP than a RC-51 so yes Ducati does have the most powerful twin. Of which I was referring.
Yep. Looks like the 999R is the most powerfull one (There might be strange exotics out there, but I'm not going to count them)


Twins spin slower due to the reciprocating mass of the pistons and rods, not the valve train. I-4' don't need as much displacement for an equivalent power output because they can spin faster due to the lighter pistons/rods. RPM=HP, the key is piston speed, the slower the piston speed the faster the engine can rev, basically short stroke, high RPM =more power. Again engineering fact. Yes, the companies that produce twins want them to spin faster, Ducati’s answer is the new Terrastratta (narrow head short stroke) engine that can rev higher than the old desmoquattro engines. But they still can’t match the I-4’s lower piston speed. So Twins are tuned to put out power at the lower RPM, Your own post show that. I never said top end power wasn't useful, but a wider power band is more useful to the average street rider. It’s a different story on the track where you spend most of your time at high RPM.

Inlines dont get higher red lines just due to lighter pistons, but due to the balancing effect possible there. Obviously if you add the mass of 4 pistons and connecting rods, they will be heavier than the duc's 2 larger ones. However, inlines can use some of the pistons to counter the recipocating motion of the other pistons (on hitting TDC as another is hitting BDC, since they are on the same plane the vibrations cancel out, leaving minor torque force longitudnal to the bike).
The hard limiter on rpm is ussually listed as linear piston speed, but that factor is easily controlled with stroke. Valve float is still a problem for the engineers to deal with, however, its anticipated now, and can be worked in.
Ok lets get to usable power. If you look at the bottom of the two graphs I posted it shows '05 999R in comparison to the rest of the liter bikes. (Its good that we are using the same source too).
The 999 produces more power in the 3.2-3.8krpm range. it then lags untill the 5k range. The engines produce almost the same power untill 7k, when the duc gets a boost overpowering to 8k before dropping again at 9k. The duc mulls a bit more before redlining. The inlines continue to produce power for another 2k.
My point is not that the inlines have a higher top end, that much is obvious. Its that the gsxr and zx10 are doing equally well (and possibly better, due to linearity) in the midrange, which is supposed to be the domain of the twin. Based upon that I would have to say that the duc's are using outdated tech. Very very refined outdated tech.
The twins have (or had) another advantage over inlines, in the form of better traction. This is starting to be immitated by the big bang firing order, so I guess we may not see many twins in the future.
The motors used to be cheaper to produce (less parts), however they actually become more difficult, once you consider sophisticated valve trains, and having to build 2 heads instead of just one. Then there is the fact that they are heavier.


There is more to useable power than pure HP, the thing you not addressing is Torque, twins are torque monsters and that torque hits lower down in the rev range and holds longer in a twin. The RC-51 has the same rep for loads of torque and it's justified, it is one, so is the 999 series. Yes the I-4 liter bikes out there produce more power and some more torque. Never said they didn't, but it all comes on higher in the rev range
Oh the old torque comment. Check out a thread in daily drive "qustion for vash" or something like that. We have discussed it to death. My personal belief is that what you are really referring to is low end power, but everyone likes to call it torque (which it isnt). As I demonstrated the inlines have as good of a low end as the twin over 4k rpm. Under 4k niether engine is worth a sh*t really.


I wonder when they start doing something really fancy with a single?
 

·
Registered
Joined
·
9 Posts
Vash said:
You are absolutely right on energy losses. No argument. But the additional rotating mass isnt negligable. While it has little effect on energy losses, its yet enother eccentric spinning part, adding vibration, and robbing from the redline. And as we both agree Revs=HP.


I guess I'm confused as to the "additional Rotational Mass" you are talking about. There isn't any additional rotational mass. Two cams per head operate the rocker arms. The arm are pivoted and stationary and do not rotate. Except for the small keeper springs the system is very light and easy to turn.


Inlines dont get higher red lines just due to lighter pistons, but due to the balancing effect possible there. Obviously if you add the mass of 4 pistons and connecting rods, they will be heavier than the duc's 2 larger ones. However, inlines can use some of the pistons to counter the recipocating motion of the other pistons (on hitting TDC as another is hitting BDC, since they are on the same plane the vibrations cancel out, leaving minor torque force longitudnal to the bike).
Not really true, the mass of the twin system is necessary larger and heavier to handle the increased loading from the heavier power pulses. That is why historically twin sport bikes are heavier than I-4's. Also do to the heavier lower end necessary to handle the bigger power pulses. The balancing effect is used in BMW twins and they can't rev either, BMW has gone to and I-4 for it's performance bikes. Piston/rod weight is a huge factor. The spinning mass is lighter so it can change direction faster. Even if the total rotational mass were equal between the two bikes. The reciprocating mass is much lighter per cylinder makes that unit (piston/rod) lighter and faster. Short stroke motor = slower linear piston speed I agree, but it sure is easier to change that direction with a lighter piston/rod. That is why every year the factories are finding ways to lighten the piston/rods lighter reciprocating mass. That way they can increase redlines and that way increase power.

The hard limiter on rpm is ussually listed as linear piston speed, but that factor is easily controlled with stroke.
Actually short stroke it is a compromise and it entails heat problems, but as new materials come on line the problems are being worked out. Too much to go into here.

Oh the old torque comment. Check out a thread in daily drive "qustion for vash" or something like that. We have discussed it to death. My personal belief is that what you are really referring to is low end power, but everyone likes to call it torque (which it isnt). As I demonstrated the inlines have as good of a low end as the twin over 4k rpm. Under 4k niether engine is worth a sh*t really.
I understand torque quite well and have been tuning engines for both HP and torque output for many years. I'm referring to the mid range pull of a modern sporting twin. Granted modern liter bikes have monster midranges and the traditional advantage of the twin had eroded. Still twin power still manages to be easier to control and use due mainly to what you refer to. The big bang motors where originally developed for the 500GP two stroke bikes and have been tried in fact by Ducati in it's MotoGP bike. No one is using the system at the moment in a production street bike that I know of. So the advantage still stands if not as prominent as it once was.


I'd love to see a modern single, unfortunately It most likely wouldn't sell here. Remember the STX?
 

·
Registered
Joined
·
8,798 Posts
First of, let me say that its a pleasure making your aquantance. Nice to talk to someone that really knows their stuff (But still likes ducs :confused: :D)

BCebrian said:
I guess I'm confused as to the "additional Rotational Mass" you are talking about. There isn't any additional rotational mass. Two cams per head operate the rocker arms. The arm are pivoted and stationary and do not rotate. Except for the small keeper springs the system is very light and easy to turn.
I should have said "Additional recipocating mass" Another rocker arm adds yet another vibration wave. Vibration is the enemy of all things mechanical.


Not really true, the mass of the twin system is necessary larger and heavier to handle the increased loading from the heavier power pulses. That is why historically twin sport bikes are heavier than I-4's.
Really? I always thought that 4 piston+4rods would be heavier than the larger 2 combo. I attributed the overall higher weight of the engine to housings (more complicated and heavier block, 2 heads instead of 1) But I have been wrong before.
Also do to the heavier lower end necessary to handle the bigger power pulses. The balancing effect is used in BMW twins and they can't rev either, BMW has gone to and I-4 for it's performance bikes.
Or whatever it is they call that strange thing....
Piston/rod weight is a huge factor. The spinning mass is lighter so it can change direction faster. Even if the total rotational mass were equal between the two bikes. The reciprocating mass is much lighter per cylinder makes that unit (piston/rod) lighter and faster. Short stroke motor = slower linear piston speed I agree, but it sure is easier to change that direction with a lighter piston/rod. That is why every year the factories are finding ways to lighten the piston/rods lighter reciprocating mass. That way they can increase redlines and that way increase power.
They arent just reducing piston rod, but valves, cams, etc. the the need to get rid of that extra rocker arm.

Actually short stroke it is a compromise and it entails heat problems, but as new materials come on line the problems are being worked out. Too much to go into here.
Not sure I follow. Short stroke results in lower torque, that much is obvious but heat problem? I can see an indirect case (Short stroke>higher redline> more heat) but nothing direct (like higher comp ratio>more heat)



I understand torque quite well and have been tuning engines for both HP and torque output for many years. I'm referring to the mid range pull of a modern sporting twin. Granted modern liter bikes have monster midranges and the traditional advantage of the twin had eroded. Still twin power still manages to be easier to control and use due mainly to what you refer to. The big bang motors where originally developed for the 500GP two stroke bikes and have been tried in fact by Ducati in it's MotoGP bike. No one is using the system at the moment in a production street bike that I know of. So the advantage still stands if not as prominent as it once was.
Virgin yamaha tried using the big band firing order on an R1. they were disqualified from the race, however newer rules specify that big bang order can be used so long as it was stock on the bike. So I expect to see production big bangers by '08.
 

·
Registered
Joined
·
9 Posts
Thank you also for this discussion. Lots of fun to find someone willing to talk intelligently. Kind of rare on the internet. I've enjoyed this very much. :thumb:

The heat problems are in the combustion chamber. the shorter the stroke the larger the piston surface but smaller total cylinder surface to absorb the heat. Plus more power equals more heat. I can find the math somewhere but as pistons get wider and flatter the less material is avail to cool the chamber, chamber get's too hot and you get detonation. One of the reasons you don't see cylinder liners anymore besides weight, the new coatings transfer heat better. Suzuki's Advanced cooling system on the older air/oil cooled GSXR's, Oil jets under the piston crown to cool the piston due to the short stroke design. As the GSXR's stroke shortened and the bike became even more powerful Suzuki had to finally go to liquid cooling to control the chamber temps.

I like my second rocker arm, thank you very much, even if I'm not so thrilled by having to measure everything every 6K. You're not the only one who'd like Ducati to go to a conventional valve system. There are a lot of Ducati owners who would trade in their old bikes for one. But Ducati going away from the Desmo system is kind of like asking Harley to build a inline 4. Even if it most likely would make the bike more attractive to buyers with a less arduous maintenance schedule. Besides, It's a bunch of fun to get intimite with your bike every few months.
 

·
Registered
Joined
·
8,798 Posts
Ok I see it. Proportionally more of the combustion surface area is made of the piston which is not really cooled. Well aside from the oil jets, more on that in a second. So with roughly 40% of the available surface area made of piston, another 30% or so made of uncooled valves, only a very minor part is left for the cooled sidewalls. Possible solutions?
Ceramis pistons, or at least ceramic coated pistons. The stresses on the piston are actually relativly low (lots of pressure, but no localized twisting forces) The only part that would have a problem would be the rings where the pin for the conecting rod slides in. (Dont know my terminology). A Steel (Or Ti) insert should solve that.
Then there is the problem of detonation. I suspect that it could be dialed out. If the piston is capable of withstanding the extra stress, all one needs is higher octane fuel (hey that e85 thing...) and some tuning of the timing system.
Advantages:
Hotter possible combustion chamber=Efficiency, Power
Disadvantages:
Cost.

With the reliability being the big unknown. Hmmm, I could probly get some piston coated with SiNi if you got somewhere to test the theory.

BTW, the oil jet to the bottom of the piston thing isnt new, nissan has been doing it in the 80's. I was under the impression all major manufacturers were using that trick.
 

·
Registered
Joined
·
9 Posts
Yep, to all, that is why most designs are a compromise in some way both in engineering and in cost. (plus government regs to deal with)

It's never as easy as it sounds to design what we can buy at the local dealer for about $10k would have cost $50K, 20 or so years ago. Your ZX10 is every bit as fast and capable as a works racer from just a few years ago. My 749 puts out about the same HP as the origninal 916 and is much more reliable. There is a reason they pay those design engineers so well. Who would have thought 10 years back that we would be getting 160 hp from a 400lb motorcycle, or 100hp from a 750cc twin. Good Stuff. One of the reasons I'm very timid as to modifiying what some guy who's a lot smarter than I am designed .
The ceramic coated piston is a popular performance mod in the Ducati circles but is a very expensive one.
 
1 - 20 of 23 Posts
Top