[email protected] (Dave Williams)
mc-chassis-design  26 Jan 2000
- -> extremely elegant design / mechanism -  it is said that with all
  -> those arms and pivots there is a limit to the valve acceleration that
  -> desmo can achieve.
- On the other hand, you can't float a valve either.  Formula 1 cars were 
  changing springs every race before the move to pneumatics.  Some of the drag 
  race guys in the USA change springs every day. 
- I've been predicting a comeback for the desmo systems for a while, though.  
  As various governments ratchet their mandatory fuel economy regulations ever 
  tighter, desmo would offer a hair more efficiency than springs.  Cost isn't 
  really an issue any more, not with "basic" automobiles over US$15,000 and 
  with more standard features than you could buy on a Rolls in the '60s. 

[email protected] (Dave Williams)
mc-chassis-design  27 Jan 2000
- -> Does anyone know how the volumetric efficiency of the desmos and
  -> springers compare?  This would be a better measure of the efficacy of
  -> the desmo design, as least from a performance perspective.
- *Theoretically* you could design a 'squarer' cam profile for a desmo engine, 
  though it's not really an issue as long as valve/piston clearance is 
  adequate. 
- The problems you're mainly addressing with a desmo layout are spring 
  fatigue, spring heat, spring surge and friction.  Valve springs are highly 
  stressed in most high RPM applications; in some drag racing cars their life 
  is measured by the run, and the number is sometimes down in the single 
  digits.  Springs also generate lots of heat all by themselves, just from 
  flexing.  Surge is often a factor, requiring dampers or interference springs 
  (with attendant friction and wear), and sometimes compromises to the cam 
  profile.  All heat generation outside the cylinder is lost as far as we're 
  concerned. 
- Friction is still an issue even with roller followers, not to mention the 
  additional stress the spring puts on all the parts.  A desmo system also has 
  some friction, of course, but not necessarily more than a non-desmo layout - 
  remember, due to clearances, either the opening or closing mechanism is 
  under load, but not both at the same time. 
- Springs work well enough for most circumstances.  Desmo layouts are more 
  complex and more expensive.  Racing engines are now getting into regimes 
  where conventional spring technology is barely able to keep up. Production 
  manufacturers will look to desmo valves for less friction, less wear, and 
  for marketing reasons.  The cost of motor vehicles has climbed so high, and 
  profit margins so wide, that the additional cost of a desmodromic valve 
  system isn't really a factor any more.