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Steps In CVT Operation

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RonGeorge View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote RonGeorge Quote  Post ReplyReply Direct Link To This Post Topic: Steps In CVT Operation
    Posted: Sep/19/2010 at 10:18pm
Condensed and to the point, without all the nonsense and mumbo jumbo in a 200 page book. I wrote this to myself last year from observations and plenty of reading, and I figure this would help someone now who still hasn't figured out how this 1950's technology works.

Steps in operation.

1. FREE RUNNING
Driving clutch is disengaged to permit the engine to be started and to idle when the baja car is at a stand still. Idle speed is the rotational speed the engine runs on when the engine is uncoupled to the drivetrain and the throttle pedal isn't depressed (generally measured in revolutions per minute, or rpm, of the crankshaft) of a combustion engine. At idle speed, the engine generates enough power to run reasonably smoothly and operate its ancillaries (water pump, alternator, and, if equipped, other accessories such as power steering), but usually not enough to perform useful work, such as moving the baja car. For a passenger-car engine, idle speed is customarily between 600 rpm and 1,000 rpm. 1700 is our recommended idle speed, atleast this is what I saw in the 2009 Baja Rulebook somewhere.
Primary Flyweight Force < Spring Pretension

2. ENGAGEMENT

The engine speed is high enough that it produced useful torque to start moving our baja car.  When the sheaves engage the belt, the flyweight force is just overcoming the pretension of the pressure spring.  Engagement speed depends on the pretension the spring has been compressed to when installed in the engine. Flyweight Force <= Primary Spring Pretension

3. Full Engagement/Clutching
After the flyweight force overcomes the spring tension, the RPM has to increase along the low ratio line and give more sheave pressure on the belt until there is no slippage and full engagement. The clutching phase begins when the flyweight force overcomes the pretension, and lasts until the flyweights have generated enough side force to transfer the engine torque without slippage.  Flyweight Force > Primary Spring Pretension

4. Low Ratio Acceleration (Theoretical)
Baja Car will accelerate along low ratio line and continue to do so until the flyweight force has become large enough to overcome both the pressure spring and the side pressure of the belt on the driven sheave.   Flyweight Force > Primary Spring Pretension, Flyweight Force = Driven Clutch Belt Pressure

5. Shift Out Point
Centrifugal force from flyweights will be large enough to equalize tension on the belt from the driven clutch. Ratio starts to change from here-on with RPM change. Ideally shift out point should be on the power peak of the engine for maximum performance.  Flyweight Force > Primary Spring Pretension & Flyweight Force = Driven Clutch Belt Pressure

6. Straight Shift
Ideal shift curve is straight line between low and high ratio at peak power engine RPM, while MPH increases. What affects the straight shift - loads and rates of the pressure spring and flyweight curvature. Straight shift is obtained by matching curvature and spring rates; correct engine speed is dependant on the mass of the flyweights. Flyweight Force > Primary Spring Pretension, Flyweight Force > Driven Clutch Belt Pressure

7. Over-run
In over-run, the clutches are shifted all the way out and the flyweight forces have no more influence on the engine speed.

Disclaimer : In theory, all this sounds good. In reality, nothing happens according to paper. The belt wears out with time which affects performance significantly and the engine power and torque curves are very different from what's on paper. To tune the clutch, dyno the engine and don't put blind faith in manufacturer graphs. We learnt it the hard way.

Here's a handy calculator to figure out centrifugal force. http://www.calctool.org/CALC/phys/newtonian/centrifugal   Enjoy tuning!
-Ron George
Systems Engineer (Cummins Turbo)
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Vladmir123 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Vladmir123 Quote  Post ReplyReply Direct Link To This Post Posted: Dec/29/2012 at 8:23am
Great post. Sorry I'm a bit late with this one. But I had one query .. How did u guys finally go about tuning it ? All of the able mentioned sounds very good. But in practice, very hard to implement. What are the tools and techniques u guys used ? Is it absolutely necessary to use a dyno ?
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ballast View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ballast Quote  Post ReplyReply Direct Link To This Post Posted: Jan/06/2013 at 12:00am
i ve read in a cvt tuning book that the 'shift-out point' depends on the weight of the vehicle. that is if the vehicle is heavy for the motor the shift out point has to be set higher than the max engine power so that the cvt settles at the max power rpm in straight shift.Similarly if the vehicle is lighter for the motor, the shift out point should be lower than the engine max power.

my question is that "how to determine if the engine is heavier or lighter for the vehicle?"
as far as i figure, it can be determined only outdoor while testing. 
Is there any analytic method ?

thankyou.
jacking with the screw
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