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  • CVTs -- push belts or tension belts?

    Subaru has replaced the gearbox in its non-turbo boxer engine with a variable-pulley CVT. It appears to use a tension belt between the pulleys: http://www.subaru.com/engineering/transmission.html.

    The CVTs in the Saturn Vue, the Honda Civic, and many other cars use a van Doorne push belt (illustrated here http://auto.howstuffworks.com/cvt2.htm) between the pulleys.

    I thought the push belt (which is rather counter-intuitive) was invented to solve a problem with tension belts. What's changed?

    Last edited by aostling; 07-07-2009, 04:59 PM.
    Allan Ostling

    Phoenix, Arizona

  • #2
    You're right, that looks like a tension belt. Knowing Subaru, I'm sure they have a good reason for using it.

    That cutaway of the Subaru CVT doesn't look like it's been simplified very much. It's hard to tell which shaft the lower pulley is attached to. Any idea what the white/beige thing is that's riding on the belt?

    I can see how the push belt works, and it looks like the advantage is a larger contact area with less relative motion between the segments. Either one seems to fall into the category of "proven technology".

    I recently drove a Nissan Altima with the CVT. I don't know which type belt they use, but the performance was flawless.

    The experience was only marred by the stupid salesman who drove the car for the first part of the test drive. He was obsessed with the "manual shift" mode of the CVT, and he over-revved the cold engine several times showing it off. What a total doofus!!

    Roger
    Last edited by winchman; 07-08-2009, 06:42 AM.
    Any products mentioned in my posts have been endorsed by their manufacturer.

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    • #3
      Ells Belles, Sorhubarb have re-invented the Daf Daffydildo.

      Does this one go just as fast in reverse as the original????

      Regards Ian.
      You might not like what I say,but that doesn't mean I'm wrong.

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      • #4
        Jeez, Subaru has re-discovered the Reeve's Drive.

        Is there a plastic bushing you need to replace on a regular basis?
        "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

        Comment


        • #5
          Just when you thought Subie's drivetrain couldn't get any worse,
          They are by far the most inefficient drivetrain of the Japanese auto makers with all their 90 degree conversions and now they've added the magical frictional qualities of the CVT along with a dork converter ---- sweet

          Lets just hope the subie engineer's got their act together with a nice sized trans cooler this time cuz that puppies gonna be running its fluid through the "high speed blend mode" all the time, yes, even with a dork conv. lockout.

          What's the sales pitch on that thing anyways - "now you can waste fuel while being bored"
          Or ----------- "CVT's - the next best thing to staying home."

          Comment


          • #6
            Originally posted by A.K. Boomer

            What's the sales pitch on that thing anyways - "now you can waste fuel while being bored"
            According to the article in Drive (the quarterly magazine sent to Subaru owners), the benefits of the new CVT are:
            (1) Improved fuel efficiency, due to the engine spending more time in its optimal power range.

            (2) No gear hunting when driving uphill.

            (3) More passenger leg room resulting from the elimination of the gearbox.
            Allan Ostling

            Phoenix, Arizona

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            • #7
              Originally posted by aostling
              the benefits of the new CVT are:
              [indent](1) Improved fuel efficiency, due to the engine spending more time in its optimal power range.
              Reeve's Drives are not very efficient -- the Bridgeport Vari-speed head has a 2HP motor, and the step pulley version has a 1 1/2 HP motor. Same deal with my Clausing lathe (which has a hydraulically operated Reeve's): 2 HP motor, and the step pulley version has a 1 1/2 HP motor.

              If you've ever made the mistake of putting your hand on the variable-width sheave on a CVT after it's been running for awhile, they get really hot.
              "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

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              • #8
                They've been using similar drives in snowmobiles for decades and in ATV's for the last 20 years or so. Inefficient? The belt drive system in my ATV gets pretty warm. However, a "regular" automatic transmission runs fluid at a couple hundred degrees through a radiator to remove all the heat it makes. If its heat, it's not motion, so by definition, that's a lot of ineficciency.

                As opposed to a Reeve's type drive where the user controls the spacing of one of the sets of sheaves to control speed, the typical CVT uses flyweights and resistant springs to establish a typical acceleration curve. The spacing of the two sets of sheaves is continuously changing during acceleration. This the side effect of allowing the engineers to keep the engine in its "power band" all the time....so there's where you gain efficiency.

                Paul
                Paul Carpenter
                Mapleton, IL

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                • #9
                  Originally posted by pcarpenter
                  They've been using similar drives in snowmobiles for decades and in ATV's for the last 20 years or so. Inefficient? The belt drive system in my ATV gets pretty warm. However, a "regular" automatic transmission runs fluid at a couple hundred degrees through a radiator to remove all the heat it makes.
                  Sure, but a traditional automatic transmission is not efficient either, because of the torque converter. Like you say, the heat exchanger is dumping waste heat from the fluid coupling.
                  "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

                  Comment


                  • #10
                    Originally posted by lazlo
                    Reeve's Drives are not very efficient -- the Bridgeport Vari-speed head has a 2HP motor, and the step pulley version has a 1 1/2 HP motor. Same deal with my Clausing lathe (which has a hydraulically operated Reeve's): 2 HP motor, and the step pulley version has a 1 1/2 HP motor.

                    If you've ever made the mistake of putting your hand on the variable-width sheave on a CVT after it's been running for awhile, they get really hot.
                    One tenth of a horsepower = 75 watts. Put your hand on a 75 watt light bulb after it has been running for a while and see which is hotter.

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                    • #11
                      CVT's work great on things like snowmobiles, where you have enough power not to worry about efficiency or fuel economy.
                      When I worked for Ford, we had the Mazda Tribute Hybrids (prototypes of the Ford Escape hybrid) for testing at the proving ground. They all had CVT transmissions. Acceleration was leisurely, fuel economy was in the low 18's and they messed up constantly. About that time, Ford was offering CVT 's in the Ford 500 and they soon earned the nickname of "Ghost Shifters". Technicians were going crazy trying to diagnose the sometimes erratic and mysterious antics of the CVT. I found it amusing because the alternate transmission was the six-speed automatic built by none other than GM's Hydromatic division.

                      True, Lazlo. they do build up considerable heat in the torque converter, but modern auto transmissions are equipped with torque converter clutches, which eliminate any heat buildup in the converter by locking up mechanically,
                      like a "direct drive"...One nice thing about torque converter clutches....they allow an automatic to get the same or better fuel economy than a manual transmission.
                      No good deed goes unpunished.

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                      • #12
                        Sure, but a traditional automatic transmission is not efficient either, because of the torque converter. Like you say, the heat exchanger is dumping waste heat from the fluid coupling.
                        That was my point. I was responding to Boomer's comment about how the friction of a belt *added* inefficiency. As compared to what? A traditional automatic which is *horribly* inefficient as evidenced by the heat load being high enough that it requires a radiator to remove heat from the transmission?

                        I think it would require some careful analysis to know for sure the real difference in efficiency, but CVT's are air cooled as far as I know, and some like the one in my ATV are even a closed system (mostly closed with only a tiny vent for pressure changes). If it tells you anything, the belt housing is plastic.

                        They are a neat system in that it requires no control other than throttle application. All control is at the input, and the load seen by the engine is much closer to constant than nearly any other system. The weak link is the belt. The ones used in snowmobiles and ATV's are highly tunable as well. You have both a primary and secondary clutch (input side and output side) and you can change the spring rate for either as well as the flyweights and their profile on the primary clutch to produce different acceleration curves. I have a kit in mine that allowed for much faster lockup at low speeds. They tend to be mushy on the front end to allow the engine to get up into its power band quickly. Given plenty of engine torque, its actually desireable when you are crawling slowly to get the belt grab much tighter, sooner. This makes for less heat and longer belt life.

                        Paul
                        Last edited by pcarpenter; 07-08-2009, 05:08 PM.
                        Paul Carpenter
                        Mapleton, IL

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                        • #13
                          One tenth of a horsepower = 75 watts. Put your hand on a 75 watt light bulb after it has been running for a while and see which is hotter.
                          You are comparing a filament which is almost 100 percent inefficient to a transmission. That filament is heating a thin glass shell with almost no mass. Try heating a transmission to 200+ degrees (Fahrenheit) with a 75 watt lightbulb. Trust me, an automatic transmission is wasting well more than 1/10th of a horsepower.

                          Paul
                          Paul Carpenter
                          Mapleton, IL

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                          • #14
                            Originally posted by aostling
                            According to the article in Drive (the quarterly magazine sent to Subaru owners), the benefits of the new CVT are:
                            (1) Improved fuel efficiency, due to the engine spending more time in its optimal power range.

                            (2) No gear hunting when driving uphill.

                            (3) More passenger leg room resulting from the elimination of the gearbox.


                            Those guys are hillbillies --- They may be right if its an archaic engine with a hillbilly behind the wheel who doesnt know how to drive for optimum efficiency,
                            But now we have things like variable valve timing and duration, honda nissan and yodertoy all have proven dependable units that have been in production vehicles for quite sometime now, their matched to the transmissions to fill in the gaps far past the high and low shift points (high and low shift points when someone is driving for efficiency) and therefore have made the quest for the CVT all but obsolete. Why not take care of the problem where it originates? the engine, then you can run a free flowing drivetrain and surpass anything a CVT can do... common sense is a wonderful thing.

                            As far as the no gear hunting when going up hill, I don't "hunt for gears" --- I use the appropriate one for the task at hand and get the job done, If I happen to be in an automatic and the speed I want is close to the shift points causing it to seek then I use it manually and once again don't "hunt" -- just use the appropriate one, Please people - stop trying to take the last little crumb of thinking out of my driving or I really will fall asleep at the wheel (more times than I already do)

                            And taking a look at that trans and comparing the bell-housing to the length -- there's no more room for "passengers" It doesnt eliminate a gear box, it still is and its not only just as big, its got a higher hump in the middle, Junk.

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                            • #15
                              Originally posted by pcarpenter
                              That was my point. I was responding to Boomer's comment about how the friction of a belt *added* inefficiency. As compared to what? A traditional automatic which is *horribly* inefficient as evidenced by the heat load being high enough that it requires a radiator to remove heat from the transmission?



                              Paul



                              Actually that's why I threw in the clause;

                              "Lets just hope the subie engineer's got their act together with a nice sized trans cooler this time cuz that puppies gonna be running its fluid through the "high speed blend mode" all the time, yes, even with a dork conv. lockout."

                              Any automatic worth its weight in shop rags is going to have a lock out torque converter, So how much of a difference does this make? depends on design and the mode the trans is in but here's a little cue for you, a properly designed automatic can achieve better fuel economy than a manual trans on the highway, The reason for this is the fact that todays automatics are basically a manual trans when the torque converter locks out at highways speeds, Now couple that with two other factors that outdue the manual, one is the auto runs in thinner fluid --- the second is that it uses a superior design in gear loading -- its called the planetary system and instead of pushing gears into bearings and trying to force them apart they equally load them in at least three 120 degree directions, This is a great way of transmitting power, this is why you can go to www.fueleconomy.gov and research certain vehicles and actually verify that certain autos get 1 or 2 mpg's better than their manual options.
                              Like my dad used to say when pointing to his head and tapping on it with his finger "kidneys"...
                              Last edited by A.K. Boomer; 07-09-2009, 10:00 AM.

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