By on May 6, 2015

acceleration. shutterstock user Sensvector

Jesse writes:

Hello, Sajeev.

My 2013 Outback 2.5i is fine and I don’t have any questions about it. Instead I wonder:

1. Why do car reviews measure acceleration in time but deceleration in distance?

2. Why do high performance electric cars need conventional brakes? I think there was a Mini concept a few years back that had 4 in-wheel electric motors that did all of the accel/decel.

3. Why don’t cars with CVTs have a ‘downshift’ button? Is it too hard on the transmission? Should I stop using the paddles to do so?


Sajeev answers:

Perhaps simple queries deserve simple answers. So let’s do this thang…

1. Relevance: people want to know how many seconds it takes to get to a certain speed, but not how many seconds it takes to stop.  A certain stopping distance is far safer for them. Even if reaction times vary and press cars come delivered with varying levels of brake degradation (from previous journos cooking the pads and glazing the rotors), apparently stopping distance and acceleration times still means something to readers.

2. After driving Code Brown, I can guarantee you that traditional brakes are still important in the world of regenerative braking. Nothing stops like a disc brake, both in terms of speed retardation on the highway to the ability to modulate from a high rate (for safety on the highway) to a low rate (for comfort at a red light). Even when you put Code Brown’s regen brakes to full power, they can’t stop in a panic situation. I suspect the G-forces created via panic stop would spike hard enough to damage some component of the propulsion system. No surprise, Wikipedia has a great article on its limitations.

3. Why? Because CVTs don’t need one.  You downshift (so to speak) via throttle inputs. You want maximum acceleration? Just go right ahead and bury the throttle in the carpet. Otherwise let the CVT work its fuel economizing magic. Downshifting (and upshifting) algorithms in CVT gearboxes exist to acclimate drivers to CVTs: a stepping stone (get it?) to the future. I mentioned the parallels to motorized seatbelts, I still believe this is true.

Off to you, Best and Brightest!

[Image: Shutterstock user Sensvector]

Send your queries to [email protected]com. Spare no details and ask for a speedy resolution if you’re in a hurry…but be realistic, and use your make/model specific forums instead of TTAC for more timely advice. 

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28 Comments on “Piston Slap: Hat Trick!...”

  • avatar
    heavy handle

    Q2: regenerated power has to go somewhere, which means that regenerative braking is limited by the battery’s ability to accept a charge. You could get around that by using huge resistors (heating elements, in effect), or capacitors, but why bother?

    • 0 avatar

      The bigger issue is that the braking force is proportional to the speed of the motor. Once it gets too slow the force is next to nothing and it is zero at zero rpm. So you absolutely need the friction brakes to come to a complete stop and to hold the vehicle stopped.

    • 0 avatar

      +1. I’ve been caught off-guard in my EV when trying to slow by ‘shifting’ to L, which makes the Focus EV charge more aggressively, effectively doubling deceleration. You get used to this kind of shifting pretty quickly. But when the battery is topped off and you shift to slow for the first stop sign of the day, there’s no regen possible because the power has no where to go.

    • 0 avatar

      A better solution to get around it is incorporating a large capacitor into the battery system. They excel at high power charge/discharge, so let the regenerative braking energy go into it.

  • avatar

    I thought he meant downshift to use engine braking going down a steep hill or something.

    • 0 avatar

      3. What he (I) meant was if the CVT can constantly adjust for acceleration and hold rpms steady, why can’t they do the same for decelerating? I currently use the paddle to ‘downshift’ for planned stops but why can’t CVT downshift ‘infinitely’?

      For clarity the 2013 is stepless(normal CVT) until paddles are used in which case there are 6 pretend gears.

      How many seconds does it take a P85D to stop from 100kmh?
      That motor can spin either way, right? Yes, that would be powered (not regen) but could it beat discs (or assist them?)

      4. In your reply you said they were good questions :)

  • avatar

    “Downshifting (and upshifting) algorithms in CVT gearboxes exist to acclimate drivers to CVTs”

    This annoys the everloving s— out of me. My 2015 outback “shifts” under acceleration because apparently that’s what people expect (2011-2014 didn’t do that I’m told). So basically the marketers told the engineers to make it worse on purpose because that was an easier obstacle to overcome than giving the buyer 20 min to get used to a superior technology.

    • 0 avatar

      I’ve been wondering if it was buyers doing the complaining, or if it’s just the motoring press.

    • 0 avatar
      heavy handle

      I doubt that there’s much of an efficiency difference between having the engine rev from 2000 to 3000 and having the gearing vary while keeping the engine at 2500. Certainly the first option can provide much better feedback to the driver.

      I guess the most efficient option would be to mimic the noise of a revving engine in the cabin, while keeping RPM constant.

    • 0 avatar

      In theory it should be possible to reprogram it to not “shift”.

      Don’t know if a dealer can do it, or it’ll have to be aftermarket, or if the aftermarket offers it yet… but it *ought* to be possible.

    • 0 avatar

      Similarly it drives me nuts when vehicles without torque converter transmissions exhibit creep. Completely needless and nonsensical.

      • 0 avatar

        That’s the only thing I don’t like about my car. The creep feature makes it more difficult to come to a really smooth, like you can’t tell when you’ve stopped, stop.

  • avatar

    When I had an Impreza CVT for a week, I found it easy to tap the left hand paddle to “downshift” for slow deceleration like coming down a hill to a stoplight. It worked extremely well, the same as the 5 speed auto in my Legacy GT does.

    The floor shift lever can also be placed in manual position, and will hold the “gear” you have selected by paddle, or alternatively, you can do your further downshifts/upshifts with it.

    The paddle change to another gear last only as long as you don’t give the car gas for several seconds – the floor shift holds them until you either put the car back in Drive or manually change gears.

    So keep on using the downshift paddle – I’ve been doing it steadily for 7 years, and using the left pinky while holding the wheel is simplicity itself.

    Sajeev’s response just shows he hasn’t driven a Subaru automatic, CVT or 5 speed.

    • 0 avatar

      100% correct. In theory, I like CVT tech as-is, mostly on the merits of efficiency. Maybe I’d just love Subaru’s execution.

      I can’t imagine liking the fake gears hunting for something…no matter how well they work. It’s a waste relative to the simplicity of a “normal” CVT.

  • avatar

    I’m still fully against CVTs. Don’t nobody got time for REEEorrrreeeRRRRReee.

    • 0 avatar

      Try one. My Nissan experience has shown me that it is superior to 7-9 speed auto transmissions for routine “A to B” driving. Once you get past 6 gears, these transmissions never stop shifting (very annoying) and you can catch them flatfooted easily. In the CVT, for any slight movement of my right foot there is a corresponding change in RPM and ratio with no dead spot or need to downshift 2-3 gears. I thought I would hate it being a manual transmission driver, but it really has pleasantly surprised me.

    • 0 avatar

      I remember when Jack did a story about driving a used Ford FiveHundred long distance he reported the best method was to simply stomp the accelerator until you reached your desired speed.

    • 0 avatar

      If it makes you feel any better, I think CVT’s are nearly as old as hydraulic automatics. I don’t know exactly, my dad said the combine harvester on his grandad’s farm, presumably a Gleaner from circa 1965 but possibly much older had something like two or three manual forward gears plus vari-drive, which was a manually operated CVT, sort of gears between gears. I think they’re still common on modern combine harvesters.

      • 0 avatar

        I’m not well versed in the Gleaner models, but I know that the M2/L2 and comparable John Deere 6600/7700 were available with a full hydrostatic transmission. Our 6600 is gear-drive, but hydrostatics were available.

        Today, I think that they’re all hydrostatic, but I can’t say for certain….

  • avatar
    Arthur Dailey

    The answer to #3 is simple: replace it with a manual, so it is always in the gear that you want.

  • avatar

    You nailed one of my pet peeves Sajeev. You “shift” an automatic through throttle input vs road speed/load/etc.

    For some reason I find this much harder on modern, drive by wire cars. I find they wait wait wait! to downshift on moderate inputs, then you’re in a panic and floor it (which you shouldn’t need to do to summon a lower gear) and now it jumps three gears and surges forward.

  • avatar

    You don’t need a shifting function on CVTs under acceleration, but you do need some kind of way to change the ratio if you want to use engine braking to avoid cooking your brakes on long downhill stretches. Nissan’s CVT has a “sport” mode which works pretty well for this purpose.

    I like CVTs for cars with no pretense of enthusiasm. If they are well executed they allow very smooth driving. And I completely don’t understand why people apparently need the engine speed to go up and down at a constant level of acceleration. Maybe it’s because I drove hybrid buses (with a Toyota-style 2-motor, planetary gear setup) professionally, but having a constant RPM level when you request a constant power level seems totally natural to me.

  • avatar

    AFAICT, deceleration is measured in distance because it’s a test of the car’s braking ability, not necessarily the engine (I don’t know of many new car owners who use engine braking with an auto trans). It’s an issue of safety rather than performance.

  • avatar

    I think braking performance measured in time would be harder to understand and less practical. When you see something ahead of you, I think it is much easier to estimate its distance instead of estimating it’s speed relative to your car and therefore how many seconds it is ahead of you. Further, it is much easier to use distance to illustrate the difference in braking capabilities of two or more cars. Oh, look, this car stopped just in front of the tree, but THAT car takes 5 more feet to stop, which means the tree is in your face.

    • 0 avatar

      Bingo. Nobody cares how long it takes to stop as long as the distance is less then whatever is in front of you. 0-60 gives you idea of how quickly you reach your goal which is pretty much the whole idea of acceleration. To offset this most reviews include the time and speed at the 1/4 mile, so you get the distance there as well. What I wish you got with braking numbers where how repeatable the distance was. IE: how long before fade sets in? Can you get two short stops in before they overheat or can you get six, maybe seven?

  • avatar

    Curiously, that Lincoln had the stove-pipe hat Photoshopped on to its nose. Old Abe never wore his hat that way!
    For proper effect, it should have been near the front edge of the roof.

  • avatar

    I thought the “B” was for engine braking with cars with CVT. That’s what I had in a Corolla rental- it worked fine but did give you a bit of artificial feel of downshifting of gears but was simply engine braking.

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