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Please explain how this CVT trans thingy works...


angie

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OK, I am enjoying the new BR9 wagon. Has CVT trans. Takes a bit of getting used to in that it behaves completely differently to the 'conventional' transmission in our 6 cylinder auto BMW. I understand the concept, broadly, in how CVT differs. But can someone explain the following please... in easy simple terms... :)

1) Why does the car roll backwards on a hill? Is this because it lacks conventional clutches? I have used the hillstrart button to hold the car... but will the car roll back uncontrollable when on a steep hill if I were to let it? Or is there a point where the transmission will 'bite' to stop this?

2) When driving very slowly, say 40 km/h and one takes the foot off the gas pedal there seems to be a 'bite' from the CVT transmission that I can feel. In a regular auto transmission you just decelerate but dont feel a 'bite'. Whats happeninf in the CVT in this instance as I dont understand how the pulley systems produces this resistance (the rev counter hardly moves if at all but the petrol useage gauge moves and you feel a distinct resistance).:confused:

Thanks.

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 angie said:
Thanks for taking the time to provide an informative answer to my question. Much appreciated.

Hahaha well played.

Not many people on here have CVTs. We're more of the short shifter manual types, and as such theres a combination of distrust and ignorance when it comes to CVTs.

Plus few of us can afford the late model cars with the CVTs anyway.

Your best bet might be looking around Wikipedia and some of the US forums.

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 lachlan']typical female . your stupid and its everyone elses fault

[quote name='lachlan said:

well i was going to answer but dont know what you mean by "resistance "

fuk head

What the ****, man... grow up.

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Ahhh...thank goodness for that... normal friendly people... amongst the cranky middle aged fella... As for my question... it was more to see if someone could explain a few of the nuances on the finer points of how the CVT tranny works... not concerned that is faulty... it works perfectly fine... thanks...

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Even though my new bestie 'lachlan' tried his best to scare me off from ever using this forum again... I will share an answer I received by Subaru NZ to explain my question... Hopefully this will be of use to other forum users... Thanks again to Subaru NZ for answering.

"

Thank you for your email. I will try to answer your questions in understandable terms (difficult as the questions you’ve asked involve some fairly technical components, but I’ll do my best).

1. The car will roll backwards on a hill as at idle in a stationary position the transmission is effectively ‘decoupled’ from the transmission via the torque converter. This is good for fuel economy as there is no ‘load’ on the engine. As you apply throttle and therefore engine speed/torque, the torque converter transfers this drive gently into the transmission itself. There needs to be a specific rotational speed difference between the engine and transmission for this to result in forward motion (different depending on engine/transmission combination and engineers design). When the car is stationary, there isn’t the rotational speed difference to provide drive – this is the same on a hill, hence the ‘hill holder button’ which uses the parking brake system to hold the car.

Will the car roll uncontrollably down the hill? – difficult question. If the directions and cautions in the owners manual are you should use either use the park brake, foot brake or hill holder on a steep hill as using the accelerator to hold the car would result in a possible transmission overheat situation. I’d say that at some stage the engine/transmission speed differential would slow the car but this is not how the system is designed to be used and the onus rests with the driver ultimately to operate the vehicle correctly when stopping on a hill.

2. Difficult to answer this question exactly, but I believe the ‘bite’ you are experiencing is the ‘lock-up clutch’ inside the torque converter engaging or disengaging. This is the component that changes the forward motion of the car to and from a fluid coupling type (as described in the previous question), to a 100% mechanical type. Hope this makes sense? Not sure of another way to describe it.

"

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Hello, fellow BR9 CVT owner.

1) No, you can hold the car in place with the accelerator if you want to, although the owner's manual probably recommends against this somewhere. This has more to do with the torque converter than the CVT itself - note that the CVT also has a much weaker "creep" (rolling forward when idling in gear) than a car with a traditional automatic. As far as I'm aware, this is because the Subie CVT has a torque converter lockup clutch. Therefore the torque converter is only needed for starting off, after which point the fluid coupling locks and the CVT input is directly connected to the crankshaft.

2) Again, lockup clutch. Since the CVT is mechanically connected to the crankshaft it engine brakes. Particularly noticeable if you shift the CVT manually.

Disclaimer: I don't know the specifics of the BR9 CVT as I've never serviced, replaced, or dismantled one.

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 Zenki said:

2) Again, lockup clutch. Since the CVT is mechanically connected to the crankshaft it engine brakes. Particularly noticeable if you shift the CVT manually.

Disclaimer: I don't know the specifics of the BR9 CVT as I've never serviced, replaced, or dismantled one.

What are they like from a drivers point of view? I've never liked normal auto's, partially due to the lack of engine braking among other things, but can a manually shifted cvt be fun to drive?

Only cvt I've driven was in a Nissan Murano and it was worse than a normal auto out on the open road. Especially up the twisty parts of SH1 near Kaikoura.

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I also dislike normal autos, although in my case it's because I hate the transmission hunting for ratios when driving in hilly areas (see: half of New Zealand).

The best explanation I can give you is that the Subaru CVT is somewhere between an automatic transmission and a DSG if you're shifting manually. It's pretty obvious that it's pretending to be a manual - slight lag, ratios are a bit funky (6th is incredibly long), doesn't like rapid downshifting - but otherwise it's not bad. Feels better than a normal automatic.

I leave it in automatic most of the time, though. It does a pretty good job of being in the right ratio.

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