Recently, traffic has become a huge pain. One day, the weather has been bad and people drove very slow, causing my commute home to take well over an hour. Another day, a road problem caused a lane closure, causing my commute to the office to take, once again, well over an hour. Typically, this commute is only about 40 minutes or so. For the bulk of these slow commutes, my average speed has been around 20mph, and I discovered a flaw in my otherwise wonderful Nissan Xtronic CVT (continuously variable transmission).
First, let me explain how the Xtronic CVT works.
The simplest way to describe it is the concept is basically the same as the automatic transmission used in road scooters. There are two variable pulleys, between which a strong belt connects. The sides of the pulleys are disconnected and angled. When squeezed together, they make a large pulley. When pulled apart, they make a small pulley. In their lowest gear, one pulley will be large, and the other one small. As you accelerate, the large pulley will separate slowly, and at the same time, the small pulley will squeeze. The entire time, they will do this while maintaining tension on the belt so that there is traction to drive the wheels. In a scooter, the small pulley will close, causing extra tension on the belt to pull the large pulley open. This process changes the gear ratio similar to shifting gears in a normal transmission, but there are an infinite number of gears within the provided ratio range. The difference between the Xtronic CVT and scooter CVTs, however, is the Xtronic is computer controlled, while scooters use weights and rollers.
The result is completely smooth acceleration and a constant pull. There is no shift shock. The feeling when accelerating is actually quite strange.
However, this supposedly completely smooth transmission was programmed for average driving and doesn’t seem to compensate for very slow driving, nor hard driving.
I discovered that when attempting to maintain 20mph, with slight variance, the transmission can become less than smooth. The ratio at that speed provides a lot of torque, and the computer seems unable to compensate quickly enough. When pressing the accelerator and maintaining as steady a foot as possible, it is okay. However, inevitably in traffic, you have to press slightly harder, or release slightly. This results in a such a drastic change that the computer overcompensates before normalizing, causing a very slight shutter.
In short 20mph trips, such as through school zones, you probably wouldn’t notice. However, driving ~20 miles at 20mph is a completely different story.
It’s not a major concern, really, but it really takes away from all of the marketing made about this transmission. So much for no shift shock, smooth shifting, etc.
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