Why PRNDL?

June 14, 2006 6:28 AM

Something I have been thinking about lately: why PRNDL? Why is the selection order on an automatic transmission PRNDL?

To my mind, PDNRL or PDRNL makes more sense. Park and Drive are the most commonly used selections, so why not put them at the top of the list? Is it just a common convention? Is it done for safety reasons? Anyone that works in the auto industry have any insight into this?

posted by internal to travel & transportation (14 comments total)

It's so that the car can't just be bumped from Park to Drive.

It also keeps the transmission from being shifted into Park while the car is in motion.
posted by bshort at 6:33 AM on June 14, 2006

Most of the time, Reverse is the first thing I do when I start up my car, to back out of my parking space.

And "Neutral" makes sense to be in between Reverse and Drive, since the transmission is already transitioning between backwards and forwards, and Neutral is right in the middle...
posted by jozxyqk at 6:34 AM on June 14, 2006

internal, your selections would require coming to a full stop to get into low gear
posted by exogenous at 6:39 AM on June 14, 2006

It's done for *mechanical* reasons. You may not realize it, but your little handle is actually moving a bar, pulling a wire, causing physical changes in the configuration of your car. You're disengaging one set of gears in a planetary gearset and engaging another.

You've got to read up on how a planetary gearset works, which is the sort of thing that can't be explained without diagrams, to understand what's actually happening.

It may be that in the future, your automatic transmission and the rest of the vehicle controls are all fly-by-wire - no more mechanical linkages. Your controls would then just be signalling a computer chip, much like a Nintendo controller does, and the computer would then signal the mechanical bits of the car to do things. When that is true, the controls can be put anywhere the designer wants and work anyway he wants (and we'll undoubtedly get some really bad user interfaces out of it). But for now, in most cars the controls are still directly connected to the appropriate mechanical bits of the car. The brake pedal pulls on a cable. The gas pedal pulls on a cable. The steering wheel turns a gear. This leads to a very direct form-follows-function design.

(The Prius is fly-by-wire. Gas pedal - no cable, just signals a computer chip. Transmission is a lttle lever on the dash, just signals a computer chip.)
posted by jellicle at 6:41 AM on June 14, 2006

in fact the little lever inthe prius apparently doesnt even have switches - its all done through induction! crazy.
posted by joeblough at 7:00 AM on June 14, 2006

You're disengaging one set of gears in a planetary gearset and engaging another.

Which just makes the question "Why are the gears in that order?". They're in that order because of human factors. It's form-follows-function, but the function isn't the way transmissions happened to be built; they were built to support the gear order. You don't have to know how a planetary gear works to understand why the positions are in the order they're in.

First, the whole thing is built around D, not P. P is the one you could completely do without: put the car in neutral, put on the parking brake, and you're done, but you need all of the others to drive.

As for the others, it's a clever bit of safety engineering. You need to be able to do two things when you're moving forward: safely disengage the engine, and force a shift to a lower gear. So that means you want N and L to be on either side of D.

But you have to prevent accidentally shifting to a lower gear, because shifting into L at 100 mph is not so good for the engine. On the other hand, you have to prevent accidentally being unable to disengage the engine because the shift lever's button fails. That's why you can shift to N without having to push the button, but you have to push the button to shift into L.

So now there's R. You need to make sure that you can't accidentally shift into R at a high forward speed. So it's going to go either above N, or below L. If it's below L, then there's a chance of accidentally finding R when you want L, because you've pushed the button in. But if it's above N, then you're fine -- you don't push the button to go from D to N, so there's a built-in safeguard to allow D-N shifts without hitting R. That's also why you can shift from L into D without pushing the button -- if you overshift, you rev the engine, but if you overshifted with the button, you'd be in reverse at high speed again.

And lastly there's P. You need to make sure you can't accidentally shift into P even more than R, so the same principles applied -- it couldn't go below R because of the button thing, but since shifting from anything into R needs the car to be stopped, it doesn't matter that you have to have the button pressed because there's no danger of breaking things if you shift into P accidentally.
posted by mendel at 7:04 AM on June 14, 2006

Incidentally:

The brake pedal pulls on a cable. The gas pedal pulls on a cable. The steering wheel turns a gear. This leads to a very direct form-follows-function design.

I don't think you'll find many cable-actuated brakes in cars in the last eighty years or so; it's all hydraulic (and the parking brake is the cable-actuated system in case of hydraulic failure).

Oh, here's something else I remembered: The difference between automatic shifts that require a button-push and those which don't was even more noticeable with a steering-column-mounted shifter; I remember being taught to pull on the lever to shift from park to reverse or to downshift, for instance, but to push on the lever to shift from drive into neutral or to upshift.
posted by mendel at 7:11 AM on June 14, 2006

If you're curious as to how the automatic transmission works, Howstuffworks.com is brilliant.
posted by antifuse at 7:14 AM on June 14, 2006

What mendel says is right. One also should be aware that the order wasn't always this way. I think I remember a chapter in Unsafe At Any Speed that discussed an alternative, then-prevalent automatic shift pattern that was dangerous primarily due to human factors concerns; this chapter advocated widespread adoption of the PRNDL pattern.

At least that's how I remember it -- I read it over 25 years ago.
posted by Opposite George at 8:35 AM on June 14, 2006

Also, this thread [Google Groups] has some discussion of varying auto shift patterns in use before 1968.
posted by Opposite George at 8:56 AM on June 14, 2006

My parents used to have an old F-85 that was PNDLR. Also, lots of cars have electronic throttle control nowadays.
posted by rfs at 9:39 AM on June 14, 2006

Several cars were built with pushbutton-controlled automatic transmissions: several years' worth of Ramblers, Packards, Chrysler products (1956-65), and the Edsel.
posted by Kirth Gerson at 11:08 AM on June 14, 2006

Oh yeah, my old Valiant convertible push-button automatic. The Car That Would Not Die. Nor Attract Chicks In The Image Obsessed 1980s. Why can't Chrysler build rock solid cars anymore?
posted by Rumple at 12:00 PM on June 14, 2006

rfs writes "My parents used to have an old F-85 that was PNDLR"

A truely dangerous pattern. As much as I hate that the SAE killed the push button transmission by standardizing shift patterns I'm glad I never have to drive a car with R below L

mendel writes "So now there's R. You need to make sure that you can't accidentally shift into R at a high forward speed. So it's going to go either above N, or below L. If it's below L, then there's a chance of accidentally finding R when you want L, because you've pushed the button in."

A, uh, "friend" backed a '62 Olds 4DRSDN into more than one object because of this very problem with R below L. This was because the 3 speed auto in that car had this pattern and wouldn't shift into 1st unless you actually put the selector into "L". So off the line starts with a transmission in "D" were glacial because they were in 2nd gear. The trick street racing thing to do was to manually shift between L and D. If you got caught napping at the line it was pretty easy to attempt to slam the shifter into L, overshoot, and end up running into whatever was behind you. Usually at full throttle.
posted by Mitheral at 6:07 PM on June 14, 2006