There's a scene in almost every movie set in Medieval times where the hero is stretched out on a rack in a dank dungeon somewhere. The villain, invariably dressed in black and sporting a goatee, grills our uncooperative hero about a secret plan he won't reveal, tightening the rack with each unanswered question. The hero writhes in pain but won't give in. Another unanswered question, another click of the rack, and pretty soon our hero -- well, let's just say he ends up a lot taller by the end of the film.
Other than the hackneyed script, this is pretty much the way rack-and-pinion steering works on your vehicle. Except the bad dialogue occurs inside the passenger compartment. Let's listen in.
"Will you look at that barn, Wilbur."
"I seen it, Martha."
"I never seen such a barn afore."
"I wonder how they built that barn."
"I don't know, Martha."
"It sure looks different."
"Will you look at that tractor?"
"I seen it."
"I never seen such a tractor afore."
And so on...
Believe it or not, rack-and-pinion steering is a lot less boring than that conversation. Actually, watching paint dry is a lot less boring than that conversation. Martha and Wilbur will talk like this all the way to Oklahoma City and back, and when they return home a substance resembling guacamole will be coming out of their ears.
But not us. No. Because we're going to learn about rack-and-pinion steering. Then a substance resembling guacamole will be coming out of our ears.
What is rack-and-pinion steering? Why is it so popular? What makes it different or better than other steering systems?
In a recent Tech Center we took a look at power steering systems. You may want to read that as a refresher before going any further. Here's the URL.
More Power to Your Steering Okay, since that didn't scare you away, let's talk about rack-and-pinion.
As the name implies, rack-and-pinion steering consists of two major components -- a rack and a pinion. (We're known here at Edmunds.com for tackling really challenging technical questions.)
The rack -- also known as a steering rack -- is a long piece of metal that is flat on at least one side. The flat side contains teeth running the length of the rack. The teeth are cut perpendicular to the edges of the rack, meaning they run side by side from one end of the rack to the other, like teeth in a smile.
The other major component, the pinion -- more correctly, the pinion shaft -- is a round rod that also has teeth on it, although these teeth run parallel to the length of the shaft, not lengthwise as on the rack. The pinion shaft comes into the rack at a ninety-degree angle, held in place by a collar, and the teeth on the pinion mesh with the teeth on the rack. The pinion is connected directly to the steering column, so when the steering wheel is turned to the left, for instance, the pinion rotates counter-clockwise (from the driver's perspective). In simple language, the rotary motion of the pinion is changed to transverse motion by the rack. The rack moves to the right, making the wheels go left. The car turns left.
This is pretty dense stuff. Let's take a break.
"Will you look at that horse, Wilbur."
"I seen it."
"I never seen such a horse afore."
"I wonder how God made that horse..."
Of course there are other components that make up a rack-and-pinion steering system, and we'll get to those in a moment; but realize that one of the reasons rack-and-pinion is so popular these days -- it's safe to say that it's the most commonly used steering technology on the planet -- is because of its simplicity. Fewer parts generally mean less breakage; and unlike other systems, which use pitman arms, idler arms and center links, rack-and-pinion contains only two linkage components -- inner and outer tie rods. Also, because of the lack of parts and its unique and simple design, this form of steering offers superior road feel, accuracy and responsiveness. So let's take a look at the rest of the system.
In addition to the pinion and the rack, several other components make up this type of steering gear. The pinion is housed inside an assembly which contains a series of bearings, seals, adjusting screws and lock nuts. The bearings allow for the free turning of the pinion, while the seals prevent dirt and dust from invading the system. The adjusting screws and lock nuts permit fine-tuning of the pinion.
The lower system, where we find the rack, is held in a steering gear rack housing. As with the pinion, a number of bushings, bearings, guides and adjusting screws allow for fine-tuning. At either end of the rack, an inner tie rod extends out toward the wheels. This section is sealed over with a rubber boot and clamp on either end -- again, to prevent dirt and dust from entering the system. The inner tie rods attach to outer tie rods, which in turn attach to steering knuckles. The steering knuckles connect the system to the wheels.
Let's review this one more time. A driver wants to turn left. He/she turns the steering wheel in that direction. The pinion shaft, which is directly connected to the steering shaft, rotates counter-clockwise (from the driver's perspective). The teeth on the end of the pinion shift mesh with the teeth on the rack, causing the rack to move to the right. The tie rods and steering knuckles shove the right tire out and pull the left tire in, causing both to rotate to the left. The car turns left.
We trust this helps you understand the basics of rack-and-pinion design. If you're like us, you've been hearing this term bandied about for decades. Next time you hear "rack-and-pinion," you'll know what it means.
Please note that our basic discussion here does not distinguish between power and manual rack-and-pinion. However, the design is essentially the same. For more information on power and power-assisted steering, click on the link above.
We've placed some other Edmunds.com links below that you may find useful.
"Will you look at that pig, Wilbur."
"I seen it."
"I never seen such a pig afore."
"Ever wonder why God made the pig?"
"Am I boring you, Wilbur?"
"Woman, you're driving me nuts..."