What is a Steering Ratio, Gear Ratio and Variable Gear Ratio in a Rack & Pinion type automotive steering system?

Let us discuss about a Rack & Pinion type steering gear, which is manual type (that is not a power steering system)

So, a rack-and-pinion gearbox does two things:

  1. It converts the rotational motion of the steering wheel into the linear motion needed to turn the wheels.
  2. It provides a gear reduction, making it easier to turn the wheels.

It takes three to four complete revolutions of the steering wheel to make the wheels turn from lock to lock (from far left to far right).

Understanding the terms like Steering Ratio and Gear Ratio:

The steering ratio is the ratio of how far you turn the steering wheel to how far the wheels turn. For instance, if one complete revolution (360 degrees) of the steering wheel results in the wheels of the car turning 20 degrees, then the steering ratio is 360 divided by 20, or 18:1. A higher ratio means that you have to turn the steering wheel more to get the wheels to turn a given distance. So, there is less effort required because of the higher Gear Ratio.

On the other hand, lighter, sportier cars have lower steering ratios than larger cars and trucks. The lower ratio gives the steering a quicker response which means that you don’t have to turn the steering wheel as much to get the wheels to turn a given distance, which is a desirable characteristic in sports cars. These smaller cars are light enough that even with the lower ratio, the effort required to turn the steering wheel is not excessive.

What is a Variable-Ratio Steering, then?

Vehicles that have variable-ratio steering, use a rack-and-pinion gearbox that has a different tooth pitch (number of teeth per inch) in the center than it has on the outside. This makes the car respond quickly when starting a turn (the rack is near the center), and also reduces effort near the wheel’s turning limits.

2 Comments

  1. I recently installed a manual rack and pinion with tne Mustang ll kit in a 50 Chevy coupe. It takes way to much effort to turn the wheel. It’s like steering a power rack with the engine not running. My question. Are these manual racks available in different ratios?

    1. In India, we do not see the older Mustangs and the Chevys however I would like to answer your question. But I need to understand a few things. Was the Chevy Coupe you are talking about fitted with a Re-circulating Ball Screw (RBS) type steering gear earlier before you made the modifications for fitting a Rack & Pinion (R&P) type steering gear.
      If that was the case, then you might have altered the vehicle dynamics because these are two different gear systems thus altering the fitment geometry. An RBS type gear has a pitman arm which is connected to the drag links while an R&P steering system has two tie rods fitted at both ends of the Rack Bar (the bar which has teeth and mated to the Pinion helical gear) connected to the knuckle joints directly. Minimum the connections, lesser the drag and play in the joints which adds to make the steering heavier. So, please elaborate the situation.
      Now coming to the question whether these manual racks available in different ratios? The answer is yes! For a sports car (say a Ferrari) the steering ratio will be higher (say 40:1) while for a normal passenger vehicle the ratio will be lower (say 14:1). Higher the number of turns of the steering wheel, lighter the steering. Then the Front Axle Weight (FAW) has also to be taken into account. But in your case, the FAW seems to be higher because the 50 Chevy must be a heavier vehicle. Consider tire sizes also to play a role in making the steering heavier. The more the wider, more the friction so more the steering torque. Additionally, kindly note that there also are steering systems (manual) which comes with variable gear ratios. Early 1960s power steering gearboxes were originally manufactured with a wide turn ratio. Depending on the manufacturer, most were in the area of 16.0:1 to as high as 24.0:1, requiring a full four to five turns of the steering wheel to complete their range from lock to lock (wheels locked from the extreme right to the extreme left). These units were also designed with variable pressure, meaning they have slow steering response around the center point, but improved response as you approach the ends of their maximum range.

      However, in your case, the type of steering replaced with the type that was originally fitted seems to be the case.

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