Here’s Why Car Wheels Are So Flat These Days (And No, It’s Not Just Aerodynamics And Styling)

Hello Autopians! Have you ever wondered why designers show sketches of concept cars with massive deep dish wheels, but when those cars actually make it to production the wheels end up being fairly flat? Adrian Clarke talked about this recently from a designers point of view, but I’m going to tell you why this is true from an engineering point of view. In other words, I’m going to tell you why we just can’t have nice things!

Years ago, in days of old, cars came with wheels that had very deep dish styling. Life was good, cars looked cool and everyone was happy (okay, maybe that’s a stretch). Over the years, as technology marched on, deep dish wheels got shallower until finally, starting about 20 years ago, they became essentially flat on the outside. Why did this happen? Well, in a word, “steering” is what happened. The change from deep dish wheels to flat wheels can be traced back to improvements in the steering system — in particular, to the popularity and advantages of rack and pinion steering.

[Editor’s Note: This story was originally published on September 13, 2022. It’s being re-upped as The Autopian celebrates its two-year anniversary! -DT]. 

Why Rack And Pinion Steering Requires Reducing The ‘Scrub Radius.’ And Why Flat Wheels Help

Let me explain. All front suspensions have something called a kingpin axis, or steering axis. It is the axis about which your front wheels rotate when you steer the car, and can be visualized as a line going through the upper and lower ball joints (you can think of these as pivots/hinges) in a double wishbone design (or through the lower ball joint and the upper spring mount in a MacPherson strut). It defines one of the central characteristics of the suspension: the caster angle, though we won’t get into that. For now, just know that the location of this line that represents your steering axis, relative to the tire contact patch and the center of the wheel, is critical to understanding why the look of wheels has changed so much over the years. Let’s look at a cross section of a wheel and tire, looking from the front of the car:

As you can see, the kingpin/steering axis about which your front tires turn goes through the lower ball joint (the upper ball joint is not shown here). If we extend the kingpin axis with an imaginary line all the way until it intersects the ground, we can measure something called the “scrub radius” which is the distance from the point at which the kingpin axis intersects the ground and the center of the tire contact patch.

You can visualize scrub radius by watching the video below; notice how the tire isn’t just spinning about a vertical axis going through its center (like a coin would if you flicked it on a tabletop — this has a zero scrub radius), it’s making a “sweeping” motion, the nature of which can be described by the scrub radius:

 

A Big Scrub Radius Sends Lots Of Forces Through The Steering Wheel. This Isn’t Good

When you apply the brakes in a car, the braking force together with the scrub radius creates a torque around the kingpin axis which tries to steer the suspension. What stops the suspension from actually steering is the steering system itself and the hands of the driver holding the wheel. You can visualize that in the picture above or below; if the ground pushes the tire rearwards at the contact patch, which is a few millimeters outboard of where the kingpin axis intersects with the ground, that tire will have a tendency to rotate, which will pull on the steering tie rod and send forces to the steering wheel. 

In a similar way, we can measure the distance from the kingpin axis to the center of the wheel. This is called the “kingpin offset.” When you drive over a bump in the road or through a pothole, the force acting on the suspension actually happens at the center of the wheel (see the image above) and this force coupled with the kingpin offset also tries to steer the suspension. Again, the only thing stopping the suspension from actually steering is the steering system and the hands of the driver holding the wheel. This is felt as kick-back to the driver and if it’s bad enough can rip the wheel out of your hands. Some of you may have had that experience in the past.

How Reducing The Scrub Radius Leads To Flat Wheels

Of course, a suspension engineer can’t stop people from driving through potholes and over bumps in the road so the only thing the engineer can do to minimize the kick-back is to make the kingpin offset and the scrub radius as small as possible so that the overall torque around the kingpin axis is as small as possible. But, as with everything else, other things get in the way of achieving this. In particular, the brakes get in the way. The brakes also have to fit inside the wheel and this limits how far outboard we can put the lower ball joint. Let’s look at our cross section again but this time add in the brakes:

Notice how close the lower ball joint sits to the brake rotor. If we wanted to push the kingpin axis further outboard to get a smaller scrub radius or kingpin offset, we would need to push the lower ball joint outboard which would push the brake rotor outboard as well. But look at the caliper. It sits right behind the spokes of the wheel. Moving the caliper outboard would mean pushing the spokes of the wheel outboard. You can see how trying to get a small kingpin offset and scrub radius has pushed the wheel outboard and made the outside of the wheel very flat.

By now you’re probably saying “But hey! You said it was the steering system that caused flat wheels, not the brake system!” And you would be right.

Why Old-School Steering Systems Actually Want ‘Scrub Radius.’ And How Deep-Dish Wheels Help

Many years ago, before rack and pinion steering became popular, cars used a steering system called ball/nut, or sometimes called a steering box. This system worked on the principle of a worm gear to turn a sector shaft which was connected to a pitman arm. This was then connected to the steering tie rods with a track rod (sometimes called a center link) and an idler arm like this:

And here is a look at the inside of an early steering gear, this one from a 1930’s era Packard:

In the early days of cars, before the advent of power steering, worm gears were a very effective and simple way of achieving the torque multiplication needed to convert the driver’s steering effort into the force needed to turn the suspension. Here is a good explanation of how worm gears work:

Unfortunately, while worm gears are very effective at torque multiplication, they do not like to be back-driven, meaning it is very difficult to turn a worm gear type steering box by pushing on the pitman arm (the output shaft). This is because the same concept that gives us torque multiplication when turning the input shaft (this, and big-radius steering wheels, is why old cars could make do without power steering) gives us torque reduction when pushing on the pitman arm. In other words, while it takes very little torque to spin a worm gear by turning the input shaft, it takes a massive amount of torque to spin a worm gear by turning the output shaft. It can be so difficult in fact, that any friction in the input shaft and between the teeth of the gears can be enough to lock the gears in place even when you let go of the steering wheel.

This characteristic can be very useful in a situation where you want to drive a mechanism to a specific position but don’t want it to be able to move back by itself. In a steering system, however, this principle is not very useful because it prevents forces from the tires and road from getting back to the driver. These forces represent the tires pushing back on the steering system and are what “steering feel” is all about. This is one of the reasons why the steering in those old cars often felt numb and devoid of any “feel.”

What saved the steering in those cars from being completely numb was the fact that the kingpin was very far inboard which made the scrub radius and kingpin offset very large. This had the effect of amplifying the braking and pothole forces and gave just a little bit of steering feel to the driver in spite of the numbing effect of the worm gear. Having the kingpin axis far inboard meant the brakes could be farther inboard which meant we could have nice deep dish wheels. 

A significant development of the worm gear steering system was introduced by Cadillac in 1940 which was the recirculating ball steering.box. This design replaced the worm gear with a series of balls running in a track which formed the teeth of the gears. Here is a cross section of how this works:

 

Notice how the balls form the teeth between the input shaft and a “nut” which slides back and forth. As you turn the steering wheel, the balls would roll inside their track and cause the nut to move with very little friction. The nut has teeth similar to a worm gear which engage with teeth in the output shaft. Since there is so little friction in the system it means it takes very little force to turn the gear normally, and forces coming back from the suspension have very little friction to overcome. The result was significantly better steering and steering feel, although by today’s standards it was still pretty abysmal. Still, it has been around ever since and is still in use today in some vehicles like the Jeep Wrangler.

For a long time, everyone was satisfied with the worm gear type steering box until someone decided to invent the rack and pinion steering gear. The rack and pinion gear has a simple gear, attached to the steering shaft, moving a toothed bar back and forth inside a housing, like this:

 

As you can see, there is no worm gear in this design. The result is that forces coming back from the tires can very easily move the rack back and forth and turn the input shaft which can be felt by the driver without the numbing effect of a worm gear. Of course, there also wasn’t torque multiplication going on like there is in a worm gear so at first they were only used in very lightweight cars to keep the steering efforts from being too high. With the advent of power steering this situation changed, and now you see rack and pinion steering used all the way up to full size pickups.

With the loss of the torque multiplication, there was also an increase in the ability of the forces coming up from the tires to back-drive the steering gear, which is a main reason why rack and pinion was invented and why it is so dominant today. The ability of forces to feed back to the driver meant there was significantly more “feel” for what the tires were doing and where the limits of adhesion were. This made the act of driving much more engaging and made it much easier to drive at the limit. You could actually feel what was happening at the contact patch. For the last 40-50 years, all self respecting drivers cars have had rack and pinion steering as a result.

And So Wheels Became Flat When Rack And Pinion Came Around

At first, rack and pinion gears were being applied to existing suspension designs but since the tire forces were being “amplified” by the large kingpin offset and scrub radius in those old designs, they were too much for the driver to take and were ripping the steering wheel out of their hands. Something had to be done and since there will always be potholes and braking forces, the only thing that the engineers could do to reduce the forces coming back through the steering system was to reduce the size of the kingpin offset and scrub radius. This meant the lower ball joints had to move outboard, the brakes had to move outboard and all the dominoes started to fall which spelled the end of deep dish wheels.

An excellent example of this is what happened with the Ford Expedition when it changed from ball/nut gear to rack and pinion in the early 2000’s. Here you can see the difference in the wheels before and after that change:

2000 Ford Expedition

2004 Ford Expedition

Look at how deep the wheels on the 2000 model are and how flat they are on the 2004 model. This is because the rack and pinion steering in the 2004 model demanded a much smaller kingpin offset and scrub radius.

[Editor’s Note: If you look at Jeep’s first application of a rack and pinion steering system, the 2002 Jeep Liberty, you’ll notice that it, too, was accompanied by a transition to flatter wheels with more backspacing (5.5 inches versus 5.25). 

Here’s the Liberty, and here’s its predecessor, the Jeep Cherokee:

-DT]

Aerodynamics Sometimes Plays A Role, Too

There is one final aspect of deep dish wheels that we need to consider to fully understand why wheels are now so flat: aerodynamics. In the age of maximizing efficiency both in internal combustion engines cars as well as EV’s, the need to smooth out the flow of air as a car moves forward has become of paramount importance. When a car is moving, air is being forced up, down and to the side by the shape of the car. The act of pushing air aside like this takes energy and that energy has to come from the fuel that is powering the car, whether that is gasoline, diesel, or electricity. 

The amount of drag or resistance a car has to moving through the air is measured by something called the Coefficient of Drag or Cd. Normal values of Cd for cars is on the order of 0.20 to 0.50, with 0.20 being exceptionally good while 0.50 is poor. The lower the Cd, the lower the aerodynamic drag and the less energy it takes to push the car through the air. A car’s Cd can be calculated from measurements made in a wind tunnel or calculated by a mathematical method called Computational Fluid Dynamics or CFD. Today, there are numerous software packages that perform CFD analysis and will calculate the Cd of a car and all OEM’s use this in their design process.

In order to reduce the aerodynamic drag of a car, the surface of the car must be as smooth as possible. Any minor protrusion or indentation can upset the airflow as it moves along the car and add to drag. This includes the wheels. Deep dish wheels present a pretty big indentation to the flow of air which the air must fill and flow around. Even with very flat wheels, the shape of the wheel spokes can make a large difference in how the air flows over and around the wheels. Here is an analysis of Tesla Model S wheels showing how filling in the space between the spokes reduces the Cd, in this case by 0.03. That may seem like a very small number, but it is a big deal. The total Cd of a Tesla Model S is 0.21 so a change of 0.03 represents a 12% reduction in the aerodynamic drag. That’s huge!

Unfortunately, the wheel in the Tesla example above didn’t find much favor with customers because of its appearance and many similar aerodynamic wheels have what I would consider questionable looks, but I’m confident that the design community will fix this in due time and bring out flat wheels that are both aerodynamic and attractive. Here are some examples:

You can decide for yourself how attractive these wheels are but the point is that we now have another aspect of vehicle design that is pushing us to flat wheels.

So now you know why we just can’t have nice things. Personally, I hope Adrian Clarke and his designer colleagues never stop sketching cars with deep dish wheels. You never know, maybe some bright future engineer will figure out a way of making them a reality once again.

[Editor’s Note: Not all designers prefer dished wheels. Some commenters were wondering why the Jeep JK/JL Wrangler, which has an old-school steering box instead of a rack and pinion setup, has such flat wheels. As far as I know, this is largely a styling decision. -DT].