How Center Lock Wheels Work, And Why They Suck For Most People

Centerlock wheels are a technology most closely associated with race cars. They’re key to executing pit stops as quickly as cleanly as possible. They’re also available on some elite road cars, and they have a history on certain classics as well. But how do they work?

It’s obvious that a single, large center nut should be able to hold a wheel onto a car, assuming it can generate enough clamping force.  It’s less obvious how it stops the wheel from sitting still when the driver applies power, or from spinning when the driver hits the brakes. You might also wonder what stops the nut from coming undone and the wheel from falling off the car. Don’t worry—we’ll get to all those questions, and more!

Perhaps you think centerlocks are simple and elegant, and should be on every car. Alternatively, you might believe they’re racing nonsense that has no place outside of the track. We’re going to examine the engineering behind centerlock wheels. We’re also going to look at their strengths and weaknesses, and we’ll talk about why they’re probably a terrible choice for your daily driver. By the end of this piece, you’ll be well-educated to make either argument, and you’ll be backed up by real engineering knowledge!

One Nut

Centerlocks were developed in the early 1900s by a British company called Rudge-Whitworth. Making its business in both the bicycle and nascent automotive industries, it developed “knock-on” or (“knock-off”) centerlock hubs. These were so named for the mechanism by which the fastening nut was applied. The nut featured large wings which could be struck with a soft-blow hammer to tighten the wheel to the hub, or to loosen it in turn. Initially referred to as “Quickly Disconnectable” wheels, they quickly gained popularity in the field of motorsports. They also became common on road cars, and are closely associated with British classics and wire wheels.

The basic design is relatively straightforward. The hub consists of a round shaft that carries the wheel, which is threaded on the end to receive a knock-on nut. The shaft has a section with an external spline and a taper at the base. The wheel in turn has an internal spline and an inverse taper to match. The tapers are key in aligning the wheel properly on the hub. The spline locks the rotation of the wheel to the rotation of the of the hub. It is aided by the clamping force of the nut and the friction between the taper of the wheel and the base of the hub.

The knock-on wheel nuts typically use reverse thread on the right-hand side of the vehicle. This is to ensure that the forward motion of the wheel does not loosen the nuts as the vehicle drives along.

Sadly, the design was not to last forever. The wings of the knock-on nuts were seen as a safety concern by the mid-1960s and was soon phased out for road use. Many manufacturers simply just replaced the knock-off nuts with hex nuts instead, while others explored fancier designs for fastening the wheels to the hub.

Simple knock-ons don’t have much protection against the wheels falling off, either. While the nuts should be self-tightening in use, there’s no additional retention mechanism on most classic knock-on hubs. If the nut isn’t tightened properly, or something goes wrong, a wheel can fly right off. That happened in spectacular fashion at the Goodwood Festival of Speed just a few years ago, when a classic Jaguar sent a wheel flying into the spectator area. Thankfully, nobody was hurt.

Losing a wheel on track can pose a great safety risk, and it’s the same story out on the road. Incidents like these inspired the development of more advanced center lock systems.

Today, there are a great many different types of centerlocks out there. Ultimately, they still use a taper to mate the wheel and hub together, and they still use threads and a large nut to hold the wheel on. Where they differ is the details.

Some centerlock wheels came to rely on drive pins or drive lugs to help transfer torque. These are basically protrusions on the hub, not dissimilar from wheel lugs themselves. The drive pins interlock with holes drilled in the back of the wheel. It allows a centerlock hub to transfer a great amount of torque to the wheel. Compared to splines, drive pins are much larger and can transfer torque with much lower risk of stripping. It’s a key design feature seen on most modern centerlock designs, whether used on the street or in a racing context. High-performance engines can deliver enough torque that it’s prudent to rely on drive pins rather than splines alone.

Another point of difference is the methods used to stop a nut from loosening or to stop a wheel from coming off in the event the nut does come loose. This is considered crucial in both motorsports and on-road contexts, though the methods used to deal with it differ. The Ferrari F40 is a great starting point for this discussion. Examples like this one for sale at Amplified Listings wear their centerlock wheels with pride.

R-clips are one of the simplest solutions in this regard. This involves using an R-shaped wire clip with a hub that has a hole drilled through it. Once the nut is tightened down, a wire “R clip” is placed through the hole in the hub to stop the nut from backing off. This is an effective method of retention, though it is typically too slow for motorsport use. This is because of the time required to remove and replace the R clip, which is a fiddly manual process.

In motorsports installations, pop-out tabs are often used as retainers instead. The spring-loaded tabs protrude from the axle ahead of the nut, preventing it from loosening beyond the tabs. When the socket that fits the nut is slid over the axle, the tabs are forced inside the hub, and the nut and wheel can be removed. A new wheel and nut is then installed and torqued down, with the socket again sliding over the tabs to retract them in the process. The tabs pop out once the nut is torqued and the wheel gun is removed. When functioning properly, these tabs make it impossible for the nut to slip off the hub even if it begins to loosen, as the nut cannot pass the tabs. This retains the wheel on the hub which is a key safety requirement.

Porsche uses a slightly different design on its centerlock wheels for road use. There is a sliding spring-loaded nubbin the center of the centerlock hub, which is splined on the outside. When the special Porsche tool is used to tighten the wheel nut, the nubbin is pushed into the hub. When the tool is removed, the nubbin pops out and its splines slide into the wheel nut. Thus, even if the thread wasn’t tight, the wheel nut would not be able to come undone as the splines would prevent its movement.

These retention methods don’t prevent the need for proper torquing of centerlock nuts. They merely serve as a retention method and a way to stop nuts from loosening due to vibration, knocks, or other causes.

It’s also worth noting the unique shapes found on many modern centerlock nuts. Simple hex designs work okay, but not great. The problem is that a centerlock design necessarily needs huge torque to keep the wheel mounted. This is because the clamping force is all on the one nut, instead of spread across four or five separate lugs. Trying to apply big torque to a hex nut can damage it if the tools used don’t fit absolutely perfectly.

To get around this, alternative designs are often used for centerlock wheel nuts. In the case of motorsports, this often involves the use of wheel nuts with flat radial protrusions. A mating tool is then used on a wrench or impact gun, and it can apply torque directly to these radial faces. These designs avoid any risk of stripping the nut or damaging the mating faces unless used improperly.

Why They Rule

The benefits of centerlock wheels are obvious to anyone who’s watched modern motorsport. You can take the wheels on and off very quickly. Indeed, the fastest F1 pitstops now take place in under two seconds flat. Well-trained pit crews use a ton of specialist equipment to achieve that, but that’s the prime benefit of center-lock wheels.

Theoretically, an advanced center lock design could offer less weight than a traditional multi-lug setup. Modern wheel nuts used in motorsports are very light. This is an ancillary benefit, however. The smaller central hub could offer more area for a brake disc, though this benefit is minor.

In the supercar world, perhaps the biggest benefit is that centerlock wheels look cool. Outside of that, though, centerlock wheels don’t offer much for cars on the street.

Why They Suck

So why don’t all cars have centerlock wheels? Well, the simple fact is that centerlock wheels can be a massive pain in the ass if you’re not running a race car with a full team. Indeed, YouTuber Rob Ferretti called centerlock wheels the “worst factory option ever” back in 2016.

A lot of it comes down to torque. Automakers typically recommend around 80-150 foot-pounds of torque for lug nuts on four-and five-stud hubs. However, since a centerlock wheel has just one nut to provide all the necessary clamping force, the required torque is much larger. For Porsche’s modern centerlock wheels, you need to tighten them to over 440 foot-pounds, then loosen them, then tighten them back up to the same spec. This requires a huge breaker bar or a specialized impact gun that can apply the correct torque. Indeed, HYTORC manufactures an impact gun that performs the correct tighten-loosen-tighten regime that Porsche recommends. It’ll cost you almost $5,000.

Making life harder is that manufacturers demand you jack the car up before you tighten or loosen the wheel nuts. When tightening, if the vehicle is sitting on the ground, the load on the wheel can prevent it from seating or torquing properly. There’s also the concern that if loosened on the ground, the wheel could shift under the car’s weight, and potentially damage or gall certain parts of the centerlock hub, like the splines. This obviously isn’t a problem for race vehicles with air jacks, but it’s a big pain in the ass for the home gamer. Many owners have posted videos on YouTube showing how much this complicates the process.

In the case of Porsche centerlocks, swapping the rears isn’t so bad. The handbrake will usually hold the wheel tightly enough for loosening and tightening the nuts. For the front wheels, most owners either use an assistant to mash the brake pedal, or an extendable clamp to do the same if they’re working alone.

Modern centerlock hub assemblies aren’t as simple as the early knock-on versions, either. Porsche’s current design has a pop-out locking spline to avoid the wheel coming off accidentally, for example. The automaker requires regular inspection and lubrication of the wheel hub and nut components with messy molybdenum lubricant, a further complication not typically seen with regular multi-lug hubs.

In summary, centerlock wheels are great if you’re trying to change wheels in a hurry. However, they come with some drawbacks that make them kind of impractical without a racing team and lots of equipment behind you. In any case, now you know what makes them tick and why they’re so special. And if you’re so lucky to be pursuing a supercar purchase, you’ll be able to decide whether you want to go the centerlock route or stick with a five-lug setup with all the facts at hand. Ultimately, whatever hubs are on your car, just make sure your nuts are snugged up tight!

Image credits: Mercedes Classic, Lewin Day, Jeff Strimel via YouTube screenshot, Porsche, Lamborghini, Superspeedersrob via YouTube screenshot, Mercedes-AMG F1 Team, Amplified Listings