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

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

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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!

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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!

Retro Classics 2024 – Starker Auftritt Von Mercedes Benz Retro Classics 2024 – Strong Performance By Mercedes Benz
The Mercedes-Benz W 25 750-kilogram formula racing car from 1934. Note the knock-on wheel nuts. Also known as “knock-offs,” these were the first style of “center lock” wheel to be developed. Credit: Mercedes Benz Classic Archives

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.

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A basic diagram of a representative “knock-on” wheel hub. Torque is transferred via the splines, as well as the friction between the tapers on the wheel and hub. 
Mercedes Benz W196r (exploded View) Front Brake Mercedes Benz Museum (1)
The Mercedes-Benz W196R used Rudge-Whitworth style centerlock hubs in the 1950s. Note the obvious taper at the base, as well as the thread and spline. Credit: Morio, CC BY-SA 3.0 

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.

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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.

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Two- and three-winged designs are most common. These types of wheel nuts were used into the 1960s.

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.

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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.

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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.

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The Ferrari F40, like this example on sale at Amplified Listings, used centerlock wheels.
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Note the octagonal wheel nut and the R-clip used to stop it from backing off.

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.

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The centerlock assembly used by a Lamborghini Huracan GT3 race car.

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The retention tabs used on the 2014 Mercedes-Benz F1 car

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.

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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.

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F1 wheel nuts have radial tabs that engage with the wheel gun. These allow the transfer of massive torque—over 3000 foot-pounds—without stripping the nut.

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.

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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.

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Using centerlock wheels can be a pain on a road car. Serious tools are required to install, remove, and tighten them to spec.

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

 

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Aravind Margam
Aravind Margam
26 days ago

Motorcycle’s with centerlocks are sexier and more practical than automotive centerlocks.

JamesRL
JamesRL
27 days ago

Fun fact:

The reason the Knock On British wire wheels fall off occasionally is because of the splines and the hub design.

The hubs are supposed to be greased regularly to prevent the splines from wearing… but over time they wear down and eventually they become weak. Once that happens eventually the brakes are applied during driving and splines will roll over and hub will stop but the wheel will continue to roll… and due to the “self tightening” design of the nut in normal operation they turn into “self loosening” because the wheel spinning on the hub in the direction that loosens the nut. This basically loosens the nut from the inside and then once the nut is gone the wheel flies free.

TaylorDane > TaylorSwift
TaylorDane > TaylorSwift
28 days ago

Never would have guessed the F40 has the same low tech center nut R-pin solution as most lightweight towed trailer axles’ cotter pin design.

Knowonelse
Knowonelse
28 days ago

My ’67 Sunbeam Alpine had knock-off wheels. The wire wheels needed to be trued and balanced, and finding someone to do that, even in 1984 was a challenge. Finally got them balanced and drove off. Hitting the brakes I heard a zzzzipppp sound. The sound of the nut not being tight enough and the splines slipping. The wheel didn’t come off, but hitting the brakes loosened a nuts due to the loose spline. By this point I was really pissed off. Just got the wheels to a point where I could exceed 60MPH, and one nut was loose and I was miles from home. So, screw it, I got that sucker up to 80 MPH on the way home, knowing that I could not hit the brakes much at all. It was smooth and nice on balanced wheels! I managed to get home using very little braking. Oh, just remembered the loose spline was on one of the front wheels, so I could only use the handbrake on the rear wheels. Fun car though. I sold it not long after for reasons that had nothing to do with the car. We both quit our jobs and moved out of the state.

Slow Joe Crow
Slow Joe Crow
28 days ago

I think 3 lugs is the right number because it means you’re driving something weird and French.

TheDrunkenWrench
TheDrunkenWrench
28 days ago
Reply to  Slow Joe Crow

Mercedes sitting proud with the fleet of Smarts.

Regorlas
Regorlas
28 days ago

Entertaining and educational, thank you! I would love to see a follow-up: How F1 Prevent Cross-Threading Wheel Lug Nuts During Those Pit Stops.

(With the likely subtitle of: “And Why It’s Too Ungodly Expensive For Your Car“)

Mike F.
Mike F.
28 days ago

My dad’s MG had those really cool knock-off wire wheels. One of the coolest things with them was when he was driving down the street one day and suddenly noticed someone’s wheel bouncing down the road and into the Hobo Joe’s coffee shop parking lot. This observation coincided with a sudden leaning of the car and a loud scraping noise. Fortunately, the wheel hit the statue of Hobo Joe which kept it from bouncing through the window and into the restaurant. Hobo Joe was pretty big and stout, so he emerged unscathed.

OK, so maybe that wasn’t the coolest thing about those wheels.

Last edited 28 days ago by Mike F.
Spikersaurusrex
Spikersaurusrex
28 days ago

“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.”

This is why I have my pit crew follow me around in the Suburban when I go for a drive.

Collegiate Autodidact
Collegiate Autodidact
28 days ago

Presumably it was an issue with the centerlock nut assembly that led to last summer’s incident at Goodwood Festival of Speed with a Jaguar Mk1 losing its left rear wheel? It flew off and bounced into a crowd of spectators though apparently nobody was injured.
https://youtu.be/wPeTIK1Mh8k?si=DCrPOsg1VJErOqsC (viewpoint from the right side of the Jaguar)
https://youtu.be/eHMxjY9km3U?si=gCx4oQYkBwW1cJTs (viewpoint from the aforementioned crowd of spectators)

Ruko64
Ruko64
28 days ago

Worth remembering that with modern centrelocks the torque is designed to be enough to transmit the entirety of the drive/braking load purely through friction. In normal running the drive pegs should never be loaded, they’re just there so you can still brake to a halt if the nut backs off

HumboldtEF
HumboldtEF
28 days ago

“In summary, center lock wheels are great if you’re trying to change wheels in a hurry.” unless you’re not a race team!

It seams like it would take the average joe much longer than with a standard lug nut setup and you’d be much more stressed out doing it.

HumboldtEF
HumboldtEF
27 days ago
Reply to  Lewin Day

Apologies, Yep reading comprehension isn’t my strong suit apparently. I looked back after posting and saw that yes I’m simply repating wht the article says, Doh. Is there a way to edit comments?

Ronan McGrath
Ronan McGrath
28 days ago

Just don’t get a flat.

For instance there is no spare in the centerlock Porsches.

I have a few of them.

So, a flat basically means a flatbed to get to the dealer and if you are in the middle of nowhere there is an approximately zero chance of finding 335-21 rear tires for a 992RS if damaged ; even if you carry a torque wrench the length of a broom handle it will not be any help.

Taco Shackleford
Taco Shackleford
28 days ago

I wouldn’t kick a F40 out of my garage for having center locks.

MATTinMKE
MATTinMKE
28 days ago

Damn skippy!

Andrew Daisuke
Andrew Daisuke
28 days ago

NUBBIN IS RACIN’

Logan King
Logan King
28 days ago

The thing that stuck out to me the most when watching the BPR seasons was how often the typically fairly dominant McLaren F1s would just randomly have to retire from a race because one of the wheels would go flying off when the center lug wasn’t tightened correctly when pitting.

Last edited 28 days ago by Logan King
AssMatt
AssMatt
28 days ago

So happy with F40 content!

Pro Engineer
Pro Engineer
28 days ago

Properly engineered splines are more than capable of transmitting tremendous amounts of torque, however, they are much more expensive to manufacture than drive nubs.

Scoutdude
Scoutdude
28 days ago
Reply to  Pro Engineer

Pretty much every car relies on splines to transmit the engine torque to the wheels, from the clutch plate on manual transmission vehicles, various bits in an automatic, to the axle shafts whether FWD or RWD.

Sklooner
Sklooner
28 days ago

Worked in a British car repair place, still have my copper and leather Thor hammer to beat the centre locks on-, they were marked near and off for RHD cars

UnseenCat
UnseenCat
28 days ago
Reply to  Sklooner

As someone who rides horses, I understand that reference!

ES
ES
28 days ago
Reply to  UnseenCat

yeah, but if they were only marked so for the RHD cars, were near & off reversed from their equestrian meaning? seems like the terms would be counterintuitive for a driver on the right side. unless he or she carried a mechanic with them in the passenger seat…ah, now i get it.

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