Why Did The Wheel Come Off That Chevy Pickup That Launched A Kia Into The Air? Let’s Look At The Tech

You’re all wondering what the hell happened with that Kia Soul that launched way up into the air after running into the tread of a rolling wheel that fell off a pickup truck. Specifically: How did the wheel fall off the pickup in the first place? I just pored through dozens of parts diagrams and repair videos/photos to come up with what I think is a strong theory explaining what sent that wheel into the path of that poor Kia Soul.

By now, many of you have likely seen this video of a Kia Soul using a rogue wheel as a ramp:

Witnessed and recorded the most INSANE car crash yesterday, you can see Autopilot also swerve and avoid the rouge tire for me $TSLA pic.twitter.com/csMh2nbRNX

— Anoop (@Anoop_Khatra) March 25, 2023

If you’re wondering “How the hell did that happen,” the answer is simple: The tire’s ability to deform and grip allowed it to grab hold of the Kia Soul’s front fascia such that the Soul began to “ride” up the tire. That happening at highway speeds made it appear as though the Kia was launching off a jump.

How did the wheel fall off the adjacent pickup truck? Well, the answer to that is a little less clear.

The California Highway Patrol says its officer’s initial assessment implied that lug nuts or studs had broken, but the official investigation hasn’t yet been completed, and even when it’s done, we may never know what the actual cause of the tire-detachment was — that’s at least what the California Highway Patrol’s media contact told me over the phone. (This issue is a private matter between the people involved, so the police may never publish the root cause of the failure).

But even if we never hear it from the police, there’s enough info in the video to help us determine some possible causes of the wheel detachment. So let’s talk about them.

Before we start, it’s worth pointing out a key bit of information in that clip. The screenshot above shows a shiny silver surface on the backside of the errant wheel. This will come into play later.

(With that said, some of you who pointed this shiny disc out when you first saw the clip, and think it’s now obvious what the cause was, may end up surprised by my conclusions).

Let’s Talk About The 2011 Chevy Silverado’s Front Suspension Setup

We need to talk about what the 2011 Chevrolet Silverado’s front suspension looks like — specifically, how the wheel is held onto the vehicle.

How Your Wheel Is Held Onto Your Car

Let’s begin by having a peek at a typical “spindle” design on an older vehicle. A spindle is the part of the suspension that holds the “hub” that contains the threaded studs to which the wheel mounts. On an older car, like my Jeep J10, the spindle is a pointy pipe that bolts to the steering knuckle, which is attached to the suspension control arms (or in my case, the axle) via ball joints. You can see all of that below.

Those ball joints are just greased steel balls that allow the steering knuckle to pirouette when you turn the steering wheel (turning the steering wheel pushes and pulls the tie rod shown below, which yanks and shoves the arm on the knuckle), thus turning the front wheels.

Onto that spindle sits a brake disc, which has inner and outer tapered roller bearings inside of it. The inner race of each bearing rides on the spindle; the outer race is pressed into that rotor. Here’s a look:

Tightening the castellated nut under the grease cap is what sets the bearing “pre-load” (i.e. made sure there wasn’t too much play so the bearings last).

Here’s what those bearings look like in my Jeep J10:

The point is that, on an older design like the one above, the rotor itself is what houses the replaceable wheel bearings, which rotate on the spindle (called the “stub axle” above) as the wheels rotate. The spindle itself does not rotate with the wheel, but it does pivot/pirouette on the ball joints as you turn the steering wheel left and right. The vehicle’s front wheel bolts up to the studs on the rotor (labeled in the diagram above).

The Hub Assembly/Unit Bearing

The way wheels are mounted to cars changed sometime around the 1990s with the implementation of the “wheel hub assembly,” also called the “unit bearing.” This is a single part that incorporates a lot of the functions done by multiple components in an older spindle/wheel bearing system like the one presented before.

Instead of the brake rotor carrying the wheel via studs, and that rotor housing two bearings, with those bearings riding on a stub shaft/spindle hooked to the knuckle, the hub does a lot of this work.

A unit bearing contains two bearings built in (see below), it mounts directly to the knuckle (no stub shaft/spindle required) via three bolts in the outer case of the hub assembly, and it features studs that hold both the brake rotor and the wheel. The rotor now only has to handle braking loads, not the entire load of the car.

Let’s look specifically at the hub assembly of the 2011 Chevrolet Silverado involved in the now-viral video. The hub is shown in the 1A Auto YouTube video above, but here are some screenshots:

You can see that in the image above, the steering knuckle is attached via ball joints to the suspension control arms, which hook up to the vehicle’s frame.

In the Silverado, the knuckle has three holes in it; bolts enter those holes from the back (inboard) and thread into the three threaded holes in the wheel hub assembly, holding the hub (and that thin brake dust shield) tightly against the knuckle, perfectly flat so that the hub face/flange is situated vertically.

You can see the three bolts holding the hub assembly to the knuckle here:

Here’s the hub assembly all tightened up. Notice that the hub’s face/flange is basically vertical:

Onto that hub face and over the hub’s studs slides the brake rotor:

Once the rotor is on, the brake caliper is mounted to a bracket that is bolted to the knuckle via two beefy bolts:

Below is how the caliper looks from the front: Notice that the caliper and caliper bracket are both slid over top of the rotor. The rotor isn’t yet fastened tightly to the hub flange just inboard of it, since the lug nuts (which go over the lug studs) aren’t squeezing the wheel against that rotor yet.

Still, even though the rotor can wobble a bit in this setup, it isn’t going to slide off those studs since the caliper and bracket are mounted over top, holding it all in. This will become relevant when we talk about how that hub and rotor (that’s the shiny thing shown in that screenshot early in this article) might have escaped — that rotor wouldn’t have easily moved axially outward from the truck, as the caliper and bracket would have held it in.

To finish off the Silverado’s wheel setup, let’s peek at the image below. It shows that the wheel goes on over the six studs, and lug nuts squish that wheel and and the rotor below tightly against the hub face.

Why Did The Wheel Fly Off?

Now that we’ve got a good understanding of the Silverado’s front suspension/steering setup, let’s talk about possible causes of the truck’s tire evacuation:

1. The lug nuts came loose/the lug bolts sheared
2. A wheel spacer failed
3. The ball joints failed
4. The knuckle failed
5. A wheel bearing separated
6. A wheel bearing failed catastrophically (sheared)
7. The bolts holding the unit bearing to the knuckle failed

Let’s just go through these one by one:

The Lug Nuts Came Loose/The Lug Studs Sheared

Take a look at the image just above, which shows all the wheel’s lug nuts. Imagine if those came loose. What would happen is the wheel would start to wobble with a severity proportional to the number of loose lug nuts and the extent to which each was loose.

It’s safe to say that a scenario where all nuts come loose at the same time is unlikely, and it’s even safe to say it’s unlikely that they’d all shear at the same time. And if they somehow did, the wheel would fly off, and the rotor would flop around between the hub flange and brake caliper. Even if the rotor somehow fell off the car, there would certainly be nothing holding it to the ejected wheel, since the rotor simply slides over the same now-sheared studs that the wheel slides over.

A Wheel Spacer Failed

One thing I haven’t yet mentioned is wheel spacers. These have become fairly common in the automotive world as a means to improve a vehicle’s “stance.” Specifically, they push wheels outboard to make a car look wider and more aggressive.

Though it’s not clear that the truck in this video used wheel spacers, it is clear that it has had its stance widened — something that can only easily be accomplished with new wheels that have a different offset/backspacing or with wheel spacers.

The way wheel spacers like the shiny aluminum ones at the top of this section work, is they bolt to the wheel hub’s studs. This squishes the rotor against the hub assembly. The wheel, instead of being bolted to the hub assembly’s studs like it normally is, now bolts to the spacer. In this way, the spacer just acts as a middle-man, creating more space between the brake rotor hat (and thus the hub face/flange against which it sits) and the wheel, pushing out the wheel. Here’s a look at the setup courtesy of eBay seller brakeautoparts_20:

As you can see in the image above (the spacer is black in the image above, whereas it’s silver two images up), the studs that normally tighten your wheel up against your hub, squeezing the rotor, are now just being used to squeeze the spacer and rotor against the hub. The spacer has its own studs built in; the wheel slides over those, and the lug nuts tighten that wheel against the spacer.

Here’s another look:

The question many people have is whether the spacer failed.

If the spacer’s own studs failed, the wheel would have just fallen off. The brake rotor/disc would still have been attached to the vehicle via the nuts in the spacer’s bored holes. Now, if the spacer itself had failed, it would have come off and remained attached to the wheel. The rotor, though, would still be attached to the wheel hub.

And though I’ve looked at this screenshot a number of times, I have concluded every time that what fell off the truck is a wheel with an attached shiny brake rotor, not a shiny wheel spacer. A wheel spacer would have a smaller bore at its center, and there would be holes in it to accommodate nuts:

So thus, I don’t think a failed wheel spacer failure is what caused the wheel to fall off.

The Ball Joint Failed

Failure of ball joints holding the knuckle to a vehicle’s control arms is one of the primary causes of wheel detachment on modern vehicles, but I don’t think it’s what happened to the Silverado in the video.

A ball joint failure means the knuckle detaches from the suspension, which is bolted to the chassis. The wheel would still be carrying the unit bearing and the knuckle, since the six lug nuts would still be creating a clamp load between the wheel, rotor, and. hub, and the three bolts would still be holding the hub to the knuckle from the backside.

Given that there’s no evidence showing the knuckle still attached to the rogue wheel assembly (the screenshot clearly shows that there’s no big steering knuckle still attached to that wheel — there’s just a rotor), I don’t think this was the failure mode.

What’s more, a ball joint failure rarely happens suddenly, and it certainly won’t happen to both ball joints simultaneously (typically only one ball joint fails at a time, causing the tire to simply “tip”). Even if one failed and put increased stresses on the other joint, causing it to fail shortly thereafter, the steering tie rod from the steering rack would still hold the wheel in place.

In any case, ball joint failure is rarely something that happens suddenly without warning, and when it does, the wheel usually doesn’t fly completely off. This just isn’t likely what happened to the 2011 Silverado in the video.

The Knuckle Failed

What if the steering knuckle fractured? This isn’t unheard of, especially if a vehicle has been in some kind of collision, either with a curb, another car, or some off-road obstacles.

But a failed knuckle isn’t likely to violently throw a wheel out into the road. As mentioned before, the knuckle is held to the vehicle via three joints — two ball joints and a tie rod end. The wheel is attached to the hub (via its lugnuts) and that hub is bolted to the knuckle through those three holes shown above. I see no single fracture that would completely release the wheel.

This was clearly not the issue.

The Wheel Bearing Separated

 

 

 

 

 

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A post shared by David Tracy (@davidntracy)

Have a look at the clip above, and you’ll see what a typical wheel bearing failure looks like. The wheel will start becoming loose as the inner and outer parts of the wheel hub (called inner and outer rings — I’ll show a labeled wheel bearing in a moment) want to separate, since the bearings that typically wedge between those parts to keep them tight have ground into dust.

If a wheel bearing gets bad enough, the center part of the hub (the part with the flange that the wheel bolts to) will actually be able to pull itself out of the bearings and outer ring, which is what’s attached to the knuckle. Have a look:

Here’s a screenshot of me pulling the inner ring out from the hub flange:

And here it is completely extracted:

That’s scary even on a driven axle where the axle’s outer shaft helps hold the bearing together via a big nut; on a two-wheel drive vehicle (like I suspect this truck is), it’s even scarier.

Indeed, since the detached flange (shown above) includes the hub studs, if this were to fall apart, it would take the rotor and wheel with it. So you’d end up with a sandwich made up of this hub flange, the rotor, and the wheel, which is being squished by lug nuts that thread onto that hub’s studs. This is not unlike what we see in the screenshot.

So this is it, right? The wheel bearing just failed — that explains the wheel coming off completely, and the rotor remaining attached.

Possibly. You see, for that inner bit of wheel bearing to become detached from the rest of the hub, it has to slide out axially, and that means it has to push the rotor out axially. And remember, that brake caliper and bracket are the way:

Not only that, but when a wheel bearing fails in the way I just described, it usually causes the wheel to wobble quite violently before failure. The video does not show the truck’s tire wobbling prior to it falling off. Here’s another look:

Witnessed and recorded the most INSANE car crash yesterday, you can see Autopilot also swerve and avoid the rouge tire for me $TSLA pic.twitter.com/csMh2nbRNX

— Anoop (@Anoop_Khatra) March 25, 2023

But it’s sometimes hard to tell from the outside if a wheel is wobbling at speed. So this failure mode isn’t entirely implausible; it’s possible that the unit bearing could have separated, and either snapped the caliper bracket right off or the rotor could have slid radially out from the caliper’s pads.

A Wheel Bearing Failed Catastrophically (Sheared)

My primary inkling is that the wheel bearing failed catastrophically. Specifically, I think the inner ring sheared, possibly as a result of fatigue — but in any case, I think it broke into two pieces.

I’m going to assume that the truck was two-wheel drive for this specific example (since it was in California, and since the screenshot does not show an axle shaft attached to the escaped-wheel and rotor), but a four-wheel drive wheel hub isn’t much different anyway, so this all still applies.

Imagine if the inner part of the hub (green) fractured, especially if it happened far outboard, not far from the hub flange. Now the wheel is no longer supporting the rest of the vehicle vertically (in the previous section where I assumed the ball bearings had worn out, you’d still have that inner tube running up against the outer ring, thus supporting the truck’s weight). This could help explain why the wheel launched the way it did:

If you look at the video, it’s not like the wheel just detached and rolled just beside the pickup — it was launched sideways. And as you can see, the bottom of it was shoved out first and the tire seemed to turn towards the left lane. Why would this happen? I have a theory.

Let’s say the wheel bearing fractures like I just mentioned. Now the bearing is no longer supporting the weight of the car. This means the caliper, which is bolted to the knuckle (which is bolted to the truck’s suspension), is going to want to fall down relative to that rotor. Again, at this point, the rotor (and the broken-off hub flange to which it’s bolted and of course the wheel) is not attached to the car at all, it’s just got a caliper hovering over it and a knuckle (with a broken off hub section) behind it.

If you look at the image above, and imagine the caliper wanting to push down, it’s going to create a load on the edge of that brake disc with a component pointing downward and a component pointing towards the front of the truck. Watch this little clip below and you’ll see that the downward force of the caliper pushing the rotor (which is inboard of the tire — the pivot point) creates a moment arm that wants to “tip” the tire such that the top moves inboard. Since it can’t move inboard due to the knuckle being there, the bottom moves outboard, just like in the clip.

The caliper also pushes the rotor forward, and since the rotor is inboard of the tire tread (again, the pivot point), this makes the tire want to pirouette about its axis, rotating outboard towards the Kia. This also would cause the wheel/rotor/broken hub to be “squeezed” and violently shot out of the truck.

The Bolts Holding The Unit Bearing To The Knuckle Failed

Another very plausible cause of this wheel leaving the Silverado is the failure of the three bolts holding the hub assembly onto the knuckle. If this were to have happened, the wheel hub would still have had to move outward towards the caliper and caliper bracket (since the back part of the unit bearing bearing sits inside the center bore of the knuckle about an inch), which would have wanted to hold the rotor in.

But I could see the back of the unit bearing escaping the center hole of the knuckle just enough to slide out radially away from the caliper in a way similar to what I described in the “sheared unit bearing” scenario above.

If all three bolts were broken, and the unit bearing were just sitting in the knuckle’s hole, the weight of the vehicle would, via the knuckle, push down on the back of the unit bearing, and this would create a moment arm around the tire’s contact patch that would tend to want to rotate the top of the tire inboard and the bottom outboard. There would be a lot of force that could break the caliper off (the caliper isn’t with the wheel nor is shown flying down the road, but it could stay with the vehicle since it’s attached via a brake hose), or – with the unit bearing now out of the knuckle’s bore – the caliper could just shove the wheel/rotor/entire unit bearing out and away and towards the Kia like I described in the previous example.

How would three bolts break at exactly the same time? Wouldn’t you expect some kind of chain reaction? Well, it’s possible two had threaded out all the way already (or broken), and the hub was just being held in by one. That would be enough to let the vehicle drive smoothly, since the back of the hub slots into that knuckle’s center bore, keeping things centered. It’s possible that what we’re seeing is the final bolt letting go, as there is substantial stress on it from the aforementioned moment arm.

My Conclusion

My initial hypothesis is that the unit bearing sheared. I spoke with The Autopian’s own suspension engineer, Huibert Mees, and he agrees, telling me:

Here is my take on it. I think it is a 2WD truck and the spindle failed. You can clearly see the brake rotor still attached so it is definitely not a lugstud failure. The rotor is still connected to the wheel by the lugnuts so it must be the spindle that failed. It is possible the bearing retaining nut came off, especially since it is the left wheel (which means the rotation of the wheel would want to unscrew the nut), and the wheel simply came off the spindle. I’m fairly sure it cannot be a 4WD truck since the driveshaft (or at least the outer CV joint) would have had to come out with the wheel if the bearing failed.

OK. I just looked up what the front bearing for the Silverado looks like and it is a bolt on unit like the one in the picture you sent (called a HUB III style). A failure of that type of bearing is very unusual. I’ve never seen it before but since the rotor was clearly still inside the wheel, it is the only explanation I can think of. The hub flange that holds the studs might have fatigued off. I still think its a 2WD truck though.

Last month Huibert wrote the article “Our Suspension Engineer Talks About The Effects Of Wheel Spacers And Wheels With Different Offsets.” In it, he talks about how a wheel spacer (or a wheel with different offset/backspacing) can affect bearing durability by increasing a key moment arm, so that may have played a role in causing the bearing failure in the first place. From that article:

…look at where the bearing load line now sits. It is way outside the ball bearing races and will cause a significant increase in the loading of the bearing — loading that it was not designed for. The life of the bearing will be reduced, and you can expect to have failures in a part that would normally last the life of the car.

And if you look around the internet, a number of Silverado owners (not of the same generation, but with the same basic suspension design) have had similar wheel failures happen.

Check out the post on Chevrolet Forum titled “Wheel assembly fell of while driving down HWY…” It includes the images above and reads, in part:

As we are on vacation driving down the HWY going 70mph, we left as if we got a blown tire. Luckly my husband had good control over the truck, which is a 2007 chevy silverado 1500, we got the truck pulled over and the tire, gas sideways under the truck! The lugs and tire with brake assembley were all together in the wheel! It looks as it the axel just broke completley in half. The man who towed us said he has never seen anything like this before.

Looking at the images, it appears that the unit bearing sheared on what looks like a two-wheel drive truck.

Another Chevrolet Forum post titled “wheel assembly came off at 60mph” is even more dramatic, features the images above and this description of events:

as i was driving home i was going 60 mph down a 3 lane road and i was in the middle lane. all of a sudden it felt like the front left tire blew out…the truck slowed down and started pulling to the left. i mashed the pedal and it straightened back out, but when i left of the pedal again it started to drift to the hard left again. i thought i had a flat so i decided to pull over and see whats up….as soon as i hit the brake pedal the truck dropped down to the ground and sparks were flying and i looked over and saw my tire flying next to me about 10 feet in the air. the tire took off, bounced over 2 cars, hit a stop sign, took some guys chain link fence out and went through his trailer and ended up in his living room. (trailer park was right next to the road) i had no brakes considering my brake line was ripped and all the fluid came out. so i was skidding down the road and hit the median…(luckily it was only a curb!) then bounced off and finally came to a stop in the turn lane.

Notice how this truck is four-wheel drive, and the bearing doesn’t appear to have sheared, it appears to have fallen apart, unlike my initial hypothesis. I don’t think the 2011 truck was four-wheel drive, and I’m not entirely sure the bearing “fell” apart.

As this forum post shows, if the bearing does fall apart, the rotor tries to move outward but is held in by the caliper; the truck straightens up when the driver touches the brake (see in bold), which clamps the caliper and pulls the rotor and hub back inboard, straightening the vehicle. Eventually, enough of this actually tore the caliper off with the rotor.

Though I can’t completely discount this type of bearing-separation as a possibility, there’s no obvious wobbling happening to the 2011 truck in the initial Twitter video. I’d expect that if the bearing came apart (whether due to an axle shaft breaking and no longer holding the bearing together in the case of this 4wd truck or just wear and tear) instead of shearing, the outer bit of the unit bearing would try to move outboard and hit the caliper; the driver would hit the brakes, it would straighten out, and this might happen a few times before the caliper just snapped off the knuckle and the wheel went rogue. That behavior is not obvious in the Twitter clip.

So based on these forum posts, and based on my look at the system in question, I still think the unit bearing sheared, though it’s similarly likely that the three bolts holding the unit bearing onto the knuckle were the culprits. These are just theories, of course, so let me know what you think in the comments below.

UPDATE (March 30, 2023 6:30 PM ET): We have new footage from the Tesla driver who released the original video clip! Check it out:

Found some additional interesting footage pic.twitter.com/OV9aeeZ1jU

— Anoop (@Anoop_Khatra) March 30, 2023

A close look shows the rotor attached to the wheel, and a small circle at the axis of rotation:

That circle looks silver, indicating that it’s clean. What would be this clean underneath a vehicle — in an area that sees dirt and grime regularly? The brake disc is clean because it gets wiped by the brake pads, but what about that little circle? The answer could be that this is a shear surface that is clean because that bit of metal is being exposed to the elements for the first time. This could be a sheared axle (but again, I think this truck was two-wheel drive), though I think it’s a sheared unit bearing.

The shiny bit in the screenshot could also be the shiny cap on the back of the unit bearing (in the case of the three bolts holding the unit bearing to the knuckle being the culprit), though that would get dirty not long after installation. Also, the the triangular-ish flange with the three threaded bolt holes — the part of the back of the hub bearings that actually touches the face of the knuckle — is nowhere to be found in the grainy screenshots. But again, they’re grainy screenshots. 

UPDATE (March 30, 2023 10 PM ET): This story has been updated to discuss the likelihood of the three bolts that hold the unit bearing to the knuckle being the cause of the failure. Hat tip: VermonsterDad!