Home » NASCAR Truck Loses Its Entire Rear-End During A Race: How Is This Even Possible?

NASCAR Truck Loses Its Entire Rear-End During A Race: How Is This Even Possible?

Andretti Accident
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The closing laps of the NASCAR Truck race at the Circuit of the Americas saw a yellow flag come out as the entire rear differential assembly departed the No. 04 truck of Marco Andretti. This is an incredibly unusual incident. So how does this happen?

Race trucks that compete in the NASCAR Truck Series still employ a solid rear axle that is mounted to truck arms with u-bolts much like we would have seen in production trucks of decades past. The illustration below shows the rear components of a NASCAR Truck and the important thing to note is that many of these parts are fabricated by the teams or purchased from other teams.

Vidframe Min Top
Vidframe Min Bottom

The truck trailing arms, which are commonly just called “truck arms” in NASCAR, are the basis of this suspension and what allows the rear axle assembly to move around.

NASCAR Truck Rear Suspension
Image: NASCAR

Those front end of the truck arms attach to the chassis using monoball mounts that allow the truck arm to pivot up or down. The rear of the truck arms is a bit more complex with spring seats that hold the spring assemblies along with mounting saddles that the rear axle assembly sits in which use u-bolts for fastening. The very end of the truck arms also contains the shock mounts and the track bar mount. The basic idea is that the truck arms are what help absorb everything between the tires and the chassis of the truck.

NASCAR Truck Arm
Image: NASCAR

Cutaway views show us a better look at the design of the I-Beam type truck arms that are employed in the NASCAR Truck Series and the various attachment points for the suspension components. These truck arms are typically fabricated by teams and often customized based on track. Due to the nature of the budgets in the NASCAR Truck Series, these components are often used for multiple years and traded between teams.

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Suspension Closeup2
Photo: Author

The most likely scenario in this situation is that the welds failed towards the rear of the truck arms just ahead of the u-bolt mounting saddle that is labeled above. We can come to this conclusion because the recovered rear axle assembly still shows the u-bolt attached to the rear portion of the truck that includes that saddle area along with the shock.

Weld Break
Cutaway: NASCAR

Once one of the truck arms snapped in that area, the rear axle assembly would start departing the truck as the momentum of the truck would be moving the chassis forward and away from that axle assembly. The force of that can break the other truck arm along with the connection to the driveshaft which results in that whole assembly hopping out from under the truck as we saw on the broadcast.

Now that we understand how it broke getting into the reasons why leads us to how these trucks behave on road courses. Wheel hop is a common occurrence for NASCAR trucks on road courses as the rear axle assembly tends to hop up and down when drivers lock up the tires under heavy braking.

Wheel hop usually happens under heavy braking because the weight is shifted towards the front of the car so the rear goes up and the tires lose contact with the track surface. In addition to weight transfer, wheel hop can also be induced by setting brake bias too heavily to the rear. NASCAR drivers can adjust brake bias from the inside of the race car and often will try to shift as much as possible to the rear in order to improve handling but going too far can induce some of those lock-ups that cause wheel hop.

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Andretti No. 04 NASCAR Truck at COTA
Photo: Author

In this situation, that wheel hop likely proved terminal because it was sending all of those forces through older truck arm that likely had compromised welds. These truck arms are often built with the bare minimum of welds in order to keep them as light as possible and to allow them to flex when necessary. Over time, these weld can sustain damage from situations like wheel hop that can cause cracks and if they are not inspected they can result in a portion of the truck arm fully breaking away as we saw here.

Suspension Closeup 1

Photos: Top imagine Bozi Tatarevic, NASCAR, NASCARonNBC

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Freelivin1327
Freelivin1327
21 days ago

I love the top pick w/ the guy staring w/ WTF…it’s hilarious- “what the fuck just happened?!”

Chris D
Chris D
21 days ago

This comes from saving a few bucks by using cheap Amazon-quality imported suspension components.

Manwich Sandwich
Manwich Sandwich
22 days ago

So how does this happen?”

IT HAPPENS BECAUSE NASCAR SUCKS!!!!

/jk

Dan Manwich
Dan Manwich
22 days ago

The back fell off

DysLexus
DysLexus
22 days ago

Oh I see now…
It’s the rear axle that fell off and NOT that the whole rest of the truck and driver that disappeared when they were “raptured” to the heavens with an over-zealous NASCAR Andretti fan praying in the stands.

Last edited 22 days ago by DysLexus
Totally not a robot
Totally not a robot
22 days ago

Is it my imagination, or is that ducting connected to the brakes to accommodate cooling airflow?

Stones4
Stones4
22 days ago

That’s exactly what it is, brake vents. They have ducts in the grille that feed the fronts, sometimes with fans for additional cooling too. Not sure where the rears are fed from

Black Peter
Black Peter
22 days ago
Reply to  Stones4

Could just be in the open air facing forward? I actually thought those were rubber “gators” for the shocks and the mounts were all twisted.

MEK
MEK
22 days ago

From what I’ve read, NASCAR keeping with the smaller 15″ wheels limits the brake disk size compared to some other racing series. The smaller disks can’t absorb/dissipate as much heat necessitating the huge cooling ducts you see on the NASCAR cars/trucks. Other racers use them as well, obviously, but I think the NASCAR ones are the largest, at least from what I’ve seen.

Totally not a robot
Totally not a robot
22 days ago
Reply to  Bozi Tatarevic

Wow, I really wasn’t expecting a purpose-built tweet like that, but it sure answers my questions! Thanks, Bozi!

Mr. Asa
Mr. Asa
21 days ago

You can find the ducting fans on eBay fairly regularly (TBH, you can find most NASCAR parts on eBay fairly regularly if you don’t mind stuff that’s 3-4 years old) and by god those things push a lot of air.

Old Busted Hotness
Old Busted Hotness
22 days ago

During wheel hop (the wheels may or may not actually leave the ground) the axle housing wants to rotate back and forth. With truck arms there’s no accommodation for this, so the axle’s twist becomes a bending motion. Do that a couple thousand times and you get metal fatigue. Combine that with making the arms as light as possible and it’s not hard to see how this happened.

The trucks are actually recycled Xfinity/Busch cars, which in turn are recycled Cup cars (or were prior to the Next Gen Cup car) so this chassis could be 15 years old, and no telling how old the arms were. Their use in NASCAR goes back to the 1960s.

Rafael
Rafael
22 days ago

That’s not very typical, I’d like to make that point. There are a lot of these trucks going around the track all the time, and very seldom does anything like this happen. I just don’t want people thinking that those trucks aren’t safe.
Well, not that one, obviously, but the other ones are. Some of those trucks are built so the back doesn’t fall off at all.

My Goat Ate My Homework
My Goat Ate My Homework
22 days ago
Reply to  Rafael

There are very rigorous standards about material requirements. Carboard is out, cardboard derivatives, paper. And the pit crew must be at least one, at a minimum.

MEK
MEK
22 days ago

What strikes me about this is just how primitive the whole thing is. Solid axel, a couple trailing arms, coil springs, shocks and a control arm. Yikes. I mean I know its NASCAR and the whole “if it’s not broke don’t fix it” with respect to change and all, (and it’s the truck series which I can guess is even more so) but I think there are lawn tractors out there with a more sophisticated suspension design that this.

Andreas8088
Andreas8088
22 days ago
Reply to  MEK

Honestly, that kinda makes me more interested in watching. Part of the reason I find NASCAR to be such a boring racing series is that it’s so far removed from “stock cars”.

Old Busted Hotness
Old Busted Hotness
22 days ago
Reply to  MEK

Very early (when these arms were actually used in production trucks) NASCAR engineers figured out that the design was well suited to circle-track racing, much more so than 4-link or leaf springs that were in use on production cars. 1960s 4-links had horrible axle-steer characteristics and leaf springs were subject to axle winding. Acceptable when cruising down the street but really hard to make work for racing. Primitive maybe, but suitable. Sometimes a stone axe is the perfect tool for the job.

Maxzillian
Maxzillian
22 days ago
Reply to  MEK

You’d be surprised at how good a truck arm actually is for a solid axle suspension. Compared to a four link all it really lacks is as much anti-squat adjustability. Otherwise it offers some anti-squat without too much risk of wheel hop under braking and plenty of torsional flexibility for road/track. In my opinion the only real reason you don’t see more of it is due to packaging limitations.

BolognaBurrito
BolognaBurrito
22 days ago
Reply to  MEK

It’s basically the same design that Chevy used on 2WD pick-ups from ’60 to ’72.

Last edited 22 days ago by BolognaBurrito
Hondaimpbmw 12
Hondaimpbmw 12
22 days ago
Reply to  MEK

The trailing arms are barely different from 60-72 Chevy C-10 rear suspension arms. It would not surprise me if the major difference is the “monoball” pivot at the fron end of the arm. NASCAR was using Chevy truck arms in the early 70s.

Last edited 22 days ago by Hondaimpbmw 12
Jack Trade
Jack Trade
23 days ago

A great article, but what really struck me was…I didn’t realize that Marco was racing trucks now. Cool and thanks for the heads up!

He’s a good driver, perhaps somewhat unfairly judged b/c of his last name. He won the SRX championship two years back, and took pole at Indy once even.

Mechjaz
Mechjaz
23 days ago

Lousy truck welds. Should’ve used truck nuts!

Crank Shaft
Crank Shaft
23 days ago

I don’t remember doing those welds, but the quality sure seems like they might have been mine.

Urban Runabout
Urban Runabout
23 days ago

“Racing Trucks” + 1960’s rear truck suspension tech that’s been “fabricated by teams and often customized based on track” and “often used for multiple years and traded between teams” + Wheel Hop = an incredible amount of stupid.

Huja Shaw
Huja Shaw
23 days ago

“NASCAR Truck Loses Its Entire Rear-End During A Race: How Is This Even Possible?” . . . um, maybe the team employed former Boeing mechanics and engineers?

Drive By Commenter
Drive By Commenter
23 days ago

That’s not how to do tray skids…

Shooting Brake
Shooting Brake
23 days ago

What? Guns N’ Roses made an appearance? You know,with Axl Rose

(Axle Rolls) ◔_◔

Last edited 23 days ago by Shooting Brake
Cal67
Cal67
23 days ago

This was the entire rear axle, not just the “entire differential assembly”. The differential assembly is the red part and the parts bolted to it prior to bolting it into the rear axle differential housing. (Pinion gear, ring gear, spider gears, bearings, housing and other components required for setup and assembly.)

BentleyBoy
BentleyBoy
23 days ago

Marco Andretti drove the wheels off that thing…” should be an honorary CTOD candidate.

BentleyBoy
BentleyBoy
23 days ago
Reply to  BentleyBoy

Rats COTD nothing like a little dyslexia in the morning.

Crank Shaft
Crank Shaft
23 days ago
Reply to  BentleyBoy

And they still haven’t extended the comment etid window. 🙁

10001010
10001010
23 days ago

You picked a fine time to leave me loose wheels…

Eggsalad
Eggsalad
23 days ago

They saved a few bucks by using discount U-bolts, made from steel recycled from a 1977 Dodge Aspen.

Turbo Quattro CS
Turbo Quattro CS
23 days ago
Reply to  Eggsalad

You can see in the pictures that it was not the ubolts that failed.

TOSSABL
TOSSABL
24 days ago

Anyone else think of the scene in American Graffiti with the cop car axle jerked out from under it?

This could have been deadly—glad it was just vehicular damage

Rust Buckets
Rust Buckets
24 days ago

I’m somewhat confused about this: “…employ a solid rear axle that is mounted to truck arms with u-bolts much like we would have seen in production trucks of decades past.”

So, pickups(not of decades past) still use a solid axle almost universally. This trailing arm arrangement was on 60s Chevy pickups for about a decade, but I would not say that pickups of decades past often used trailing arms.

TOSSABL
TOSSABL
24 days ago
Reply to  Rust Buckets

what I took from that sentence is that the u-bolts themselves are what were on production trucks, not the track arms. And we’ve all seen similar u-bolts holding axles to leaf springs

Last edited 24 days ago by TOSSABL
Vee
Vee
23 days ago
Reply to  Rust Buckets

I think RAM uses them on certain models but I’m not entirely sure. Regardless it is a weird decision considering trailing arm suspension was more popular on full size vans than trucks because vans often don’t need a lot of articulation, but do need a lower load floor and cost of manufacture that other types like multilink won’t afford.

JumboG
JumboG
23 days ago
Reply to  Vee

I just looked to confirm, my RAM has a 4 link rear end.

Michael Beranek
Michael Beranek
23 days ago
Reply to  Vee

Not weird, this is how stock cars were designed for decades. When they added the trucks in the 90s they just aped what was normal. Since then Cup has moved on, but the trucks are throwbacks.

Rust Buckets
Rust Buckets
23 days ago
Reply to  Vee

Ram uses a four link rear, but it’s pretty different from this three link trailing arm setup and doesn’t use u bolts. Raptors and Tacomas also use a four link I believe.

Hondaimpbmw 12
Hondaimpbmw 12
22 days ago
Reply to  Vee

What van had similar trailing arms like that?

Vee
Vee
10 days ago
Reply to  Hondaimpbmw 12

Bit late I know (I forgot my login again), but the Ford Transit does, as did previous Ford of Europe vans and certain variations of the Volkswagen Transporter/Vanagon (though that one only halfway counts because while the T4’s front-engined, the T2 and T3 are rear-engined).

Phantom Pedal Syndrome
Phantom Pedal Syndrome
24 days ago

So…
I watched the linked video in its entirety and I’m seriously confused as to how the cartoon bears talking explicitly about toilet paper explain what happened.

Also…
Where am I?

Last edited 24 days ago by Phantom Pedal Syndrome
JoeJoe
JoeJoe
24 days ago

It’s not “both wheels”, it’s the whole axle and it should be talked about as such. There is no “one wheel or the other” or “both wheels”, it’s the whole rear axle with differential and brakes. Saying “both wheels” could mean so many more things, starting with them not beeing screwed on correctly.

Boži, falio si naslov ovde 🙂

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