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.
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.
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.
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.
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.
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.
Marco Andretti drove the wheels off that thing…
Literally. 😳 pic.twitter.com/CjmPJl4rTX
— NASCAR on NBC (@NASCARonNBC) March 23, 2024
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.
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.
Photos: Top imagine Bozi Tatarevic, NASCAR, NASCARonNBC
I love the top pick w/ the guy staring w/ WTF…it’s hilarious- “what the fuck just happened?!”
This comes from saving a few bucks by using cheap Amazon-quality imported suspension components.
“So how does this happen?”
IT HAPPENS BECAUSE NASCAR SUCKS!!!!
/jk
The back fell off
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.
Is it my imagination, or is that ducting connected to the brakes to accommodate cooling airflow?
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
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.
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.
Yep. Those are brake ducts. Dropped a labeled illustration for you here: https://x.com/BoziTatarevic/status/1772354445428326477?s=20
Wow, I really wasn’t expecting a purpose-built tweet like that, but it sure answers my questions! Thanks, Bozi!
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.
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.
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.
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.
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.
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”.
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.
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.
It’s basically the same design that Chevy used on 2WD pick-ups from ’60 to ’72.
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.
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.
Lousy truck welds. Should’ve used truck nuts!
I don’t remember doing those welds, but the quality sure seems like they might have been mine.
“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.
“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?
That’s not how to do tray skids…
What? Guns N’ Roses made an appearance? You know,with Axl Rose
(Axle Rolls) ◔_◔
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.)
“Marco Andretti drove the wheels off that thing…” should be an honorary CTOD candidate.
Rats COTD nothing like a little dyslexia in the morning.
And they still haven’t extended the comment etid window. 🙁
When a driver literally listens to a crew chief.
You picked a fine time to leave me loose wheels…
They saved a few bucks by using discount U-bolts, made from steel recycled from a 1977 Dodge Aspen.
You can see in the pictures that it was not the ubolts that failed.
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
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.
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
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.
I just looked to confirm, my RAM has a 4 link rear end.
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.
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.
What van had similar trailing arms like that?
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).
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?
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 🙂