Broken drive shaft
 

Fortunately it failed on the dyno and not on the road. One year old DDS aluminum drive shaft. No warning NVM. Tech said the bolts had worked loose. Hard way to learn a lesson: Check and retorque bolts.

Did you reuse nuts and bolts?

Use blue locktite

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Damn, what does the underside of the car look like? Any major damage?

looks like to me its a bad driveshaft no way loose bolts cause that but where is the carrier bearing at?

I'll go with the "bad driveshaft" theory.

Is that blood all over the floor? - :eyebulge:

Yikes! I have a stock driveshaft if for free. You just gotta pay for shipping from California. :)

Oooof. The red stuff on the floor makes it look like the driveshaft died a horrible bloody death.


How's the underside of the car look?

First of all, I'm not answering any questions about possible blood on the floor.


There was no damage to the underside of the car. The tuner shut it down as soon as he heard/felt it go.


PTuning already sourced a replacement shaft. Thanks for the offer



It's a one piece DDS aluminum shaft.


Yes, I reused the bolts.


And yes to the Loctite.


I'll add checking driveshaft to my pre-track check list.

...pending litigation and all that ;)

i'm sure tommy had it coming.

I bet when it went the drivers seat got totaled!

Subaru notes these as one time use items but I never understood why, if its not rusty/striped what's the mechanical reason they can't be reused?

Looks like a two piece shaft to me!?

Maybe the bolts are TTY?

It certainly is 2 piece now.

Why bolts when all the damage is in the middle? Not saying anyone is wrong, just trying to learn something.

Once the rear u-joint loosened enough for it to spin off axis, it was doomed. Still being fastened at both ends, it buckled like a bent straw. It looks like it may also have gotten scored rubbing against maybe the exhaust. Once a crack started, torque quickly finished the job.

Whole lotta guesswork based on those pics.

I can agree with the off axis rotation causing stress but there was absolutely no vibration. No rubbing noise either. Scoring most likely happened when the broken ends dropped on to the dyno frame.

No vibration you were aware of. If spinning on the Dyno it's conceivable that it started vibrating and then failed completely in such a short amount of time you didn't even notice. That aluminum tube is strong when twisted perfectly on is axis but start bending it(with centrifugal force) and it will fail very quickly. Drive shaft is spinning very quickly

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Yeah. It's a head-scratcher but, like Ash said, it all happens in a blink of an eye. I see your point about the scoring. I'd have to get up close, maybe even with a loupe to really examine it.

Shit breaks. Super lucky it was on the dyno. :thumbsup:

Close up pictures of the fracture could give us a clue if a crack progressed into a failure or it just failed. Plus you need to look at the scoring at the failure point. It almost looks like the scoring came before the failure.

Yup. Chicken <-> Egg

I am glad it didn't happen on the street, this just reinforces my notion of putting a safety loop when going to an aftermarket one piece drive shaft.

Yep without close examination it is all a guess. And even then it may not be conclusive. Now get to the microscope level we could figure it out.

Many bolts are designed to deform in specific ways when tightened. If they're pulled out and then put back in, they no longer meet the spec and can have issues. Tesla recently had a recall over this, with factory workers torqueing, loosening, then retorquing. The bolts needed to be taken completely out, replaced, and torqued properly ONCE.

I've probably pulled and reused the driveshaft bolts about 6 times on my car :iono: Always German torque. I should probably buy a crows foot and do it proper.

Edit: Now that I think about it a bit harder, it is probably more like 12 times.

Turning it 4 times to try to get rid of vibration is def PITA having to get new bolts each time. But also there were a couple of threads a while back about the bolts being too short with aftermarket shafts and results in only half your nuts thread engaging. Solution was to get longer bolts so that all threads engage. I still haven't done it yet on mine @blsfrs do you remember if all the threads engaged on yours?

It's probably gonna be fine 999/1000 times in the first year.
But that 1/1000 is bad enough for a safety recall. 1/1000 * 100,000 cars is 100 failed cars that could kill people in the first year. And then the car companies look at a car's life over 20-30 years, and they start to sweat, because that 100 failures can become 10,000 or more if things go badly. Hence we get our valve springs recalled.

But you? In a car you probably inspect often? Fuggetaboutit.

I'll get some close up pictures of the break and mounting flanges of the 2 drive shafts.


Thanks for all of the input.


I did find out that there is a 1 year warranty on DDS aluminum shafts. My receipt is dated 12/20/2020. It broke on 01/14/2022. I've emailed Vivid racing and DDS but have not yet heard back.

On my DSS AL I had full thread engagement.

Fair enough point.


@blsfrs I would call, not just email. If they don't help you out let me know. I have another spare stock drive shaft and a DSS AL that are both used. You can have either one for the cost of shipping. I only need to keep one as a spare and I have both these laying around.

The DDS flange is quite a bit thicker. If I ever again use a non-stock drive shaft I'll use longer bolts. @NoHaveMSG: Thank you for your kind offer. PTuning sourced a shaft while they had it in the shop.

Pictures will probably lead to more guess work but here they are. Rubbing enough to cause scoring would make noise and I ain't that deaf yet. I would have thought that loose enough to deform would have been loose enough to cause perceptible vibration and my butt ain't that numb either.

Not enough definition in the pictures to tell. We will need to see grain structure in order to make a guess. What you are looking for is part of the separation being shiny and another part not, this is an indication of a fracture that was there prior to the failure.

Other thing you will be looking for is the rough fracture, Is the scoring folder over the fracture. An indication that the scoring happened after. Is the scoring not folder over, an indication the scoring happened prior to the fracture. This may be visible with a magnifying glass.

Even with the blurry pic, I clearly see a perfect circumferential cut which coincides with the failure point. Where your rear mounting screws really loose? Perhaps there was some intermittent contact with the exhaust leading up to the failure. Regardless, that's it. Doesn't take much to create a stress riser. It's like cutting glass. Sharp corners fracture easily.

IMHO, What appears as scoring is probably because the broken ends of the shaft were rotating on the deck of the dyno for several seconds after it let go.

Because bolts actually stretch when they are tightened to reach their torque value. Almost all driveline and suspension bolts are single use and should be used/torqued only once.

If you flip the right side around and look at where the broken 'tab' fits into the left side, you can see a clear mark around the circumference that continues on both pieces. Definitely got scored prior to failure.

MS Paint skills below:

https://i.imgur.com/V6LTDWX.png

I guess the next step is to jack the car up and look for evidence of rubbing. The car is at stock ride height and has the oem midpipe. I don't recall running over anything that would have hit the driveshaft.

An unfortunate contact with a speed bump is as likely as damage from the undercarriage.

That would leave a helical mark.