YIKES! Carbon fiber driveshaft goes bang...
 

An expensive weekend on the dirt.
Very suddenly let go, (luckily) on the entry to a moderately slow "chicane".
Initially thought it might have been due to a rock strike, but where it fractured is well protected by a large exhaust mid mount muffler (I think the vulnerabilty to impacts would be much further along the shaft closer to the diff).
It was fitted about 2 years ago so I'm not really sure how to properly or realistically diagnose the failure.

Which brand is it?

Driveshaft shop.
(NB not from Verus, as I previously wrote)

heat soak failure from the muffler?

Yeah, I guess thats very possible too.

In truth I'm not sure what the temps are like there, although it has never really struck me as a super hot zone, and I have some home made rubber exhaust hangers there that seem to have coped well over time.

But you make a good point.

Excessive pinion angle?

with the rear diff being fixed, unlikely. pinion angle really has more issues at extreme axles in solid axle, extended travel builds.

i don't know what he's got for mounts/bushings on the diff or engine/trans, but anything oem or even if if he's got all poly, should not cause any extreme issues to the drive shaft. unless there were a failed bushing that allowed either assembly to get clocked off center.

Well that sucks, especially now since getting a replacement is going to difficult to impossible. You could probably do some hardness testing just outside of the damage and compare it to then end at the diff ti see if it was heat. My initial guess would be it got damaged in that area then failed.

I'd consider reaching out to Verus to see what their thoughts are. That sucks, sorry to see

CF imho is very light & strong, but brittle material, no? Where metal would bend a bit, it shatters. And rally use over rocky roads .. there might be numerous hits over this through 2 years of rallying .. and might be from long time ago and eg. developing small crack, which ended up with failure at this race. My choice probably would be for rally use, maybe one piece, but more conventional material choice of metal. Unless whole underside is completely guarded by extra protection pans, including tunnel where shaft runs (may cause overheating issues due closed off with reduced venting exhaust in same tunnel though)

^^^this


How much design factor to account for scratches/divots? Stress risers lower actual critical speed.

Also interesting but might be coincidental, the failure occurred approximately where the stock driveshaft is divided.

Any pictures of this? (assuming this is a custom setup)

Yes that was the initial thought at the time by everybody, but we all became less sure that it may have been the (only) factor when we removed the shaft.

I'm confident that it hasnt had a "big" rock strike in that particular area (and even the more exposed portion closer to the diff is in very good condition).

The area surrounding the failure is quite well protected.

However of course I cant rule out some sort of impact at some point.

I have refitted the alloy driveshaft that it replaced.

An engineer who helped me with the car afterwards made a similar point about the location of the fracture along the shaft, saying it was at/near the shafts maximum stress point (or words to that effect),

Our first reaction was some simple sort of impact fracture (at some point), but our gut feeling is that there may be some other factors at play.

Somewhat counterintuitive is the overall condition of the carbon, which is almost completely umarked (to the eye) along its length. * aside from the large scrapes and marks which correlate to hits taken as it spun free of the gearbox, and then being towed off the track.

Of course having said all that, it was always a risk running carbon on this car!

The mid muffler.

The break was midway above this.

Heat could definitely be a factor.

Is it also possible that the muffler could have bounced or been bumped into the drive shaft?

Haven't seen harmonics mentioned so I'll throw it out there as a possible contributing factor. If you are racing around off road it's possible you hit some weird frequency between the front and rear bouncing around at different rates and moving around relative to each other. If it's a track, that might be repeated each lap.

More likely heat or muffler related though.

At least it was spectacular. :thumbsup:

There's evidence of a polish mark on the muffler (cover?) and the weld looks like it's missing some material?

...it was spectacular when my wife asked the actual cost... :confused0068:

I like your thinking, but there is not enough upward play in the exhaust system to get close enough to the shaft (it would hit a solid mount that I have fitted off the gearbox cross member first).

Well spotted.... although that pic was from an earlier escapade, pre break :)

I was looking at the same thing. My DDS AL shaft broke exactly in the middle due to exceeding its critical speed.

Ah, that was yours! Now I remember. You were on a dyno and felt no warning.

Since you already have the machined end parts have you considered buying a CF driveshaft blank and bonding up a new one?

I don't trust anything DSS makes after your failure. That was a huge oversight by a company that has "drive shaft" in their name.

Alloy? Chrome moly or Al or ? 2 piece? At this point I'm a big fan of the stock drive shaft.

Wasn't stock 2-piece made such, so that to allow it to buckle in case of frontal crash impact, so that engine can be pushed under passenger compartment?
It's drawbacks .. i guess slight (but probably not that immense) play in extra joints, slightly more expensive .. and a bit heavier .. hmm .. i wonder if it's not worth to leave it as is, if it's designed/engineered for extra safety ..

Yes. Also with all other properties being the same, shorter driveshaft will be able to spin faster without failing. I'm not a fan of any of these single-piece driveshafts. The only upside is a few kilos trimmed. Not worth it.

True, the twin shaft was a solution to a number of engineering issues for car manufacturers, including official safety test requirements , NVH and packaging.

Initially my main concern was trying to keep overall weight to a minimum (the supercharger and other upgrades was a painful excercise in adding weight), so the single piece helped claw some valuable kilograms (5) back.

But the first alloy shaft ended up adding (strangely, not immediately but after a few months of use) an annoying high pitched whine on a trailing throttle, which was solved by changing to carbon.

For me (personally) the possible secondary safety benefits in specific types of impacts is not persuasive... but living with an annoying whine from the alloy prop shaft again, that could be the final straw! :)

Ugh that sucks. I daily drove on a DSS AL driveshaft for 2 years and loved how amazing the shifting was, but the NVH was insane and the car felt noticeably slower when accelerating at highway speeds in higher gears (due to larger diameter maybe?). I took it off eventually

I'm not a fan of 2 piece driveshafts. I could always feel the play in the center bearing at low speeds. Changing to one piece made it much smoother. I did the same thing with my Isuzu Rodeo years ago after having to replace the center bearing twice. Had a custom one piece made. Never had a problem after that. Huge upside in my book.

This is what gets me. I don't like the presence of a center bearing. My VR4 was 3 piece and it was like playing telephone vs going 1 piece. I love the CF driveshaft on my FRS, hopefully Grump figures out what works best for him

Yea, I like mine too.

I'm sure off road racing in one of these cars will always have more than its share of damage and mechanical challenges.

Mine was really loud with tons of vibration. I'm just happy it didn't fail at triple digit speeds.

Funny, My AL DS was quiet and vibration free until it failed. At least DSS owned up to the problem.

I should mention I had Whiteline diff inserts installed at the same time, so it would vibrate at around 85-90 mph (which incidentally came on sooner if the car was traveling uphill). So I removed the inserts and it stopped vibrating. I did some googling and the diff inserts are also responsible for DS vibration on other platforms (Mustangs, BMWs, etc). Now the question is, did it stop vibrating simply because the inserts were no longer telegraphing the vibration to the rest of the structure, or did the angle change. Doesn't matter now of course, I'm back to the OEM driveshaft :brokenheart:

I have whiteline diff outrigger bushings, cusco diff brace, and SPL solid subframe bushings. Mine sounded like a metal box full of vibrators was in my trunk.

don't judge

well I still have the whiteline subframe inserts and after I took off the DS, I went and installed both front and rear diff bushings again haha. I love the feel and it reduced wheel hop a bit