Sheared driveshaft
Discussion
Morning all,
I'd appreciate any comments from the resident experts about anything they can determine from the following pics:
They show the driveshaft and outer CV joint from my Ariel Atom, as you can see the shaft has completely sheared leaving the splined portion stuck in the CV joint. What I'm curious about is the metal appears to be "soft" ie. the pics appear to show a twisting of the metal shaft. As the driveshafts are custom made for the Atom and not an "off the shelf part" from a donor vehicle, I am both surprised and disappointed that this has happened. Surely if the metal was hardened and specced correctly then it would not be able to twist The factory have been unable to give me an explanation as to why this has happened, and have sold me a replacement shaft and inner/outer CV joints for £600. Someone on the owners club has pointed out this company:
https://h-tune.co.uk/insane-shafts-performance-dri...
They sell a PAIR of driveshafts for a K series (same engine as the Atom, running circa 300bhp) rated to 500bhp for £450, so clearly an uprated set of driveshafts should easily be able to be sourced for a lot less than the standard factory ones at £600 EACH side. Clearly food for thought....
It's worth pointing out that I've owned my Atom for 5 years and in that time I have not subjected it to "launches" or other abuse that one would expect to lead to premature driveshaft failure.
Cheers
I'd appreciate any comments from the resident experts about anything they can determine from the following pics:
They show the driveshaft and outer CV joint from my Ariel Atom, as you can see the shaft has completely sheared leaving the splined portion stuck in the CV joint. What I'm curious about is the metal appears to be "soft" ie. the pics appear to show a twisting of the metal shaft. As the driveshafts are custom made for the Atom and not an "off the shelf part" from a donor vehicle, I am both surprised and disappointed that this has happened. Surely if the metal was hardened and specced correctly then it would not be able to twist The factory have been unable to give me an explanation as to why this has happened, and have sold me a replacement shaft and inner/outer CV joints for £600. Someone on the owners club has pointed out this company:
https://h-tune.co.uk/insane-shafts-performance-dri...
They sell a PAIR of driveshafts for a K series (same engine as the Atom, running circa 300bhp) rated to 500bhp for £450, so clearly an uprated set of driveshafts should easily be able to be sourced for a lot less than the standard factory ones at £600 EACH side. Clearly food for thought....
It's worth pointing out that I've owned my Atom for 5 years and in that time I have not subjected it to "launches" or other abuse that one would expect to lead to premature driveshaft failure.
Cheers
Edited by Toilet Duck on Wednesday 28th June 09:06
As you can see, your driveshafts are case hardened, ie they have a hard outer "skin" which in this case has experienced a clean brittle fracture, and then the softer inner core has had a long, slow plastic shear failure (twisted into an "mr whippy icecream cone" kinda shape!
So why are driveshafts made like this you ask?
Well, there are lots of good reasons, and it's a bit of a complicated story, so you'd probably be best googling it for yourself rather than us re-typing a load of stuff!
But broadly, and in simple terms, because the larger diameter the lower the load, we try to make shafts carry the majority of their load at the largest diameter, so a soft core to give some torsional springyness (to help avoid sudden shock loads) and a hard shell works well. Really trick shafts are hollow btw!
In terms of what caused the failure, i'd suggest cleaning both faces of the fracture joint and then posting up some pics, and we'll see if we can see the crack initiator. Basically, a small crack occurs at a place of high stress, which then grows, which transfers that high stress onwards, causing the crack to continue to grow, eventually, the shaft fails.
It's certainly possible to have shaft failures due to four main mechanisms:
1) immediate gross overload - if you put a load through the shaft that causes it to plastically deform and exceed the materials shear strength, it'll break. On the WRC cars we painted straight white lines down the shafts when new, and it wasn't uncommon to have them come back with a complete twist in them........
2) Crack propagation from a high stress point caused by surface damage. Things hitting and pitting the surface of the shaft cause a localised area of high stress, starts a crack, and then snap, the shaft fails as that crack propagates.
3) Crack propagation from a material defect. Materials aren't perfect, if you have an undetected inclusion or poor localised grain alignment a crack can form here under high load. It might take some time to fail, but fail it eventually will when there have been enough cycles (fatigue failure)
4) And the one people often forget about - bending loads on top of pure torsional loads! The driveshaft carries a high torsional load, but it also carried bending loads as the suspension moves. Combine these things and you can fail the shaft due to the superposition of both those loads. And ideal CV joint applies no bending moment to the shaft, but an ideal CV joint doesn't exist. if you have lots of suspension travel, and you apply high load at high deflections, this can become a serious problem.
So why are driveshafts made like this you ask?
Well, there are lots of good reasons, and it's a bit of a complicated story, so you'd probably be best googling it for yourself rather than us re-typing a load of stuff!
But broadly, and in simple terms, because the larger diameter the lower the load, we try to make shafts carry the majority of their load at the largest diameter, so a soft core to give some torsional springyness (to help avoid sudden shock loads) and a hard shell works well. Really trick shafts are hollow btw!
In terms of what caused the failure, i'd suggest cleaning both faces of the fracture joint and then posting up some pics, and we'll see if we can see the crack initiator. Basically, a small crack occurs at a place of high stress, which then grows, which transfers that high stress onwards, causing the crack to continue to grow, eventually, the shaft fails.
It's certainly possible to have shaft failures due to four main mechanisms:
1) immediate gross overload - if you put a load through the shaft that causes it to plastically deform and exceed the materials shear strength, it'll break. On the WRC cars we painted straight white lines down the shafts when new, and it wasn't uncommon to have them come back with a complete twist in them........
2) Crack propagation from a high stress point caused by surface damage. Things hitting and pitting the surface of the shaft cause a localised area of high stress, starts a crack, and then snap, the shaft fails as that crack propagates.
3) Crack propagation from a material defect. Materials aren't perfect, if you have an undetected inclusion or poor localised grain alignment a crack can form here under high load. It might take some time to fail, but fail it eventually will when there have been enough cycles (fatigue failure)
4) And the one people often forget about - bending loads on top of pure torsional loads! The driveshaft carries a high torsional load, but it also carried bending loads as the suspension moves. Combine these things and you can fail the shaft due to the superposition of both those loads. And ideal CV joint applies no bending moment to the shaft, but an ideal CV joint doesn't exist. if you have lots of suspension travel, and you apply high load at high deflections, this can become a serious problem.
btw, casual appraisal of your failure suggests the crack has formed on the outer shell, right next to the spline. This is typical for three reasons:
1) smallest diameter shaft at this point (highest load)
2) Spline machining can leave surface defects that are stress raisers
3) The portion of the shaft in the spline of the CV joint is supported, and so carries much lower stress and much lower deformation, the main portion of the shaft winds up, and these torsional vibrations run up and down the shaft going "boing" <<< technical term ;-) and are reflected off those stiff ends of the shaft. At the point of reflection, the loads can be concentrated. ie a soft bit of shaft next to another soft bit can just pass on the "twist", but the bit at the end, is stuck up against a bit it can't twist, so it gets more twisted itself. ( Imagine being in a queue of people, where each person pushes the person in front of them. For the people in the middle of the queue, no problem, they just move forwards and push on the next "soft" person, but the poor guy standing at the queues end, in front of a concrete wall can't do that so get's his bell wrung against that immovable wall!)
1) smallest diameter shaft at this point (highest load)
2) Spline machining can leave surface defects that are stress raisers
3) The portion of the shaft in the spline of the CV joint is supported, and so carries much lower stress and much lower deformation, the main portion of the shaft winds up, and these torsional vibrations run up and down the shaft going "boing" <<< technical term ;-) and are reflected off those stiff ends of the shaft. At the point of reflection, the loads can be concentrated. ie a soft bit of shaft next to another soft bit can just pass on the "twist", but the bit at the end, is stuck up against a bit it can't twist, so it gets more twisted itself. ( Imagine being in a queue of people, where each person pushes the person in front of them. For the people in the middle of the queue, no problem, they just move forwards and push on the next "soft" person, but the poor guy standing at the queues end, in front of a concrete wall can't do that so get's his bell wrung against that immovable wall!)
Max_Torque said:
copious amounts of detailed information
Jesus Christ, if that's not a concise answer from one of Pistonheads resident oracles then I don't know what is!!
Many thanks for the info and explanations etc
Max_Torque said:
In terms of what caused the failure, i'd suggest cleaning both faces of the fracture joint and then posting up some pics, and we'll see if we can see the crack initiator.
Here are some more pics, I wrapped a bit of tape around the shaft and numbered it for ease of referencing specific points.
One reason why I'm not too happy about this is because it's the second driveshaft failure. The other driveshaft failed towards the end of last year whilst on a track day at Bedford. Thank God it didn't let go down the back straight as I would have ended up in a nasty crash. Here are some pics taken at the time:
The outer CV joint sheared off leaving the spline inside the upright/wheel bearing assembly. The wheel was free to flap around, the only reason it didn't fly off was because, thankfully, I have two separate brake calipers on the rear (one for handbrake, one for foot brake) so they held the wheel loosely in place by the brake disc. I contacted the factory at the time with the pics etc but they purportedly "hadn't seen that before" either.
If there is some inherent flaw, whether it be in design or material/manufacturing quality, I would really like to know as if this happens a third time I might not be so lucky......
Cheers
I'm not familiar with that setup and struggling to visualise those broken parts in their unbroken state. Isn't there a stub axle between the CV joint and the wheel? It looks almost as if the hub is bolted directly onto the end of that half shaft in some sort of swing axle arrangement, but I can't square that with your mention of an outer CV.
GreenV8S said:
I'm not familiar with that setup and struggling to visualise those broken parts in their unbroken state. Isn't there a stub axle between the CV joint and the wheel? It looks almost as if the hub is bolted directly onto the end of that half shaft in some sort of swing axle arrangement, but I can't square that with your mention of an outer CV.
Here is a pic of the new one before I fitted it, the left hand side goes through the upright/bearing assembly (there is a threaded portion on the end after the splined section just out of shot where the hub nut screws onto) and the right side into the diff/gearbox
Edited by Toilet Duck on Wednesday 28th June 16:04
So the part that failed previously is the stub axle, and it failed outboard of the drive spline?
I can see how that would be pretty hair raising. I can't see how it would happen other than due to faulty parts or faulty fitment - this seems quite unlike the second failure which was presumably caused by excessive drive torque.
I can see how that would be pretty hair raising. I can't see how it would happen other than due to faulty parts or faulty fitment - this seems quite unlike the second failure which was presumably caused by excessive drive torque.
Second one looks like an over torqued driveshaft nut? Or a stress raiser from a sharp transition from splined shaft to the threaded end; from a faulty CV.
The CV doesn't look like anything special, it's probably from your average family saloon.. Infact the whole shaft looks like your standard thing really.
The CV doesn't look like anything special, it's probably from your average family saloon.. Infact the whole shaft looks like your standard thing really.
bgunn said:
Second one looks like an over torqued driveshaft nut? Or a stress raiser from a sharp transition from splined shaft to the threaded end; from a faulty CV.
The CV doesn't look like anything special, it's probably from your average family saloon.. Infact the whole shaft looks like your standard thing really.
I'd agree with that, i've seen the same type of failure with an overtightened nut, the latest fail looks like it's stemmed from the end of the splines - the weakest point.The CV doesn't look like anything special, it's probably from your average family saloon.. Infact the whole shaft looks like your standard thing really.
Apart from missing the helical grooves it looks like a Vivaro N/S driveshaft.
It is difficult to tell from the photos, and because of the amount of plastic deformation. But, because of the section that has fractured right on the end of the spline, you can send the end of each individual spline. My bet would be on a manufacturing defect with the spline, such as spline cut to deep, spline run out incorrect, etc. Something along those lines.
Megaflow said:
the section that has fractured right on the end of the spline, you can send the end of each individual spline. My bet would be on a manufacturing defect with the spline
I had one fail with very similar characteristic radial cracks from the splines down into the core of the shaft. I believe mine was a fatigue failure caused by insufficient material strength relative to the torque applied. A slightly more expensive replacement made from stronger material is still going strong.The noticeable surface layer on that shaft makes me suspect it's relatively mild material that has been surface hardened. If so, there are probably better materials available which will be a lot stronger.
Thanks for the continued replies gents, very much appreciated.
I have NEVER over torqued the driveshaft nuts when working on this car, I've always tightened them to spec, and the only other place that works on this car is the factory...
I know all things go wrong eventually, but I don't think two driveshaft failures in approx 6 months is acceptable, there must be an underlying reason. Unfortunately the factory can't offer an explanation yet won't admit to this being an "issue," and just expect me to buy replacement driveshafts at £600 a side
If I didn't have another charity day giving passenger rides coming up soon I would have looked to source a better engineered driveshaft from one of the specialist companies out there. I think I will wait until the winter hibernation and then pull both driveshafts and send them off to get matched replacements made using decent materials etc.
Cheers
I have NEVER over torqued the driveshaft nuts when working on this car, I've always tightened them to spec, and the only other place that works on this car is the factory...
I know all things go wrong eventually, but I don't think two driveshaft failures in approx 6 months is acceptable, there must be an underlying reason. Unfortunately the factory can't offer an explanation yet won't admit to this being an "issue," and just expect me to buy replacement driveshafts at £600 a side
If I didn't have another charity day giving passenger rides coming up soon I would have looked to source a better engineered driveshaft from one of the specialist companies out there. I think I will wait until the winter hibernation and then pull both driveshafts and send them off to get matched replacements made using decent materials etc.
Cheers
GreenV8S said:
I had one fail with very similar characteristic radial cracks from the splines down into the core of the shaft. I believe mine was a fatigue failure caused by insufficient material strength relative to the torque applied. A slightly more expensive replacement made from stronger material is still going strong
Can I ask who you approached to make a replacement?What surprises me is the Atom only puts out around 200lbs/ft of torque so relatively speaking it isn't anything ridiculous, it's obviously the low weight of the car that makes the difference in performance.
Cheers
227bhp said:
bgunn said:
The CV doesn't look like anything special, it's probably from your average family saloon.. Infact the whole shaft looks like your standard thing really.
Apart from missing the helical grooves it looks like a Vivaro N/S driveshaft.For six hundred quid a side, I'd be walking that round your local factor's stockroom... Somebody's having a laugh.
TooMany2cvs said:
<nods> Looks very off-the-shelf.
For six hundred quid a side, I'd be walking that round your local factor's stockroom... Somebody's having a laugh.
£600 is daylight robbery. I posted a link up top to a firm selling uprated driveshafts rated to 500bhp for a K20 Type R engine (same as Atom) for £450 for a PAIR.For six hundred quid a side, I'd be walking that round your local factor's stockroom... Somebody's having a laugh.
The factory obviously use bits out of parts bins where possible to keep the costs down, I understand that. So I'm guessing the the CV joints are an "off the shelf" part, and its just the shaft linking the inner/outer joint that is (probably) custom made by an engineering shop somewhere to take into account the Atoms unique track etc (as an aside, the offside driveshaft is considerably longer).
This is a pic of a replacement CV joint I bought from the factory back when the stub axle sheared off:
The part number stamped on it is GKN DOENA 4S41 EA 3484 A14
I did try emailing GKD to find out what it's originally off but after an initial reply I didn't get any further.
Toilet Duck said:
Can I ask who you approached to make a replacement?
Cheers
I initially approached Prolinx but they didn't have a standard part matching my requirements so they referred me to GB Engineering. GB made me a one-off pair of half shafts in maraging 350 or equivalent for about half what you were quoted for one side. I had to buy CV joints on top of that of course. These were for a much bigger heavier vehicle but I doubt that would affect the cost much.Cheers
I say that the second shows that it has broken where the shaft has an abrubt change of diameter, and the stress has concentrated there.
The first is "taffy-like twisting, just that the crack has propagated in the fashion.
The first is a common design at the outer end, and usually gets away with it. If it's inside of the wheel, then it doesn't.
John
The first is "taffy-like twisting, just that the crack has propagated in the fashion.
The first is a common design at the outer end, and usually gets away with it. If it's inside of the wheel, then it doesn't.
John
Don't pay them £600. Talk to this bloke:
http://davemacprops.co.uk/products.html
He's made me custom shafts for applications vastly heavier duty that that. Unconditionally guaranteed full stop, and they were a lot cheaper than that!
http://davemacprops.co.uk/products.html
He's made me custom shafts for applications vastly heavier duty that that. Unconditionally guaranteed full stop, and they were a lot cheaper than that!
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