Front axle sheared through. A fatal design flaw??

[MikeParker]

Had a very close shave a few days back. I had just driven back up the A3 and as I turned the corner into my road I noticed a strange scratching noise coming from my front right wheel. Turned into my drive and the car seemed to have the brake partially stuck on. Managed to drive into the garage and when I tried to rock the car back and forth, the front right wheel appeared locked. Imagine my surprise when I jacked up the front and the wheel dropped off complete with the hub! The front axle had sheared through. See attached pictures which show the broken shaft and the new one. The shaft has broken just where you would expect, that is at the point where there is the maximum bending moment and the shaft abruptly narrows. Surely it should get thicker if anything as you move in towords the mounting point? If not, a much more gradual narrowing to spread the strain. It seems to me that this is a major design flaw. Does anyone know of any redesigned parts that rectify this? Although my old axle shaft lasted 150k miles, it does make me less sure of the car.

Strangely, a similar thing happened with a rear shaft on the car about 30 years ago! I do not remember the design of that, but maybe it has the same issue also.

[mole42]

Since this same stub axle has been used on thousands of Triumph Stags, 2000/2500 and others, I’d suggest it is age rather than a design flaw. Our cars are old, mine is 48 this year, and I’d expect parts to fail. I once had a front wheel fall of a 2.5PI on a fast roundabout and had a front tyre blow out at 70mph on the M1, both required replacement underwear but the emergency was controllable.

I won’t stop rallying my Stag or driving it hard.

Richard

[alan_thomas]

There have been a few of these failures recently and this has prompted me to replace them for my peace of mind before I have any trouble. Chris Witor sells a CNC axle machined from a high grade steel much stronger than the induction hardened, forged, original. Probably the other suppliers are selling the same? but only Chris gives a full writeup of the remanufactured part - Alan

https://www.chriswitor.com/proddetail.php?prod=138425RM

[barkerwilliams]

Isn't old age wonderful, bits of me keep snapping as well?

I think that such a failure that did not result in either flesh or metal damage is a win-win; though you are obviously unhappy with the failure.

I had one break on a two year old Cortina back in the day. I'm sure when designed no one specified that it should last at least fifty years, most cars then never even got to ten years and 100,000 miles was unlikely.

Sorry if I am too philosophical but old metal fatigues especially when designed to keep weight down on unsprung components to improve ride quality.


Alan

[Motorsport Micky]

Had a very close shave a few days back. I had just driven back up the A3 and as I turned the corner into my road I noticed a strange scratching noise coming from my front right wheel. Turned into my drive and the car seemed to have the brake partially stuck on. Managed to drive into the garage and when I tried to rock the car back and forth, the front right wheel appeared locked. Imagine my surprise when I jacked up the front and the wheel dropped off complete with the hub! The front axle had sheared through. See attached pictures which show the broken shaft and the new one. The shaft has broken just where you would expect, that is at the point where there is the maximum bending moment and the shaft abruptly narrows. Surely it should get thicker if anything as you move in towords the mounting point? If not, a much more gradual narrowing to spread the strain. It seems to me that this is a major design flaw. Does anyone know of any redesigned parts that rectify this? Although my old axle shaft lasted 150k miles, it does make me less sure of the car.

Strangely, a similar thing happened with a rear shaft on the car about 30 years ago! I do not remember the design of that, but maybe it has the same issue also.

Mike,

I've been banging on about these stub axles and also the hubs on our cars for the last 24 months.

They are dying, just as Richard says the mileages are not the relevant point, it's the last 50 years and numerous work hardening impacts from sliding into kerbs in winter (when used as everyday car) or the pounding from our disintegrating roads which are doing them in. Todays design engineers would recommend you have your bumps read if you asked any Audi, BMW, Ford etc etc design shop to supply materials and make components that could be used nearly 50 years later with no control on how they've been used but still remain in use...not feasible. Yet we've done it with our old cars and it's a positive gold star and a tick for the Triumph engineers who specified and costed these units together.
However, forget your multi piston calipers with ventilated discs and all the other bling we fit persuading ourselves this is for safety reasons, replacing the hubs and stub axles front and rear before an immediate roadside incident causes a very bad and dangerous occurrence is well worthwhile. Link the rear hubs together with CV driveshafts or your choice of rear drive components will take the safety forward another level, forget reconditioned hubs, the very act of stripping these aged hubs imposes such forces (often includes a 20 ton press and heating the hubs up) that cracking is caused in hidden places where crack testing cannot reach. Driving the car and sub units to destruction should be done by manufacturers not their owners 50 years later.

Micky

[DJT]

I recently had the same failure with the NSF axle. From my records this shaft had covered something like 220,000 miles. Design flaw? I don't think so, just age related failure.

I changed both mine using the newly made ones from Chris Witor: https://www.chriswitor.com/proddetail.php?prod=138425RM. Nicely made, but check the size of the ball race surfaces. I had to dress mine down slightly using 1400 wet 'n dry paper to allow the (Timken) bearings to fit.

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[MikeParker]

I think I am with Micky on this. While I get that certain parts of of a car will wear out after 150K, I think there are some key, safety critical components that simply should not break. I have had several BMWs including a 530d that did 220K and nothing broke. It needed a new prop-shaft (gone anti vibration damper) and of course new tyres, discs and pads, but nothing that would kill me, snapped.

I have always wanted to try to make my Stag as good as as a BMW (mechanically) . I drive to the south of France every year and expect it to get there and back. I have re-enforced the differential nose, as that is a commonly accepted failure point, so I am not sure why a front wheel falling off is not in the same category. It may be the new part is better but I still think the design just looks wrong. We need a mechanical engineer to have a view on this.

[022Dave]

I changed my front & rear hubs to the CDD designed units https://classicdrivingdevelopment.co.uk/index.asp. They're not cheap, but are far superior to the originals.

Dave

[davidf]

This is a fatigue failure. Fatigue cracks start with a small surface defect, which creates a raised stress at its root. Repeated load applications cause this crack to slowly propagate. In the original stub axles, the area where it fails is a rough forged finish, full of potential surface defects. Even many years ago, replacement parts were fully machined, which eliminates the start point for the fatigue crack.
Following a series of similar failures in our local area (Solent and New Forest) in the 1990's, I removed both of mine and had them dye-penetrant tested. One was fine, so on the basis that a crack had not started in 20 years, I put it back on. The other was cracked about half way through, and I replaced it with a fully machined one (probably from SOC Spares).

[SaskStag]

Thanks for the link to Chris Witor. Now ordered. The stub axles looked a bit beaten up when I did the wheel bearings. I wondered about replacement then. I already have the strengthened diff extension, new half shafts and Goodparts (USA) up rated rear hubs on my car. Makes sense to deal with the front too. I even tossed the original wires for modern alloys.

[KOY 23]

This is a fatigue failure. Fatigue cracks start with a small surface defect, which creates a raised stress at its root. Repeated load applications cause this crack to slowly propagate. In the original stub axles, the area where it fails is a rough forged finish, full of potential surface defects. Even many years ago, replacement parts were fully machined, which eliminates the start point for the fatigue crack.
Following a series of similar failures in our local area (Solent and New Forest) in the 1990's, I removed both of mine and had them dye-penetrant tested. One was fine, so on the basis that a crack had not started in 20 years, I put it back on. The other was cracked about half way through, and I replaced it with a fully machined one (probably from SOC Spares).

David.
Thank you for your clear explanation of the problem. Chris Witor will wonder why his stock is flying off the shelf. Generally,
will a bearing puller free it or is a hydraulic press needed?
John

[Phil S]

I agree with Dave, I have noe got the CDD front hubs/stub axles fitted, these went on with the 4 pots and vented discs.
not cheap doing the lot at once, but do it once and do it right

[DJT]

Generally,
will a bearing puller free it or is a hydraulic press needed?
John

John,

The only bit that needs a bit of force is getting the taper free from the vertical link. With the broken one, it was easy to squeeze it out using my bench vice, but the unbroken one I took to the local garage that I’ve used for 30 years. The foreman got it out using their 20-ton press. No charge.

[wilf]

I see Rimmers do a "front hub and axle overhaul kit" code RS1465

Same as/as good as the Witor parts?

[davidf]

As a footnote, to indicate how much redundancy there is in the stub axle design, if you look at the picture in DJT's post 6, the fatigue crack has progressed over many years to about 90% of the section of the axle. Finally, with only 10% remaining, and highly stressed along the root of the crack, it has failed.
So, the design flaw was not in the sizing of the component, it was in the specification of the surface finish. If they had turned or ground the section, even though that woudl have made it thinner, it would have eliminated the weakness. As long as the new ones are of a suitable material with suitable working and heat treatment, they are more than adequate.
Remember the classic story of the Comet 1 crashes in the 1950's, which they eventually traced to the square-shaped windows providing stress raisers at their corners, leading to fatigue failure of the cabin structure. All aircraft since have oval windows and very rounded corners to doors. For those not familiar - see this film https://www.youtube.com/watch?v=aHbyOqE0q3o