What have we got here then?
Ian gets to the bottom of his Tatra’s rear wheel loss
In my previous report (PC, January 2020),
I left unexplained the total loss of our trusty Tatra 603’s rear wheel, hub flange and brake drum, a mere half mile from our house just a few days after its extensive club rally exercise in Shropshire. Safely back home, eleven-and-a-half hours after our misfortune, we were none the wiser. Plainly a halfshaft had sheared but, depending what I was driving, I might have expected that to result in anything from an undramatic loss of drive in a truck to a loose shaft thrashing about underneath a rear-wheel drive car, not a hub, wheel and brake coming adrift, but that’s to under-estimate the vulnerability of the dreaded ‘semi-floating axle’.
This big rear-engined official transport from old Czechoslovakia is a potpourri of eccentricities, over-engineering and, as I seem to make myself unpopular with some of my club associates by highlighting, the occasional design weakness, but it’s worth recording that ours isn’t some weird old clunker that’s just embarrassed us on a rare outing. One of six Tatras we’ve owned, it’s been restored on two occasions and has carried us 66,000km in at least seven countries over two decades. And, all that said, don’t imagine it couldn’t happen to you. Do you have a Lotus Elan? Anyway, the following day Tim Bishop was able to talk me through his post-mortem. The Tatra’s driveshafts live in tubular swing axle casings, their inner paddle ends floating within the final drive and pivoting between half-segment fulcrum plates in simple carriers, just like a Volkswagen Beetle’s. At the outer end of the axle-tube is a self-aligning twin-row ball-race.
A stressful situation
All the cornering side forces are taken by this bearing, and the halfshaft itself takes all the bending stresses imposed by the full weight of the vehicle, plus pot-hole impacts on the poor roads we suffer from today. Part of a swing-axle suspension on this tail-heavy car, it is also stressed by track changes that generate their own in/out loading on grippy radial tyres for which the car wasn’t designed.
The outer 18cm of each shaft is tapered and must be a precise interference fit with the matching socket in the hub flange. Despite a parallel keyway that resists twisting, the outer castellated hub nut must be
wound on tightly enough to achieve almost a cold weld, so that the key is not being relied on. And this is where our issue has arisen.
Tim held the hub up to the light, explained that he’d not wiped the socket and showed how a tight fit had been achieved on only the outer third of the taper that was grease-free… and it was that 6cm that had broken away. He drew my attention to the classic snail-shell fracture of our shaft and observed that this failure had likely been many years in the making. I now realise that we could be talking about two or even three decades.
Consistent… or not
Taper and socket had not been consistently and tightly engaged throughout their life, and the keyway had been the initial focus of stress. The inner two-thirds of the shaft taper hadn’t been supported and a fracture had gradually worked around from the shaft’s keyseat. Some vague recollection prompted me to break out my maintenance records and there it was.
Back in 2005 a ‘loose hub’ diagnosis had prompted dismantling and reassembly on one side, and a year later the other, work that I’d farmed out. In each case a couple of attempts had been made to lap the tapers into engagement but, plainly, that hadn’t been completely achieved. In fairness, it’s highly likely that damage had already begun, but we hadn’t ended up with the assemblies safely stabilised. If the tapers are not fully supporting the hub, the keyway will be taking all the torque (it shouldn’t be taking any), and a smaller shaft diameter taking the weight of the car. A stress crack will begin, particularly if the shaft keyseat is sharpangled rather than U-shaped, and it is.
What to make of it all? Well, probably the most worrying thing is how improbable would have been a timely diagnosis. Our local garage and MOT station is run by the most honest man in the business, who understands our desire for rigour and certainly doesn’t need our encouragement to report any issues.
He couldn’t possibly have anticipated the imminent failure. In a classic case of finding wisdom after the event, my wife Kirsten and I can both recall hearing a sort of regular rubbing just a couple of times en route to our rally. ‘Yes, I heard it too’, I’d said. ‘So, what do you think it was’ she responded, and it’s embarrassing to recall saying ‘Maybe a slightly binding brake, or a wheel bearing on the way out? Not mission critical, so don’t worry. We’re not about to lose a wheel!’ Not for a few days anyway. Famous last words!
practicalclassics@bauermedia.co.uk
‘The failure had been many years – or even decades – in the making’