Many, many years ago we had an Austin Healey delivered to the shop on a flat-bed. The car, a 100/6, was missing the right front wheel; not only the wheel, but the stub axle, the brake drum, the wire wheel splined hub and spinner were also MIA. The owner/driver reported that he had hit a substantial pot hole while spiritedly driving down a country road and immediately the right side of the car dropped to the ground and everything went “pear-shaped”, as he put it. Fortunately, he hadn’t been travelling too quickly and, although he had no brakes, had managed to bring the car to a halt before hitting anything significant.
Despite a fairly intensive search, no sign of the missing parts could be found and it was concluded that the wheel had bounced off into the Greater Canadian Wilderness never to be seen again!
The cause of this incident was a failed front stub axle which had broken off just inboard of the inner wheel bearing.
This all happened years before the appearance of the internet but, when I raised the subject at a club meeting, a very interesting discussion ensued regarding the use of bearing spacers between the inner races of Austin Healey front wheel bearings.
The crux of the discussion was whether or not these spacers were necessary and, after all viewpoints were put forward, we were unable to come to a consensus. The pertinent points raised were:
- Many makers of rear-wheel-drive cars do not use this type of spacer, including most American and several British manufacturers.
- If the designers of the Austin front hub assemblies had decided to use spacers then they MUST have had good reason.
- We had all encountered incidences of cracked stub axles.
I have given this subject much thought over the ensuing years and concluded that there most certainly is a reason for using these spacers.
During the process of supplying the required parts and repairing the 100/6 we decided to crack test all the used Healey stub axles of the same type using a penetrant dye method and we were surprised to find that at least 50% had cracks on the lower side just where the other had broken. Fortunately our used parts were all identified as to which car they had been removed from and almost all the cracked ones had been removed from BN2’s. We also checked the left side of the 100/6 in for repair and discovered that not only was it missing the bearing spacer and associated shims but it was also cracked.
The obvious question is, why BN2’s? Because, unlike BN1’s and 100/6’s, BN2’s were not originally fitted with the bearing spacers and shims!
So why is it desirable to have those spacers?
Well, in simplified terms it is all to do with the diameter of the base. Consider a 4-foot-high stack of bricks. Apply a force to the side of the top brick and it is pretty easy to push over the entire stack.
Now try the same thing with an empty 40 gallon drum which probably weighs about the same and you are going to have to push a lot harder to topple that.
Effectively, what the spacer does is increase the diameter of the base of the stub axle so that when a force is applied the stub axle does not flex or bend as far thus decreasing the possibility of fatigue failure which is defined as: “The tendency of a material to fracture by means of progressive brittle cracking under repeated alternating or cyclic stresses of an intensity considerably below the normal yield strength of the material and is a function of the magnitude of the fluctuating stress.”
You may be able to tell that I didn’t write that mouthful, but in simple terms the further the stub axle bends the more rapid is the onset of fatigue failure.
There is a very interesting photograph taken by Daniel Rubin of Stirling Moss in a 100S retiring from the 1955 Nassau trophy race with a broken stub axle.
The 100S used the same front hub design as the BN2 i.e. no bearing spacers. I often wonder if this incident had anything to do with why B.M.C. reintroduced them.
Incidentally, why didn’t Stirling’s wheel take off into the undergrowth like the one on our 100/6?
The answer is that unlike the 100/6, the 100S had disc brakes and the disc brake caliper held onto the wheel.