Thread: what axle/diffs 130 defender

Originally Posted by MLD

Since we on that topic. Can anyone clarify if the axle housing for the P38 under a 130 is stronger, weaker, comparable in strength to a salisbury from a 110. If the 110 salisbury and P38 130 axle housings are comparable strength it might be all a bit academic that the 130 salisbury is stronger again. I'd like to know the answer because i'm in the process of building a 130 and i have a 110 salisbury with maxi locker under a 110 ready to be swapped over if the numbers for the Ashcroft P38HD conversion don't stack up.

Cheers MLD

IMHO it would be cavalier of anyone to make such comparisons. The construction of a Salisbury housing is not remotely similar to a welded Rover or P38 housing.

Regarding strength of axle housings,there are 'Static Strength' and 'Fatigue Strength'.

I have little doubt the static strength is an issue. All will have a reasonable 'Factor of Safety' designed in, and replacing the axle housing with one having a lower static strength, will simply reduce the 'factor of safety'.

It should take a gross overload such as a collision to exceed the static strength. In circumstances of such high overload, there will most likely be other damage or even injuries to completely ruin your day.

Fatigue strength is a very difficult matter to grapple with. 'Stress Raisers' don't affect static strength but they have a huge impact on fatigue strength.

Fatigue failure usually takes the form of a crack that starts at a small defect in a region of fluctuating stress (considerably lower than the static stress), at a stress raiser. The crack propagates during the fluctuating stress cycles, until the amount of material resisting the load is reduced so much that it fractures. Most have heard stories that go like, "it broke in my driveway ...".

The critical regions and stress raisers and the cause of the stress raisers are what are so different between a Salisbury and Rover/P38 axle housing, that it is like comparing the old "apples and oranges".

With the Salisbury, the issue is the transition from a massively rigid cast housing for the diff, to the significantly less stiff axle tubes. This creates a stress raiser in the axle tube at the point where it exits the diff housing.

Increasing the thickness of the axle tube does two things; it increases the stiffness of the tube, thus reducing the stress raiser to some extent, and it also reduces the magnitude of the stress by virtue of spreading the load across a greater cross sectional area.

With the Rover/P38 housing, the transition in stiffness from the 'Banjo' diff housing, to the axle tubes is gradual, resulting in no perceptible stress raiser. The load carrying material in this region is continuous (no join). However the parts of the pressed housing are welded together and it is these where fatigue cracks are likely to form.

The static strength of welds is often greater than the parent metal, due to use of higher strength filler material in the welds. However the fatigue strength of welds is much lower than that of the parent metal, and varies greatly depending upon the location of the weld compared to the direction of the principle stress. Transverse welds being the worst case.

Identical (even to 'forensic' examination) items have widely varying fatigue strength, and so it is predicted with 'statistical chance'. This statistical nature means that one item can have a much lower Fatigue Strength than predicted, another can be be much higher, but we 'hope' 90% will exceed the predicted strength.

The affect of 'Fatigue Damage' incurred during a particular 'Load Cycle' depends very much on the 'Magnitude' and 'Direction' of the 'Stress' during each 'load cycle'. The 'fatigue damage' is 'Cumulative'. So one high magnitude cycle is equivalent to numerous lower magnitude cycles.

The fluctuation in the stress is of the utmost importance, not only the magnitude but whether the stress direction is in the same direction or if it reverses direction during some cycles. Reversing is much worse.

What this all means is that many can drive around for the entire life of the vehicle and not experience a problem. However a vehicle heavily loaded and driven over many kms of corrugations will have a reduced 'Fatigue Life'. A few heavy load cycles, e.g. hitting an unexpected wash-away at to high a speed will take away a significant portion of your fatigue life.

What I am certain of is that Land Rover state a higher rear axle load capacity for the 130 than for all but the 'Heavy Duty' option for some 110 Defenders. Furthermore we know that they increased the strength of the Salisbury axle for the 130 and the 'HD' 110.

If you hardly use the 130 loaded up, for many kms of rough roads, and you are lucky enough to get a 110 Salisbury that has had an easy life and is one of statistically fortunate ones, then you may have a happy future after installing it.

Edit: there is nothing to stop you from replacing the axle tubes in a Salisbury with tubes having a thicker wall. Another alternative which has been used is to weld another tube inside the existing axle tube (so called laminating). I appreciate both options are a pain with the Maxi-Drive locker.

As usual John, thank you for taking the time for such an imformative reply.