Thread: Life of Puma Rear Diff's

Originally Posted by slug_burner

Here are a couple that came up.




If this is a bridge or truss what is meant by laminated?

Hi Slug_burner

Thanks for finding those photo's, I wanted to keep this in this thread as it relates to the strength of Land Rover diffs in general, (Fifty lashes shall be received by yours truly if I am out of order).
All of those mods are heavy without addressing properly the issue of the tube breaking where it enters the diff centre casting.

I will write this next part as a demonstration for youngsters, so please don't feel annoyed at the simplistic description, instead grab a twelve year old and go through doing the model with him/her.

For way of an illustration, play with a milk carton, a couple of pencils and a sheet of paper.

Lay the carton down, puncture the carton each side with the pencils so the holes are opposite.

Try and lift the carton using a finger under each pencil and the pencil's will bend up, the weakness is the wall of the carton, relate that to the Salisbury where the tubes enter the diff centre housing.

The next thing to try, is to strengthen the joint on our milk carton model using a sheet of paper, lay the paper over the top of the carton, emulating the bracing using in the photo's, pull the pencils out and puncture the paper and reinsert the pencils back into the carton, then move the paper so that the holes in that paper are moved away from the carton sides and the paper pulls tight across the top of the carton.
Now repeat the lift and you will see that that paper doesn't offer any resistance to the upward motion of the pencils.

The last part of this experiment is placing that paper under the carton, doing what I have already explained, but pulling the paper tight under the carton. If your model is working correctly, the paper should offer resistance to the upward motion of your pencils.

So for a truss to work on the Salisbury, it should be underneath the diff centre and needs not to have the weight or size of those constructions shown in the photo's.
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Originally Posted by wrinklearthur

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So for a truss to work on the Salisbury, it should be underneath the diff centre and needs not to have the weight or size of those constructions shown in the photo's.
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Sorry but you are incorrect. A truss on top is just as strong as one on the bottom. Steel has the same strength in tension or compression.

Originally Posted by wrinklearthur

[snip]

So for a truss to work on the Salisbury, it should be underneath the diff centre and needs not to have the weight or size of those constructions shown in the photo's.
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Yes, a truss works much better in tension, and that's how it's been done for a very long time (I avidly devoured US Off Road magazines as a kid in the seventies and trusses were common on Dana 44's and 60's way back then) but you dramatically reduce the ground clearance which isn't great on a 4wd.
You can make an over the top version work but it must be substantially larger/heavier to have some sort of influence.

Originally Posted by isuzurover

Sorry but you are incorrect. A truss on top is just as strong as one on the bottom. Steel has the same strength in tension or compression.

Ben, can you draw a picture of what a good over the top truss would look like on a Sals, taking into consideration the A-frame and having to be between the coil springs.

Originally Posted by isuzurover

Sorry but you are incorrect. A truss on top is just as strong as one on the bottom. Steel has the same strength in tension or compression.

What about the cables on s suspension bridge or in a post tentioned slab....

Originally Posted by rick130

Yes, a truss works much better in tension...

Sorry Rick but you are also wrong. Steel behaves exactly the same in tension and compression, therefore a properly designed truss will be just as strong on either side of the axle.

Of course if you get plastic deformation it may be another matter, but at that point your truss (and axle) have failed.

Serg - yes the a-frame mount is a problem. A truss would need to be designed to run either behind or in front of the a-frame ball joint. In front may have cle3arance issues with the a-arms, Behind may have clearance issues with the fuel tank, though the truss doesn't need to sit further proud than the back of the diff pan, so it should be fine.

edit - no need to extend out further than the coil mounts. As mentioned, sals only ever fail close in - to the centre - e.g. where a md actuator block has been welded on.

Originally Posted by ashtrans

I haven't even looked at costs yet but from other parts we make I could take a guess, depending on how many we make :

ring and pinion £ 225-250
casing £ 200-250
bearings, seal, flange, no more than £ 100

if you want a locker £ 475
if you want HD shafts £ 260

Dave

Dave,is there a reason you don't make 3.54 HD CW&P for the P38?. Pat

Originally Posted by isuzurover

Steel has the same strength in tension or compression.

I must have went to a different school! As design plays a far greater part in steel work than simply adding more material to the cross section.

Sorry but you are incorrect. A truss on top is just as strong as one on the bottom.

I disagree, we are not talking about fabricated building girders here.
The problem here is, where the connection of the tube to the Salisbury's centre housing is, it fails under tension. When a strengthening plate is used under the centre housing and welded to the axle tube just beside the insides of the spring mounts, you form a triangulation on both sides of the diff centre housing. The result is the flat plate is now in tension and the axle tubes are in compression.
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