Lets talk axle swap

[kennyw]

On a SF axle design you have to combine the two sources of stress (the full weight of the vehicle plus the torque being applied to the shaft) then account for the cantilever loading which is bending the axleshaft while the torque is being applied. That is probably closer to 10X the stress of the FF axle design where you only have to look at the torque applied to the axleshaft that does not see any bending. The larger diameter of the 14SF axleshaft vs/ the 14FF axleshaft is compensating for the inherent weakness, but overall is a weaker axle in the end.

It is however a very decent axle for smaller tires (33-36") and would not break under most situations. I ran an 8 lug 14SF in my '79 K10 with 33" tires and hauled many heavy loads and still used it for mild wheelling with no problems. I'd trust them over a 10 or 12 bolt any day.

The Dana 60 FF compared to a 14SF however, the D60 is going to be a better choice and still give you the better ground clearance and close to equal ring gear size. Plus when you do finally break an axleshaft on a full floater you pull out the broken piece, cap off the outside and drive home on front wheel drive. On a semi floater you better have the correct spare.

 

[Dabba]

yeah, dont think im gonna go 3/4 ton swap. but just for fun what vehicles can i grab a 6lug 14bsf out of without moving the perches?

 

[AJMBLAZER]

None of them.

 

[blazerbones]

On a SF axle design you have to combine the two sources of stress

Yup. SF axle shafts undergo bending as well as torsional stress. FF shafts just torsional stress. Since bending stress is a combination of tension and compression (at opposite sides of the shaft)... the SF shaft CAN NOT be hardened to desireable levels... otherwise the side of the shaft in tension will crack!

FF shafts can be and I believe are hardened since they only support torsional loads.

The different hardness levels of the axle shafts is a huge contributor to the strength difference between FF and SF axles. It, along with that discussed above makes the FF MUCH stronger.

 

[MaxPF]

On a SF axle design you have to combine the two sources of stress (the full weight of the vehicle plus the torque being applied to the shaft) then account for the cantilever loading which is bending the axleshaft while the torque is being applied. That is probably closer to 10X the stress of the FF axle design where you only have to look at the torque applied to the axleshaft that does not see any bending. The larger diameter of the 14SF axleshaft vs/ the 14FF axleshaft is compensating for the inherent weakness, but overall is a weaker axle in the end.

That sounds good in theory, but it doesn't fit with the observed facts. If the major stress concentration was due to loading+torsional the shafts would break at the outer bearing when you exceed it's torque limits. This never happens - SF shafts subject to torsional overload break at the area of smallest diameter, just like FF shafts. On every SF shaft I have seen this area is on the inboard portion of the shaft, and the loading here is essentially 100% torsional.

Anyway, I know I will never change the CK5 closed-mindedness. I just like to occasionally remind folks that there ARE alternatives.

Max-you can't wheel that 1/2 or 3/4 ton $#!t-PF

 

[MaxPF]

Yup. SF axle shafts undergo bending as well as torsional stress. FF shafts just torsional stress. Since bending stress is a combination of tension and compression (at opposite sides of the shaft)... the SF shaft CAN NOT be hardened to desireable levels... otherwise the side of the shaft in tension will crack!

Man, there is so much metallurgical BS in that post I don't even know where to start.

Perhaps you can explain how crankshafts can be hardened when they are subjected to cyclic bending forces. Better yet, explain how torsional stress lacks a tension component?

In fact, I would like to see pictures of the piles of broken 14BSF shafts. Heck, lots of guys here wheel 1/2 tons with big tires, and failures aren't common. When they do fail, it is often the stock carrier rather than the shafts. The only common SF shaft breakages are D35's on Jeeps...

 

[Dabba]

screw it then

 

[blazerbones]

Man, there is so much metallurgical BS in that post I don't even know where to start.

Perhaps you can explain how crankshafts can be hardened when they are subjected to cyclic bending forces. Better yet, explain how torsional stress lacks a tension component?

Hahahaha. He's on the rampage. I can dig it! Crackin' me up man.

Well, I'm not in metallurgy, but they did run a couple of theories by me during my recent Engineering education. So I'll give a try. Here comes a bunch more "BS":

Regarding shaft (crank, axle, whatever) design: Designers consider the laods that will be applied (shear, tension, compression, combinations thereof) and the package space available. They then optimize cost and weight by choosing the best COMBINATION of geometry (radius very important in shaft design of course), material, manufacturing methods, and treatments (heat, carburization, etc.) that will perform in the application and fit in the space.

So when designing crank shafts, apparently some heat treatment will optimize the design... but if the shaft becomes too brittle, it will fail in tension (bending) as is always the case with brittle materials.

Regarding torsional loading and axle shaft design: TORSIONAL loading causes SHEAR stress. The maximum amount is at the surface of the shaft and PERPENDICULAR to the RADIUS of the shaft. BENDING loading causes COMPRESSIVE and TENSILE STRESS (varies linearly across the diameter of the shaft with max tension opposite max compression). The maximum amount at the surface of the shaft and PERPENDICULAR to the LENGTH of the shaft. Combined, these two stresses limit the heat treatment possible MORE than just torsion would alone... becaue most heat treatment methods result in hard but BRITTLE material especially at the SURFACE of the shaft... brittle plus tension is baaaaad.

Untreated steel is weekest in SHEAR (by a factor of about 2/3rds as a rule of thumb). But very hard steel is weekest in TENSION... just like a piece of concrete... all brittle materials are.

Wow... said alot more than I intended or probably should've...

That sounds good in theory, but it doesn't fit with the observed facts. If the major stress concentration was due to loading+torsional the shafts would break at the outer bearing when you exceed it's torque limits. This never happens - SF shafts subject to torsional overload break at the area of smallest diameter, just like FF shafts. On every SF shaft I have seen this area is on the inboard portion of the shaft, and the loading here is essentially 100% torsional.

Anyway, I know I will never change the CK5 closed-mindedness. I just like to occasionally remind folks that there ARE alternatives.

Max-you can't wheel that 1/2 or 3/4 ton $#!t-PF

Makes sense to me. Sounds like the designers beefed up the area of the shaft that is seeing combined loading out near the outer bearing. Also sounds like you have alot of experience with the SF applications... I'm not questioning your experience AT ALL.

Heck, I run 3/4 ton stuff and haven't had issues... yet... I'm expecting the front to go boom someday.

 

[MaxPF]

In both applications ductility is desirable, but so is low cost. Shafts for both axles are made from medium carbon steel, probably .40-.50% carbon. Heat treat would be similar for both with one exception: the SF shaft will have the bearing surface carburized to provide a hard bearing surface. As you probably know, carburization provides some residual compressive stress, which helps inhibit fatigue cracking. Thus, this surface hardening will not make the material more fragile.

I think if you were to break a sf shaft you would have broken a ff shaft under the same circumstances. That's just my opinion, but unless the ff shafts are made out of a much stronger alloy the sf shafts should be able to hang with them. Certainly anything up to and including 40" tires should be no problem.