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Full Float vs. Semi Float

Hogback Fabrication

Hogback Fabrication

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I brought up the argument in another post that an axle being full floating doesn't add a whole lot to the torque that the shafts can sustain. This is from my own engineering knowledge (I am a senior engineering student) and things I have picked up from Pirate (not necessarily correct). No one argued with me and I was kinda surprised because it seems everyone thinks that FF axles add tons of strength over SF. That argument said, I do run 1 tons under my Blazer so I realize that a FF 14 bolt is probably a little stronger than a 10 bolt :wink1: . So, using real logic and strength of materials knowledge, lets hear some arguments for either side. Don't post up "I switched to a FF and I quit breaking shafts" or "everyone always says ...". Real tech with real facts to back it up. Let's hear it. :D
 
Oh yeah, part of my argument is when solving Strengths problems, a vertical load (i.e. the weight of the truck) has no effect on the moment load the shaft can take.
 
Don't most people run FF because if you break something the wheel won't fall off, whereas with a SF it could? I prefer my wheels stay on the rig at all times. :wink1:
 
The wheel staying on is one benefit of having a FF axle. Most people switch to FF because the FF axles are generally the heavy duty axles and everything about them is usually much stronger than SF stuff. FF axles can handle much more payload above them as well. The original argument that spawned this post was that a FF Dana 60 (30 spline, 1.20" shafts) is not stronger than a 12 bolt (30 spline, 1.31" shafts), at least from an axle shaft perspective.
 
The disadvantage of the SF axles is that the axle shaft must support the whole weight of the vehicle and creates the most pressure at the wheel and transmit the torque to the wheels. Whereas with a FF axle, the weight is distributed across the whole axle (and bearings) and the shafts only have to transmit the torque. At least that is my take on it!
 
I'm gonna have to say, a SF axle, with carrying the weight on the axle, has to utilize a bearing in the end of the axle tube to support the weight of the truck. There are limits to how strong and reliable you can make that type of bearing, and seeing how the bearing does not provide any resistance to movement of the axle in and out of the carrier, this leaves it up to the C-clip. How thick do you think a c-clip would have to be to hold the axle inside the diff so it won't break when turning a corner? In the end, it is the ability to retain the axle inside the diff while turning with heavy loads, and bearing durability.

A FF uses bearings like a front axle...two cone bearings in cups that prevent movement in all directions when adjusted correctly. This helps the hub stay in one spot under heavy loads. Now, how could you design a SF axle that can utilize cone-style bearings? Ahhh, you can't, because you need some style of nut to hold the bearings on and adjust them, or if you can make it, it makes no sense because it would be such a complicated system. Instead, make a spindle like the front spindles on the rear....install studs on the hub, and place an axle in the center of the spindle that can transmit torque....voila! And since the axle bolts onto the hub and is automatically centered by the splines in the carrier, it eliminates any worries of a c-clip. :D

You going to the school of mines?
 
your only calculating a few items, in which case, it would be true.

Example:
Given the same size diameter ring gear/etc etc etc axle shaft strength, etc...

a SF will not be any different in its theoretical ability to transmit energy, vs a FF.

However, as someone mentioned above, any energy transfer (side to side shock loads) that want to rip the tire off sideways (such as, while turning, cornering, etc) has to go somewhere.

the difference between the FF and SF is simple: the FF transfers this sideways stress to the axle housing, while the SF transfers this energy to the C-clip.

If you can find the 10B FF conversion pictures, you'll see that they basically attach a flange to the outside of the axle housing, thereby transfering the stress of the sideways motion from the c-clip, to the axle housing.
 
forgot to add- because there are forces in every direction, having a FF is kinda like having a RWD race car, vs a SF FWD race car.

A RWD race car is inherently more suitable- the front wheels control steering/braking, while the rear provides power. Remember, there is only a few sq inches of contact. The coefficient of friction determines the overall power that can be transfered, whether its braking/acceleration or steering. Thats why you always brake before the turn, so that you have all of the tires energy transfer to be used for turning. if you brake during the turn- you'll exceed 100% capacity, and the tires will cease to grip. Because the rear wheels can still apply power, you can technically do more.

a FWD race car, has all the disadvantages listed above- its easy to understeer the vehicle, exceeding the maximum traction.

This long winded explination, is because a FF axle lets individual components handle their jobs properly, and not make any one unit have all the work.

a SF has most of the energy transfering through the shaft. This makes it easier to overload the axle shaft and cuase breakage.

there are two other comparisons you can think of, and it has to do with the axle tubing having more surface area to transfer the load, rather than the smaller c-clip. (the other one has to do with which side is pulling, and which side is pushing.
 
Are those numbers backwards? A rear dana60 could have 1.31" and 1.5" shafts. A long time ago one of the mags did an article on upgrading a rear dana 60 from the 1.31" axles to the 1.5" axles, it involved drilling out the spindle to a little over 1.5". And the 12 bolt is the other size.
 
If you look only at pure torsional strength then you need only know the material, length, and the diameter.

With the SF you also need to know the exact bearing distance from the last point of spline contact. The SF's outer bearing doesn't carry all of the wheel's load - some is transmitted down the shaft to the diff. You also need to figure in the bending moment. You'll also need to determine the spline fit to know if you are dealing with a simply supported shaft or a one end fixed shaft. I'll guess the former, but I can't say that for sure. Once you have that you'll need to add it into your cyclic fatigue calcs.

And just to make things fun, the OEs nearly always taper their axle shafts, be they FF or SF.

Since stress in any form adds into the total stress on the axle shaft, going to a FF design makes the FF stronger, even if the shaft size remains unchanged, b/c one whole set of stresses has been eliminated from the axle shaft as compared to the SF design.
 
exactly!!!

Blackchevy said:
The disadvantage of the SF axles is that the axle shaft must support the whole weight of the vehicle and creates the most pressure at the wheel and transmit the torque to the wheels. Whereas with a FF axle, the weight is distributed across the whole axle (and bearings) and the shafts only have to transmit the torque. At least that is my take on it!
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theoritically, lets say they both are the exact same stength, ok now part of this strength is holding the weight of the truck up on a sf.....so it is already using some of its strength just sitting still, therefore it cant handle as much as the ff , which is not using any of its strength holding the truck up...therefor all of its stength can be used in the rotation of the axle, therefore making it stonger in transmision of energy, altho they are both the same strength. but most people do run them so that when they do break an axel it stays on their ride instead of coming off in a giant mud hole and the next guy thru gets an axel up thru his oil pan or radiator or any where it shouldnt be, which is anywhere except on the end of the axle tube!
 
Yeah, I go to Mines. Second, the replies I have seen are pretty much what I expected. I was not saying that FF is not any stronger, just that it does not make much difference in the torque the shaft can take. FF axles are stronger, but I still think that FF 60 shafts (30 spline) are not going to hold up much better than 12 bolt shafts. Of course the SF shafts have a bunch of other stresses on them (the shaft length from the wheel bearing to the wheel makes a lever arm which will want to bend the shaft, c-clip slop, I would imagine that a SF sees more shock load when on very bumpy conditions because the tire/wheel are connected directly to the shaft, whereas with a FF the hub, wheel bearings, spindle, and housing will all soak some of that up. Finally, the measurements I gave were taken by me from my old 12 bolt shafts vs. a rear dana 60 shaft my brother had.
 
Agree with what was said above about loads in a non-vertical direction being the SF'ers weakness. The only SF axle that was ever worth a poop was probably the Ford 9". Wouldn't personally run anything else in a truck... if I was forced to run a SF of course. I understand that you're trying to figure out which would be better and why, but as someone else said, its really a moot point, because the really heavy duty rear ends are of the ff design... the lighter duty stuff is SF. Because of this, the SF has all kinds of disadvantages (smaller ring gear, shafts, housing strength yada yada) compared to the ff housing which make it superior even if SF and FF were theoretically of equal strength.

j
 
ntsqd basically explained the situation. You definitely have to add in the bending moment on a SF axleshaft when calculating the total load on the axle shaft.........this is all required when calculating items like the endurance limit, fatigue life, safety factor, etc, etc.

Regarding a sideload and a c-clip..........it would take a helluva lot of force to actually break a c-clip by a sideload. If side loading was such an issue with a c-clip retainer than there would be absolutely no way you could make a flanged axle setup work (pressed on bearing) like a rear Dana 44, 9 inch, or even a SF Dana 60 because it takes much more force to shear a c-clip then to rip off a pressed-on bearing. Besides, the c-clip theoretically only sees forces acting on it in a way that would pull the tire assembly away from the vehicle (the center pin sustains any forces with the tire assembly pushing inwards). Simply using basic vehicle dynamics will show you that it would be very difficult to obtain very high stress levels of the tire assembly trying to pull away from the vehicle.
 
I believe that the FF are considered stronger because of the shock loads an axle receives. Apples to apples comparing metals is not the point. We are talking about durability on the trail. Axle strength, on its own, is meaningless in that situation.
As far as your comment about the 60 shafts not holding up better than the 12 bolt shafts, that is incorrect due to the things you, yourself, said. It is the enviroment thay are in, that dictates how long they last.
 
so what if u had a 10b with the ff conversion? would the axle be stronger? i know ir would be pointless in doing since 14b's are dime a dozen, but its out there so apparently ppl run it.
 
muddybuddy said:
so what if u had a 10b with the ff conversion? would the axle be stronger? i know ir would be pointless in doing since 14b's are dime a dozen, but its out there so apparently ppl run it.
Click to expand...

Yes, in the real world the axle SHAFTS would hold up to more abuse and be less likely to break, and if they did break it would not chuck the tire/wheel assembly. However the remainder of the axle assembly such as the ring and pinion, differential, housing, etc... are not any better than before and quite susceptible to failure.
 
Leper said:
I believe that the FF are considered stronger because of the shock loads an axle receives. Apples to apples comparing metals is not the point. We are talking about durability on the trail. Axle strength, on its own, is meaningless in that situation.
As far as your comment about the 60 shafts not holding up better than the 12 bolt shafts, that is incorrect due to the things you, yourself, said. It is the enviroment thay are in, that dictates how long they last.
Click to expand...

Exactly. A SF 12-bolt will hold up pretty well to even a very high horsepower muscle car that gets abused on the strip, but most of that stress on the shaft is from torque as it's not supporting a 44" Bogger and bouncing off the ground which results in to a large bending moment in addition to the torque load.
 
Yeah, you're right that I argued for the FF and then made the argument that the 60 is not really stronger. BTW, I am not trying to "find out" which is better (because I already know :laugh: ), but I am just trying to get some information out there on a subject that I think there is some misinformation about. Here's a question: if my measurements from the 12 bolt and 60 are correct (the 60 definitely necked down to 1.20", I kinda doubt that 1.31" is at the smallest part of the 12 bolt shaft but that is the measurment I want to use) i.e. using 1.31" for the 12 bolt and 1.20" for the 60, and assuming both are made of the same metal, both are in the same truck in the same real world scenario, which axle's shafts would hold up better?
 
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