U Bolt Torque Thread (Rusty Calling You)

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805moparkid

805moparkid

Slant and AFX Guy
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Ok so everyone i know and myself have always used 75-90lbs on the u bolt tq due to the 1/2 hardened bolts and hardware.

recently i checked the FSM due to a tip from rusty and it says 45 ft lbs.

So here is the question...

With a center locating bolt holding the leaves together how could the leaves move in any direction regardless of tq. The U bolts are fixing the axle to the shock plates.

So i want to hear the final cut if you will on this, them i think it would make a good sticky.
 
In my opinion, no movement. I don't know why the specified torque is so low unless engineers were afraid of crushing axle tubes. This would not be the first time that there was a mistake in a shop manual for years on end. Seems to me the rocker arm bolts torque was wrong for "several" years.
 
67Dart273 said:
In my opinion, no movement. I don't know why the specified torque is so low unless engineers were afraid of crushing axle tubes. This would not be the first time that there was a mistake in a shop manual for years on end. Seems to me the rocker arm bolts torque was wrong for "several" years.
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hmm i hadn't thought of the tubes... i recall them being at least 1/8" - 3/16" though, i dont think 90lbs would bend it?
 
805moparkid said:
hmm i hadn't thought of the tubes... i recall them being at least 1/8" - 3/16" though, i dont think 90lbs would bend it?
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It's 90 lbs of twist on the nut, but how much down force is that on each U bolt?

A lot more than 90 lbs.:D

There is a sort of gear reduction at work there, that I think you are not taking into account.
 
with the U-bolts torqued down that much, it doesn't allow the leafs to move freely as much. That's pretty obvious stuff right there. When the leafs are at proper torque spec, it allows proper interleaf friction which creates a damping affect. By overtorquing, you are taking away the leafs abilities to slide around as much and create friction against each other to create a proper damping affect, you are now creating(more of) a completely different animal called stiction, or static friction/static cohesion. This is caused when two objects are pressed against each other, not sliding or traditional friction causes. This requires more force force parallel to the surface of contact in order to overcome static cohesion.
 
90 ft/lbs is about 5.4 tons clamping force on a 1/2 bolt. Thats a bit harsh on a 3" tube. 45 ft/lbs is about 5400 lbs.
 
............I for 1 think 45ft lbs is not enough, I usually go 60-70, anymore then it seems like ur streaching the bolt but may infact be crushing/collapsing the tube..........kim.......
 
PanGasket said:
with the U-bolts torqued down that much, it doesn't allow the leafs to move freely as much. That's pretty obvious stuff right there. When the leafs are at proper torque spec, it allows proper interleaf friction which creates a damping affect. By overtorquing, you are taking away the leafs abilities to slide around as much and create friction against each other to create a proper damping affect, you are now creating(more of) a completely different animal called stiction, or static friction/static cohesion. This is caused when two objects are pressed against each other, not sliding or traditional friction causes. This requires more force force parallel to the surface of contact in order to overcome static cohesion.
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there is a bolt holding all the leaves together, before they ever get on the car the leaves cant move.
 
805moparkid said:
there is a bolt holding all the leaves together, before they ever get on the car the leaves cant move.
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The centering bolt is there for just what its named for. To center the leaf springs together and into the perch. They also don't bind the leaf springs together 100%. center bolts are not meant to be a strong link at all in the leaf spring, that's why if the U-bolts are negligently undertorqued, the center bolt will break. It is simply there to keep the leaves unified and centered. As its name states.
 
PanGasket said:
The centering bolt is there for just what its named for. To center the leaf springs together and into the perch. They also don't bind the leaf springs together 100%. center bolts are not meant to be a strong link at all in the leaf spring, that's why if the U-bolts are negligently undertorqued, the center bolt will break. It is simply there to keep the leaves unified and centered. As its name states.
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out of all the sets of leaf's ive rebuilt/refurb'd, non have had any slop around that bolt, there for it couldn't move.
 
805moparkid said:
out of all the sets of leaf's ive rebuilt/refurb'd, non have had any slop around that bolt, there for it couldn't move.
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It's not supposed to. Its supposed to unify the leaves and the axle as one unit and center the springs and hold them center.

If a leaf spring compresses, it flattens out some. When it does that, the leaves inch up each other, rubbing against each other and dampening the movement. When you clamp them down super tight it keeps them from inching up each other and binds them together more so.
 
PanGasket said:
It's not supposed to. Its supposed to unify the leaves and the axle as one unit and center the springs and hold them center.

If a leaf spring compresses, it flattens out some. When it does that, the leaves inch up each other, rubbing against each other and dampening the movement. When you clamp them down super tight it keeps them from inching up each other and binds them together more so.
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i agree they move, just not in the middle, but on the ends.
 
take a deck of cards and fan them out forward like half a leaf spring. Now grab the "center of the spring"(the end of the deck) firmly, but not too tight. Now pull up on that bottom "leaf". It flexes pretty good right? Now tighten your grip real tight on the center of our gambling spring pack. Like real right, like that guy whose hand you hate to shake and wanna just crush it. Now pull up again on that bottom leaf. Notice the difference? You should. That bottom leaf isn't flexing near as much as it just was.
 
As stated above, leaf springs don't move in the middle, but the ends of them do rub against eachother. Look at where the arch is, not much in the middle where the u-bults go.... think about it! If, as pishta said, 45 ft/lbs is 5400 lbs clamping force, how much do you really think they are rubbing against each other in the center part under the u-bolts? Tightening them down more is gonna cause them to rub less in the middle where they are not rubbing at all anyway?

Anyone ever clamped them up and measured them when applying a certain load? That would be interesting to see the results!
 
69B-Cuda said:
As stated above, leaf springs don't move in the middle, but the ends of them do rub against eachother. Look at where the arch is, not much in the middle where the u-bults go.... think about it! If, as pishta said, 45 ft/lbs is 5400 lbs clamping force, how much do you really think they are rubbing against each other in the center part under the u-bolts? Tightening them down more is gonna cause them to rub less in the middle where they are not rubbing at all anyway?

Anyone ever clamped them up and measured them when applying a certain load? That would be interesting to see the results!
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:cheers:
 
805moparkid said:
:cheers:
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That's exactly what I have been getting at. Hence the deck of cards example. I spent a good bit of time thinking of something you have around to show you what I mean. The centering bolt holds them together yes, but that's not the focus here. It's the U-bolts.
 
PanGasket said:
That's exactly what I have been getting at. Hence the deck of cards example. I spent a good bit of time thinking of something you have around to show you what I mean. The centering bolt holds them together yes, but that's not the focus here. It's the U-bolts.
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re read what he posted... sorry i dont have a deck of cards handy, but your also trying to compare paper to spring steel.
 
According to my Mopar Performance 9th edition, it clearly states NOT to over tighten the U bolts. It can cause spring friction. Where does friction come from?

Hint: movement
 
805moparkid said:
re read what he posted... sorry i dont have a deck of cards handy, but your also trying to compare paper to spring steel.
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Yes it would be very interesting to see the comparison. I'm sure you might be able to do such a thing with a modified hydraulic press or something of the sort.

If the U bolts don't affect it at all, then why are we discussing this?
 
PanGasket said:
Yes it would be very interesting to see the comparison. I'm sure you might be able to do such a thing with a modified hydraulic press or something of the sort.

If the U bolts don't affect it at all, then why are we discussing this?
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i dont think it does. But if you or anyone else does im open to hearing it, i have no problem saying im wrong, i just havn't seen anything to make me think otherwise.

That said the tube's crushing could be an issue. As far as movement i dont think there is anything on the inner area of the leaf (inner meaning maybe a couple inches either way past the u bolts) for two reasons, one the center pin and two the u bolt's even at 45ft lbs.
 
I still maintain that this alleged movement does not happen, and I think I might have found proof

http://www.suspensionspecialists.com/tech0004.html

A partial quote:

"
We have emphasized U-bolt nut tightness because the center bolt area of a spring is the area of the least cross section in multi-leaf springs, hence the weakest area. If the U-bolts are loose, the center bolt area absorbs the greatest stress. When the center bolt area is clamped solidly against the axle pad, as in a vise, then the middle of the spring cannot bend and, if it cannot bend, then it also cannot break at this point. Because the middle of the spring is the place of greatest hazard, it is necessary to clamp it securely to prevent breakage at this point.
It is possible for worn or improperly fitted axle pads to be a contributing cause of center spring breakage. Since many modern springs are flat, under normal full load, pads for such springs should also be flat, and only lightly rounded at the ends."


and further:


"Wear of spring pads can only be caused by movement of the spring on the pads. If the U-bolts have been holding the spring so tightly to the pad that no movement occurs, there can be little wear."

and from here:

http://www.acespringservice.com/u-bolt.html

"
What does a U-Bolt do?

A U-bolt serves in four critical and related roles:
  • Primarily, the U-bolt provides the force required to clamp the leaf spring and related components firmly together. In addition to the leaf spring, these components include the top plate, axle seat, axle and bottom plate.

  • The properly installed U-bolt eliminates any flexing of the leaf spring in the area between the U-bolt. This is particularly critical since the hole for the centerbolt in each leaf acts as a stress concentration which would lead to rapid leaf breakage if spring flexing was not totally eliminated by the U-bolt clamping force.

  • By firmly clamping the spring to the axle seat the horizontal forces acting on the centerbolt are greatly reduced which in turn prevents shearing of the centerbolt.

  • Proper clamping of the spring by the U-bolts provides the desired spring stiffness and contributes to maintaining the vehicle ride height and handling characteristics as originally specified for the vehicle."
and further:


"

Inspect for signs of loose U-bolts

Virtually all leaf spring failures through the center hole are caused by inadequate U-bolt clamping. Also look for signs of movement within the area between the U-bolts. Worn or polished surfaces on axle seats or top plates are sure indicators that unwanted movement has occurred. Closely inspect and if necessary replace each damaged component.

and one last

http://www.performanceonline.com/blog/2009/09/top-14-spring-questions-answered/

Q: I just broke a center bolt. What happened?
A: There is only one cause of broken center bolts, and that is loose U-bolts. The center bolt is necessary to hold all the leaves together and to locate the axle in correct position. The axle is located by the center bolt head that fits into a hole in the spring seat. Then the U-bolts are used to bind the spring and axle into one unit.
The U-bolts must be tightened to the proper torque specs to eliminate any movement between the spring and the axle and between each leaf of the spring. Should the U-bolts be loose, two things can happen—broken center bolts and broken leaves.
 
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