It's a 20 yr old discussion but a couple weeks old to me. I found no obective input to help determine one way or the other what is going on with these bolts.

What I've detemined is that the vast majority of people including pro mechanic have been deciding without knowing what they are doing to reduce the effectiveness of the bolts by a factor of four.

I like to figure out why something is done before I reengineer it in case there's a reason for the design in the first place.

I'm planning to do both a torsion and parallelogram experiment before I put wife's plate back on.

@bc: doesn't have to be the same bolts to have the same rules, any bolt that is over torqued would be in the same line of reasoning.

Correct

No problems *yet*; the 56 N·m is pretty close to advised torque. When torquing to 90°, I noticed that by the 3rd or 4th test, i could feel the bolt go plastic deformation pretty close to 45°.

I think you came up with a very solid compromise that I would likely come up with but basing on some actual testing not just guessing.

The 56+45 will get the bolt past proof load every time so it just means it will take more times to break the bolt; the problem is this: if you didn't get the bolt to snap during install it may just snap when you hit a pot hole.

As i've covered these bolts are not mission critical, the the worse case scenario NOT involving a crash is that the sway bar gets loose and will start banging around like hell; you'll know you'll pull over, it won't do any damage.

I'm going to test the 45° TTA and see just how much less clamp force is generated. It will surely be above spec for the bolt which is 7500# and going full 90° the math worked to 9000, so it should be somewhere in between those two which I am good with.

The 'standard practice' of 'two three taps of impact' is very close to worthless in getting these bolts to do their intended job (hold the aluminum from sliding sideways), The people that do that are installing a skid plate not a shear plate.

The point of installing them TTA/TTY is to get consistent forces in the field. BMW wants to have 9000# force each bolt when installed. Using seat-of-pants methods, as demonstrated when i removed my wife's plate, achieved somewhere between 9000 and maybe 1000 if that; her car was not protected against an offset impact; both the front bolts were from a 10MM bolt perspective 'finger tight'

as you mentioned the torque similar to wheel, that's exactly right and as mentioned above on relatively new bolts, I measured over 100 N·m close to even 120 in one case; a simple way to confirm your bolts haven't hit their useful life span is to use a torque wrench when going through the 90° swing; set the torque wrench to 100 N·m (75 ft‚lb), and if you get a click before you get to about 60° of the swing, the bolts are still usable.

I could feel the bolts start to fail 2 uses before they actually failed. On the time it broke, it broke at about 120° but it was spongy by about 45°. Using a kinder, gentler 45° the whole time, i suspect the re-use count might be as high as 10 or 20 and that should get anybody through the lifetime of their X5, so as much as it was a seat-of-pants guess to use that method, I'm pretty confident that once i actually test how many times it takes to fail a bolt with that method, it will be proven to be a damn fine guess. (It also was my first thought of how i would re-use them).

Also, odd are VERY small that any previous mechanic ever re-tightened to spec, so basically if you ever start to use 56+45 it's almost certainly 'the second use' no mater how old.

I have 12 to test just from mine and wife's cars, I re-tightened all four i put back on wife's car to 90° since I'm replacing them anyhow it would be a good test and all four held well over 100 N·m during the TTA phase and are doing their job as designed. One is broken and i lost one nut so i'll have about 10 to use for destructive testing i'm going to do half at 90° and half at 45° to get some solid numbers figured out.

The torque didn't drop below 100 until the 3rd re-tighten, so i think that's a really solid # to work with and i'll test with new bolts at some point to figure out if 95 is even ok, that seemed to be the point where the bolts started to fail. For now, a dry torque of 100 N·m at 90° worked great for a 2nd use, and I prefer to keep the design spec of full clamp force myself, but don't feel bad about de-tuning a bit to extend the life either.

The bolts start to stretch at 7500#, they are full stretch at 9000# and NEW will break about 10000#. You'd literally have to crash cars to even know the answer to how much difference 7500 and 9000 would even matter, the key to get above 7500 is you NEED to stretch the bolt if even just a little so the 45° idea is the best way to do that. The fastenal paper showed that torque value is worthless to achieve clamp force on re-use of a bolt; it took 60% more force to get to the same torque by the 4th or 5th use! tap-tap-tap with an impact is so random it'd be far better to just use a ratchet and a 'good hard tug'.

case-in-point; the four bolts i just re-used and torqued to 90° varied from about 105 to 120 N·m torque to achieve the same 9000# ish force. Each re-use the cross-section of the bolt gets smaller so maybe the first time it's 9050 the second time 9000, the third time 8050, you get the picture; it's in the same ballpark until the cross section narrows quickly and the bolt stretches very easily.

Anybody with a spare hour on their hands and wants to learn a bit more about bolts, repeat the experiment with any bolt doesn't need to be this one; I put a bolt through a socket for something to squeeze, i used a breaker bar in a vice to hold one side and another breaker bar and my digital torque adapter on the other side, and did the tighten process a few times, and what became clear was that when you get to the point of yield you can clearly feel the force drop off; every bolt will have a different set up of pretension and TTA to get you to yield but you'll feel it; the torque will climb linearly until you get to yield and then it takes the same torque to keep going, it feels very odd that it stops getting harder.

The happened at just about 75-80° the first three tests, then at maybe 60 then maybe 45, the pre-failure was very obvious and dramatic, it felt like the bolt turned to lead from steel on the last test.

So, the summary if, sure the bolts can be reused. they can even pull their design force, just about 4 or 5 times, but i'd pay attention to the torque during install, and consider the 45° method especially if you remove the shear plate frequently.

As always, thanks andrew. Very informative. Will order up some of those clone bolts and new nuts after one last plate drop as I've had mine off around 4 or 5 times since I've owned it already.

I'd be interested if someone made a X brace that replaced the steel plate as it seems it could save some weight.

Not enough weight to make it worth it.

You could take some 1" steel tube crush it flat on the ends and middle and make a functional brace you'd also want to go across the back. Then you can just leave it on. (and have instant access to engine bottom. If you don't go off road not much to loose just a little more under belly turbulence.

The plate is not two dimensional so there is some flex resistance that would be lost going to x brace.

True, maybe 10lbs going from the heavy steel plate to a lighter aluminum? But greater access is the more driving force. Would take a few removals of the bolts/nuts out of the equation for normal service I would imagine.

Thanks for reading carefully. I was hoping a few people would benefit from this discussion one day.

A few of mine can be finger threaded together, so maybe the oval self locking gets worked out too. ??? But one way or another if you've got a nut and bolt that can be threaded together very easily, then there will be no extra torque needed to overcome any misfitting while torquing it down, and it should be like new in this regard.

The few people that actually do the specs and design the manufacturing processes to meet them make it so that for example a Class 10.9 bolt and Class 10.9 nut are matched to provide the extra compliance in the nut. Use them together and you're fine. So roughly the 10 there refers to the ultimate tensile stress and the 9 refers to the yield stress being 90% of that, but actually things are a little different.

The nut will also have a bit of a bevel on the first (or more?) threads, which makes it less likely to fail. Also, the nut threads have a bigger radius from the center, so more volume, so more strength than the bolt threads they match up with.

The key is to prevent the first thread from stripping out, because if it does, it is likely for the rest of them to follow as the load that is no longer carried by the first thread is passed on.

In general though, if you consider the total effective area of the nut that would have to yield (roughly the circumference of the nut/bolt interface times the length of the nut = pi*r*2*h) vs. the corresponding are of the bolt (the cross sectional area of that = pi*r^2) you'll see the odds are stacked in favor of the nut. Those areas are equal at around nut height (h) = bolt diameter / 4 (= r/2). So forgetting the variable loading, a pretty thin nut should work fine. Vs. it's more typical for steel to have the nut height close to the same as the bolt diameter. So with the extra nut height, the variable nut thread loading issue turns out to be manageable.

And that all means that it is much more likely for the bolt to fracture than for the nut threads to strip. And that would likely happen during tightening ...

even more so because of another factor that has not been mentioned ... hate to throw fuel on this fire, but ...

When torquing the bolt, the bolt will be in torsion as well as being stretched. Those two stresses combine as an equivalent, "von Mises" stress, and will cause yield at a point before just the stress due to stretch will. After the tightening operation is finished, somehow this dissipates (mystery to me).

And when considering the torsional stress, the surface friction between the bolt or nut head and the clamping surface does not count. So the torque wrench value would need to subtract out that surface frictional torque, but not the torque due to the threaded surfaces.

But that's another factor that makes it more likely for a bolt to fail.

Still, matching Class or Grade for nut and bolt is the safest practice when there is any doubt about failure.

And if you're wondering where all of these factors go in those nice geometric-looking formulas you see in the torque tables, well it's in those coefficients that appear pulled out of the air. I love when they do stuff like that - all mathematical and physical and geometric and precise and oh yeah, here's a factor of 0.18. move along, nothing to see here. Don't ask don't tell.

Found this thread while looking for directions on how to remove the stiffener plate so I can change my gearbox fluid.

Can someone summarize the findings of this massive debate about the 4 bolts?
What torque do they need? I am going to reuse mine.

Also, are there nuts on the other side of the bolts?