Stiffening plate removal / reinstallation - Page 18

andrewwynn · #1 04-06-2019, 10:52 PM

I guess like "free shipping" fcp has to decide what items they can sell without losing money like those as an example.

If FCP it ok with it that's just very unusual but if it's a loss leader to get you to keep you coming back good on them. I may have to check them out.

I just did check 'em out and I stand corrected they do literally say "even the parts that wear out".

Nice thanks for pointing out the craziness of FCP.

andrewwynn · #2 04-07-2019, 05:38 PM

on the 2013, they call it the 'Structural reinforcement panel' and it's part number 31106786568

The bolt is: 33306772888

The stiffening plate on the 2013 model does not also share the bolts with the sway bar which is good, it uses the same diameter bolts just shorter, so i have a strong suspicion that BMW will have spec'd the same rules for install but maybe with different values since it's a shorter bolt (35 v 55 mm).

I haven't found the directive yet but in the link above it should show.

andrewwynn · #3 04-07-2019, 09:49 PM

The *proper* method to re-use if you can't TTY would be to use a torque wrench and use lubricated torque values for M10; you'll have a decent clamp force and won't damage the bolt, it will be on the order of 7000 vs 9000# clamping force, which clearly has been working 'ok' for 'most everybody'.

(the reason for using lubricated torque, other than the bolt will likely be auto-lubricated on a 17 yr old car, is that without lubrication the torque to clamp force changes each re-use, so you either have to add torque or you'll lose clamp force).

My personal estimation is that the 9000# spec is more than needed and that if everybody just torqued the re-used bolts to normal spec or close, maybe a little higher, but not close to yield.

In my testing, I didn't get to yield until about 105 N·m so it would probably be pretty safe to use a dry torque of 80 N·m or such (again, i plan to experimentally derive some useful numbers).

Here's the idea: until you get to yield there will be a little bit of internal deformation but not much; if you pull to say half the way to yield strength, (10% over normal torque), especially if you start with a new bolt, coincidentally the math just worked out to 54 N·m to achieve that level of tension. (almost identical to the 56 N·m from design spec).

I'm very curious about the 56+45° solution it's a great hypotheses and definitely deserves testing. I'm sold on the just use bigger bolts properly torqued though it's too nice of a coincidence to get the same clamping force as design with normal torque, i call it kismet.

andrewwynn · #4 04-08-2019, 02:49 AM

On MY car, when i put the bolts back in, i think i used 4 of the original bolts and 2 new SAE 3/8 i had laying around. I can't remember how much i torqued them but will measure when i remove them to get an estimate. I did NOT TTY/TTA as I was unaware of the procedure when I did it, My 'M.O.' however is to look up the torque spec on my reference app and use that value, so that's almost surely what i did.

There is a problem with the 56 N·m plus 45°, which is that at enough applications it will still damage the bolt to the point it can break apart (from a big bump in the road for example); it's not 'mission critical' even if the one of the bolts holding the sway bar breaks (which would likely cascade to the other), you'll know immediately if it happens when you get the 110dB morse code of the sway bar slamming into the subframe. (experimentally confirmed).

What i noticed during my re-tighten to destruction experiment was that the yield not only was met at a lower N·m torque, it ALSO too less degrees of turn, and on the 5th and final tighten it was already at yield by about 45°, now the previous were all 90°, so it would take more times at 45 and if the 45 only gets you from 7500# to 8000# what's the point might as well just stay at proof load and advised torque of 49 ft·lb.

For holding the sway bar the bmw spec torque is actually less strong, since the bolt is pre-stressed to 9000#, it needs less of an impact to hit the tensile strength and break, and if you torqued them normally, it would be a stronger joint.

The TTY is all about the diagonal strengthening of the subframe.

Here's the thing: the front subframe has ZIP for rigidity as a parallelogram as made pretty obvious if you look at the drawing ;

The subframe is like the letter "U" it's open on the rear side and has nothing to keep it from twisting as a parallelogram other than the stiffening plate.

Here is where it comes into play: hit the brakes hard and have the right tires hit a patch of ice with the left tires on dry pavement; with a reasonable braking force of 0.5G now you have about 3000# force pushing backwards on the LEFT side of the car only. This is the condition for which this plate is designed. There are TWO circles about 1 sq in. on the back and one in the front that will get yanked with that force. It would also work to keep the back of the "U" from pulling apart but i'm not sure what forces would get applied there.

I finally remembered exactly what it felt like when i drove my X5 without the subframe stiffener; i likened it to a minivan wobble above but a couple days ago i remembered exactly what it felt like:

When i removed my stiffening plate, it was *exactly* like when i took the top down on my Z28 Camaro! every time i hit a bump i could feel a little extra jiggle like a harmonic; think tuning fork; perfect analogy considering the sub-frame is a big "U" like a tuning fork.

Others may not feel it, but just like I felt the difference every time i drove top-down with the Z, it was immediately obvious to me when i drove the X without the plate. (and oops when i forgot to put the rear bolts in, that was a very loud few miles, it sounded like i was driving a jackhammer).

So, at least a LOT of the mystery is resolved. I don't have the exact specifics but i've found out most of the 'why' and that's what was driving me nuts. I have a plan that suits me and have put out plenty of info that will let others find their comfort zone with respect to how they abuse their bolts. (not meant as an insult, the bolts are abused by design).

-awr

andrewwynn · #5 04-08-2019, 03:12 PM

15 pack of clone bolts are here

Quote:

The first few threads of the nut will support the majority of the load. Research has shown in some cases involving UNC threaded nuts that the first thread will have to support nearly 35% of the load. The second thread will support about 25% of the load, and the third thread about 18%. In this case the first three threads support 78% of the load.
To allow this distribution, nut threads are designed to be softer than bolt thread and will conform to the contour of the bolt threads when tensioned. If a nut were reused, there would no longer be a "ideal" thread match. This will create more friction between the threads during installation, which will significantly alter the installation torque.

So, probably should replace the nut even if you keep the bolt or it throws off the torque to tension ratio dramatically.

Quote:

n a demonstration with a 1/2-13 zinc plated SAE J429 Grade 5 hex cap screw and zinc plated SAE J995 Grade 5 hex nut with an installation torque of 70 ft-lbs to obtain a clamp load of 9000 lbs (without any added lubrication). On the second installation, this torque had increased to 95 ft-lbs to obtain 9000 lbs. By the fourth installation, we required 145 ft-lbs to reach a clamp load of 9000 lbs.

So it would seem the factor that changes the torque required the most is the nut. Learning new things every day. So, i'm thinking i'll be using NORMAL nuts and locktite and i'll replace the nuts rather than lubricate them, but the final solution will depend on the with or without lubrication and using new nuts.

On the other hand, i just found the T-slot nuts that are 12.9 to match my upcoming 12.9 bolts, i'm hoping i can get them to fit up top without room to turn so they are self-stopping no need to reach up to hold the nut.

oldskewel · #6 04-08-2019, 06:28 PM

https://www.youtube.com/watch?v=fvzdehnJA9k

I'm trying to stay out or at least limited here, but here are some general and specific comments. Not trying to give any answers, just information for those interested.

There is a ton of bad info on the internet. It's a challenge to sort through some of it.

Stress-strain of steel (and other similar materials) is a very important and pretty well studied and understood science. Important for it to be well understood for building bridges, pressure vessels, pipelines, etc.

That science can be extended and applied to help understanding of the specifics of threaded fasteners - nuts vs. bolts, yield, proof load, torsional vs. axial stress, etc. That's where things can get a little complicated and it may seem to be purely empirical and more black magic than it really is. Thinking back to the basic materials science of stress-strain helps bridge the gap.

For example, "yield" means some of the steel plastically deforms, which is a permanent change in shape, and strengthens the material. From that point on, it can still function elastically, including returning to the previously reached deformation point repeatedly in an elastic state, with the same elastic modulus but a now higher limit before yield will occur. But pushing beyond that point has its limits, and will eventually lead to fracture.

Those were the general comments. Now for some specific stuff:

Be careful if torquing with a non-flanged nut and bolt. The BMW ones are both flanged. Due to the increased area and torquing radius vs. non-flanged fasteners, and the fact that about 50% (depends, of course on surface prep, smoothness, etc.) of tightening torque is used to overcome that surface friction, using non-flanged fasteners will provide more stretch (and I use that term without distinguishing between plastic and elastic deformation) than will flanged ones.

Similar to how using really well lubed surfaces and threads might result in excessive tightening.

So that could mean if you're using non-flanged fasteners, even on the initial installation you are over torquing the 56 Nm stage, possibly causing plastic deformation where it would otherwise not occur.

Different thread pitch
If you try using SAE fasteners or a fine thread, the bolt stretch you get from applying a 90* angle will be different. The torque will be different too, but that is less clear.

Variable nut-thread loading
That is a basic fact of threaded fasteners. It happens regardless of any plastic deformation.

Fasteners are generally designed to one one hand have the nut be more compliant than the bolt to allow for better load distribution, trying to reduce this effect.

And on the other hand, the nut is always designed to be stronger than the bolt. The reason for that is that a nut would fail by having the threads progressively strip out, vs. the bolt would just fracture. You need to worry about the first nut thread stripping out, because if it starts to strip it will pass that load down the line and all the threads will strip out. Vs. the bolt fracture would most likely happen during assembly which is not such a big problem, vs. having a nut progressively strip out would happen literally "down the road" when it would be more of a problem.

For similar reasons, you should never use a weaker nut than bolt if there is any concern for nut failure. So using a 10.9 nut on a 12.9 bolt is not something to do casually. Doing that, you will be ensuring that the nut will fail before the bolt, which is the opposite of what is safer. Here's a pretty good reference for that and other things: Frequently Asked Questions on Bolting Matters

On the other hand, my M54 engine uses Class 12.9 head bolts, which do yield along their whole 110 mm length, threaded into an aluminum engine block ... or during a typical head gasket repair threaded into a ~15-thread long steel insert, which is probably not Class 12.9. So if it's designed carefully, it can be made to work.

When terms like plastic deformation, yield, proof, ultimate, etc. are used, they sometimes refer to total system failure (like "ultimate tensile strength" tells you when the bolt will snap [system failure] when increasing load is applied).

And sometimes they refer to localized material effects - where for example, the first thread in a nut may plastically deform very slightly - and it's not a problem at all, but it did deform. BTW, by the time the nut threads noticeably plastically deform, the bolt will have deformed a lot more.

True "stretch bolts" or TTY bolts - by which I mean bolts that are made specially for a TTY application, rather than regular bolts that are used in a TTA/TTY(?) application (as we have here) - are typically designed with relatively oversized heads and threaded sections, but with a long, thinner, uniform portion that is designed to yield, and to have all the yield there, and none anywhere else. Those are easier to analyze.

On figuring out whether your previously used nuts and bolts have deformed enough to significantly affect reinstallation by requiring more torque to overcome the bad fit caused the deformation ...

One pretty simple test is to see if you can finger-thread the nut onto the bolt. If you can do that, your torque wrench will probably not be fighting too hard to overcome that, and the measured tightening torque will have the same clamping effect as when they were initially used.

bcredliner · #7 04-09-2019, 03:11 PM

Quote:

Originally Posted by oldskewel

https://www.youtube.com/watch?v=fvzdehnJA9k

I'm trying to stay out or at least limited here, but here are some general and specific comments. Not trying to give any answers, just information for those interested.

There is a ton of bad info on the internet. It's a challenge to sort through some of it.

Stress-strain of steel (and other similar materials) is a very important and pretty well studied and understood science. Important for it to be well understood for building bridges, pressure vessels, pipelines, etc.

That science can be extended and applied to help understanding of the specifics of threaded fasteners - nuts vs. bolts, yield, proof load, torsional vs. axial stress, etc. That's where things can get a little complicated and it may seem to be purely empirical and more black magic than it really is. Thinking back to the basic materials science of stress-strain helps bridge the gap.

For example, "yield" means some of the steel plastically deforms, which is a permanent change in shape, and strengthens the material. From that point on, it can still function elastically, including returning to the previously reached deformation point repeatedly in an elastic state, with the same elastic modulus but a now higher limit before yield will occur. But pushing beyond that point has its limits, and will eventually lead to fracture.
https://en.wikipedia.org/wiki/Torque-to-yield_fastener

Those were the general comments. Now for some specific stuff:

Be careful if torquing with a non-flanged nut and bolt. The BMW ones are both flanged. Due to the increased area and torquing radius vs. non-flanged fasteners, and the fact that about 50% (depends, of course on surface prep, smoothness, etc.) of tightening torque is used to overcome that surface friction, using non-flanged fasteners will provide more stretch (and I use that term without distinguishing between plastic and elastic deformation) than will flanged ones. https://www.nde-ed.org/EducationResources/CommunityCollege/Materials/Structure/deformation.htm

Similar to how using really well lubed surfaces and threads might result in excessive tightening.

So that could mean if you're using non-flanged fasteners, even on the initial installation you are over torquing the 56 Nm stage, possibly causing plastic deformation where it would otherwise not occur. Lubing bolts is no longer recommended because of potential over torquing and wider variations of torque.

Different thread pitch
If you try using SAE fasteners or a fine thread, the bolt stretch you get from applying a 90* angle will be different. The torque will be different too, but that is less clear.

Variable nut-thread loading
That is a basic fact of threaded fasteners. It happens regardless of any plastic deformation.

Fasteners are generally designed to one one hand have the nut be more compliant than the bolt to allow for better load distribution, trying to reduce this effect.

And on the other hand, the nut is always designed to be stronger than the bolt. The reason for that is that a nut would fail by having the threads progressively strip out, vs. the bolt would just fracture. You need to worry about the first nut thread stripping out, because if it starts to strip it will pass that load down the line and all the threads will strip out. Vs. the bolt fracture would most likely happen during assembly which is not such a big problem, vs. having a nut progressively strip out would happen literally "down the road" when it would be more of a problem.

For similar reasons, you should never use a weaker nut than bolt if there is any concern for nut failure. So using a 10.9 nut on a 12.9 bolt is not something to do casually. Doing that, you will be ensuring that the nut will fail before the bolt, which is the opposite of what is safer. Here's a pretty good reference for that and other things: Frequently Asked Questions on Bolting Matters

On the other hand, my M54 engine uses Class 12.9 head bolts, which do yield along their whole 110 mm length, threaded into an aluminum engine block ... or during a typical head gasket repair threaded into a ~15-thread long steel insert, which is probably not Class 12.9. So if it's designed carefully, it can be made to work.

When terms like plastic deformation, yield, proof, ultimate, etc. are used, they sometimes refer to total system failure (like "ultimate tensile strength" tells you when the bolt will snap [system failure] when increasing load is applied).

And sometimes they refer to localized material effects - where for example, the first thread in a nut may plastically deform very slightly - and it's not a problem at all, but it did deform. BTW, by the time the nut threads noticeably plastically deform, the bolt will have deformed a lot more.

True "stretch bolts" or TTY bolts - by which I mean bolts that are made specially for a TTY application, rather than regular bolts that are used in a TTA/TTY(?) application (as we have here) - are typically designed with relatively oversized heads and threaded sections, but with a long, thinner, uniform portion that is designed to yield, and to have all the yield there, and none anywhere else. Those are easier to analyze.
Do we know for sure the bolts are regular bolts? TTA is a process. TTY bolt uses TTA process and are for one time use.
On figuring out whether your previously used nuts and bolts have deformed enough to significantly affect reinstallation by requiring more torque to overcome the bad fit caused the deformation ... We have two questions to answer. Do the bolts break after some number of TTA sequences. The other question is does the clamping power change with each torquing sequence. Clamping power is only about 10-15 percent of torque. At some point of decline in clamping power the bolts may loosen over some period of time.

One pretty simple test is to see if you can finger-thread the nut onto the bolt. If you can do that, your torque wrench will probably not be fighting too hard to overcome that, and the measured tightening torque will have the same clamping effect as when they were initially used.

There are formulas to answer these questions without tests but we would have to know the specifications of the bolt and nut and have someone that can wade through them.

One other thought--what we are posting is not necessarily about the same bolts. I suspect not all models/years used the same bolt?

andrewwynn · #8 04-08-2019, 08:58 PM

great stuff, especially how of course the angle changes directly with respect to the thread pitch when using TTA; the specs are only for one exact bolt. Also with the details about lubricate torque; my 'torqueometer' shows both the dry and lubricated torque.

re: hand thread nut; of course with these nuts you can't hand-thread them because they are oval in the center self-locking nuts, so you won't be able to tell that way. I was concerned that the squashed nuts would affect the torque numbers but i guess once the nut gets started it's much easier to turn because i got no reading on the torque adapter from the nut before snugging tight.

I'm a little confused by the 'stronger nut' vs. Fastenal quote above saying that the nut is 'softer' I believe that means that the metal of the nut is a lower ksi than the bolt, but because the nut 'melds' into the bolt it holds stronger or something, i would love to have some clarification on the seemingly contradictory things.

oldskewel · #9 04-11-2019, 11:09 PM

Quote:

Originally Posted by andrewwynn

great stuff, especially how of course the angle changes directly with respect to the thread pitch when using TTA; the specs are only for one exact bolt. Also with the details about lubricate torque; my 'torqueometer' shows both the dry and lubricated torque.

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

Quote:

Originally Posted by andrewwynn

re: hand thread nut; of course with these nuts you can't hand-thread them because they are oval in the center self-locking nuts, so you won't be able to tell that way. I was concerned that the squashed nuts would affect the torque numbers but i guess once the nut gets started it's much easier to turn because i got no reading on the torque adapter from the nut before snugging tight.

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.

Quote:

Originally Posted by andrewwynn

I'm a little confused by the 'stronger nut' vs. Fastenal quote above saying that the nut is 'softer' I believe that means that the metal of the nut is a lower ksi than the bolt, but because the nut 'melds' into the bolt it holds stronger or something, i would love to have some clarification on the seemingly contradictory things.

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.

andrewwynn · #10 04-09-2019, 11:19 AM

On a 17 year old X I would absolutely start with new bolts. I would not buy official BMW bolts I will be getting black oxide 12.9 and washers. If they fit I will likely get the T nuts and epoxy them in place to convert my e53 to act like the new e70 that has threaded inserts in the axle support.

Once replacing the bolts I'll be simply set I'm opting for the forever 12.9 bolts.

If I got the 10.9 bolts I would torque to spec and keep track of the use count with a center punch and dents in the plate, replace after five uses or when torque drops below about 97 N·m before 90°

(That number might change higher or lower once I test with new bolts).

I linked above to a 15 pack of decent bolts for $17.

My determination is that the high force on the spec of the design is mostly for the well being of the CAR in a crash. The bolts can be at 1/3 of design spec and handle anything you can possibly subject them to by driving. That is why "everybody" gets away with treating the metal at if it's just for under belly protection and not an important part of the frame