This is a very interesting point and also greatly added to the mass-confusion on the stiffening plate bolts; many of the bolts that are NOT overly stressed are also recommended for replacement, in what seems to be a clear-cut case of CYA; there is no logical reason to replace a 16mm bolt that isn't stressed to deformation.

I had been fairly certain that the bolts weren't stretched until i actually measured a couple of them. When i did the first test of re-tighten to spec and actually measured it grow by 0.08mm then after 4 more it snapped.

FYI; the advised torque for M10-1.5 10.9 is 49 ft·lb = 67 N·m and I am willing to bet that the 56+45° will not cause a loss of strength for well into the double-digits because there is a 20% margin in the design of bolts and that may be right in the middle.

For those like me with a digital torque adapter with live torque reading, it's darn simple to determine when the bolt goes plastic; the torque number just stops climbing it's bizarre! Like i mentioned right after my experiment, I noticed Also since i did one after the other after the other, that each successive time, the angle where the plastic deformation kicked in was sooner at each time, i will pick up one of the TTA tools for my detailed experiment i was only using 'start vertical, end horizontal' with the breaker bar for the initial test.

If you use a torque adapter, and follow the TTA specs, As long as the torque gets to 100 N·m before 90° There is still going to be life in that bolt. One of the bolts I put back in was over 115. I'm going to do a handful of new bolts to get a better scatter plot of what numbers to use, but with the right knowledge you can definitely use the bolts a few times at least.

Too bad the OEM bolts aren't $3.53 like real-oem says. The thing is, even if they were $3.53 + 78¢ (nut) that's still over $25 that need not be spent just to check on the oil leaking status.

I just thought of another biggie; in the example of braking while on ice, it only generated a sheer force of 3000#, in that example but it would be more than that because the lever ends are the tires, the bolts are in the middle there will be leverage factor so it could be double that in moderate braking and quadruple if you get near a 1.0g panic stop.

HOWEVER; bump into something on ONE side of the bumper and now you could easily get into the 10g range; if you put a 10g deceleration on one side of the bumper, now you are putting 50,000# force twisting the car into a parallelogram!

NOW it's coming together to make sense. When i did my early math to determine that the sheet was thick enough to hold back 50,000# of force it felt pretty serious and 'why that much' but it will GREATLY reduce the risk of totaling your car from a fender-bender.

Now i wish i had a couple end-of-life X5 to bump into something solid at 10 mph. The reason people aren't having problems with not tightening their bolts properly is they are good drivers :-D

Get into a fender-bender with your plate removed, there is a better chance you will total your car.

So, it's becoming clear that a large part of the point is to keep the car from twisting into a pretzel from an offset impact, the shear strength of an M10-1.5 is just about 8000#; but hardened aluminum has a tensile strength on the order of 40-50 ksi, so by using a strong 'pinch' they can get 10s of 1000s of # of resistance. just 'snug' that bolt and especially in the case like my wife's where one corner was 20 ft·lb, it doesn't drop to 8000# force, it drops to the sectional size of the cross of the bolt and the aluminum. That works out to 1/16 of a sq. in. so approximately 45/16 = about 4000# force to pull the bolt sideways through the aluminum.

I have to measure the clamping point on the car, but it was maybe 1.75", so that is about .27 sq. in. multiply by 45k and get 12,400#.

These are obviously a lot of educated guesses but they have sound backing in science. basically the idea is that with 9000# of clamp force, the raised donut in the steel about 1.75# in diameter will lock the alum. plate in place so the aluminum would have to tear out sideways it can't slip since it will actually be slightly 'dented in'. Don't have enough force to keep the alum from slipping and then you drop down from 0.27 to 0.062 sq. in. of cross section of alum holding things back. a nearly 80% drop in the capability.

Anybody else starting to see why BMW decided to use a TTY solution to ensure the clamping force? The design (to me is clear now) is to protect the car/frame from damage in an uneven front impact (though would also help from rear impact as well).

I only have gear that can pull 1T, maybe with a sheave (pulley) i can double that, i might see if 2T will pull the plate sideways at what torque on the bolt. I also can't wait to examine the plates on mine and wife's car to see if there is a permanent 1.75" dent from the clamping force. (if I was bmw I would stamp in a dent and then that greatly reduces the need for the extreme clamping force).

There is more function to the plate than above, clearly since i can feel the wobble in the suspension when i don't have the plate on it tightens up the front end, but i'm pretty confident that for that purpose the bolts can be FAR FAR less tight; even if the bolts slipped under stress of cornering etc, they will act like a shock absorber taking energy out of the equation.

So BMW could have just explained in a sentence "we require replacing the single-use bolts to ensure the maximum clamping force required to ensure the design strength is maintained during an accident".

Minor irony alert: the plate makes your life more at risk; anything that makes the car more rigid to protect the car pushes that force onto the contents of the car; deformation of the frame is the best way to save people. So now ya don't know what to do! loosen the bolts to make you safer in an accident or properly tighten them so you don't have to replace your car from a minor bump into a post.

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2011 E70 • N55 (me)
2012 E70 • N63 (wife)