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  #141  
Old 04-03-2019, 05:17 PM
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  #142  
Old 04-03-2019, 06:22 PM
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Quote:
Originally Posted by cn90 View Post
In my 1998 528i, FRONT wheel bearings bolts...BMW manual says replace them.

All BMW mechanics I know re-use them. I re-use them, they are as big as the wheel lugs.
That was 10 years ago, the bolts still hold the bearings just fine.
Why does BMW state not to reuse the bolts and what can happen if they are reused?
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  #143  
Old 04-03-2019, 06:38 PM
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Quote:
Originally Posted by oldskewel View Post
I don't see any doubt about them being special TTY bolts. But I think there may be some doubt about whether they yield when the TTA spec is applied properly. While responding to another thread right now on oldmactech's head bolts, I was reminded of this thing I noticed when examining my old vs. new head bolts last summer.

https://xoutpost.com/bmw-sav-forums/...formation.html

Should be an easy test for someone who actually buys new ones to compare vs. the removed ones. And if you line up old vs. new and the threads match all the way, you can pretty safely conclude that they did not yield, and reuse them with even more confidence.

Also, regarding applying the TTA, that's only part of it, since the factory, the owner, the PO, the dealer, etc. might not do it accurately. And then once applied, stresses from driving could cause some yield. So different people might find different results, even among their 6 bolts.

I've had my plate off a few times; first was for front differential oil change, last was for top-end engine rebuild. I paid attention the first or second time, noticing some deformation based on the fact that the nuts would not all spin on easily with nut and bolt in hand. So as I've said, I re-use with the conservative assumption that yield has occurred, and not wanting to yield it more, I torque accordingly to a higher but conservative value, without doing the final angle.
You should have doubt if the bolts are TTY bolts. There have several posts saying they aren't . However, I think so far nobody knows for sure.

85 or so percent of torque is friction. The other 15 percent is clamping power. If there is deformation there is no torquing procedure that will get to the original clamping power.

Why does BMW state not to reuse the bolts and what can happen if you do?
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  #144  
Old 04-03-2019, 08:02 PM
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Quote:
Originally Posted by andrewwynn View Post
from BMW:



(they didn't include the protection from the garage floor, but maybe that is attributed to the foam padding above the panel).

I put my 'restriction plate' (restricts you from getting to anything) back on with some SAE bolts i had handy and am soon going to be removing the plate and putting it back on.

Since it's the way i'm built, i plan to do a ditch-witch maneuver with the plate off with some steel strap snug by not overly tight to measure how much the subframe will twist when i lift one wheel up in the air, probably drive onto a stack of cement blocks to lift it 8".

It's pretty clear that the primary job of the plate is in-fact to keep the front suspension true and not twist; 4 or 5 mm thickness of aluminum times 200mm width (my estimate) means 800-1000mm cross section that's the equivalent of a solid rod of 16-18mm diameter and would substantially hold quite a bit of force. (40-50,000# before yield with mid-grade aluminum alloy; could be double with high strength aluminum) You start with making an assumption that it is pretty clear why the plate is there and then throw out an estimate. Anything following is meaningless.

Since i used under performing bolts and they've been on there a while, if they didn't do their job and allowed motion, it will be very obvious. If they on the other hand, held the plate from moving in spite of being much lower clamping force than the design, it will be solid proof that bmw over-engineered the part and give people piece of mind that they can definitely use normal bolts.
Now you have taken the estimate and assumption and drawn a conclusion, that's flawed. Even if it weren't you don't know what driving circumstances are needed for a proper test. The exercise proves nothing.
The bolt i found online just now clearly is stamped 10.9. There is no reason you couldn't use a similar bolt from a hardware store except it won't be galvanized. Steel-to-aluminum will set up an ugly galvanic reaction when it gets a little salt spray on it.

The TIS for replacing the plate is here

The torque spec is [URL="https://www.newtis.info/tisv2/a/en/e53-x5-3.0i-sav/repair-manuals/31-front-axle-front-suspension/31-10-front-axle-suspension/5df26uS"]here[/UR

My reference for bolt torque shows 67 N·m and the tech spec above shows to use 56 N·m + 90°. I will also see how a re-used bolt vs a new 10.9 bolt behaves when it's torqued. I have a strong suspicion that the extra 90° will go past the 67 N·m advised torque and puts you from TTA into TTY.
.0888 in² cross section and yield of 136k psi, means yield force of 12,100# roughly and torqued normally (67 N·m / 49 ft·lb), is 'only' 7531 # of force. Another measurement I will take is what is the actual torque applied to achieve the 90° TTA.

The whole point of this endeavor is to determine if the bolts are even close to their yield strength and there is any need for this debate to continue. If the bolts aren't close to even the proof load of 113kpsi, the debate can be concluded. No damage to the bolt and no reason to not reuse them. Torque to Angle is a torquing procedure. The angle is determined by the torquing specs for that particular bolt. The angle could be 90 degrees but it could be more or less. TTY is a type bolt that when torqued to specs for that particular bolt it elongates to plastic deformation and regardless of the torquing procedure will not reach the original clamping power a second time.


If, on the other hand the bolts are being stretched to yield, then my recommendation would be to slightly under-torque to keep them from stretching or oversize the bolts one size bigger so you can get the same clamp force without damaging the bolt. If you torque the bolts to a lower number the clamping power is reduced. A larger bolt will not necessarily reach the specified clamping power even if torqued to a higher number. You would have to know what torque is needed on that specific larger bolt that delivers the specified clamping power of the original bolt.

A little quick math achieves this factoid: it takes about 89 N·m to get a 10mm 10.9 bolt to proof load, which won't hurt the bolt at all, and about 107 N·m to get that bolt to deform. So, if the 90° after 56 N·m is in the range of about 100 N·m the bolt won't deform and you can reuse it as many times as you'd like; until you hit the limit of elasticity steel acts like a spring and you can basically use it countless times (think of coil spring on a car how many millions of compressions; yes they can fail, but wow it's not likely).

So here is another interesting factoid: switch to fine pitch thread and you can bump the clamping force from 7500 to 8400# per bolt. That would likely be easier than drilling out a hole to 11mm (and finding 10.9 M11 bolts!).

So my summary is that if it can be determined that 56 N·m + 90° ends up at less than 100 N·m of force you did not deform the bolt and it can be reused without any concern.

I am almost certain that the design of the system is that it uses the bolts beyond the advised torque and clamp force on purpose to achieve two significant results: 1; that it will keep the plate from slipping sideways so it can do its job and 2; that it won't wiggle loose; the added forces are simply acting like a lock washer. Similar to the concept of head bolts; using a pre-tension and angle gives you a very precise force based on the elasticity of the steel and simple math (how much longer did you stretch the bolt).

I scoured the 'net to see if anybody ever did any actual tests or experiments on this stiffening / reinforcement plate and found hundreds of posts posing the question and not a single example of finding an answer. It won't be that difficult to figure out a definitive answer, which will certainly give 'seat of pants' guessers some satisfaction that they guessed correctly.
I'm not guessing. I have said many times that I don't know the answer if they should be reused or not. I don't know why BMW states they shouldn't be reused or what happens if they are. All I have been doing is challenging conclusions extrapolated from wrong assumptions or estimates, statistics that are not valid samplings, etc.
I'm planning to remove my underbelly plate and wife's as well in the next couple weeks to do some hard-core engine cleaning and finding some wayward leaks, so i will do a couple tests when i do so to determine the answer people have been pondering for years yet nobody took the HOUR it will take to answer them.
Based on assumptions, roughly, about , almost certain and estimates. Further, if you don't know the answer to the questions of why does BMW states to no reuse the bolts and what happens if you do it's a waste of time. As an example---are the bolts supposed to stretch? If so, how far, to plastic deformation? What clamping power are they trying to achieve? Finally, if you aren't using industry accepted methods and equipment for testing there is no way to be sure your results are accurate. As I have said many times, I'm not trying to convince anyone of anything. I am challenging the methodology thus far presented to conclude it is fine to reuse the bolts.
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Last edited by bcredliner; 04-03-2019 at 08:10 PM.
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  #145  
Old 04-03-2019, 08:33 PM
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Quote:
Originally Posted by bcredliner View Post
Why does BMW state not to reuse the bolts and what can happen if they are reused?
You can ask BMW.

Personally, I think BMW has its strength and weakness.

- Strength is in esthetic "curb appeal", driveability, the "name BMW" etc.

- Weakness is: reliability and repair procedure is overly cumbersome.
Technical manuals were written with liability in mind. This bolt discussion is somewhat dumb simply b/c the crazy (paranoid) engineers wrote that sentence of "replacing the bolts" and not-too-smart people follow it like the Bible. Remember "it takes 2 to tango" and I am not part of this tango.
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  #146  
Old 04-03-2019, 08:51 PM
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Stiffening plate removal / reinstallation

Because they set themselves up for liability. By err on the side of caution they can't be responsible for anybody using anything outside their specs.

The simple math shows that the corners are held with about 20,000# force in the back and 10,000# in the front. It wouldn't surprise to discover that is just enough or to find out it's 4x as much as needed.

BMW guys must see the threads asking and choose not to answer with authority on the matter. It feels like only the guy that designed it knows why and chose not to divulge why so now the only reason anybody does it is "because somebody said so". Since childhood that is not an acceptable answer for thinking people so it drives thinking people crazy when the reason is not given.

I've seen nothing of authority on the subject. (only the generic torque spec sheet with what but not why).

I have driven the X5 without the plate and can absolutely feel the difference in the front end so It definitely does it's job. I can also say that with much smaller bolts it firmed right up so for the majority of the function it does not need nearly the spec clamping force. (I will measure the current torque/clamp force on my plate now to get a sense of what might actually be required).

I will not be too surprised to see that my plate has signs of sideways slip but it won't surprise me if there isn't either. There is a good amount of surface area clamped together so there is likely some force multiplication e.g. with 7500# clamp force it might take 12000# to move it sideways.

When you hit a big bump on one wheel it could easily generate forces on that scale and I'm sure that went into the why In the original design.

Unless someone beats me to it I will be getting some very useful data that can be used to decide which route to take.

The "easy way out" is to assume BMW is infallible and follow them blindly. That solution will have you paying $920 for replacing the four door lock motors (parts only) vs $24 for the actual motors.

If there was even one single paragraph with authority explaining the why, it could end the debate.

There are people that recommend leaving the plate off. Terrible idea, the other extreme is to always replace the bolts. The most logical solution I guarantee is in the middle. And I diagree that it's all about cost: if the bolts were an appropriate $2 each I would still want to replace only if it's logical to do so: we don't replace our bolts each use but nobody worth their weight in cheese will reuse a head bolt that is just moronic.

I contend that it's not much smarter to replace a bolt "just Because" if it works out that the bolt has no wear from normal use. It's much smarter to get some informed data and proceed on a logical path.

If the 56+90° ends up taking 120 N·m and is well into plastic deformation I would not reuse that bolt. I would probably get a new bolt and pay attention while installing and stay in the non deformation zone as long as I can back it up with some math.

Eg if the spec is 7500# clamp force but you can achieve 7000# without distortion you can believe that is what I would do (or switch to fine pitch bolt etc).

This story will continue. Withing a couple weeks I will be doing some measurements that will give us some actual values not striaght up guess work.
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Last edited by andrewwynn; 04-03-2019 at 09:06 PM.
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  #147  
Old 04-03-2019, 09:36 PM
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I will take a new bolt and TTA while measuring the torque applied to get a decent estimate of clamp force. From that I can reverse engineer what straight torque can get a similar clamp force without going into plastic deformation.

If not possible with the stock bolts I will be swapping out the bolts to a size that will achieve the factory design of clamp force without damaging the bolt.

One other thing to consider: the back corners have a PAIR of bolts to hold the sway bar. I would be comfortable with reengineering to have the back bolts backed down to normal torque since that will net 15,000# clamp force and double the surface area on the plate.

This may have just gotten stupid simple. Class 12.9 bolts have a higher yield strength than the tensile strength of the 10.9 bolts. You can buy FIFTY of them for less than the SIX if you buy an official BMW bolt.

I'll be doing some measurements and if simply upgrading to class 12.9 will achieve the same clamp force without going to a bigger size you can bet a dozen donuts that I will have a dozen of them in short order.

The only conceivable risk is they could be brittle enough to break vs stretch more but two things about that : whatever maneuver could cause that, a corner of my restriction plate coming loose will be the least of my worries and it will be nowhere near the clamping force maximum I'm betting.

There are also grade 14.9 which are far above and not expensive "each" but the smallest order I found in a quick search is for 1000 quantity.

So: I'm quickly leaning toward get a beefier bolt so I cab achieve the same clamp force of the design spec but not cause any wear on the bolt. With any luck I'll discover that the TTA causes no plastic deformation and I'll be able to report a torque value that approximates the TTA without hitting deformation. I myself would have no problem torquing to 80 N·m vs 90 N·m if the former caused no damage and the latter meant single use on $60 of bolts.

To be continued.
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  #148  
Old 04-03-2019, 09:50 PM
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Originally Posted by bcredliner View Post

As an example---are the bolts supposed to stretch? If so, how far, to plastic deformation? What clamping power are they trying to achieve? Finally, if you aren't using industry accepted methods and equipment for testing there is no way to be sure your results are accurate. As I have said many times, I'm not trying to convince anyone of anything. I am challenging the methodology thus far presented to conclude it is fine to reuse the bolts.

Using basic tools (peak reading torque adapter) and micrometer it will not be difficult at all to determine several things:

Are the bolts stressed to plastic deformation mostly is the key. If they are not simple formulae exisit to determine the clamping force.

Taking a new bolt and tighten TTA per spec and measure the torque to get there. Remove the bolt and meausue it's length before and after to determine if it's been stretched. If not plug the torque value into the torque to force calculation and you'll get the clamping force within 10-20%.

You don't need anything particularly NASA grade to figure this one out it's not mission critical like a head bolt.

It's my contention that it's an over enginereed part and I plan to test that in a real world way and report the findings. If I'm wrong I'm completely fine with that I'll just use a bigger bolt to achieve the factory design without self-destructive bolts.

It's the blind faith following with no explanation that bothers me and surely tons and tons of others (based on the thousands of posts easily found on the internet on the topic).

If we knew for example that the plate moved a mm every pothole if you attempted to reuse the bolts that's information we can use to side with BMW. If we knew that you could use a 3/8" unhardened bolt for a year and have no sign of sideways slip, that's hard evidence to support single use on the bolts is a crock of crap.

The reality is somewhere in the middle I'm betting.
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  #149  
Old 04-03-2019, 10:04 PM
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Quote:
Originally Posted by bcredliner View Post
... If there is deformation there is no torquing procedure that will get to the original clamping power...
Completely false. Keep it simple here. Figure that one out before going any further. That is a very basic concept that is part of the first-day lecture on the stress-strain diagram. Really, it is. If I had to give a 5 minute lecture on stress-strain, I'd include that.

You can loosen the bolt and then tighten it to the exact same strain (torque angle, bolt stretch, however you want to measure it) and it will have the exact same clamping force. Theoretically of course. Nothing is really exact. But practically true as well.
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  #150  
Old 04-03-2019, 10:06 PM
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Originally Posted by cn90 View Post
You can ask BMW.

Personally, I think BMW has its strength and weakness.

- Strength is in esthetic "curb appeal", driveability, the "name BMW" etc.

- Weakness is: reliability and repair procedure is overly cumbersome.
Technical manuals were written with liability in mind. This bolt discussion is somewhat dumb simply b/c the crazy (paranoid) engineers wrote that sentence of "replacing the bolts" and not-too-smart people follow it like the Bible. Remember "it takes 2 to tango" and I am not part of this tango.
Same reason why BMW tells owners that the ATF is lifetime, but if you go on ZF's website they state that ATF should be changed every 40,000 miles, not unless it is revised by the car manufacturer. So who are you to believe, the company that sells the trans to the manufacturer or the manufacturer that buys the trans and install it in the car.
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