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E39 (1997 - 2003)
The BMW 5-Series (E39 chassis) was introduced in the United States as a 1997 model year car and lasted until the 2004 when the E60 chassis was released. The United States saw several variations including the 525i, 528i, 530i and 540i. -- View the E39 Wiki

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  #226  
Old 09-16-2009, 11:20 AM
TheStig TheStig is offline
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25 mph is pretty fast to go over a speed bump....IMO
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  #227  
Old 09-16-2009, 11:48 AM
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Quote:
Originally Posted by bluebee View Post
You know, I realize belatedly that I had read that the wheel was up in the air, and immediately discounted it. I've NEVER had a wheel in the air in my life. Not even on a bicycle. I'm going so sloooooowly over those speed bumps (maybe 20 or 25 mph) that I just don't inherently feel my wheel is "in the air".

Does a heavy wheel & tire assembly REALLY get airborne on a 25mph speed bump when the shocks don't dampen the sprints as much as they used to?

Really? I've been "airborne" for the first time in my life? That's the part I find so very difficult. Is it true?
Well, it's not mid air like you see in the cheesy 80's TV cop car chase in San Fransisco.... haha

The wheel bumps over the road bump, and jumps in (up). I am not saying the whole car becomes airborne, or that the wheel jumps like 1 foot up. Nothing crazy like that. Just that the wheel, at that point and at that moment, will lose (traction) - no, sorry, I meant to say "contact" - with the road. ... And when that happens, that wheel will stop turning. It will stop turning because the car is not pushing it forward on the road and your brakes are engaged.

So at that moment, you ABS will recognize that not all of the wheels are turning, and will start pulsating, thinking - wrongly - that you are sliding on a slippery road.

mw
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Last edited by MatWiz; 09-16-2009 at 01:06 PM. Reason: MEANT TO SAY "CONTACT" NOT "TRACTION"!
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  #228  
Old 09-16-2009, 03:39 PM
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Originally Posted by TheStig View Post
25 mph is pretty fast for a speed bump
It always bothers me when people practically stop for speed bumps. Isn't that why they put a suspension on the vehicle in the first place? I barely slow down for 'em. Always have. Never before had this violent shudder.

Quote:
Originally Posted by MatWiz View Post
Well, it's not mid air like you see in the cheesy 80's TV cop car chase in San Fransisco.
OK. Lemme take it step by step again ... (please doublecheck my math):

- BMW E39 approaches speed bump ...
- As the BMW "hits" the speed bump, the coil spring compresses (dampened by the shocks) ...
- On the ramp of the speed bump, the suspension is still compressed and the wheel travels some angle upward as it climbs the front ramp of the speed bump ...
- However, the speed bump is shaped such that there is a drop off on the back ramp ...
- So at the top of the speed bump, the wheel is still traveling on an upward force vector (essentially "lightening" the load on it) and the suspension is still compressed with potential energy ...
- KEY POINT (1): Just past the top of the speed bump, there is now barely any tire rubber touching the road (as there's that drop off on the other side of the speed bump yet the wheel force vector doesn't know that yet ... and ... worse yet, the coiled-spring suspension is now beginning to unload as the wheel sees nothing underneath it (sorta' kind') as it gets to the other side of the speed bump ...
- KEY POINT (2): Since one or more front shocks are "bad", the less-dampened suspension on one or both front wheels unloads the potential energy coiled up in the spring for that particular shock more than the BMW designers intended it to ...
- In effect, this additional springiness perhaps "pushes" one or both front wheels even further off the road contact surface than the engineers imagined ...

- At that point, the front wheel or wheels is, effectively, pushed up into the air by the three forces:
a) the force vector is upward at the speed-bump angle ...
b) there's only a thin stripe of road surface (traction) under the tire ...
c) the less-dampened springs are unwinding too rapidly just at the worst moment ...


- At this point, you purposely brake rather firmly ... (You see, they're right: it's all your fault!)

- Since there's very little to keep the front wheels moving, the front wheels slow down a LOT during this braking (relative to the rear wheels) ...

- If only one shock was bad, then that less-dampened front wheel is more unloaded than the other front wheel and now the ABS is REALLY confused since the two rear wheels are going at one rpm, the good front side is spinning at another rpm, while the bad front side (the one with the bad shock so the coil spring is unloading it even more) is spinning at yet a third (slower) rpm.

- This differential spinning makes the ABS dizzy and it reacts violently to the situation by shivering violently. Its equilibrium system had fully expected two of the three forces, but, the third force, i.e., that of the unduly undampened coil spring "pushing" one or both front wheels up in the air was unexpected, in effect causing the BMW ABS to puke violently.

- The solution is to give back to the ABS the three forces it expected (by removing the additional force of the undampened coil spring on one or more front wheels).

Is this the scenario?

I know you don't think it's that something called "bushings" ... but ... just to edify me ... (and others) ... can you show us on this RealOEM diagram for my 2002 525i where that "bushing" that some people think is also part of the problem lies?


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  #229  
Old 09-16-2009, 03:58 PM
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OK... The Bushing is not there, it is part of the control arm. I posted the parts diagram in the next post down.

Quote:
OK. Lemme take it step by step again ... (please double check my math):
/..../
Is this the scenario?
Yes pretty much with just a few points.
1. The wheel can definitely go up and separate itself from the road. No contact whatsoever to the surface. Maybe if somebody else can drive your car over these speed-bumps and you can look from the side of the road at exactly what happens, you will get a huge "aha!". Hey, do you own a slow motion video camera??? haha! Somebody should do it!

2. I would not say that the ABS system reacts 'violently'. But rather that the suspension system reacts violently to the [expected] ABS system behavior. It is the combination of the two that creates the problem. I don't see how the ABS system will pulsate the brakes in any different manner than it pulsates it over slippery road. In other words, (forget a design problem for a moment), there is nothing that you can fix in the ABS system. You need to fix the "broken" suspension.

Question: Let's say that as an hypothetical experiment, we will go and drill holes in the cylinders of the shocks, basically removing all the gas from them, and allowing them to move freely up and down. We will then go and drive over the bumps and hit the brakes in an attempt to re-create the problem.

1. How would you think a car will behave now? Answer this question.

2. How differently will the car behave if we now install brand new shocks? Answer this question.

That is what I mean by saying that the shocks are completely gone, and you are riding on your springs only.

The wheel will bounce too much. The range of motion up and down will be much more than it suppose to be. With nothing to absorber the shock of hitting a bump, the wheel will go shooting up and then shooting down at a speed much faster than if a new shock will be in existence.

Now add the natural harmonic oscillation frequency rate of the system, that just to our bad luck, is in the same harmony as the ABS pulsation frequency, and the bouncing and the oscillation and pulsation will feed upon itself and will get amplified. There is noting to dampen it, so it will just continue. And NOW you have the problem.

You want to solve it? Make the system less bouncy. Put a shock absorber in it. It will slow the bouncing rate and will also reduce the velocity of the wheel shooting up, and will reduce the range of motion.

mw
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  #230  
Old 09-16-2009, 04:05 PM
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The bushing is part #5.


mw
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  #231  
Old 09-16-2009, 06:44 PM
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Ramming over speed bumps is OK, depending on the size and shape. If it's a steep one it stresses your entire suspension system and will knock out the alignment. I slow down unless I know the bump isn't something to be concerned about. Many bumps are a concern….

It amazes me how someone who is so incredibly precise about getting the repairs done 110% right can ignore their filthy car (which is causing the paint to wear down prematurely, causing rust to form prematurely, causing the car to be junked prematurely with amazingly installed brakes and well-drained A/C) and bash it into bumps.
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  #232  
Old 09-16-2009, 07:22 PM
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[QUOTE=bluebee;4522809]
Does a heavy wheel & tire assembly REALLY get airborne on a 25mph speed bump when the shocks don't dampen the springy motion as much as they used to?


You bet it can.
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  #233  
Old 09-16-2009, 07:22 PM
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Quote:
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It amazes me how someone who is so incredibly precise ...
Naaaaahhh ... it's not all that amazing. Everyone else simply washes their bimmer just before posting their pictures! Or they photoshop in that lemon-smelling shine so you _think_ their E39 is clean. Or maybe it's the angle (I hate it when the underside of my rocker panels show)...

Quote:
Originally Posted by MatWiz
The bushing is part #5.
uh., oh, ok. I'm not sure if that helped or not ... I will need to crawl under the bimmer and take a look for that "bushing" to see it with my own eyes ... (Luckily it's not the culprit).

Speaking of culprits ... a friend whom I forwarded this thread to for an engineering explanation explained that I got it backward in the description above.

The BAD SHOCK WHEEL actually hits the ground at something like 200 G's (assuming roughly 20 mph and no shock). Here's what he had to say ...

"I have a book on it if you're interested, "Vehicle Dynamics", written by an Italian so the Germans won't read it.

Basically F = M A.

The front left of your E39 weighs about 840 lbs. The tire and rim etc.
weighs about 50 lbs, so the spring accelerates the rim/tire down at about 160G (holy cow!) in the absence of shock absorber action.

If you want to drop off the back of a speed bump, let's say 3 inches, and you're going 25MPH which is about 40 ft/sec then you have to drop 3 inches in about 50msec.

Solve .5 A t squared = .25 and you have A = .5 / (.05**2) = 200, which means that the tire will hop! Of course, I'm ignoring the flex of the tire (which is WHY you have a tire).

So it's plausible that the non-shock tire will stay in contact with the speed bump, but the one with the working shock absorber will definitely hop over the bump."


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  #234  
Old 09-16-2009, 07:45 PM
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Just remember that in almost all cases, both struts will be worn out to the same degree.

I am saying that because you are referring a few times to only one shot strut.

mw
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  #235  
Old 09-17-2009, 02:44 PM
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The tire has to accelerate from zero or even from a speed in the opposite direction, a negative velocity, that can affect your numbers.
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  #236  
Old 09-17-2009, 07:51 PM
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Ok, got the car back from the shop, they replaced front struts said they were gone completely. But here's the strange thing and I would really appreciate your input on this - my car is a my2003 530i w/ SP and the invoice lists part # 31311096862 as the replacement, did they put standard non sport part in by mistake? I went on to realoem and it lists two options - m sports suspension and m sports suspension II. Neither part number matches what they installed. How do I know which suspension I have m sport or m sport II?

Here's the diagram

thanks!
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  #237  
Old 09-17-2009, 08:12 PM
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Oh my. It appears you haven't been doing your homework. All my whore posts on Bilstein sport struts and shocks lately and you let the shop install oem replacements? You prolly got Sach's crap parts. Yikes.
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  #238  
Old 09-17-2009, 08:15 PM
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Originally Posted by wfn View Post
Ok, got the car back from the shop, they replaced front struts said they were gone completely. But here's the strange thing and I would really appreciate your input on this - my car is a my 2003 530i w/ SP and the invoice lists part # 31311096862 as the replacement, did they put standard non sport part in by mistake? I went on to realoem and it lists two options - m sports suspension and m sports suspension II. Neither part number matches what they installed. How do I know which suspension I have m sport or m sport II?

Here's the diagram

thanks!
After 5 years of talking to the walls, a damn proof I was 100% right. That this symptom is a 100% indication that the shocks are completely gone.

wfn, I can check your VIN and see if I can get the information about your suspension option, if you want to send me your complete vin number.

Is it possible that they installed the right part and then just put the regular number on the invoice? Who was the shop? A dealer or an Indy? If it's an Indy, run back there tomorrow, and ask for your old struts, and as to put them in the box of the new struts for convenience, and see the part number on the box... A little sneaky, but I am afraid that otherwise they might try to hide their mistake?

Anybody knows if there is any indication of a number printed on the strut? And where? If so, maybe you could get under your car and see with you own eyes.

mw
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  #239  
Old 09-17-2009, 08:19 PM
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Mack,

The part the put in is 31311096862 sachs part according to the invoice. Realoem lists other part numbers 31311096857 for m-sport and 31311096857 m-sport II. I have no idea which one I need but I was under the impression the shop is supposed to know this. What do you recommend?
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  #240  
Old 09-17-2009, 08:31 PM
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MatWiz,
Yep, they were completely befuddled said the shock has all leaked out on each side. It's an indy but they do a lot of higher end car repairs and I haven't had any issues with them before. I'm going to check and see if can see any visible markings on the ones they put in.

Edit: tried to get under the car and see if i can get a p/n off the strut assembly somewhere.. no luck, the shocks are just black unbranded.

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  #241  
Old 09-17-2009, 08:45 PM
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MatWiz,

Thank you for putting this together in a single post, I'm dropping the car off for service tomorrow first thing and in addition to what's been done a year ago I'll have them check out and replace the rest of original suspension parts. I will ask to replace the shocks regardless of whether they think they're bad or not. My wife drives the car now primarily so I'm not taking any chances. /....../ ...
wfn, aren't you glad you told them? You have potentially saved your wife from a car accident that could have gone very bad for her. I am so glad you've done it!

Mack, Bilstein or OEM, there are pros and cons to each brand of suspension. You have the Bilstein - you prefer the performance over comfort. I have the Dinan - I sacrifice comfort and get lots of performance. Sacks gives a balance of comfort and performance that BMW got praised for by most car experts. I am sure his wife is VERY happy with the car now... ...To each is own... I have learned that there is no right or wrong, as long as you know what you are intending to get, and make the right choice accordingly.

mw
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Old 09-17-2009, 09:00 PM
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MatWiz,
You bet and it's because of your posts I insisted they'd swap them! And now I don't have the problem, I drove for awhile over speedbumps I used to reproduce the problem before and the issue is now gone for sure. I probably took off a good chunk of my brakes between this and other times I wanted to diagnose this. Now all I need is to find out what the hell the shop put in there, I'm not doing aftermarket I want OEM.
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  #243  
Old 09-17-2009, 09:25 PM
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MatWiz,
You bet and it's because of your posts I insisted they'd swap them! And now I don't have the problem, I drove for awhile over speedbumps I used to reproduce the problem before and the issue is now gone for sure. I probably took off a good chunk of my brakes between this and other times I wanted to diagnose this. Now all I need is to find out what the hell the shop put in there, I'm not doing aftermarket I want OEM.
I checked that part number here. You are not going to like it. http://bmwfans.info/parts/catalog/31311096862/

Click on the little "+" bottom right side of the screen to open the menu, and it tells you that this part number is for a "front spring strut" for the 530 Turing model.

I will check your vin.

mw
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Old 09-17-2009, 09:30 PM
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Originally Posted by MatWiz View Post
wfn, aren't you glad you told them? You have potentially saved your wife from a car accident that could have gone very bad for her. I am so glad you've done it!

Mack, Bilstein or OEM, there are pros and cons to each brand of suspension. You have the Bilstein - you prefer the performance over comfort. I have the Dinan - I sacrifice comfort and get lots of performance. Sacks gives a balance of comfort and performance that BMW got praised for by most car experts. I am sure his wife is VERY happy with the car now... ...To each is own... I have learned that there is no right or wrong, as long as you know what you are intending to get, and make the right choice accordingly.

mw
Don't forget Bilstein also makes an HD model which are going to give a more comfortable ride. The main concern I would have with Sachs however is their short lifespan.
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  #245  
Old 09-17-2009, 09:42 PM
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Quote:
Originally Posted by MatWiz View Post
I checked that part number here. You are not going to like it. http://bmwfans.info/parts/catalog/31311096862/

Click on the little "+" bottom right side of the screen to open the menu, and it tells you that this part number is for a "front spring strut" for the 530 Turing model.

I will check your vin.

mw
Yes, I saw this before. In US they only use strut 31311096862 for 525iT and 528iT according to realoem.. I think i have "S704A M SPORT SUSPENSION" on my car and therefore need the 31311096857 strut. If you could check that out for me I would greatly appreciate it!
Thanks for everything.
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Old 09-18-2009, 05:27 AM
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If you decide to go Bilstein go for the HD model, the Sport model SP is much stiffer. I've had both on my car, a claim few can make.

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  #247  
Old 09-18-2009, 09:24 AM
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I am confused (and probably a bit naive when it comes to handling):

FACT:
a) When the ABS is working, I have the classic "violent low-speed brake shudder" ...
b) When the ABS is not working, the violent shudder does not occur

PROBABLE FACT:
c) The violently active shudder is caused by the COMBINATION of one or more 'bad shocks' and the ABS working normally
d) The 'bad shocks' may not be discernible from the classic "123 bounce-the-corner" shock test

CONFUSION:
e) If I have "bad shocks" ... why don't I feel that in normal driving?
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  #248  
Old 09-18-2009, 09:33 AM
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I have no idea how you cannot feel the bad shocks in normal driving. The ride has been terrible and nobody could tell me why, the front was vague and bouncy over bumps. There was longer suspension recovery going into turns and the car no longer felt solid at high speed. You know how secure you feel when it just sticks to the road and feels tight - that feeling was no longer there. There is no doubt in my mind that this is the problem, like I sad before I drove the same exact circuit that I used to reproduce the problem every time and not once I got the shudder. The old shocks that were taken out have no more dampenining than a scotch brite pad, you can easily compress them by hand.
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Old 09-19-2009, 05:29 AM
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I have no idea how you cannot feel the bad shocks in normal driving
I probably don't drive like you guys do ... and I probably don't even know what it "should" feel like ...

Having said that, we should probably reference people to a shock-and-suspension DIY when they (like me) come to the realization that it isn't the ABS but the SHOCKS that probably need to be replaced when we have this violent speed bump shudder.

I'll research shocks but any pointers for the newbies (including me) is in order.

The things that need to be pointed to are the following:
1. How to diagnose bad shocks in your E39 (normal methods might not work!)
2. How to choose the best shock replacement for you (I tend to go with OE or OEM unless someone convinces me logically otherwise)
3. How to install shocks yourself (I don't know if this is something we can do as a DIY or not)

Any advice is always appreciated!
Bluebee

Last edited by bluebee; 09-19-2009 at 11:42 PM.
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  #250  
Old 11-10-2009, 10:12 PM
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Mein Auto: 525iT
When I go over a speed bump, pot hole, I feel like the front tire is gonna shake itself off. But it does stop shaking after I release the brake pedal.

You guys all think it's struts? I have no clue. Sure doesn't seem like ABS issue.
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