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Discussion Starter #1
I'll say up front that I don't know a lot about car design and modifications - however I am an engineer and feel like I have a valid question/comment about strut braces.

Looking at pictures of the front brace on one of the after-market guys it occurred to me that there was no benefit to bolting one of those things on. The question I have is, what possible direction is it strengthening the car ??

The only thing I am sure of is that it keeps the tops of the struts the same distance from each other, but I doubt if that's the problem it's advertised to fix.

Any architect will tell you that the triangle shape is what you use for strength, but the strut brace completes a square-ish shape which still allows all kinds of deformation.

I'm very skeptical of the benefits and it clearly adds weight. Now if you could bolt an "X" shaped piece (four triangles) you might be getting somewhere. As for me, when my car gets here ('01 - December build), I'm keeping it as-is.


I'd appreciate any comments from the group.
 

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Depends on the car.

If the front strut towers are beefy and directly attached to the firewall, a strut tower brace doesn't do much.

If they're dangling out in the front sheetmetal somewhere, it should help somewhat.

Your point about design is well taken, though: a simple bar that extends across the strut towers isn't nearly so much help as one that triangulates into the cowl area.
 

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Discussion Starter #3
Gem,

Thanks for the reply. I guess there are too many variables to make a good decision... did BMW put the right amount of support in the frame for the towers, did they account for flex under load when they designed the suspension geometry and how hard are you intending to drive the car.

Oh well, I can't wait to have a "test bed" of my own to play around with.
 

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call me naive, but I cant imagine BMW making this kind of design "mistake"; I strongly suspect the strut bars are there for looks/ pride of ownership, and if thats the case, thats fine- its all fun.

also, consider that they try hard to get the car to 50/50 weight distribution, but many people say the aftermarket exhausts are lighter- normally lighter is better but, the weight distribution on the m5 is alreadt 52/48, now you reduce the rear weight a little more....

ME
 

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While admiring the stuff under my M5 I noticed a big X brace. Is that standard for all 5 series or is that a M5 thing. I recall the 5 series has an outsanding chassis resonance figure

BTW there is a lot of cool stuff under there; rear dif cooling fins, big beefy suspention parts...

Terry
 

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A geeky thought in progress here


Instead of putting on the real strut bars and get subjective performance, is there a way to put on something more like a "shock absorber" where the springs have very very small k (not hard to press at all) and mark how much displacement difference has been made between two points that the brace is supposed to stiffen?

That way we'll know, objectively, the effect of the strut braces in stiffening the chassis.
 

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Discussion Starter #7
OK - I'VE GOT IT !!!


Somebody (I don't have my car yet or I'd do it) stretch a piece of string or tape across the two posts and go driving like a maniac.

If there is significant flexion/relaxation of the frame, the tape/string should break.

Heh,heh...
 

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kuzibimun - BRILLIANT! This would answer all our questions once and for all. OK - here's an el-cheapo design for someone else to build. A rod - anything - threaded rod or fishing pole or whatever affixed to one strut tower. Another rod affixed to the other, welded or otherwise affixed to a tube that can slide over the first rod.

Place O-rings on the rod on both sides of the tube, and slide them up against the tube.

Now drive and thrash in some corners. Stop and look at the O-rings. The distance of the O-ring from each side of the tube should be equal to the strut tower displacement on the opposite side.


Who volunteers?
 

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The fishing line trick works I have heard. The strut bar is more to keep the shock tower straight during hard cornering (track conditions). It prevents the top of the tower from pulling away from the other one and decreasing negative camber on outside tire. It is not really to stiffen the chassis.
 

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Well, gee - I would -- but I'll be traveling starting tomorrow for two weeks. I expect you gentlemen to have completed and documented your study by the time I return....
 

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The fishing line method would only work if the two towers break away from each other and would not measure much if they move closer to each other. The fishing line will also be subject to some extreme temperature right above the engine, which may change its length. Please correct me if I'm wrong as I am not very knowledgable about fishing lines.


I like Greg's suggestion, and that's kinda what I had in mind, but I'm not a well trained mechanical engineer like he is.


I'll definitely want to see the objective results, especially for a car like M5 under hard cornering. It would be nice if this prototype "stiffness measurement" is used on many other models to establish a base-line on car manufacturer's chassis/body stiffness quality.
 

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I kinda like old style tests. A local E39 540i club member was changing his tires at an autocross and insterted the jack in the factory hole at the back. The back tire and front tire almost came of the ground at the same time. That was a pretty good indicator of chassis stifness. My E34 has to go a lot higher in back to also get the front wheel off.

As for strut tower braces, if your towers deflect even a little you could lose up to .5 degrees negative camber which would give you a lot less grip. I would say they are more useful on older more stressed cars. My friend with a 635 actually had to put a jack under the tire to fit his bar (by pushing up the towers moved closer together) on as the towers had flexed away from each other over time.

I would also agree about the appearance thing. Dinan charges $500 for their bar and they are aluminum like everybody elses that are $150-$200 less.
 

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Although it may be true that you loose some negative camber due to chassis effects, the suspension settings (i.e. positive caster) will introduce additional negative camber while cornering... which would overcome the tendancy. Think about it.

Dan
 

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They may be true Richard but cars with camber plates and strut braces plow less--that is enough theoretical evidence for me. I am sick of understeer in turns 4 and 7 at Sears Point so I will eventually make those changes.

I am sure many guys on this board will also want to make some changes after taking their car on the track. Plow sucks.
 

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StupidGrin8UP said:
OK - I'VE GOT IT !!! Somebody (I don't have my car yet or I'd do it) stretch a piece of string or tape across the two posts and go driving like a maniac.

If there is significant flexion/relaxation of the frame, the tape/string should break.


Tape and or string probably has too much elasticity ... whereas an extra long guitar (B-String - 2nd thinnest string) string or maybe a double high "C" piano string from letsa say would probably SNAP under the pressure or movement.
 

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I don't have a lot of experience with the M5, but I do know that the strut tower braces that are just the straight bars that link that top of the struts do in fact help stiffen the structure some.

But for better performance, I know serious autox'rs use a brace that forms a triangle between the strut towers and the firewall... Never seen one of these for the M5 though, but then my experience with the Beast is pretty limited at the moment.
 

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I installed the Dinan front strut brace a few weeks ago. It was originally my intent to run a blind test with my wife and have her try to tell if the brace was on the car or not. I would have her make several runs through the local twisties (the ones that Gustav and I raced through when he was visiting) and then tell me if she thought the brace was installed or not.

However, when I did the installation, I made a quick run through the twisties myself, then slapped on the brace (about 20 minutes) and then went for another run. The difference was not subtle. The steering felt lighter during the hard, sweepers and the car felt more precise. I've spent enough time on the track to know it wasn't psychological, so I decided to bag the tests and keep the strut brace on. I didn't need any more convincing.

A strut brace from SHO Shop made a HUGE difference on my 1991 Taurus SHO many years back, even in ordinary driving. The strut brace on the E39 was only noticable while carving some pretty high speed turns. The effect was clearly not as dramatic. But it was there. The E39 chassis is much stiffer than the Taurus, but there's still some room for improvement.
 

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Loads

Dave,
Great info! I guess the front strut brace does make a difference.

Do you have any susp. mod's on your car that would accentuate the difference, or is the susp. stock?

I have a question about the loads that the strut brace sees... I guess I always thought that the strut brace would be in compression, but is that right? Is it in both tension AND compression at different times? Does it see any torsional loads?

:cheers:
 

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Re: Loads

M-Fünf said:
Do you have any susp. mod's on your car that would accentuate the difference, or is the susp. stock?
Stock suspension in my 2001 540i/6 except for the M5 front swaybar and the Dinan adjustable rear swaybar set to the middle holes.
 
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