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How important is a Monte Carlo Bar and Export brace? Take a look and you decide.

A

Anonymous

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Thought I'd document the give in a 68 Mustang chassis as it's loaded with the engine and such. The engine is in the car but I have jacked it back up so the mark when the weight is off the chassis, the first mark was made before anything was installed. Also the chassis sprung back as the weight was removed to almost the original mark (1/16" less). With a little pushing and bouncing I was able to get it back to the original. I'm sure if this vehicle was driven on the road without these braces the flex would be even more. On the front to rear flex I noted very little movement (or give) between the firewall shock tower, it still shifted about a 1/16" though.
 
I am constantly telling people to use em, but its funny how many people put a 5.0 EFI motor in and then cannot get good bracing to fit so they just leave it out.

And dont get me started on swivel joints on an export brace....
 
Just from a purely geometric stand point, IMHO, the export brace isn't so much to control fore and aft movement as it is to add more lateral stiffness. Coupled with the Monte Carlo bar it actually triangulates the stress forces. A triangular shape is supposed to be the strongest geometric shape for structural elements (or so I've read). While you may have seen a small amount of movement in your measurements, keep in mind that while driving (especially in a competition environment), the loading would be immense. The suspension arms would act as levers, applying a significant greater amount of force through the shock towers (trying to bow them in), than what you applied. Also don't forget, it only takes fractions of an inch in movement with suspension settings to throw them out of alignment. That alignment change would translate into less than optimum settings for handling, and conversely, a slower car at the track and increased tire wear. Just my two cents! :pbj
 
"67 evil eleanor" said:
With a little pushing and bouncing I was able to get it back to the original. I'm sure if this vehicle was driven on the road without these braces the flex would be even more.
Click to expand...

Yep, you answered your own question. . .

Robert
 
I've often wondered if there is a big difference between a curved monte carlo bar and a straight monte carlo bar? The curved bar is surely substantially less effective, right? Someone spell it out to me before I have to buy another bar since it appears NPD sent me one for a 67-68 years ago. Doh.
 
"Coupe" said:
And dont get me started on swivel joints on an export brace....
Click to expand...

I am not an engineer but would like to explore this theory and see if I am missing something.

Does not the export brace keep the towers from bowing out (spreading) or falling in (collapsing)? If so, why would it make a difference if there is a heim or not, since the heim will not allow the bar to compress or expand? If your theory is that the heims will allow twist/vertical motion, have you checked how much the one-pieces can flex if you have them out of the car? Granted, it's not as flexible as a heim but it will still flex.

So, if the basic question is whether it prevents contracting or expanding of the tower, would it not be irrelevant if the bar has joints or not?

Or, will the heims allow the towers to contract/expand since that appears to be a swivel point? If they contract, it would have to force the tower forward at the same time, expanding would have to force the towers too?

It seems using a tied-in Monte Carlo bar would eliminate the collapse/expansion but not the potential for some vertical/twist effect?
TWRB-01_AT.jpg
 
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"SAC69" said:
I am not an engineer but would like to explore this theory and see if I am missing something.

Does not the export brace keep the towers from bowing out (spreading) or falling in (collapsing)? If so, why would it make a difference if there is a heim or not, since the heim will not allow the bar to compress or expand? If your theory is that the heims will allow twist/vertical motion, have you checked how much the one-pieces can flex if you have them out of the car? Granted, it's not as flexible as a heim but it will still flex.

So, if the basic question is whether it prevents contracting or expanding of the tower, would it not be irrelevant if the bar has joints or not?

Or, will the heims allow the towers to contract/expand since that appears to be a swivel point? If they contract, it would have to force the tower forward at the same time, expanding would have to force the towers too?

It seems using a tied-in Monte Carlo bar would eliminate the collapse/expansion but not the potential for some vertical/twist effect?
TWRB-01_AT.jpg
Click to expand...

Have to agree 100%. I just wasn't gonna kick that dog! :hide
 
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I wouldn't mind checking the movement with heim joints, if I had them. I would guess if both a Monte Carlo bar and Export brace were used it would essentially form a equilateral triangle which is very strong. If I get a little extra time I may make a couple braces using some small angle and check the movement again. Something else I found very interesting was when a seat to trunk firewall was added (welded in) it reduced the torsion rigidity about 5 times better than sub frame connectors.
 
"SAC69" said:
It seems using a tied-in Monte Carlo bar would eliminate the collapse/expansion but not the potential for some vertical/twist effect?
Click to expand...

Heims would allow more vertical flex. A Monte Carlo bar and Export Brace is stiffer, but the weak link there is that they are 2 separate pieces. The area between the top of the shock tower and where the Monte Carlo bar mounts should be reinforced or tied together - only then it is truly superior to the heim setup you pictured, IMO, unless you're a Bling addict. . .

Robert
 
I'll add my limited knowledge since I'm a structural engineer, though not in automotive aspects. I think in the end, it is a moot point with the heim vs rigid-end export brace assuming a monte carlo bar is installed with the heim jointed export brace.

With the suspension forces on the shock tower, I visualize three force directions involved. The first force tries to tip the towers in due to the angle of the coils. The second force are forces towards the firewall at the top of the shock tower. This is the resultant of the vertical forces that try to tip the shock tower towards the firewall. The vertical forces are exactly why our frame rail depth gets larger as you get toward the firewall, to resist the moment. If you were to assume the forces to tip the shock tower toward the motor were the same as the force to tip it back toward the firewall, you'd have an resultant force vector in between these two forces (roughly a 45deg angle) which is precisely the direction of the export brace. However, while I've not done any actual analysis or physical tests, I would venture to guess the forces are not equal and would therefore have some movement of these heim joints under normal loading. The third force is a component of the second force, in that it is torsion on the front clip due to a vertical force from one side's suspension....in which it tries to move the shock tower upwards or downwards.

Now lets take the one-piece export brace by itself. It has the rigidity at the firewall and at the tower (fixed-ends, if you will) to handle some of the shock tower tipping in forces, as well as the tipping back toward the firewall. It will also resist a smal amount of the forces from the shock tower trying to move upward or downward. If you replace it only with a heim jointed export brace (no monte bar), you now can only handle only pure tension/compression forces in the struts, and it loses all ability to resist the forces of the tower tipping in. As the towers tip in, the spherical ends of the heims allow the struts to move with the towers.

If anyone ever considers a heim jointed export brace, it MUST be installed with a monte carlo bar otherwise you just bought an expensive paperweight. Adding a monte carlo bar, whether a stock one or heim jointed one, resists/takes all of the forces from the shock tower tipping in. The downfall of the monte bars that mount to the front inner fenders is that its mounted to the thin sheet metal. Ideally you want it tied to the top of the shock tower, similar to what the Meier piece does. This eliminates the flexing of the sheet metal in the front inner fenders.

Finally, the heim jointed export brace/monte carlo bars do lose the ability to resist up/down movement due to torsion. One example would be hitting a speed bump with only one of the front wheels. It will introduce torsion on the entire front clip, and try to deflect the front frame rail upwards (tipping the tower to the rear). However, based on what I've seen of the aftermarket (not stock-style one-piec) non-heim setups, their brackets at the shock tower and firewall aren' the beefiest, so the ability of those to resist up/down shock tower movement may be minimal. The stock-style one piece export brace's height isn't very tall (depth of ridges), therefore it also would likely deflect some depending on the up/down shock tower forces. It also has a single bolt connection at the thin firewall lip, so the firewall lip would likely bend way before the stock export brace would due to the up/down forces. Due to the thin depth and single bolt connection of the one-piece export brace, I would consider the heim and one-piece setup to be similar in resisting vertical forces (up/down movement). You would also need the rear inner fender to flex before you get this vertical movement to occur, FYI.

I personally like how the Street or Track's export brace mounts below the lip and has bolts going through the firewall as well, adding rigidity to this connection.

export-brace3.jpg



If I were to do something home made, I would utilize Meier's overall rigid-end configuration (with thicker tower plates) but have the firewall connection similar to Street or Tracks. I'm already using a beefier engine cross member, so this would fully box the engine compartment.

FWIW, KarKraft realized the weakness in the frame rails and shock towers with the heavier motors in a racing situation, so they added bracing the the rear inner fenders. A similar concept is the tube typically installed on track cars from the top of firewall/cowl and bending down towards the frame rail on the wheel-side of the inner fenders and shock tower.

Well that's my long winded armchair analysis. Pick it apart as you see fit :beer Cliff notes version is the heim joint setups only weakness is vertical movement, but is no worse than the one-piece export brace or the aftermarket non-heim setups due to their weak firewall connections.
 
Not a problem! May not be 100% correct, just just my quick but drawn out thoughts :thu

One other thing I left out, is that the front suspension also has anti-dive and caster, which angles/tilts the vertical forces from the suspension towards the rear. This adds more force to the top of shock towers trying to tilt back towards the firewall. While the force may be vertical, it adds a horizontal vector force. The more caster you give the car the greater this force (although we are talking very minor increases)
 
I can also see where hard braking will try to "fold" the towers over. I can see where a heim joint support would be useless in that situation.
 
Its still about "Twist". A good export brace will assist somewhat in preventing one shock tower from trying to raise or lower itself to a different height of the other one. It would be interesting to compare these stresses somehow.

If the car is going in a straight line and hits a bump squarely then the heims are fine, but a pothole or off camber curve could make the front of the car twist, and in that event the pivots can do nothing.

Imagine two guys holding the bracket with one on the shock end and one on the cowl end and them trying to twist it, they would see a huge amount of twist on the unit with the pivots without even exerting themselves, sure the demonstration does not account for the stresses found under the hood and it could be a laughable experiment, but those pivots do zero to stop the demonstrators from twisting it. A GOOD export brace does provide more strength on this plane.
 
One option would be to use solid heims (without the ball). You could sandwich the solid heim at the firewall like what Street or Track does and make it a pretty tight fit to eliminate movement of the solid heim. At the shock tower though, you are still only as strong as the stiffness of the bolt since it would be bending in single shear configuration.
 
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