strut tower brace info

[DocB]

lamont2506, could you please post up some pics of the paint wear.
Regardless of what others have to say, I would be interested in seeing this.
Also, what type of driving, street or track?

 

[Moochman4life]

Horseshit. I know tons of S197s on real rear coilovers without any issues. Please stop spreading false information. The rear shock towers are very very strong.

Actually, I was going to be nice and ask that guy to elaborate because I'm not buying it, either. Had it drafted up and decided to close it and go to bed, instead.

What makes a set of coilovers physically different-enough to make them need a rear brace? Please explain, Mountain.

 

[lamont2506]

The poor panel alignment from the factory finally clearanced itself?

Simplest explaination (Occam's razor) is that the trunk-lid panel stops were mis-adjusted, allowing the trunk to compress the weatherstripping enough to contact the rear fascia. OR that the lower support for the fascia is problematic, allowing the airflow under the car to balloon it up and out, thus contacting the trunk lid.

Face it, IF the brace does anything, it prevents the inner wheelwell from moving inward, which is the force axis applied by the inclined dampers. IF you were getting panel wear from that motion, I would expect to see it between the quarters and the trunk lid. NO motion that would be constrained by the brace would push the trunk lid down, or pull the bumper up.

I would take the car back to the dealer and let them figure out why their stock car is rubbing panels together.

Thanks for the theory. Makes sense to me except it only got worse under hard cornering. Again I am not trying to promote anything and I realize I most likely wasted the money on the brace.

Too late to take it in to the dealer. I already discovered firsthand how fast the stock brakes fade and slid the rear quarter panel along a guard rail.

 

[Philostang]

Actually, I was going to be nice and ask that guy to elaborate because I'm not buying it, either. Had it drafted up and decided to close it and go to bed, instead.

What makes a set of coilovers physically different-enough to make them need a rear brace? Please explain, Mountain.

I'm not advocating for the "you NEED a rear brace if you go to rear coil-over shocks" camp. But I think I get the concern, and since you asked, here goes my best at stating their case.

The stock spring pocket is heavily reinforced to take the bulk of the load from the weight of the car. The shock pocket is actually pretty stout, but its design intent was to take only the loads required to dampen the spring. Obviously, in a coil-over configuration, you're now seeing both loads imposed on (a) one area rather than two and (b) an area clearly designed for the lesser of the two loads in mind.

The consequence of that situation is typically cashed out in two forms of failure. First, the actual shock pocket will fail, causing the coil-over to "punch through" or otherwise exceed the integrity of the steel in that area. The second is that that towers will bow in or otherwise distort the general structural area of that part of the chassis (poss. causing chassis flex, rear quarter panel misalignment/distortion, etc.).

So much for the theoretical concern. In practice, we have anecdotal evidence that many of the genuine hard runners (FR500S and up) all run a brace and/or reinforcement of the shock pocket itself. So goes the line of "if they feel the need for it, you should feel the need for it too" reasoning.

My $0.02:

There's little-to-no chance of the coil-over "punching through" the pocket with the sort of adapter clevis Ford and many others use. These things capture a very large area of the pocket compared to the OEM shock's single bushing. You would have to literally rip the pocket out of the chassis, and it's welded into an even larger area of the rear structure.

As for bowing in the towers themselves, it's possible I suppose. We don't have any hard evidence of this. I've seen rippled quarter panels, but they were not even on coil-over equipped cars, so this concern exists with or without coil-overs (though it may be exacerbated with coil-overs...OR...it may be entirely a non-issue if the rippling was not caused at all by dynamic chassis loads in the first place). We simply don't know, or at the very least, nobody I know really knows.

For my own part, I'm going to coil-overs this year and I'm putting in a rear brace because (a) it's a "just in case" preventative measure, (b) I'm not freaked out about the weight penalty, (c) I weld so I can do it myself for a total cost of maybe $15 in steel (which I had lying around anyway), and finally (d) it looks cool and it's my car (so F.U. to anyone who feels otherwise).

As for the "genuine race cars run it" line-of-reasoning, there's too much going on for those guys to let that crude fact alone carry the day for us. First of all, they have to run a full cage just to be legal for their sanctioning bodies. Second, they as much as anyone else may fall prey to habits of old practice, so its plausible that they designed their cages using prior knowledge of what was needed before. Third, what they need may have nothing to do with what we are worried about. For example, they may have in mind the need for structural integrity in a roll-over incident after being t-boned directly in the rear quarter during race conditions (like triple-digit speeds, etc.). So, do they run a brace? Yes. What does that mean for us? Dunno...without asking their cage designers directly or knowing someone who really knows.

Best,
-j

 

[Sky Render]

Too late to take it in to the dealer. I already discovered firsthand how fast the stock brakes fade and slid the rear quarter panel along a guard rail.

So the car was in a wreck, and you didn't think THAT might be causing the panels to rub? Seriously?

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[lamont2506]

So the car was in a wreck, and you didn't think THAT might be causing the panels to rub? Seriously?

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Wreck was after the panels were rubbing. Not that stupid. And I have buffed most of the paint wear out of the rear fascia so pics wont be much use.

 

[OkieSnuffBox]

lulz at fading brakes on the street.

 

[lamont2506]

No, I admit I was an idiot and thought it would be a good idea to run up and down a mountain all day on the stock brake pads.

 

[speedfreak1000]

hey lamont whatever you do dont get butt hurt here. stick around and give the site a chance. guys here are brutal but they know their shit and are very helpful when youre in a pinch.

 

[Sky Render]

No, I admit I was an idiot and thought it would be a good idea to run up and down a mountain all day on the stock brake pads.

Either your car was assembled incorrectly, or it was in a wreck before you got it and it was repaired incorrectly. Regardless, your car shouldn't have panels rubbing.

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[Norm Peterson]

i was actually wondering what to go with first sway bars or STB. i was online reading how when "people" put on a STB they could tell a big difference so i was leaning toward that but if its not that noticable i'll go the swaybar route first. thanks for the input guys.

http://en.wikipedia.org/wiki/Confirmation_bias

I'm surprised they didn't mention Cognitive Dissonance, as well.
http://en.wikipedia.org/wiki/Cognitive_dissonance

I have done a before - after - before test with a STB on a V6 convertible Mustang. It was intolerable cowl-shake - barely tolerable - intolerable again. A convertible might benefit more than a coupe: same bar on a GT coupe, I couldn't tell any difference.

Frank's experience is exactly what should be expected.

A brace acting mostly in tension or compression between the strut towers only is not going to improve the car's overall stiffness enough to much matter. Better than zero, yes. Worth it for this reason, probably not. At best, it'll stiffen only the vertical-transverse plane that contains it, and the biggest benefit here is going to occur under hard braking, where the tops of the towers are tending to tip toward each other. In cornering they tip in the same direction, and the brace cannot make these deflections zero - only try to make them the same. Make it a 3 or 4 point brace (meaning 2 towers and rigid point(s) on the cowl, not two or more bolts on each tower) and it'd be a slightly different story. BTW, tower deflections during cornering might not be in the direction you first think.

But the local stiffness that it does provide can and does affect the structural vibrations, either by suppressing certain frequencies/modeshapes entirely or by chasing them off to where they aren't as noticeable.

How stiff the rest of the car is absolutely does matter, particularly where low frequency vibrations (like cowl shake) are concerned. That's the difference in the coupe vs convertible observations.

I really don't see a rear tower brace doing all that much except for addressing vibrations. That doesn't mean zero point zero structural help, just not much at all. The main loads from cornering are going to be coming through the PHB and PHB brace attachment points - which are already tying the two sides of the car together right down there where the load is being applied. It's a different situation from a strut-suspended IRS car, where the tower tops do see lateral loads from cornering in addition to lateral force components from changes in the spring loads. IOW, just because a rear tower brace may provide some benefit or other to some cars doesn't mean that it'll work effectively on all cars.


FWIW, STBs were fitted as OE on at least some of the Buick sedans of a decade or so ago. Does anybody here really think they were added for cornering/handling performance reasons?


Norm

 

[speedfreak1000]

Frank's experience is exactly what should be expected.

A brace acting mostly in tension or compression between the strut towers only is not going to improve the car's overall stiffness enough to much matter. Better than zero, yes. Worth it for this reason, probably not. At best, it'll stiffen only the vertical-transverse plane that contains it, and the biggest benefit here is going to occur under hard braking, where the tops of the towers are tending to tip toward each other. In cornering they tip in the same direction, and the brace cannot make these deflections zero - only try to make them the same. Make it a 3 or 4 point brace (meaning 2 towers and rigid point(s) on the cowl, not two or more bolts on each tower) and it'd be a slightly different story. BTW, tower deflections during cornering might not be in the direction you first think.

Norm

the way you described it i can visualize that in my head so that makes sense

 

[SoundGuyDave]

As for the "genuine race cars run it" line-of-reasoning, there's too much going on for those guys to let that crude fact alone carry the day for us. First of all, they have to run a full cage just to be legal for their sanctioning bodies. Second, they as much as anyone else may fall prey to habits of old practice, so its plausible that they designed their cages using prior knowledge of what was needed before. Third, what they need may have nothing to do with what we are worried about. For example, they may have in mind the need for structural integrity in a roll-over incident after being t-boned directly in the rear quarter during race conditions (like triple-digit speeds, etc.). So, do they run a brace? Yes. What does that mean for us? Dunno...without asking their cage designers directly or knowing someone who really knows.

Just to expand on this stem of discussion: I am NOT a structures engineer, and will admit to a bit of "habits of old practice" both in the initial conception of my custom cage, and in working with the cage builders. It was an iterative process, with each of us feeding off the others' ideas. My cage is SEVERLY over-built compared to the minimum requirements of my sanctioning body, and in fact is so over-built that it flat bumped me out of a class that I could have been legal for... That said, MY purpose in having the cage installed was not "meeting the rule book," but personal safety first, minimization of crash damage second, and performance benefit dead last. Mine technically has 10 points of attachment with the chassis, plus FULLY gusseted A- and B-pillar bars, and all tubes are 1.75" DOM 0.125" wall mild steel. The attachment points are (plus their reasoning), from front to rear: (1) Footwell intrusion bars, running from the A-pillar down tube to the footwells, terminating in a decent-sized plate. This was to keep a major front-end impact from pushing the motor/K-member/wheelwell into the pedal box area, thus keeping it away from my feet. (2) A-pillar down tubes (Forward Hoop design, paralleling the door opening up top and tying into the Main Hoop), plated into both the floor and the kickpanels. Required attachment point, forming the true front of the cage greenhouse. (3) Door-bar vertical supports, extending downward onto the sill, roughly 1/3 of the way back in the door opening. This not only reinforces the door-bars, which are a LONG unsupported span of tube, but also lines up with an FIA bar running from the door bar vertically to the A-pillar bar near the top of the windshield. Note that the A-pillar bar is gusseted with 3/16" plate the entire length of the windshield opening, tying into the factory crash structure. (4) Main hoop/B-pillar. The mounting pads are on top of the rear seat pedestal, and the vertical sections of the main hoop are again gusseted full-length to the inner structure of the car. This bar is required (obviously!) and forms the rear of the cage greenhouse. (5) Rear stays, plated on the frame forward of and to the inside of the shock mounts. Also required bars.

Points (1) and (3) are NOT required, and in addition, I added a few things... A required windshield bar, which ties the two Forward Hoops together laterally. This bar is additionally gusseted to the Forward Hoops with a 45* tube on the horizontal plane to further reinforce that joint. Also between the windshield bar and the main hoop, I have a fore/aft roof bar, gusseted with 3/16" steel plate, to keep the long span of the windshield bar from crushing inward. I also have REQUIRED door bars (2 tubes), which I took a little further. Mine are NASCAR style bars, extending all the way to the door skin to maximize crush space (safety!!), with two vertical support tubes tying the door bars together. The forward support tube also acts as an inline support between the sill mounted tube (3 above) and an FIA bar extending vertically to the top of the A-pillar tube, right at the windshield bar. This provides a heavily reinforced load-path for any sort of rollover where the car lands on the top of the windshield. IMO that area is the weakest point in the cage design, as you have very long, unsupported bent tubes, and can crush inward. The FIA bar is an effort to reduce that possibility. In the plane of the main hoop there are the required diagonal (high on the driver's side, low on the passenger's side) and harness bars which form a direct lateral load-path and further triangulate the main hoop. I elected to NOT include a full "X" crossmember in the main hoop, as I felt that the loss of rearward vision would not be worth the additional main hoop strength gain. With the main hoop being gusseted to the factory crash structure, I feel that there is enough vertical crush protection without the additional tube. Out back, I have a diagonal brace (not required) between the rear stays, aligned so that in the rear-view mirrors there is a minimal impact on vision. The SOLE purpose of this bar, at least in my design, is to minimize the ability of the rear half of the cage to move in a high-energy side impact behind the doors. At the bottom of the rear stays, I also have the shock-tower brace bar, just to keep this all somewhat on-topic. The purpose there is simply to triangulate the rear stays, nothing more! In essence if I get T-Boned right on the rear wheel, I just want as much steel in place as I can reasonably have to keep the rear frame stubs where they are. In the event of a "minor" crash, I hope to avoid having to pull the frame; for a "hard" hit, this forms some crash structure around the stock plastic gas tank; and in a "catastrophic" hit, it just puts more steel in place between the catastrophe and the greenhouse where yours truly will be sitting... I was on the fence about extending bars into the engine bay and tying them into the shock towers, but in the end elected NOT to, relying on the factory crumple zones to reduce impact severity in a big hit. Please note that in the event of a major shunt, it's a LOT easier to graft on a new front frame stub than it is to do the same out back. NOWHERE in my design did I really worry about reinforcing the shock towers to handle coilover loads as a necessity. Yes, my design does have that as a side-effect, but it certainly wasn't a driving concern.

My mental "design criteria" was a worst-case scenario involving 140mph car-car contact (botched pass), immediately followed by car-wall contact, with a following secondary car-wall contact, this time inverted, and landing on the leading edge of the roof. My theoretical model was based on the entry area to T12 (Canada Corner) at Road America, an outstanding passing opportunity, but with concrete on both sides, and no catch fence to the inside. Survivability was the first concern.

 

[Mountain]

Horseshit. I know tons of S197s on real rear coilovers without any issues. Please stop spreading false information. The rear shock towers are very very strong.

The basis of that are based on an engineering paper some chassis engineer wrote in regards to the S197 chassis back ('06 or '07) when it first came out. This was a PDF document I found on some chassis engineering forum/website some time ago (I do not recall the details on the individual or where I found the document). Mabe the data that was found had faults in it???

My statement is also based on some forum posts and pictures (of ripped out rear towers) from GT500 owners who modify their cars for track days/road racing.

Finally, looking at the FR500C/FR500S/BOSS 302R/BOSS 302R cars, they all seem to have a brace there and run coil overs.


I am not a structural/chassis engineer. So, my claims are what I have read and what I have seen by "pro" and pros.


If you have sound evidence and data, supported by more than a few vehicles, that indicates otherwise, that's great! What I posted is what I thought was sound information based on what is out there, but may be wrong.

... My thinking and understanding of why a rear brace may be needed in a coil over setup is in-line with what Philostang commented. Maybe, as Philostang alluded to, it isn't realted to coil-overs in general, but maybe specific designes/setups?

I do not post much because I come to these forums to LEARN. From what I have read here, you all seem very knowledgeable and experienced. If people are going to act like dicks and attack peoples' unintentionally provoking statements, I hope you enjoy your self-gratification of making others feel bad or stupid rather than help those who want to learn something. [All reasons I DON'T frequent other forums].