Pic of my custom rear strut bar

[COZY Z COLE]

DAMN .... This thread should be required reading before any new person is registered!!!!! Then let's see if they will ask" How much HP can I get from a V-8 conversion "????

 

 

LARRY

[pete280z]

Guessing that the load on this bar is too small to matter is not an opinion. It's a guess. Everyone agrees that this is a less than optimal design. So here's the point:

 

"My Z looks good painted red." - OPINION

 

"These vinyl stickers covering my Civic make it .2 seconds faster in the 1320." - FACT. Not necessarily true, but stated as a fact.

 

This whole argument is about factual data, which has not been measured.

 

Jersey - Until real measurements have been gathered, this argument is silly. You're guessing. Guess away for all I care, but please don't try to make this a subjective argument.

 

Pete - Thanks for keeping us honest.

[Drax240z]

OK Jersey... you really aren't making much sense to me here.

 

Why? because we do not have all the exact figures to make it a fact that the bar i used, in this application we're applying it to which is between the rear strut towers of an S30 is just as strong as a straight bar, or it's not.

 

It is NOT. That is a fact!!!!!!

 

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Case 1:

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Assuming a Z of 3000lbs mass, cornering at an instantaneous 1.5g/s.

 

Lower control arm pivot is 7" from the ground. (assuming a 24" tall tire)

Height to strut tower is 30" from the ground.

 

We have 4500lbf acting on the contact patch of the tire.

We have 1050lbf acting on the top of the strut tower.

 

Assume worst case scenario is that the chassis is stiff enough for the unloaded strut tower to be stationary.

 

We then have 1050lbf of compression on the strut tower bar.

 

A standard 1" OD, 0.065" wall tubing will have a x-sectional area of 0.191in^2, leading to a stress of 5500psi.

 

Assuming run of the mill mild steel, (E=29x10^6psi) there will be a compression of .006824".

 

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Case 2:

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Same loading and assumptions as above, only this time the bar is under bending loads as well as compressive. (ie: bent bar)

 

Simplifying the bent beam to being 3 sections of 12" in length, with a 1.5" (centerline to centerline) offset, and again using 1"OD, 0.065" tubing.

 

All of a sudden we have stresses of 37,500psi. (bending moment 1575lb*in, 0.5" from neutral axis, moment of inertia being 2.097*10^-2 in^4)

 

Case 1 (straight bar): Pure compression: 5500psi stress

Case 2 (bent bar): Bending loads: 37,500psi stress on bar

 

It's a royal pain in the arse to figure out the final overall deflection of the bent bar, but the stresses should give you an idea of the overall shape of the bars in these 2 situations. If the actual dimentions are even close to this I know which bar I'd be using. (oh wait, I allready am!)

[Mikelly]

Jersey, I'll be there in the Vette. The Z is months away from being back together.

 

As to your comment about not putting enough stress on the Z strut towers, you should seriously look at some older Zs that have spent lots of time autoXing and roadcourse/ track day eventing. The telltail sign is at the seam where the roof joins the rear hatch slope by the circular vent hole. That seam is often visible, and sometimes cracked due to stress from the strut tower flex these guys site. There is a significant amout of stress put on those towers. Also, check the link John M. posted.

 

I've seen first hand the effects of age and thin metal on Zs that have been abused, and "Fortefied" with stiff suspension bits. It is usually poor execution, with evidence of damage in the above area, the "A" pilar at the windshield/ roof attachment seam, as well as in the TC boxes and the frame/firewall/ batterybox/ floor pan junctures.

 

Good luck...

 

Also Do a search in Google and search for Summit Point West Virginia. There you can see the schedule, and prices for instruction for track events! $195 for the day if you pre-register.

 

Mike

[johnc]

We have 4500lbf acting on the contact patch of the tire.

 

Tires - plural. At a minimum 2 tires are taking this load.

 

The load path into the strut towers is 99% vertical; they are not designed to handle significant lateral loads. The lateral loads come into the chassis throught the lower control arms. The rear strut towers distort from the vertical loads imposed so we would have to calculate the amount and direction of distortion to determine the loads placed on the strut tower bars.

 

The largest inputs into the strut towers are bumps where the suspension bottoms out. Good examples hard cornering on a bumpy track, FIA curb hopping, and wheel hop on hard acceleration or hard braking.

 

A corresponding street driving example would be potholes, of which I've been told New Jersey is the mother load of such things...

[Drax240z]

Sorry, my 3000lb load was assuming absolute worst case, one tire on the ground. John is right that it would be very difficult to pull a 1.5G corner on 1 tire.

 

However, also as john mentions the most significant loading is shock loading from potholes etc, where suspension loads can be easily as high as those experienced in a 3G corner.

[Pop N Wood]

We then have 1050lbf of compression on the strut tower bar.

 

And the above line is also an error. You may have 1050 lbf acting on the top of the strut tower' date=' but how much of that is being absorbed by the other structural elements of the chassis? Not fair to say the whole thing is being supported only by the strut bar.

 

 

The rear strut towers distort from the vertical loads imposed so we would have to calculate the amount and direction of distortion to determine the loads placed on the strut tower bars

 

Ditto?

 

As for the suspension bottoming out, at that point haven't you have pretty much lost it anyway? With a soft suspension what would be the advantage of a stiffer chassis? For potholes seems like you would want even more give since no one here is a street racer anyway.

 

Paint cracking in the C pillars may be helped with a strut bar, but I can tell you first hand it doesn't cure it. I have a straight bar between the rear struts towers on my 1970 (a nice MSA one that didn't clear my L6) and I still get chassis creaks going down driveways.

 

One other thing to consider. Jersey's now infamous girlie (er, sissy) bar is solidly welded to the strut mounts. My MSA bar has heim joints. Is the purpose of the strut bar to keep the struts parallel, or to stop all movement altogether? Wouldn't that have an affect here?

 

I thought it was obvious I meant all other things being equal

 

This was exactly the point I was trying to make. One of JohnC's favorite answers to "this is better". But to extrapolate my conduit example, at some point if you make a curved bar heavy enough, it will be stronger than a straight bar made out of lesser material. Or else how could motorcycles function with curved downtubes?

[Drax240z]

I think you can pretty much sum up the word "engineering" as "the search for efficiency". In this case, you certainly could make a curved bar with the same stiffness and strength as a straight bar by using much more material, it just wouldn't be as efficient.

 

The 1050lbs figure is an estimate of the worst case scenario. We don't really care that much about this if we aren't considering the worst case....

 

It has been discussed over and over here that there is no advantage to a strut tower bar with rod ends on it, other than adjustability. The more constraints splaced on the tower by the bar, the better overall the chassis will feel. The idea is to resist movement, not to keep the bars parallel.

 

A stiffer chassis is ALWAYS of benefit, even with a soft suspension. (the only exception being when your chassis IS your suspension) The stiffer the chassis is, the more predictable it is, and the easier it is to make consistant changes to the suspension. This is independant of spring rate, but would be more noticeable at higher rates most likely.

[Z-TARD]

Well, I just went cross eyed. Thanks guys

[Dead Roman]

Technical "argument" aside, i think we can all agree that in a case such as this one, all variables being equal in reference to the stated dimentions and weights.....

 

 

 

 

 

Biscuits and Gravy are good

[pparaska]

Jmortensen - thanks for the link and keeping me straight - I appreciate it. But I don't believe most of what I read on the Internet and I'm not convinced that the structural analysis is really valid. As Johnc said, the lateral links take the lion's share of the lateral loads from the tires. The strut towers take the vertical reactions, and since they are angled in, they should tend to flex towards each other.

 

The FACT that I'll always stand behind is that a straight bar is much STIFFER (I didn't say anything about strength - that should always be designed to be adequate) than a curved bar with the same cross-sectional qualities. And the stiffness is linear while everything is in the elastic region of the material. That means that if I push on the bar with force F, the straight bar will compress by an amount X. For twice as much force (2*F), it will deflect by 2*X, twice as much. It matters not what the loads are - the equation shows the relationship. For the curved bar, it will be more than X for a force F, the amount more will depend on the curve, end conditions, etc.

 

The actual loads aren't needed if the material isn't stressed past it's elastic limit. But as Drax showed, that elastic limit will come about at a lower end compressive load with the curved bar - because of the eccentricity. That's another reason it's not a good solution for a strut bar, if the point is to keep the towers from moving away from or towards each other. However, I doubt the curved bar Jersey has will actually fail (that's related to strength, not stiffness). That I agree with. But strength isn't what I was discussing. Personally, I'd be more concerned with the thin section near the strut towers, since they will see flexure and are thin in the direction of the flexure.

 

I can state the above FACTS without knowing the actual loads - that's the beauty of algebraic equations.

[pparaska]

You say here:

 

Maybe you think the loads you'll apply to the curved bar are low enough that it won't matter. That's your opinion.

 

YES. Exactly what i'm trying to say. I do not believe the loads applied by the rear towers will compromise the bar i used. You understood my OPINION on this whole discussion right here. This has been exactly what i've been saying from the beginning. Perfect.

 

The sentence

"I do not believe the loads applied by the rear towers will compromise the bar i used."

of the paragraph above is probably where we haven't really communicated correctly yet. If by compromise, you mean cause it to fail due to insufficient strength, my gut tells me the same thing yours does. But I've been fooled by my gut feel before on structural problems - which is why I prefer to actually analyze the problem with the details of the material, geometry, and loads. In fact, I broke down in New Jersey because of an eye-ball alternator bracket design on my Z. Repeated flexure of the section near a hard point (stiffer in bending) in the bracket caused it to fatigue and fail. Thanks to a New Jersey bud who I knew that lived close by, I got back on the road that night after repairs in his garage! Teaches me for following my gut feel!

 

But strength hasn't been my point at all - it's been STIFFNESS - i.e., that it will DEFLECT appreciably more than a straight bar of the same cross-sectional properties would. That assertion (by me) is based on engineering fact.

 

Were I do agree with you is that the loads and details of the end conditions (fixity) of the bar, as well as the geometry of the bar must be know to analyze whether it will fail or not.

 

In fact, the shear fact that the curved bar is less stiff is part of the reason I think the bar it self will be fine. Softer structures under the same loading as a stiff one, in general will have lower maximum stress, with alot of variables depending on end conditions, etc. That's why tall buildings and bridges are designed to flex more than they might if a stiffer structure was designed - the operating stresses in a less stiff structure under bending loads will be lower, generally. It's hard to make general statements like that without a bunch of caveats - it depends on the details of the load conditions and details of the fixity, etc.

[john kosmatka]

Only you guys could turn a strut tower bar into a technical discussion. Interesting concetps though. Looks good Jersey, Ill let you know when I get mine done, (the swap).

[pparaska]

Do you think that the curved "sissy" bar that used' date=' which is 1/16" thick and 1" OD, will flex at all by any amount of pressure that the shock towers can apply to it? I don't. If you do, that's fine and i respect your opinion. We just may have different opinions, that's all. [/quote']

 

Yes, I KNOW it will flex. How much, I can't say because I'd need to know the loads involved, and other details.

 

Many people think that if something is very stiff that it is rigid (won't compress, stretch or bend). A very stiff structure WILL do these things.

 

Whether they will fail is another quetion entirely.

 

Again, I think the confusion has been that you were really thinking the bar won't FAIL (has to do with the STRENGTH of the design), and I was saying it will FLEX more than a straight bar will (has to do with the STIFFNESS of the design). Stiffness and strength are two very different things but people do confuse them at times.

 

I can say alot of FACTUAL things about the STIFFNESS comparisons between a straight and curved in this application, without knowing anything about the magnitude or direction of the loads (compression or tension), but nothing about the STRENGTH, without knowing alot of details.

[Drax240z]

Another factor that can be illustrated by the stresses present in each bar is the bars fatigue life. Guess which bar will last longer, the one experiencing 5500psi or the one experiencing 37500psi? Likely using steel each bar will have an infinite fatigue life, but it is another thing to consider. The higher the stress the lower the fatigue life. (a generalization, because there is a point of effectively infinite fatigue life) But an useful rule of thumb all the same.

[pparaska]

Only you guys could turn a strut tower bar into a technical discussion. Interesting concetps though.

 

That's the beauty of HybridZ. We DO get technical!

[johnc]

Another factor that can be illustrated by the stresses present in each bar is the bars fatigue life. Guess which bar will last longer, the one experiencing 5500psi or the one experiencing 37500psi?

 

Unfortunately you can't say that with such certainty without knowing the yield limit of the steel part being tested. If 37,500psi is below the yield limit of the part being tested then the test could go on forever, barring any stress risers. If that's true, then the bar being stressed at 37,500psi will last just as long as the bar being stressed at 5,500psi.

[JMortensen]

Jmortensen - thanks for the link and keeping me straight - I appreciate it. But I don't believe most of what I read on the Internet and I'm not convinced that the structural analysis is really valid. As Johnc said, the lateral links take the lion's share of the lateral loads from the tires. The strut towers take the vertical reactions, and since they are angled in, they should tend to flex towards each other.

 

This is not the first place I've heard of this. The first place was in an article in a POC magazine where they drove a 944 around with a tattle-tale dial indicator hooked up to measure the deflection. The outside always deflected out. Until someone does that with a Z and comes up with a different result, I'll go with the BMW and Porsche guys on this one.

 

BTW I appreciate not believing it because you saw it on the internet too. I try not to be easy to convince, but I have fallen prey to my own gullibility on occasion...

[Drax240z]

Another factor that can be illustrated by the stresses present in each bar is the bars fatigue life. Guess which bar will last longer, the one experiencing 5500psi or the one experiencing 37500psi?

 

Unfortunately you can't say that with such certainty without knowing the yield limit of the steel part being tested. If 37,500psi is below the yield limit of the part being tested then the test could go on forever, barring any stress risers. If that's true, then the bar being stressed at 37,500psi will last just as long as the bar being stressed at 5,500psi.

 

Yeah I covered that in the rest of that post. I should reword it a bit to be more clear.

 

Fatigue life will be shorter as stress increases, except in the case where the stress is below the endurance limit of the said material, where the material can then have an infinite life.

 

How's that?

[johnc]

Poifict.