epitrochoid

It seems to me that a strut bar with a bolt between the mounting pads and the bar itself would be totally useless compare to a solid one like our OEM one. If enough force is present to flex the chassis, I'm sure it would just pivot the whole assembly. Even the big names use this style (ie - JIC), why???

Sputnik

A tower-to-tower strut bar is merely meant to maintain a constant distance between the two towers. It's strength is push-pull, not anti-twist or anti-bend, so it is still effective.

---jps

epitrochoid

but i cant see how the towers would come together under heavy load. Seems that the inside tower would raise while the outside remained static in a turn.

takahashi

I thik you agruement makes a lot of sense. Is there a pic to illustrate your point, please?

epitrochoid

here's one of the JIC bars.

think of it this way... the point of the strut bar is to connect the top part of the box formed by the underbrace and the two strut towers. in the OEM case, everything is solid forming a strong square to prevent chassis flex. imagine the same box with the top part formed as a flexible strut bar, with those two joints i think it would really render it worthless.

Doctorr

Imagine your box is really solid at the bottom - subframes, chassis, rollbars etc.
but your have your two 'towers' that hold all the springs and shocks.

All would be fine if the wheels were directly under the tower, but they are on the OUTSIDE, so when they take a jolt, they transfer enough flex to the tower structure, that it wants to bend in to the centre.

The best strut design would be a triangle from the top of the drivers strut, to the base of the passenger side strut. This would keep it nice and solid without affecting the other tower, but since there are parts in the way, a brace from the top of one tower to the other works pretty well. It also takes advantage of the leverage afforded by the height of each strut tower.
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doc

takahashi

It is more 10 years since I touched any Physics book (although I won state award in high school). I don't understand a thing that has been discussed :p

Front strut - you are right - epitrochoid, I don't like the JIC joint. It is ok for the rear strut, coz it will be a pain to put it in otherwise.

Doctorr how about this strut: (A pass? :o :p )

Sputnik

In this car, the firewall, two sides of the engine bay, and the radiator supports that connect the two sides of the engine bay, form a box. Take a shoe box, and push in on the middle of the sides. They will push in easily, not just because it's cardboard, but because it is a weak point of the structure. When a car is cornering, it will generate lateral forces on one side in a similar manner. Putting something between those points decreases how much the structure deforms.
First, a strut-tower bar, whether 2, 3, or 4 point, is too thin to resist forces in that direction, whether they have a bolt there, or are cast or welded as one piece. Take a butter knife, for example. You can flex it pretty easily, but when you try to pull it longer, or compress it shorter, it is much more resistant to those forces. It's not impossible, but it's much more resistant.

Second, the sides of the engine bay are not connected to the firewall at one or two points, they are connected from top to bottom. This means that that connection is very strong, and the firewall and engine bay sides are actually the weaker link at that point for forces in an up or down direction (again, twist the shoeobox accordingly, and you'll see that as far as forces in those particular directions are concerned, the walls of the shoebox deform before the corners).

Remember, a chain is only as strong as the weakest link. No matter how much you strengthen the strongest link, it will be no stronger than the weakest link. The same goes for stiffening a chassis. In most cases adding a tower-to-tower bar strengthens the weakest link. Remember, we aren't trying to fix everything with one little bar, we are only trying to fix one little issue.

When you are thinking of how to go about stiffening your chassis, remember one thing: Triangles are strength. Square structures are stronger than "U" shaped structures, but triangles rule all. Look at a truss bridge. Look at a roof truss. Look at the roll-cage of a professional race car. Look at the rollbars available for the Miata (I use that example because of all of the available items, some of which have to comply with sanctioning bodies). Triangles abound. Even a basic loop rollbar helps stiffen the Miata chassis, but a rollbar with a single or double diagonal makes a significant difference in how much, and those diagonal bars create triangles. The design that Doctorr suggested creates a couple of triangles, but the engine is in the way, so we can't do that.

So, when you add a 2-point (tower-to-tower) strut bar, you've tied the two towers together for lateral forces. But even so, now both sides of the engine bay flex to one side or the other together under load. The combination of the two is stronger than with no strut tower bar, but it is still a weak point. The next thing would be to tie them into the firewall to reduce that flex even more, and that is done with the 4-point bar that Takahashi posted. The short sides of that bar create triangles with the firewall and engine bay sides, which is very effective. The optimal setup would have one bar go between the two towers, another bar go from the right tower to the left corner where the firewall meets the engine bay side, and a third bar go from the left tower to the right corner where the firewall meets the engine bay side. The resulting angles would give us the best leverage against the forces we are trying to contain (and it would create more triangles). But again, the engine is in the way, so it's not practical, and the bar that Takahashi posted would be the best bolt-on solution for a street car.

---jps

epitrochoid

i know 4 point bares are supreme, but they are often welded..maybe the JIC bar was a poor example. the JIC bar keeps the bar pretty level with the tops of the towers and low across the motor meaning at the bolt joint, the two forces (the force from the tower coming inward, and the normal force opposing it) oppose each other and in theory anyway cancel each other out meaning no movement occurs at all. on other bars, the 'bar' section is raised up above the tops of the strut towers, which puts the forces out of alignment and just rotates the joints.

consider the butter knife example...put one end on the floor and press from the top. if youre pushing straight down, you should see no flex. now imagine the center of the better knife was removed and shifted up from the centerline of the knife and two bolts connected the whole assembly.

takahashi

Very informative post, and thx for the positive comment. The end result, with the sway bar, is unbelievable. The feel is just amazing.

Lock & Load

So this strut bar posted by Petrus (forum member )could only serve a decorational purpose ?

epitrochoid

great looking piece, but yeah that'd be one of the worst as far as stiffness is concerned. the joints form a 90 degree angle meaning no force is cancelled out.

Petrus

GREAT thread and discussion!!! Lucky for me I found it before ordering something nice looking but useless.. :D

brothervoodoo

Yes very interesting but much like everything else, it has been discussed in one form or another in the past.... :p

https://www.rx8club.com/showthread.p...trut+bar+hinge
https://www.rx8club.com/showthread.p...trut+bar+hinge

heheh... epitrochoid, I noticed you asked this one before... :o

Sputnik

Not quite. In your sketch, you show the forces pushing in different directions for the different bars. A force is going to push in the same direction, regardless of how the parts of the bar are orientated. While there are different forces going in different directions during cornering, we are merely trying to contain those going laterally. And even a bar that it raised as you describe, will still be effective (as seen in the attachment). Notice that while the 4 point bar we've been talking about in this thread is welded, it too is raised above the tops of the strut towers. As long as the base is designed well, it should still be effective.

IMO, that item is not merely decorational.

---jps

truemagellen

now this is an interesting debate...epitroid has a point...raising the joint seems more of an ascethetic design...the more simple the design (the stock bar) the more effective it should be

by moving the joint up you have loaded the joints with all of the pressure instead of spreading out the entire load over the width of the bar

the 4 point is xtreme but would be fairly effective....a star pattern would be the most effective but that would be a serious overkill and would add undesirable weight

please critique me to death...I'm trying to justify NOT buying an aftermarket strut bar (or MS one)

epitrochoid

the raised strut bars are usually done to fit around the engine. not a problem in our case since the keg sits so low. sputnik is right about the forces determining the vector, not the angle of the joint, but that still places the majority of the bar off the axis of force. considering the amount of hype in this industry, im sure 90% of the bars out there are NOT strong enough to resist compression when their general shape is an arc as opposed to a straight line. i'm not saying strut bars dont do anything, i've seen people pull cheap ones out of their cars that were all bent up. the is an intense amount of force that the bar has to handle (not so much in our car, as it's a sports car chassis, not an econobox with a decent motor shoehorned in), so it's best to keep everything straight and level.

IMO any 2pt upgrade from the stock unit is purely cosmetic. If you want a good bar, get the 4pt jic or a similar design from a good manufacturer.

Sputnik

I do agree that there certainly are some STBs out there more for show than go (what percentage, I won't speculate), and some are better than others. And a STB in the shape of an arc (in any direction) will definitely be less effective than a straight one.
I don't understand what you are getting at. How would moving the joint higher change where the bar handles the load?
Could you illustrate how that pattern will look?

---jps

Kart Racer

i agree. that with a bolt on bar, there are weak points where flex will be prone.....also it is important to note that most of those types of "decoration" strut tower bars, can be adjusted to fit better, however that brings more of a concern, if they are adjusted slightly off, then the chassis is flexing anyways......so whats the point.....true tower bars should be an exact fit, thats welded.

epitrochoid

unfortunately when you weld, you can't relly unweld if you need to take it off