Snrub

iTrader: (1)

Being 4lbs or 15lbs will not improve your performance. How rigid is the 4lbs bar? Does it do anything?

1987RX7guy

iTrader: (3)

The reason I want to know the weight is because weight and keeping it off my car is very important to me since I am not planning on gutting out my Sport. yes it works and it helps a bit.

LEt me explain the arguement here:

Say 4lbs bar does X amount of work. and a 15lbs bar does x+1 amount of work. In this case I would pick the 4lbs bar simply because it isn't TOO MUCH better and I would be gaining another 11 pounds. Now say the 15lbs bar does 4X amount of work then compared to the 4lbs bar it would probably be worth it since its work(performance) is more proportional to the increase in weight. Now after I narrow it down to 2 or 3 bars i will descide upon the cost of each one and probably go with the middle one.

Santiago

PS_ I hope that was understanable.

jon88se

do you think a round vs. oval bar would make any difference you'll be able to feel in how load is transferred through the suspension? it is precisely zero this isn't NASA folks, there aren't such big real world differences in the smallest comparisons on a strut brace. A hollow bar compared to a solid bar, that makes sense...but not this type of comparison. Ebay bars suck for a couple of reasons...first, the bars are usually hollow, second the mounting plates for your strut tower bars are just plain THIN, cheap and very very flexible...second, the hardware used to hold them together is horrible...take it on and off a couple times to work on your car and you'll snap a bolt or cross thread something. Either make your own bar with some beefy hardware store stuff (not too hard) or buy Cusco...their stuff is solid and cheap

wozzoom

That bar looks very cheap. The end plates are stamped out of what appears to be a very thin gauge steel. The attachment points for the bar look like they are made out of the same material... I bet that it bends under load.

Looks like a ricer/poser piece to me.

I have the cusco bar on my 86. The build quality is 10X better.

jon88se

yep...I have the Trust/GReddy bar on the front of my S4 and the cusco in the rear...the GReddy bar is alright (CHEAP, like 80 bux) but the bolts that come with it SUUUUCK so I just picked up some nice stuff from a hardware store.

Josepi

Mine hasn't bent at all and it's been on the car for around 1.5 years. I paid ~30-35 back then...if it's hitting the alt, you're putting in on upside down.

tabularasa

Hard to know whats right with all these opionions...

jon88se

you will never notice if a strut bar is flexing or not, there aren't such forces on these pieces that they will literally twist or bend...remember, the struts are rigidly mounted into the car and we're running street tires...there isn't a crazy amount of lateral force here. the cheap thin bars WILL flex...is it better than nothing? sure...is it worth $25? sure...for what your get that sounds right on. the better bars shouldn't flex at all though. At a minimum I'd upgrade the hardware that comes with these EBAY bars, costs you nothing and you'll have much better material that will last, not strip, cross thread or snap.

MechE00

Lot of stuff being said about flex in the bar.. are you guys thinking the bars are loaded in compression or tension? What are you saying is flexing?

Check out this link for some strut bar theory..
http://e30m3performance.com/myths/St...bar_theory.htm

edit: this is with respect to a bar being used for performance enhancement rather than longevity..

theloudroom

I don't agree with that guy's analysis.

Assuming all the front cornering force is being provided by the outside wheel, all the suspension arms are going to be in compession. This is going to push the strut tower inwards, towards the engine. If there is no load on the inside tire, the other tower would typically not move, so any strut bar is going to be in compession, not tension.

Look at his axis. F3 is pointing in towards the engine and the magnitude is positive. For a strut bar to resist that motion, it will need to provide a force pointing out away from the engine. To do that, it needs to be in compression.

It's obvious if you think about it. If all the weight is on the outside wheels, the entire car is in compression. The force provided by the contact patch of the tires is fighting the inertia of the car. The forces are pointing towards each other, therefore compression.

theloudroom

Also, as for flex, at some point a strut bar must become worthless, that's why they don't make them out of plastic.

If your strut bar would deflect more under a 300 lb load than the strut towers would by themselves, it's going to be of very questionable value.

Someone should take measurements of these ebay strut tower braces, look up an approximate stiffness for steel, and calculate their deflection under load.



Does anybody with one of these bars want to measure it?

wozzoom

Most people will never drive their car hard enough to even stress the shock towers... Let me know what the bar looks like after an autocross.

West TX RX-7

Looks like a decently stout bat. Wonder what kind of steel it's made from, like the ends or is it ALL aluminum??

theloudroom

I don't think anybody's talking about it bending to the point where it stays that way. Even if the strut bar provided no strength, I doubt the strut towers would move enough to do that.

What people should be worried about is if it flexes too much to be useful. You could put a piece of coathangar between the two towers, but it wouldn't provide any extra strength, because it's too weak to do so.
When you got done with a run it wouldn't be bent into a pretzel or anything, it would look just like it did at the beginning. The thing is, having it there would be totally useless.

Take a look at this SLP strut bar for a Firebird:


"The braces are constructed with flame cut 1/4" mild steel plate and 1-1/4", .095 wall thickness mild steel tubing. "

This one looks pretty stiff. (And that's the point when you're trying to stiffen up the car's chassis.)

Now look at this one:
http://cgi.ebay.com/ebaymotors/ws/eB...tem=2439285801

Look at those arms that connect the bar to the strut tower, they're pretty much perpendicular to the axis of loading. This makes them much worse at carrying load than if they were oriented in the other direction.

Now look at this one:
http://cgi.ebay.com/ebaymotors/ws/eB...ayphotohosting

See how thing the metal plate on that mounting bracket is? How stiff is that going to be? And remember, that's what we're worried about stiffness, not strength.

Next we can look at some more expensive bars from corksport:
http://www.corksport.com/main.php3?p...3Fcat%3D214917

Note that unlike the ebay rear brace, the corksport rear brace actually has the mounting for the bar pointed in the right direction to handle the load. Even if these two bars were made of the exact same materials, that means the corksport model is going to be better.

Conclusion? I dunno. I'm not a mech e. So I can't give you a definitive answer on this, but I would hesitate to buy a strut bar, even for $25, if I'm not sure it's going to be stiff enough to actually do anything.

Trav

why is that bad? Basic physics tells us that a hollow rod will flex less than a solid rod of equal mass and material... Personally, I'm thinking the hollow one would be more desireable than a solid one. For one, you could get lighter than a solid one and retain just as good (if not better) rigidity.

theloudroom

You are correct. A hollow tube of EQUAL MASS is going to be better.

The issue wouldn't be that these are hollow, it would be that these are likely very flismy. (less mass than other bars)

I haven't seen one up close to verify it, but see my above post about how stiff the rod must be to be work anything at all.

1987RX7guy

iTrader: (3)

My bar is not hollow.

Trav

well my point was that just because it's hollow doesn't mean it's flimsy. You could even have LESS mass than a solid rod and still be more rigid. i.e. less weight.

physics still applied folks, but I'd agree with much of the above posts, that the quality may come through in the mounting points. I'd say that is the critical failure point in this part. If you have well made brackets, I think the bar is less of a factor.

MechE00

You are reading the diagram wrong. From the page: Thus, there is a force acting on the strut assembly pulling the top of the strut assembly in towards the centerline of the chassis.

Please re-read the text of the article. If you have any further questions, I'll try to answer when I have time. (Disclaimer: I did not write the article)
You forgot about torque. I think you are a bit rusty on static analysis, F=m*a, and the conventions of a free body diagram.

theloudroom

I made a sign error with F3 (pretty simple mistake), and everything was wrong from there. Or at least pointed in the opposite direction. Thanks for catching it.

Having fixed that, everything makes more sense. I'd never realized before that the forces acting on a chassis can be viewed as originating where "the rubber hits the road", not at driveshafts, etc.


Do you have any idea what the stiffness of a typical chassis is? I'm trying to figure out how much strength is really necessary in a strut bar.


I've decided to try another analysis:

I viewed the strut towers as two springs with the same k (k1) and the brace as a spring with k=k2. I came up with F = x (k1 + k1 / (k1/2 + 1) ), with F being the force applied and x being the deflection of the outside strut tower.

If the strut bar was perfectly stiff, half the load will be transferred to the other tower and the deflection of the tower would be cut in half. If the strut bar was twice as stiff as the chassis, deflection would be reduced by 40%. If it was equally stiff, 33%.

Now for the suprising part:
If a strut bar is only half as stiff as the chassis it's bolted to, it's still going to reduce deflection by 25%!

I may have to recant here, and say that even an el cheapo bar could result in a significant improvement. I'm still not sure, about some of these bars that have flimsy brackets though. If we an idea how stiff and RX-7 was, we could find the point of diminishing returns.

Bukwild

I have a front strut bar on my vert and I noticed that it stiffend up the front end right away. I also have the CP racing rear bar and IT DOES NOT FIT THE VERT.

MechE00

There aren't really any readily available numbers for chassis stiffness.. (at least that I have access to). There are so many potential aspects to chassis stiffness people might be interested in... torsional stiffness, longitudinal stiffness, stiffness in collision, etc. that there is no one number that is quoted in magazines and meaningful to lots of people.

With the proper gear, you could experimentally determine a stiffness for a certain deflection range, but I've never done it and don't know anyone who has.

I might be missing something, though...

theloudroom

Sounds about right to me, I was just hoping for an order of magnitude type ballpark estimate. I'm interested in the deflection of the strut tower in a 1g corner, with no strut bar.

There's a whole lot of "butt dyno" action going on in this thread, and I don't really trust that by itself. People aren't doing double blind tests here, so the whole thing could be purely psychological.

I'm not trying to say anybody in partiular doesn't know what they're talking about, but I have a feeling you could put a fake carbon fiber finish on a piece of flimsy plactic pipe and many people would still think there car felt stiffer. It's a placebo effect. It would probably work on me.

I don't really have any exposure to Mech. E, instrumentation, so I'm not sure how you would set things up to measure, nor do I have the equipment.

Maybe you could get a number by using two different strut bars w/ strain guages on them and measuring both of them off the car, holding up 200 lbs and on the car on a skidpad.

jon88se

forumlas for strut bars, scary...these pieces will help make for a more responsive and predictable car but they aren't paramount. If you do get one though I'd recommend one of the better companies as there is a distinct difference in quality.