Well I was going to ask for some engineering analysis of the following device....Exibit A ?

To me it seems its designed wrong for torsion loads... It should lose the top tube and gain side tubes.

Basically, for whatever idiotic reasons exists, how should a device be designed to torsionally brace the frame between those two points (rear tower mounts and ...*gag*... hatch strut mounts)? Please keep it on topic.

P.S. - hmmmm this has just given me some tangent thoughts for my Mid-Engine Subframe design.....


[Modified by Vracer111, 1:22 PM 10/8/2002]

 

comptech says you really don't need a rear strut tower bar....the loads just aren't that great.

it seems to me that the unit pictured does show some functionality. Think of the chasis twisting...

hmm...I need to think about this some more...

 

the bar actually ties the rear strut mounts to the rear seatbelt anchor points. not to the hatch strut mounts. it may be a little overkill, but with that much expanse of open area back there, isn't there some chance of deflection? other companies have developed bars that tie the rear seatbelt mounts together in a hatch.

 

It will do something. Since a hatch has no cross bracing like a trunk / seatback panel provides, this framing applied tightly will provide stiffness, mainly from the diagonals, the horizontal/vertical sections do little. The fold down seat will not due to freeplay, and also is almost useless now.

 

no one sits in the rear seats, too much potential for a head into steel tubing.

 

if you are rendering the rear seats usless anyway...why not just get a rollbar?

 

i was thinking the same thing.....

 

cause the bar i'll put in will also make it unpleasant to be in the pass side...this car is a daily sriver, and i don't just want a street hoop.

 

why is that? My roll bar is not intrusive to me or my passenger.

RJ

 

on what kind of vehicle?
does the bar extend into the front? ie; dash area? door bar?

 

on a 95 saturn coupe. The bar does not extend forward of the front seats, and it has 4 attachment points (and two diagonal braces). Hence it is a roll bar, not a roll cage. It is legal for Solo1 competition, should i ever decide to do so. The only comprimise i make is that i loose the use of my rear seat for passengers - which was never used for passengers in the first place since its too small.

RJ

 

i will be caging the car. that would be the difference.

 

for a car that sees track and street use, a cage could become unsafe on the street.... Most weekend warriors that are not going wheel to wheel stick with just the roll bar portion. And if its not going wheel to wheel, then the cage really is over kill, IMO - but you can never be too safe. I wouldnt put a cage in a car that sees any regular street use.

RJ

 

you don't have to drill to install
it may be a little cheaper than a roll bar
the front passengers won't hit their heads on it

 

a comprimise in convenience for safety and stiffness...

 

what convenience is being comprimised?

it is not a safety item... it is a stiffing item.

 

if the car is going to see track use, i'd buy the roll bar and deal with what small comprimises than buy 'stiffening' braces.

If you really wanted one on your car a local race shop could fab up a brace for you for 1/2 of what the other guy paid from japan.

you don't have to drill to install
it may be a little cheaper than a roll bar
the front passengers won't hit their heads on it

 

yes. they couls also make me an exhaust, intake, and many other pieces. this does not mean i will have them done, certain research and design elements are worked into these things, and i would rather pay for pieces that have proven they will fit/work rather than letting a shop fab them up. This does not hold true to every part, but in this instance i thought it would be less headache to buy and install the part rather than explain to a shop a design they have never seen and get a pre production quality piece.

 

Its a brace... if you use the same size material and you brace it in the same areas how is it different from the expensive JDM peice? Which i'm still not saying works, but it would 'function' in the same fashion.

RJ

 

I realize the diagonals are what carry the torsional load from lower left to upper right and lower right to upper left mounting points. But wouldn't additional members from lower left to upper left and lower right to upper right mounting points keep the cross members from flexing (providing for tension support between the bottom tube and diagonal member) better than the top member in tension...or would it be nearly the same effectiveness? Then again, the weight of two side members verses a single top member would be greater. There are pros and cons for every design....

Vracer111, who thinks that some basic structure stress diagraming as learned in a Materials and Structures class (trig/algebra based) might be "homework" tonight for fun.

 

The horiz/vert members won't do much, they're mimicking the body structure already. Best example is a bookcase or cardboard box. With a back/bottom, the structure is rigid/square. But take off the backboard or open the bottom, and you could collapse it to a parallelogram and beyond with one hand. The aformentioned bars act like the remaining sides.

 

The horiz/vert members act like a strut and they are required to work TOGETHER with a X-brace system.

Your concept of a bookcase and cardboard box is correct. The backboard acts like a diaphragm to transfer the later load - exact same purpose as a X-brace. Only difference is that struts are required for a X-brace system, hence the horizontal members at top and bottom. Without the struts, the X-brace would rotate and not provide any rigidity.

 

questions like what material to use? what to use to secure the bar to the hardware? the hardware to the secondary bracket? the measurements? don't need to be worried about by simply purchasing the product. by setting forth a little extra money, i was able to save time that is more precious. without a bar to work off of, we would need to start from scratch. this seemed like too much of a project considering i had more important things at hand.

 

Trying to stay away from the "culture" clash, street utility, and such...just playing with a little pen & paper geometry.

I guess my questions about the x bar are: what it's trying to stiffen, and how well it does it.

I can see that the lower horizontal bar would act as an upper strut bar (shock tower connector). It would keep the left shock tower and upper control arm mounting points positively located a set distance from the right ones.

I'm not so sure about the purpose of the X, though.

If it's to triangulate the box formed by the upper and lower control arm mounting points, it seems that it's putting quite a bit of weight high in the car to get that done. Seems that diagonal bracing from left top mounting point to right bottom and vice-versa would work as well or better, although it might involve some cutting. Or perhaps a box-truss type strut bar tying in to all four bolts on top of the shock towers. If nothing else, it would keep the weight lower in the car.

If it's to triangulate the box formed by the shock towers and seat belt mounting points, I don't understand why you would care if that area flexes, as long as the suspension points stay correctly located.

Apropos of nothing, I also wonder about the utility of stress bars of any type that have flexible joints at the ends. This seems a weakness in any stress situation other that pure compression or tenison.

Darn. I wish I had a good CAD program.

 

Well, I stayed out of the original thread because of all the hate that was flying around, but now that things seem to have calmed down a bit, I'll throw in my $0.02.

Got kids?

If you do, steal some of their tinkertoys and make yourself a little box out of them. Then grab the corners of one side and twist. Watch how the other sides deform. That'll give you a rough idea of how a car chassis deforms due to torsional loads.

Hey n1ek, want to do a test to see how much (if any) that bar is stiffening your chassis? Here's what you can do:

1 - Place the car on a level surface.

2 - Make a mark at the top of each fender lip with a piece of chalk or something -- use something that will not rub off immediately but you can wash off easily enough.

3 - Measure (and record) the distance from the ground to that mark.

4 - Jack up one corner of the car. Jack it from the lower balljoint or someplace similar -- you want to load the chassis due to suspension deflection, so you must jack from close to the wheel, not the frame itself. Jack it up a bit until the wheel is a few inches off the ground.

5 - Measure and record how far you jacked it up (the distance from the ground to the top of the jack is a good place to measure).

6 - Measure and record the distance from the ground to your fender marks again.

7 - Lower the car back down off the jack, remove the bar, and repeat steps 2-6, being sure to jack up the same corner by the same amount.

By comparing the heights of the four corners as measured in steps 3 and 6, you can see how much the chassis has deformed due to the suspension load.

You should repeat the test several times, using the same corner and different corners, to reduce the inevitable variation due to friction in the suspension. Another thing you can use to increase the amount of torque applied to the chassis is to jack up opposite corners (e.g., front left and right rear).

If you decide to do this test, and you don't know how to compute the amount of deformation, just post the raw numbers and I'll do the calculations.

I'm kinda curious how much difference it makes myself.