Thread: Question for techinspector1

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Hi Richard. One of these days I may have some time to get back to work on the "Z" chassis. When I do, one thing I need to do is beef up the back half.

Art Morrison recommends the brace from the main loop to the back half rails (Like the ones I have tacked in place ''purple arrow''). Then adding "X" braces that I have drawn in ''blue and yellow''. But, I took a little "artistic license if you will when I put an aluminum housing in instead of their steel 9" housing. and i also added an anti-roll bar that's mounted behind those braces. The "red arrow" shows my mounting point for the shocks and the anti-roll bar that is beyond the 1 5/8 tube I have tacked in as a support from the cage to back half rails. I've seen several cars that run the brace back to near the rear body panel as I've shown in the pic. in "green.". Art Morrison contends a brace like the "green" brace will actually provide less support than the type I have which stop at the top of the back half rail "humps". I will also be mounting a chute to the frame rails further increasing the stress on the back half rails.

Contrary to Morrison's recommendations, it seems to me without the "Green" brace all the stress of the shocks / anti-roll bar / & parachute will bear upon the area of the frame rail between the "purple and red" arrows. Tech guy "Ritch" said ''you can't build the back half strong enough''. I didn't think to ask him this particular question during tech that day. In your countless inspections of door slammers, what seems to be the best most efficient set-up for back half bracing? I want it strong but don't want to just throw braces in any ol' way making it unnecessarily heavy. ( and fuggly). Thanks for your input.
Gary

Here's a side view that might be a little better showing the stress points on the back half rails. Looks to me like there will be allot a stress on the "hump" of the rails without some extra support?

Gary, you're really putting me on the spot here, because I'd never try to second guess a chassis builder who probably has an engineering degree and access to stress analysis of his product. I have dabbled into it through the years and I've determined that you always want to install a bar using tension or compression, never putting a bend in it or introducing a bending load into it from another tube member. Like you said, I've looked at a bunch of cars through the years and have determined that the stiffest cars use lots of triangulation, the strongest geometric shape. I can appreciate your remark about fuggly, but you know Gary, there is nothing fuggly about a properly triangulated cage/support with "too many bars". On the contrary, it's a thing of beauty and a joy forever. Your remark about "heavy" is appreciated, however, each 100 pounds in a car equals 1/10th second in e.t., so what does it matter if you put another 100 pounds in the car and go 6.90 instead of 6.80? One tenth quicker won't seem so sweet if you begin to develop stress fractures in the whole mess. There's a builder here in Phoenix named Neriz who has taken the cage building craft to a new level of art form. He has triangulated bars going every which way and it is absolutely beautiful. He has had a little trouble with chassis cert in the past because he has tended to use undersize bars. In other words, where a 1 5/8" bar is called for, he might use two bars of 1 1/4", so although you can see that the car is probably stiffer than using one bar, it might not adhere strictly to the rules. Irregardless, you just have to stand back and admire one of his cages with bars every-damn where like a fine painting.

Sorry, I got off on a tangent. As far as the long bars shown in black that go from the B-bar to the rear of the main frame rail, I would try to make them straight without a bend in them and eliminate the short vertical green bars near the bend. Those short bars will be in a bending load during chassis flex. I'd leave the vertical green bars in at the chassis hump. Although they'll also be in a bending load, they'll help to support the black rear braces in their long run to the back. I'd put the blue and yellow X braces in and I'd also X-brace the black rear braces and I'd run a cross-bar between the black rear braces at the intersection of the X, like a top fuel wing strut.

Gary, I hope all this doesn't wipe the smile off your face, but I tend to go with Rich's assessment. I don't think it's possible to overbuild a car and if you vary the diameter of the tubing at different points, it'll add to the eye candy, breaking it up into a visually-appealing art form. Personally, I'd rather have a car that's stiff enough to maintain its integrity through many years of abuse that might be a tenth slower than to have a flexi-flyer with stress cracks that nobody wants to buy down the road. That's my $.02 worth.

Pro,

If you don't mind an unsolicited comment . . .

It's been a long time since statics class in E-school, but here's what I see.

I agree with what Tech said, but I would leave both of the short, green vertical braces in place. As the chassis flexes upward, you'l get compression in that top tube. If you think about a long stick in compression, it will try to bow out in the middle. Those uprights will be in tension and fight that tendency. I don't see any bending moments to worry about.

I'd also think about an "X" behind the rear axle.

Thanks for the help Jack, an educated opinion is always welcome.

Thanks Richard and Jack for the pointers. I'm hoping to build this thing right the first time so i appreciate all the advise. Good thing about not having much time to work on it ......... gives me more time to plan things out. Thanks again.

Jack, I understand your assessment of the vertical green bars preventing bowing of the main rear braces and that's why I suggested leaving them in at the main frame hump, roughly halfway down the bars. I may have been using the wrong terminology when I said "bending" load. What I was trying to convey is that they will be adding nothing to the structural integrity of the structure because they are neither in a major compression or tension load, only preventing bowing of the rear braces, but that the twisting force of the structure would tend to try to break the welds at their junctions with the main frame and rear braces. Am I looking at this wrong? And if you have the rear braces nailed down with the anti-bowing vertical tubing at the main frame hump, do you also need the anti-bowing verticals within a foot of the terminus of the rear braces at the main frame rails? Help me out here, buddy.

Richard,

Here's how I looked at it. In order to tie that long diagonal to the rear of the chassis, Pro put a bend in it. I felt that a strong upward force on the rear of the chassis would try to increase that bend, since the force was not acting in a straight line up the axis of the tube. The lower vertical brace keeps that from happening. Think about what a plain ol' steel gusset near the end of the tube would do. It effectively takes the bend out of play.

You suggested something that would accomplish about the same thing when you recommended making the tube as straight as possible.

Make sense?

Concerning the contribution to structural integrity, sometimes braces or stiffeners do as much for the structure as the compression and tension members. Think gussets again.

Bear in mind that I'm not a structural engineer (although I did a lot of similar stuff in the pipeline industry) and I didn't run any type of analysis.

There is something in that chassis that I'm going to take a further look at as soon as I copy the photo down. Film at 11.

Thanks Jack, I appreciate your input

Gary, about the cross bar in the rear braces at the intersection of the X that I suggested, what if you used a piece of oval "aero" tubing. That would be a conversation piece, don't you think?

Pro,

Not baggin' on your chassis, because I think it's plenty strong. However, a couple of joints caught my attention - black circles.

Ending a structural member in the middle of another structural (unless it is tied through) is something I tried to avoid wherever possible.

Just a thought.

Originally posted by Henry Rifle
Pro,

Not baggin' on your chassis, because I think it's plenty strong. However, a couple of joints caught my attention - black circles.

Ending a structural member in the middle of another structural (unless it is tied through) is something I tried to avoid wherever possible.

Just a thought.

No problem, everything is still just tacked in place at this point so now is the time to find the bugs. I wondewred about those circled areas too. That's the way the SFI book recomends doing it. I figgured maybe that's supposed to be some sort of crumple Zone or sumpthin' ???? although it's purdy' close to my a$$ to be crumpling??

I typed this up while I was waiting for the puter' to become available again.........
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My Son needs the internet so this might be old news by the time I get back on.

This is the side view of the green brace.(Made it straight). If I install the Green brace would the short one I have tacked in under it now be ness.? Or would I be better off eliminating the short ones tacked in now and go with the full length Green brace adding and "X" between the Green bars? Then a yellow "X" from the first pic. and as Jack suggested an "X" either on the back slope of the back half rails or maybe in the far rear flat part of the rails? I will be putting a parachute brace back there so if I put an "X" way in the back I would have to integrate the chute brace with the "X" brace.

Another thought was use the Green brace with an "X". Then replacing the existing braces with an "X" that extends from a point on the outside corner of the ''right'' main loop to the "left" hump of the back half rail (Where the existing brace is tacked on now) ........ and "left Main hoop to "Right" hump. (Pink "X" right hand pic.) This "X" would be lower than the green braces and at a lower angle creating more triangulation of the bars and maybe shoring up the cage area at the same time?

Still going with the yellow "X" from pic. #1 and the "X" Jack suggested for the back side of the frame rails.

If I'm reading the book right that I need the Green braces (If they are to be the main ''Cage to Frame'' supports to be 1 5/8" tube ......... or is this "elective" territory as far as certification? Tech guy said I can use chromoly on my mild steel chassis if I want to. I've only used chromoly tube for bolt on suspension parts so far.

If that's what SFI recommends, keep it. Better the chassis absorb energy than you.

I've been thinkin over the "green braces." When you make them straight, it really cuts down the angle of support. You might be better off going out, then down and making a couple of triangles. Check the back end of this CE chassis for example.

http://www.chassisengineering.com/chassis.html

You could use chrome moly if you want, but you don't need it. That's why the different wall thicknesses for CM and steel.

That looks like a good idea. Looks like it would double as a cage for the fuel cell too.

"or is this "elective" territory as far as certification? "

Gary, I'm now unsure which 25 spec you're using for your build. I have 25.2, 3 and 5 in print here in front of me and want to help you get this nailed down. As much as anything, I'm writing this for Jack to get him up to speed.

First, the object of the spec as read in the beginning of 25.4:
SFI Specification 25.4 is a design and construction quality assurance standard for a full bodied car tube chassis roll cage, used in the sport of drag racing competition. This specification is concerned only with the protective characteristics of the driver area and its ajoining structure in the event of a crash and not with the racing performance properties of the frame.

What this says is that other than the rear braces, the spec is unconcerned with anything forward of the A-Bar (front hoop) or rearward of the B-Bar (main hoop) and is only concerned with driver safety within the "box". They're saying that except for the rear braces, it is up to you to properly triangulate the rear and front structure so that the car will hook and remain consistent, they really don't care.

For the rear braces, this spec offers four options and I'm pretty sure your spec, if you're using 25.1 (which, again, I don't have a copy of) will say the same thing.

All required rear braces must be fully welded. Listed below are the options:

A. If the material used is 1 5/8" X 0.058" chromoly or 0.118" mild steel, two bars of any length are required.

B. If the material used is 1 1/2" X 0.058" CM or 0.108" MS, two bars, thirty (30) inches or less, must attach within five (5) inches of the main hoop. (What they're saying is that if you attach the rear braces on the vertical portion of the main hoop rather than the horizontal portion, the braces must attach no more than 5" from the top of the horizontal portion of the main hoop).

C. If the material used is 1 3/8" X 0.049 CM or 0.108" MS, a minimum of four (4) bars are required with a minumum of two (2) bars being attached to the horizontal portion of the main hoop.

D. If the material used is 1 1/4" X 0.049" CM or 0.108" MS, a minimum of six (6) bars are required with a minimum of two (2) bars attached to the horizontal portion of the main hoop.

So, as long as any of these four options are met, the rest of the structure is up to you.

Personally, I like option C with two bars the width of the main frame rail attaching the horizontal portion of the main hoop to the frame rails and the other two bars attaching at the intersection of the main hoop / halo bar and running down to the main frame hump like the hot pink bars you showed in the last photo.

I will have to say that like Jack said, I don't particularly like the black rear braces laid out flat in your application. It just seems to pinch the whole rear section too thinly at the rear of the main frame humps. Put the bends back in them where they attach to the main frame rails and use the green uprights you had in originally.

At this point, after you've satisfied the rule with your rear braces, it's up to you what else you want to do to stiffen up the rear. I've seen all kinds of different arrangements.