Racers, chassis experts, is this frame crossmember needed anymore?

Twin_Turbo said:
No it does not. His rollbar is just a big hoop, there's no diagonal brace at all. It's basically a big U shape and without any triangulating it's very easy to deform and move the "legs" of the U shape relative to each other. This means that as the suspension gets loaded up one side of the frame may distort upwards and the other will not. The bolted in batwing will do very little to keep that from happening, same for the rollbar.
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Look, That crossmember does nothing to provide support to resist flex of the kickup area the way a full cage does. So neither does his roll hoop. What we're talking about is the linking together of the frame rails after the kickup. I would say its at least a wash between the hoop as he installed it and the crossmember and probably a lot better. The hoop acts as a torsion bar between the frame rails. Do you visualize that? Its sort of intuitive to me.

The point where things are going to flex are at the kickup point. That's the weak link. That's why i installed a rollbar that attaches the main hoop to the lower frame providing some structural link between the lower frame and the rear of the frame well past the kickup point.

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Did you get any statics when you got your engineering degree? I guess i could do some calcs to prove what i'm saying or even better, you could do them to try and prove me wrong. Or maybe someone has a computer modeling program.

Let's get deep into this shit.
 
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Hey, I'm here. I can here you. My four point "hoop is more than a stock 80-82 frame gets. GM deledted that crossbar and didn't even install an autopower four point roll bar. Would a diagnal to the roll bar help. I need to add a lower mount for my shoulder harness, I think. The jury is still out on that one.
Bee Jay
 
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I have an 81, and my frame does not look like that. The tranny x-member is detachable, and has no exhaust holes.
 
Bee Jay said:
Hey, I'm here. I can here you. My four point "hoop is more than a stock 80-82 frame gets. GM deledted that crossbar and didn't even install an autopower four point roll bar. Would a diagnal to the roll bar help. I need to add a lower mount for my shoulder harness, I think. The jury is still out on that one.
Bee Jay
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I would think any tie between the frame and kick up would only help.
 
big2bird said:
Bee Jay said:
Hey, I'm here. I can here you. My four point "hoop is more than a stock 80-82 frame gets. GM deledted that crossbar and didn't even install an autopower four point roll bar. Would a diagnal to the roll bar help. I need to add a lower mount for my shoulder harness, I think. The jury is still out on that one.
Bee Jay
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I would think any tie between the frame and kick up would only help.
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Is the weight penalty worth it though? That's why engineers perform calcs and do tests.
 
Bee Jay said:
I need to add a lower mount for my shoulder harness, I think. The jury is still out on that one.
Bee Jay
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I'm not sure its all that bad. The instructions i got with my G-Force 5 point belts said to attach the mounting point above the shoulders to prevent spine compression.

I say TT did his WRONG but some people are beyond question, so nobody says anything.
 
Jeff, that's my frame, I added the earlier crossmember to it.

Turtle, do you really think I am going to answer this here? It's clear you are out to start crap again.
I have the feeling YOU can't even do the calculations for the frame stuffness for the crossmember vs the non diagonal braced hoop (upper pic that racervette posted)

I know I have it right, I have a frame model of my entire car but I'm sure as hell not gonna waste time with cosmos just to please you. I don't need to prove my point here. If you want to prove me wrong, be my guest. All you do is shoot with shot and hope you hit something,

As for the seat belts, how do you know mine are wrong? Oh sure, it's just to take a stab at me. I can tell you this, mine are positioned exactly right, I followed the simpson instructions to a tee. You are just looking at the vertical position in the car but what's important is the angle of the belt going to the mounting point in respect to the angle of the spine. My seats are inclined a lot because otherwise I don't fit in the car, also the floow is 1,5" lower than stock and the seats are mounted almost directly on the floor.

You need to stop the keyboard warrior deal. When I had you on the phone you were as meak as a lamb and now you start this crap again, is it because I gave you 10 days off for calling someone else a drunkard?
 
Bee Jay said:
Hey, I'm here. I can here you. My four point "hoop is more than a stock 80-82 frame gets. GM deledted that crossbar and didn't even install an autopower four point roll bar. Would a diagnal to the roll bar help. I need to add a lower mount for my shoulder harness, I think. The jury is still out on that one.
Bee Jay
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What is your c3 going to be used as? autox, hpde, drags, Street?
 
Guys, let's chill. Let's get along. What we need is "World peas" and a little carrots to go with that.
I only use my car on the street now, when I installed the four point roll bar I was doing a lot of Autocrossing and canyon driving. Now I drive it to work occasionally to give the kids at the bus stop something to wave at, and I love to take it up in the mountains. I also want to get the car into the 12 second quarter mile club. The car is like my Harley. I spend more time wrenching on it than driving it. It keeps me out of pool halls and bars. But when I drive it, I drive it like I stole it. Then I put it away for a week or so. The car will probably never need to pass a serious tech inspection. The local Corvette Club only inspects for Snell '94 or later helmets, working seat belts, two springs on the carb return, a tied down battery, and no loose articles in the car. I usually run without my t-top.
Bee Jay
PS If the car flexes noticeably, I will weld in another crossbar. The original was in the way while mounting my batwing.
 
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This is just like the thread about balancing the rotating assembly or not.... keep the personal stuff out of technical discussions, if you don't agree with someone else's opinion try to prove your point.
To me it is as simple as this: removing material from a 30 year old frame makes it weaker, adding material usually makes it stronger... anyone disagree ??
 
MYBAD79 said:
This is just like the thread about balancing the rotating assembly or not.... keep the personal stuff out of technical discussions, if you don't agree with someone else's opinion try to prove your point.
To me it is as simple as this: removing material from a 30 year old frame makes it weaker, adding material usually makes it stronger... anyone disagree ??
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GM removed the bar in '80 after replacing it with a batwing, making it weaker? I removed mine in 2008, but I added a four point harness and a batwing. The batwing has poly bushings now, and I'm tossing the metric bushing inserts for 1/2" inserts. The 1/2" bolts fit much tighter. Maybe GM removed the crossmember because the batwing is now a stressed member.
I will drive the car. If I feel the frame is weaker by removing the crossmember, I'll weld something better in it's place. Of course I will consult with you guys first.
Bee Jay
 
Bee Jay said:
GM removed the bar in '80 after replacing it with a batwing, making it weaker?
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Well, if you replace the crossmember it probably doesn't change anything.... I would think the batwing is now stiffening the frame just like the crossmember did... well almost as the bushing allow some flexing....

My opinion is if you can leave it in place then leave it in place, the weight saving is not worth the potential weakening....
 
MYBAD79 said:
keep the personal stuff out of technical discussions, if you don't agree with someone else's opinion try to prove your point.
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Is'nt that why most of us ended up here? Other forums (and I dont care witch one) have some people who will get personal real fast and it's anoying.

I dont agree with everything that is done here but I respect what each and everyone's research.

Did I use the word "respect"???

Lets have fun with our hobby. :thumbs:
 
I've been thinking about it some more. Simple statics analysis isn't going to cut it. We need to know the twisting and bending moments of the crossmember in question as well as the roll bar tube. An analysis with computer modeling will still be highly dependent on these parameters, so we're back to getting an intuitive feel of what's going on.

Think about the fact that there is a beam at the back end of the frame which provides similar stiffening but using the greater mechanical advantage out at the end of that longer beam. The beam in question becomes redundant to an extent. There is no question that it provides some stiffening. The question is whether the additional frame stiffening is worth the weight penalty. You could add more crossmembers and cross braces to our frames until you make the car weigh 4000 pounds, but is that an efficient design.? GM engineers were ok with removing that piece so that tells us something. We could go back and research all the old road and track skid pad and slalom performance data. I doubt the C3 ever went backward on handling.

As far as the seatbelt mounting goes. Maybe we should start another thread to discuss this important issue. Bee Jays point about the stock shoulder belt mounting point is a good one. It's important not to have any spine compression in a collision. It just looks to me like the points where the shoulder belts pass through TT's seats is a little low. He's pretty tall so i was just questioning that. The way i presented that was a little harsh. Telling someone that their seatbelt design looks like a "hangman's noose" is a little harsh too. As much as he hates to hear it, he and i are a lot alike and somtimes likes repel.
 
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Here is a picture of Yellow 73s four point roll bar. It's just like mine except the crossbar. What if I add a crossbar like that to mine. I would put it much lower so that I could still put stuff back in my 79' storage area. It would give me a lower spot to mount my shoulder harness and it would replace the cross bar I removed from the chassis. Thoughts?
Bee Jay
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turtlevette said:
GM engineers were ok with removing that piece so that tells us something. We could go back and research all the old road and track skid pad and slalom performance data. I doubt the C3 ever went backward on handling.
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Full of assumptions here.
The later C-3, say 80-82 was put on a strict diet to meet EPA and Gov. mileage standards. The alum rear end was "adequate" for the lame 190 HP L-81 engine. Between detuned and "smogged to death", it was adequate. Factor in EPA mandates, GM bean counters, GM lawyers, DOT "Crush Zone" requirements, the lack of a wet noodle convertable model, the added rear window area, (Yes, glass can be structural when glued in), the national 55 MPH speed limit and the 85 MPH speedometer, and the rear impact bar and shocks that were added, it just may have been removed by necessity, and merely adequate.
The C-3 was dying, and these stop gap measures mandated the C-4 upgarde to restore any hope of performance.
 
My new harness mount is lower than the seat holes

I'll take some pics
 
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