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Old 04-30-2013, 03:18 PM   #1
rene040269
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Default How to improve torsional strength on a C3?

I have discovered the distinguished c3 layout is not the most stiff setup ever made. My '80's frame is in good condition as well as my birdcage but i am surprised about the amount of flex in the car. When i jack it up on one place the door starts to jam and door gaps are widening almost 1/8th of an inch. I am warned to take the glass t-tops off when jackin the car up because they could shatter as the car bends when lifted.

Frankly, the frame/body of a C3 is as stiff as freshly boiled spaghetti....

I know it is an early sixties design but i am wondering if anyone has taken any effort to stiffen the frame to decrease torsional movement.

So a couple of questions:

Has anyone ever tried to ditch the rubber mounts and connect the birdcage directly to the chassis by means of bolting or welding to improve overall stiffness (make it more of a spaceframe like a c4 or later) or will this cause a lot of stress cracks in the car?

i have seen people mounting a roll cage in a c3 but how do they connect it to the chassis? Bolting it to the body will not improve anything i guess.

What can be done to improve overall stiffness of the car without welding in 200 kgs of steel. A c3 isn't one of the lightest cars ever made and i am looking for loosing weight, not adding it. I have seen the spreader bar for the front suspension towers which seems to improve riding quality a lot, proving the c3 frame is not very stiff to begin with.

I am a mechanical engineer and do a lot of strength calculations in my work and i cannot understand why GM had to use so much steel to produce such a weak structure. (no offence guys, i know it is 50 year old technology but i would like to improve things a bit if possible)

You can spend a lot of money on extreme suspension components these days but i don't think you will take full advantage of these high performance parts when bolted to a rubber ladder chassis, is that correct?

What about box-welding the frame main trusses?

I have seen people welding several steel sections in the cornes of the frame to stiffen it, do or don't?

Last edited by rene040269; 04-30-2013 at 03:26 PM.
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Old 04-30-2013, 03:31 PM   #2
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Originally Posted by rene040269 View Post
I have discovered the distinguished c3 layout is not the most stiff setup ever made. My '80's frame is in good condition as well as my birdcage but i am surprised about the amount of flex in the car. When i jack it up on one place the door starts to jam and door gaps are widening almost 1/8th of an inch. I am warned to take the glass t-tops off when jackin the car up because they could shatter as the car bends when lifted.

Frankly, the frame/body of a C3 is as stiff as freshly boiled spaghetti....

I know it is an early sixties design but i am wondering if anyone has taken any effort to stiffen the frame to decrease torsional movement.

So a couple of questions:

Can the birdcage be connected directly to the chassis by means of bolting or welding to improve overall stiffness (make it more of a spaceframe like a c4 or later) or will this cause a lot of stress cracks in the car?

i have seen people mounting a roll cage in a c3 but how do they connect it to the chassis? Bolting it to the body will not improve anything i guess.

What can be done to improve overall stiffness of the car without welding in 200 kgs of steel. A c3 isn't one of the lightest cars ever made and i am looking for losing weight, not adding it. I have seen the spreader bar for the front suspension towers which seems to improve riding quality a lot, proving the c3 frame is not very stiff to begin with.

I am a mechanical engineer and do a lot of strength calculations in my work and i cannot understand why GM had to use so much steel to produce such a weak structure. (no offence guys, i know it is 50 year old technology but i would like to improve things a bit if possible)

You can spend a lot of money on extreme suspension components these days but i don't think you will take full advantage of these high performance parts when bolted to a flexible ladder chassis, is that correct?
Why be concerned about chassis flex - design your suspension geometry around the kind of flexible parameters that the C3 chassis has - it hasn't stopped people racing them (or other cars with similar loose chassis winning) - some car designers use this flex as part of the suspension movement. Ever driven a Lotus Elise? - very much a 10/10ths car both on the road and track - initial suspension movement is surprisingly soft for such a car and gives good ride comfort considering seats are solid - step on things a bit and the suspension bites down in to progressive bump stops giving a far, far firmer ride and improved control. Works very well for such a simple basic system.
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Old 04-30-2013, 03:49 PM   #3
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A spreader bar either purchased from a vendor or DIY is a good first step. Older copies of the Chevy Power Book suggest reinforcing the welds on the frame and adding plates in certain areas. How far do you want to go? An aftermarket chassis with late model suspension is probably the best high end solution.

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Old 04-30-2013, 03:56 PM   #4
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The Chevy Power book has tons of info. You can indeed start by taking the body off and re-welding the frame and welding in support. The spreader bar mentioned will help as will a roll bar or full cage. Depends on how far you want to go. Buy the way, they cut through the body and weld the roll bar to the frame.
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Old 04-30-2013, 03:58 PM   #5
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I know you are right, the c3 line has a more than impressive racing history so i know these cars can be very, very competative. Although i don't know if the horsepower or the handling was the main reason.

I was involved as an engineer in production car racing in the past (bmw e30 and other unibody stuff) and the first thing we learned was to completely box weld the car structural components and to beef it up entirely and let the suspension do the work instead of the car. Also i would like to drive a car which will not bend like a pretzel when i lift it, just not feels right.

Any experiences?

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Old 04-30-2013, 04:15 PM   #6
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The Chevy Power book has tons of info. You can indeed start by taking the body off and re-welding the frame and welding in support. The spreader bar mentioned will help as will a roll bar or full cage. Depends on how far you want to go. Buy the way, they cut through the body and weld the roll bar to the frame.
What he said. GM didn't really build these cars to be world-beaters...they were built to give a semblance of performance, and since the only thing that even remotely resembled a Corvette in the American market was the practically-unavailable Cobra, GM felt safe continuing to market the car as it was until the early eighties when the whole world was screaming for something a little more modern.

If you go to a vintage race where there are C3's present or look back at what guys were racing 'back in the day', these cars nearly all had rollcages that were triangulated back to the frame kickups and forward to above the suspension mounts. The C3 frame can be made VERY stiff, but in unmodified form the pieces just aren't there to complete the package. And mounting the birdcage solidly to the frame won't help either-in actuality it would likely exacerbate the problem since the frame is the real problem, not the birdcage, and the rubber mounts actually help to isolate some of the frame flex from the body, so in order to properly mount the cage you've got to cut through the body to get the triangulation where it needs to go.

Trust me, if you go on at very much length here about how primitive these cars really are, you'll get hammered by the starry-eyed ones that seem to think these cars are above criticism. They were built down to a price- not up to a standard, and the bean counters said no to Zora a lot more times than they gave him what he wanted...
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Old 04-30-2013, 04:38 PM   #7
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Quote:
Originally Posted by rene040269 View Post
...i would like to drive a car which will not bend like a pretzel when i lift it...
It's your money.
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Old 04-30-2013, 05:39 PM   #8
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Hi mate, i've got a pro touring vette and the only way you'll get it stiff is a cage with 2 bars down through the rear stowage area connected to the rear of the frame connected to bars going to the footwells, i'm thinking of the door bars being the shape of the seat (kinda) to help access.

These cars are very floppy, annoying because a '70 Camaro is a much better base for a stiff car than a Vette due to the Vette having an older design.

I've owned both and the Camaro is much easier to make into a fast road car.
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Old 04-30-2013, 06:02 PM   #9
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Mines fairly stiff, I can't jack one wheel up without the other on the same side coming off the ground. If I jack the the rear wheel in the place where it it it supposed to be jacked, the front wheel on that side comes off the ground, that's the whole side of the engine to being lifted from a point way at the back.

To improve things I have solid motor mounts and a spreader bar. It actually handles pretty good especially since I put the modern ZR rated tires on it. Made all the difference in the world.

I don't see any reason for stiffening mine up anymore than it is for a street car. Rust comes from the inside as well as the outside and the frame may look OK from the outside but rusted like hell on the inside. I've never taken the roof panels off to jack the car, matter of fact I jack it from near the center cross-member and put jack stands front and rear now

You can do a body off and weld the heil out of spots all over the place but I don't think it's worth opening that can of worms
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Old 04-30-2013, 06:02 PM   #10
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There is an Australian gentleman ("Dirty Rat") with a beautiful black and white 69 that added roll bar size and wall thickness tubing to the frame of his Vette. The bars go from the front where the frame starts to come in (to make room for the front tire) to the tranny cross-member and from the cross-member to the crossbar that the diff snout mounts to. Of course, this was done on both sides of the car, with the result that jacking up by one tire lifts 3 tires off the ground.
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Old 04-30-2013, 06:08 PM   #11
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Like Shelby said about the cobra, everything flexed and it just worked!

I always thought it was pretty stiff for a no roof car. I've had convertible chevelle and cutlass. If you jack one corner up, you can't open a door. Now that's flex
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Old 04-30-2013, 06:59 PM   #12
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Quote:
Originally Posted by aaroncorvette View Post
Hi mate, i've got a pro touring vette and the only way you'll get it stiff is a cage with 2 bars down through the rear stowage area connected to the rear of the frame connected to bars going to the footwells, i'm thinking of the door bars being the shape of the seat (kinda) to help access.

These cars are very floppy, annoying because a '70 Camaro is a much better base for a stiff car than a Vette due to the Vette having an older design.

I've owned both and the Camaro is much easier to make into a fast road car.
Are you sure about that??

The Camaros from '67-'81 had a front subframe that was held to the rear half of the car by 4, 1/2" bolts, and there was NO triangulation at all, between the upper firewall and front frame rails......
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Old 04-30-2013, 07:47 PM   #13
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Gm reduced the frame thickness in the 81 and 82 cars, to save weight. Did this actually start in 1980? Is your frame rusty? Lou.
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Old 04-30-2013, 08:03 PM   #14
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Gm reduced the frame thickness in the 81 and 82 cars, to save weight. Did this actually start in 1980?
Yes. The cars were lightened for 1980. That didn't make them flimsy though.
I can jack my car up just in front of the rear tire, or just behind the front tire, and jack up that whole side of the car.
And while it's jacked up I can open and close the door, and it latches perfectly.
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Old 04-30-2013, 11:15 PM   #15
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Sure the 1980 Frame is lighter but I'm not sure any body knows how much, probably just a little. The body mounts are avalable in both Poly and Aluminum. All GM Frames from this era are known to sag in the front, some times a Frame shop is needed to get them back in alignment. The front brace is the best bang for the buck. Then comes a Roll Cage, depending on how many Points is is the stiffer it will get. But all of this Dates back to the 70's, ways of doing things. A SRIII Frame would be a huge leap forward in tech, even if a Tube frame is old tech for a real race car of today.
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Old 05-01-2013, 12:57 AM   #16
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years back, 03/04 when twin turbo and a lot of those type of guys were on here there was some talk about using expanding foam within the frame. I believe that around that time german cars were using expanding foam to strengthen/resist twisting of their frames or something to that effect. The concern that some raised was weather expanding foam purchased from home depot would be as stiff as what was used by car makers. There was also a concern of whether the foam would crack from the frame flex resulting in rattling foam in your frame.
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Old 05-01-2013, 03:04 AM   #17
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How much time and money are you willing to invest???
For my '69 i did the gussets and boxing recommended in the Chevy power book, I added the spreader bar, the solid disks to supplement the sombrero differential mounts and I'm using original solid body mounts. Just dropped the body back on, so I still have a way to go before I can compare to the ‘old’ car…
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Old 05-01-2013, 05:57 AM   #18
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...And mounting the birdcage solidly to the frame won't help either-in actuality it would likely exacerbate the problem since the frame is the real problem, not the birdcage, and the rubber mounts actually help to isolate some of the frame flex from the body, so in order to properly mount the cage you've got to cut through the body to get the triangulation where it needs to go....
In older C3:s the body stands on aluminium mounts instead of rubber mounts. Anybody know why GM went to rubber mounts with later cars? Comfort maybe?
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Old 05-01-2013, 07:10 AM   #19
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I think you're overthinking it.
The fiberglass body adds NO stiffening to the overall car, unlike all cars with metal bodies which do.
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Old 05-01-2013, 07:14 AM   #20
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I think you're overthinking it.
The fiberglass body adds NO stiffening to the overall car, unlike all cars with metal bodies which do.
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