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....:eek:

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?

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.

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.

Rob

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.

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?

:iagree: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...

It's your money.
:thumbs:

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.

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 :thumbs:

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 :cheers:

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.

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 :hide:

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......

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.

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.

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.

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.

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…
http://www.islandtimeinvestor.com/it...08_jpg_jpg.jpg
http://www.islandtimeinvestor.com/it...43_jpg_jpg.jpg
http://www.islandtimeinvestor.com/it...12_jpg_jpg.jpg
http://www.islandtimeinvestor.com/it...21_jpg_jpg.jpg

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?

I think you're overthinking it.
The fiberglass body adds NO stiffening to the overall car, unlike all cars with metal bodies which do.

:iagree: :iagree:

I understand, but i am not talking about the fiberglass body. The pressed steel sheet birdcage is the structural component for the body and should add some stiffness to the overal structure if connected rigidly to the chassis.

But that is theoretical, the car was never designed this way so just simply welding the birdcage to the frame could introduce more problems then it solves.

I do like the box welding of the frame as shown on the pictures above and i am also very interested to know why they changed from alu to rubber body mounts.

In the future my '80 will be stripped down to the bare frame and i am planning to rebould it as good as i can get it, so that is the change to modify things if worth it.

Thanks for sharing the knowlegde. The Corvette Power book is long gone out of the bookshops i guess?

I understand that some folks who have installed a "shark Bar" that connects the upper shoulder belt mounting point to the floor seatbelt mounting point with a solid bar behind the seats along with a bar on the floor connecting the lower seatbelt mounts would help. I would think that the bar along with a front spreader bar between the shock towers (which I have and DEFINITELY helps rigidity) would be a BIG improvement. Some folks say the shark Bar does not work, others WITH the bar especially in convertibles say it is a big improvement. Any Sharkbar C3's that can comment?

http://vetteworksonline.net

I do have a heim jointed bar on each side of my C3, like a mini spreader bar, that connects the upper seat belt mount to the lower seat belt mount, but no shark bar on my 78. I can say that my car with the front spreader bar and the heim joint bars on either side behind the carpet does make the car stiffer-mostly the front spreader bar and MUCH quieter-no squeaks!

:lurk:

I don't have those on my C3 but on my C4 I have a Camber brace (ie. spreader bar) and a bar which connects the seatbelt mounts together (cannot remember what its called but also from R-D Racing).

Without the bars I can jack up one wheel of the car. With them its not possible, when I am jacking up any corner of the car both tires on the same side lift off the ground at the same time. So, atleast C4 chassis is stiffened by a great deal by these bars. I know the chassis is very different but maybe (and I do believe so) they would also help with C3 stiffness.

BTW. does a '70 coupe have a shoulder belt bolt connection in the B-pilars under the panels?

I have noticed the same thing on mine as well and while it's jacked up I can open the doors no problem.

Mine as well! Must be the condition of the frame versus others!

To eliminate squeaks, rattles, and rocks-in-my-pocket complaints from 68-72 owners.

:thumbs:

The more crossmembers in a ladder frame stiffens the torsional stiffness. One way to create more of a crossmember effect, front and rear motor plate's, the only good way to fix it is a full rollcage, triangulate.

Camaros were way flexier than Corvettes in their stock form. If you had a bit of power and tires that hook, it didn't take many runs before the doors wouldn't open and close right anymore. I have even seen the windshield break on launch with stock bodied and framed Camaros. It is a must to tie the frames and body together on those cars.

The Vette Verts have no structural integrity in the body behind the doors. The top of the sides have a deck lid that opens so there is nothing tying them together from one side or the other. The doors have guides to try and tie the rear body to the closed doors so the doors don't fly open from all the flexing. A Shark bar or something like it makes a big difference on the Verts.

If you want to talk flexy, look at the 59 to 64 full size Chevys with the "X" frame. I have a 64 SS. Look at this 60 on Youtube. Talk about twist and flex!:willy:

Absolutely. Noise, Vibration and Harshness isolation. Sometimes abbreviated as NVH.

Actually in the video above, I did not see much frame flexe at all. What I saw was suspension flex. Quite abit differant scenerio.

Several years ago I bought a used frame to modify to install a C4 suspension. While I was working on it I tried a few things to try to improve the overall stiffness (torsion and beam). I anchored the frame to my lift while I was doing the torsion measurements (@ 2000 ft-lbs).

http://i96.photobucket.com/albums/l1...ps77168886.jpg

First thing I did to the frame was finish all the seam welds.

My biggest desire was to put some sort of torsion box in the tunnel, and connect that to the frame rails, but there was minimal available room in the tunnel due to the 5 spd trans I had, so I tried another option, a bolt-in torsion tube between the trans crossmember and the pinion crossmember.

http://i96.photobucket.com/albums/l1...7/IM000793.jpg

This gave me a couple hundred ft-lbs of additional rigidity.
I would have liked to have been able do something similar ahead of the trans crossmember, but obviously the engine and trans already occupies that real estate. So I tried to stiffen up the front half of the frame rails. I put some tubing inside the frame to add a bit of torsional resistance.

http://i96.photobucket.com/albums/l1...7/IM000937.jpg

With that done I cut open the rear frame "kickups" and welded in an additional vertical wall to help resist the springiness of that area. I also added tubing between the upper shock mount and the pinion crossmember to help stiffen up that area. The C4 trailing arms placement allowed me room to install that piece.

http://i96.photobucket.com/albums/l1...7/IM000977.jpg

I got about 40% improvement in the torsional readings with these mods. They weren't free (time, material, and about 30# of additional weight), but it was an engineering exercise I was interested in doing at the time. My "rules" were anything goes regarding frame modifications, but I did not want to cut up the bodywork. (If I had no restrictions on bodywork modifications I would have enlarged the tunnel a couple inches and built a "large" diameter torsion tube inside there to connect the rear 2/3 of the frame length.) Once I was done playing around with the frame and installing the C4 components I then swapped the body over onto it.

http://i96.photobucket.com/albums/l1...7/IM001052.jpg

The stock frame can be improved with a little work obviously, but IMHO unless you can install some material in the Z (vertical) direction, either by a tunnel torsion tube or a cage with correct tubing orientation, there are significant limitations on how stiff you can make an essentially "two dimensional" frame. I mention "correct" tubing orientation, as I made a few test models during the frame build, and a few members kept asking for results with different configurations, so I kept modifying the model. Some of the models incorporated significant amounts of additional "tubing" without significant attending improvements in the torsional stiffness.

http://i96.photobucket.com/albums/l1...7/IM000868.jpg

Rene, GM went to the rubber body mounts in 1973 because of them going to steel belted radial tires. The steel belts added too much harshness to the ride. Lou.

69427,

Impressive jig work on the frame modifications. In retrospect do you feel the extra effort was worth it versus simply box welding all the frame seams on the garage floor/jack stands?

I just got my 72 vert body off the frame last night, it's still on the dolly ready to be moved to storage tomorrow night, so I was planning to start frame work this weekend. After seeing your work, I guess I'm debating whether to focus on engine/driveline now (got a bare new LSX block ready to start focusing on) or devoting the next few months on that level of frame work. Your expertise and experience is appreciated.

You can make them very stiff but at the cost of added weight.

Monty what's the plan this time N/A or Turbo or S/C ? :cheers:

When I had my frame out I also added gussets, boxed the front, and went over all the weld continuously. Another problem I had were the body mounts rotted out on my birdcage. I cut out my body mount areas in the A and B pillars, cleaned up the birdcage and welded new 3 piece rocker channel inserts on both sides of the birdcage, doubling the steel down both sides and into all 4 pillars. I also have a VBP spreader bar, and a BTO trans crossmember. I used poly body mounts, actually all poly throughout. Also, my vette weighs 3,180 lbs.

A Pillar
http://i229.photobucket.com/albums/e...s/IMG_0546.jpg

B Pillar
http://i229.photobucket.com/albums/e...s/IMG_0545.jpg

Center Rocker Channel
http://i229.photobucket.com/albums/e...s/IMG_0538.jpg

The problem with the vette is the chassis rails are too long for how tough they are, you could box them, seam weld etc but fro most people putting a cage in is easier.

When I drive my vette I can feel the shock waves running from corner to corner when I hit a bump, it's the chassis flexing. I drive my car very hard and it annoys me because I know that whatever I do to the suspension it'll always be there, it's just a crap design:ack:

So for me i'll fit a cage, one that triangulates the rear of the chassis and connected to door bars that follow the contour of the seats to end in the footwells, this will be more of a stiffening exercise than a safety one, although it'll help safety too.

The '70> Camaro is a great base for a pro tourer, the one piece shell at the rear is stiff and can be made very stiff very easily.
And the front frame rails are nice and short so don't flex much, just add solid frame mounts, a set of shocks and antiroll bars and you have an awesome corner carver

The Front steer design of the Camaro is a massive improvement over the crap vette design, i'd like the change the vette to front steer although this would require a new crossmember.

The '71 Camaro I had would run rings around my vette with 1/3rd of the work done to it.

All in all the vette has a great engine, trans, brakes and suspension....but has a crap frame and steering:thumbs:


My Camaro -http://i877.photobucket.com/albums/a...psdb099983.jpg


My vette -http://i877.photobucket.com/albums/a...tteburnout.jpg

Motorhead,

Turbo, but I'm just at the bare block ( LSX 9.720" tall deck still in the box, and UPS stalking a pair of 305cc mast rec port LS7 12 degree heads) I just lifted the body off the frame last night, so I've got a ways to go (and alot of work).

:iagree:

buy a after market frame ,they start at $3999.you can custom order it ,and mount a c4 suspension to it .

That is quite an impressive build you have. I like the way you have used your lift to anchor the chassis and measured the torsional strength.

A ladder chassis is by definition a two-dimensional structure and just adding crossmembers does not make it stiffer. (that is not completely true, but you will mostly add weight and a little increased stiffness).

You have made some very interesting modifications, i will certainly take this into account when i ever remove the body.

There was a member on here that did quite a bit of "two dimensional" frame stiffening. I forget his username but he experimented with the structural foam filling. I looked into it and the one drawback is that you better be 100% done modifying your frame before you foam fill it because you can no longer weld on foam filled areas. Im waiting to figure out how to add another pickup point for my cage (among other things) before I dive into the foam filling. Seems like a very good lb per flex reduction option.

Well, for my purposes I believe it was worth it. I like to do a lot of track days with my car, and that involves tweaking the suspension geometries and spring/bar rates to get these antiques to handle decently at speed. Any improvement in the frame stiffness reduces the "fuzziness" of the suspension changes.
For a purely street usage vehicle I think the main benefit to a stiffer frame is a slight reduction in the squeak factor when driving over uneven pavement.
Fully welding the frame will increase the rigidity, although modestly, and it's an easy thing to do when the body is off. A couple days of grinding and welding will yield a slightly more pleasant personality to the car in everyday driving.

I would think that a 4 pt cage would solve most of the problems.
That is on my list.

R

I have a Sharbar on my vert and yes it helps immensely, felt like a different car 1st time I drove it after the install. I also have 550 lb front springs, a VBP 360 rear fb spring with Bilsteins front and rear. The one drawback to the SB is that is restricts access to the rear hatch somewhat due to the lower bar, but if I need to put something larger back there I just open the rear deck. Just remember that this is not a true roll cage while it does add rigidity, ease the installation of a 5 pt harness and looks nice it can not be thought of as a substitute for a true roll cage to use at the track.

Thanks for the feedback! That's what I thought. Do you have a front spreader bar as well between the front shock towers? I was shocked how much the Speeddirect,com bar helped my 78-with the stock OEM mechanical fan, not electric fans! I would imagine that a Sharkbar and a front spreader bar would be the ideal setup outside of a full roll cage/bar which is not practical on the street.

Don't have the spreader bar but am seriously considering buying one from S Direct. Shipping up to Canada is rediculous if they do not go through USPS as the couriers hit us with all sorts of addl'n fees. At least Corvette Central from time to time has free shipping and no addl'n border fees.

:iagree:

What a difference in ride comfort....before avoided all roads that were not smooth, avoided driving over R.R. tracks, it was just brutal...

In a post above, I mentioned the stiffening that "Dirty Rat" did to his 69 frame. I found a photo that he sent a while back and it's in "My Corvette Photos". The bracing is slightly different at the rear than I remembered.

So don't tie in birdcage?
 

Lots of good info here. I built a backbone frame in mine and then tied that to my kick ups to give it a 3 dimensional component. I cut up my floors, etc. I am at the point where I could weld the birdcage down to the frame but am apprehensive since I don't want to create any body issues. My frame is heavier but it's a street car so it is a trade off I was willing make. Also, all the weight is very low in the car CG is good.
I think vertical tie in to the birdcage could add a real torsional benefit to the chassis.
Has anyone ever tied them together? Results??
Thanks

That sure does sound like you have to post some pictures....:thumbs:

I am very interested in your solution. When the body of my '80 comes off i want to make the frame as rigid as possible without adding hundreds of pounds of steel, so i have to find the "sweet spots" where adding steel is the most effective.

Any pics of your progress would be greatly appreciated!

Thanks

Please post some pictures. :thumbs:

A backbone setup has always been my prime interest in stiffening a C3 frame. It puts the additional torsion strength/material right in the plane that the frame twists in. If it weren't for possible cutting and modifying of the tunnel shape to clear the backbone structure I would have done that to mine several years ago.

how much stability is a fiberglass body going to give you?

On the 70 to 75 Verts, having the door hinges tight with good bushings and pins and the door guides adjusted correctly and fitting snug will help tie the front and rear of the body. Good guides and latches adjusted correctly on the deck will help too.

I was thinking that if a person used aluminum body mounts along with a 4 point roll bar that attached to the frame and also to the soft top mounts at the sides, it would really tie the car together, short of using a 6 point roll cage. I just really don't want a full roll cage in my Vert for mostly street purposes.

I have solid motor mounts with a BTO cross-member I will use a poly tranny mount on. Probably will mount my differential solid by removing the bushings in the cross-member and welding disks in their place with a solid front mount. It will raise the diff approximately 3/4" lowering CG and raising the roll point. It should really tie the rear together and make that a lot stiffer.

I have a spreader bar and tubular upper A-arms that I am going to lower the frame mount position to raise the roll point on the front.

All this combined with welding the frame and adding a lot of gusseting should really tighten the car up. I'm not really concerned about vibration and noise form everything being mounted solid. With the top down and the pipes roaring, who's going to notice?

I also have Steeroides rack and pinion, hydroboost and 18" modern performance tires. This thing was loose as a goose when I got it. I can't wait to finish it and go for my first ride!:woohoo:

Now if my back would heal up, the weather would warm up, and work and life would get out of the way, I could get going on it!:(

First pic post
 

This is the concept. I will post pics soon. But the basic idea is to use stringers fore to aft and the tie it over the drivetrain but under the floor to the main crossmember. Then continue the top portion on back to the next crossmember. I then triangulated back to the main crossmember and stringers.
You can utilize the space behind the rear firewall panel to tie in the frame or body as you desire and keep it under the skin. Just remember I cut up the floor since there is no way I know of to do the backbone unless you are willing to cut it. I am not done yet but have a good start. I took square tubing and made a framework over the transmission and made it fit around a T56. This is welded to the stringers. I boxed it under the center console to tie it together. I plan on using a bolt in crossmember under the transmission to tie it together more effectively. I lowered the floor because I didn't like how I sat so that gave me more options on seats but created more challenges. So I don't pretend to be a great fabricator or have the tools it requires to be one. I am still trying to figure this set up out. So when you see my pics remember its a work in process....

Great explanation, i am now even more interested in the photo's. You are thinking in the same direction as i am. Only problem is i still haven't figured out a way to strengthen the front end. Your sketch show a great improvement in the rear but is leaning back to a 2-dimensional (flat) front side. Very little room to work there. Have you thought about a structure under the dash connected to the transmission tunnel reinforcement and under the hood to the front?

Keep up the good work!

A friend of mine who worked with plastic manufacturing for years, retired now, said he would fill the frame with structural plastic. Said it would make the frame several times stiffer without much extra weight. I had actually heard of this before, but what I read about was different than what he was talking about. I'll have to ask him about it again. You would still want to weld and gusset the frame though.

Chevy Power Book
 

Here is a copy of the Chevy Power Book

That's very similar to what I wanted to do if I could have fit it in the original tunnel. :thumbs:

Some pics
 

Here is my interpretation of the drawing, I added more to the front as I agreed the drawing was lacking up there too. I have decided to tie it all together if for no other reason but to seal up my lowered floors. I am also bonding 3/4" plywood on the floors of each side. I am also tying the center hump to the birdcage and frame to tie it all together. Front and back. But I don't recommend the major surgery I have done. It just keeps snow balling....... It may be years before she is on the road. It is overwhelming sometimes.... It may be a while before I can tell you if it is as stiff as I surmise. I don't have a way to anchor it and do proper testing like others have done here.
Thanks for your support. I am open to new ideas and willing to try them out- hey, it keeps me out of the house....

More pics
 

New to pics

More pics
 

More

More
 

Under side, I need to come up with a crossmember up front like the rear

Rear kick ups
 

Triangulated the rear kick ups. I thrived foolishly to put it on a rotisserie and couldn't get the back off the jack stands... POS so I beefed it more.... Oh yeah, I grafted in C4 suspension years ago, WOW did it handle after that.....

your right
 

it only happen if your running 1500 pound springs front 6 700 in the front with 335 all the way around .stock set up 69 or newer no problems with pizza slicers.

Dude, I admire how much time and effort you've put into this project. :thumbs:

The frame you are building looks similar to some British sports car frames like 70's Lotus, 90's Morgan.

:withstupid:

Absolutely, i would like to explore that road also when i lift the body of my '80.

Thanks Guys!!
 

It has definitely got a case of project creep........ Be careful...

birdsmith told it like it is. 1972 bought a 72 L48 car with 6,500 miles on it, the mild suspension last yr of the solid mounts. had 66 vert later on low miles. 73 bought a new one and again new one in 78. Drive anyone of them from brand new or close to new over any form of rough railroad tracks the T-tops did a nice sqeek for you regardless of suspension. Fast forward to today the average C3 owner may well have 50,000 on up of the frame being torsional twisted on, even if it had zero rust over the yrs and that's not the case is it. Just the welding done on it back from new is laughable to look at any welder would fall on his sword looking at it if he happened to be from japan.

The cobra 289 Shelby did all his winning with a big weight advantage on his competition. Even his R model 65/66 mustangs race weight 2,500 lbs ready to race. Try to get your vette down to 2500 lbs, good luck, his statement on flex sheep dip for the masses, light did the winning. The C5 on up 500 percent stiffer torsional then a C3 the new C7 even stiffer.

Ladder frames are good at beaming strength why there under pickups, piss poor for torsional twisting. The birdcage and body will do nothing to help you.

To fix it if triangulate it with a good roll cage if you can live with one that's the fix. The biggest weakness is known to be the rear kick up on the frame.The rear suspension design.

I admire the thinking, and I very much would like a stiffer frame... I have been following this thread some time.... I do enjoy road race, ... But I do so much to shed weight, ..... My question is, how much weight did you gain to achieve this? ... We do fab work as well, and I would guess 200 lbs.. That is a lot of weight... Unless you used thin wall box. Then 100lbs...

....BUT I DO SO MUCH TO SHED WEIGHT..... the complete a/ c system weighs how much :D How much does the heater core, radio ( 427 for good musical tunes ) power steering is it gone.

Lol:rofl: ok, funny... Let me rephrase that:-)....

Does the extra strength in the frame offset the added weight on a road corse?

:iagree:

Same here

I can get 3 tires off the ground by jacking at any corner, and the doors still open and close just fine. That's why I always wonder if people are simply masking other issues by adding gussets, spreader bars, etc. :shrug:

Any stiffening, such as fully welding the frame and adding gussets, is going to help handling. Is it necessary? It depends on what you want from your car and how you drive/use it.

I consider a spreader bar as a necessity. Jack your car up at the front under the center of the cross member enough to get the front tires off the ground. Measure between the top A-arm mounts. Now drop it back down on the ground and roll it forward and back a couple times and bounce it a couple times to get the front to settle to it's normal height. Now measure between the top A-arm mounts. I think you will be shocked at how much the frame twists in at that point.

Did I miss something, or was torsional stiffness measured beforehand without mention in order to asses the effectiveness of the mods?

I don't disagree, but you're kind of missing my point.

Which is, I have firsthand proof that C3 frames are not sloppy and twisty when they are in good condition. And I'm not the only one saying this. So perhaps the focus in many cases should be restoring a frame to its factory condition instead of going off the reservation and adding non-stock gusseting, spreaders, etc. to address a condition that many people incorrectly attribute to being endemic to the car as it came from B.G.,

It would appear that the OP is after a bit more than what may well have proven adequate stiffness for cruising/general applications, as are a good many others of us. Besides, regardless of one's intended purpose, the less chassis flex the better able are the springs, shocks and bars to do their job and be tuned to an optumum state. I'll even go so far as to recommend that a spreader bar should most probably be considered one of the top 5 improvements ANY shark owner can and should make. OK, I beleive that's $.02 worth.

If you are planning on doing any off highway competition activities (drag, road, autocross racing...) where abnormal stresses are encountered, that is where the Chevy Power Guide - Corvette Chassis Preparation is suggested.

I followed some of the suggestions in the guide myself before powdercoating the frame a light color as suggested to detect stress cracks.

Weight
 

All the talk about stiffening the factory frame and weight....
If you're racing and want light weight put in one of the Grand Sport style frames with a cage of your needs. I don't compete and I didn't have much money when I started. I got a smokin deal on Z51 1984 front and rear suspension before anyone was doing it. I grafted that in and noticed the creaks, etc. so I started welding on metal. At this point I have too much time and customizing to buy an SRIII or Paul Newman Frame. That is the way to go. Install late model parts and restomod it.
For me I am playing around and having fun with it. There is no real concern with weight on this street car. I am a corvette club governor who is going to make this thing a Danny Popp looking car that is loud and fast. With no driver aids it will be a handful to drive fast no matter the weight. If I wanted a lightweight car I would build an aerial atom.
I know it will be a tank so I want it to be solid, I am anal about stiff cars and flexy cars drive me nuts so I am making this thing stiff.
There is no magic bullet to stiffen the factory frame. That comes from good design. Look close at C5-C7 frames. We can't compete with hydro form or aluminum. Our cars are 40+ years old. If I win mega millions tonight. I will build a carbon fiber version of my car. That would be fast and light. Let's build loud, powerful, fast cars with our own personality stamped on it. That is what is so great about the group following this thread. We are Mavericks. We want to make ours cars better. We are succeeding. I see so many good ideas here. Look at Menace, this car is inspiring. Sorry for the long post. Post your stiffening ideas.... Here is mine, 3/4" plywood floors.....

Before anyone flames on the use of plywood, you'd likely be surprised to know how much more common is "nature's composite" in racing than one might think.

The floor of the C5 is a balsa sandwich.