Frame stiffness

rtj

rtj

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Nov 5, 2011
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I was watching a Factory Five sales CD the other night and they said that their cars have race car type handling because the frames are so rigid. Did some searching and found comments like this

Frame Stiffness
In order to build a house, you start with a firm foundation, right? The same is true of upgrading a car. A stiff chassis is essential for all those performance parts to do their jobs correctly. If the frame flexes too much, you'll be wasting your money on any suspension upgrades.

This was from a 60-70 mustang article.
 
I talked to the people at SRIII and they say stuff like 3x stronger. However, they can't provide a "baseline" number for a OEM frame or a number for thier frame. I'm sure it's better but would be nice to see some results.
 
The largest improvement on my '72 vert, was wheels/tires I put on '89 vette wheels 17x9.5 275/50 rear 255/50 front,

VBP 360 rear spring, ditching that metal junk was a huge ride/handling plus....

after that, putting in the front tower cross support....really stiffened up the handling.....

for ride quality I went with Bilstein sport shocks...VBP once again.....

my frame was/is in great shape....nothing broken....

:thumbs:
 
I know there is lots of flex in the stock frame. My 68 bb cracked around the headlights and again after repair. I attributed it to frame twist from the bb. My suspicions were supported by all the body de-bonding after installing a bb in the current car. It was originally a smog motor 350.
 
BBShark said:
I talked to the people at SRIII and they say stuff like 3x stronger. However, they can't provide a "baseline" number for a OEM frame or a number for thier frame. I'm sure it's better but would be nice to see some results.
Click to expand...

I'm surprised they don't make test results available, assuming it is true. I seem to remember three tubes down each side.
 
I am planning on adding a roll cage -not just because I want to take the 79 to a track ..... I am convinced the frame is too weak, especially now that it's 35 years old.... Now that my suspension is stiff I believe the frame needs more stiffness/support

Just look at how much flex you're getting by simply lifting one corner with a jack.....
 
mrvette said:
VBP 360 rear spring, ditching that metal junk was a huge ride/handling plus....

after that, putting in the front tower cross support....really stiffened up the handling.....

:thumbs:
Click to expand...
For my 68, 360 pound rear wheel composite spring...8 pound composite spring replaced 41 pound iron spring. Added the front spring tower cross support also. I think the front spring tower cross support is a really good idea.
 
MYBAD79 said:
I am planning on adding a roll cage -not just because I want to take the 79 to a track ..... I am convinced the frame is too weak, especially now that it's 35 years old.... Now that my suspension is stiff I believe the frame needs more stiffness/support

Just look at how much flex you're getting by simply lifting one corner with a jack.....
Click to expand...

This.
I've been saying this for years and it seems like I just get dismissed, but the frames on our cars are just barely stiffer than dried out spaghetti. Ok, that's hyperbole, but you get the idea. I actually cracked my windshield from A pillar to A pillar at the track about 10 years ago. That's how much the frame flexes.
I also say that increasing the spring rates is really not a great solution. It's merely a bandaid in which you give up one thing (driver comfort) for a perceived gain (handling).
Look at the newer cars. They handle far better with softer, more compliant suspensions than ours because the suspension can do what it's designed to do and the frame does what it's designed to do. In the older cars the frame is a suspension component. Not really the optimal configuration for performance, handling or comfort.
 
I've always enjoyed this quote from the 1963 Corvette SAE paper, "In the course of development, the point arose that if stiff was good, stiffer was better. Consequently, a frame was built with torsional stiffness considerably greater than the production design. Evaluation of a vehicle eqipped with the stiffer frame proved the ride to be unacceptable. This approach was quickly abandoned."

They probably shouldn't have abandoned it so quickly. :)


53538ff494c7c10.jpg
 
I did my roll cage in two steps... 6 point - helped dramatically then went to 14 point and did the power service manual frame mods. WOW... is all i can say... the next biggest WOW was swapping the stock brake calipers for Wilwood superlites on the the front and dynalites on the rear the difference in ride and suspension compliance was again dramatic. I have VPB 550 front coils and and Gulstrand did a custom rear steel spring for me... I used Delco stock replacement shocks LOL everyone who rides cant get over how smooth and supple it rides and how well it corners... Of course the only on modified parts on my car for now are the windshield, door hinges, and lower control arms... :smash::smash::smash:

Putting in Window nets tomorrow and I am getting ready to install DA Shocks and front Coilovers with 650 LB/in springs from VBP as soon as i finish my tubular lower control arms

I guess it is possible to get a frame too stiff for a few reasons... as a motorcycle racer from 1969 to 2011 I dont think very many of us push our cars that hard... :twitch::twitch:
 
i added the strut brace first and was amazed how much it helped. then i added the VBP 360 rear composite rear and 550 front with 3/4 of a coil cut out which lowered the front to perfect in my case. added QA1 shocks and it is a different animal now. i will be adding the smart struts next then the f & r sway bars. i think my frame is reasonably good as it has less than 13k miles and lived its life indoors. but one front spring was broken in half
 
a couple of other thoughts...

the frames are getting old... I snapped the pinion mount right off my frame just rolling in the throttle crusin on the interstate last year... it was paper thin and tore off... built a new one out of 0.125 plate...

I have also extensively modified the frame/crossmembers... Added a new subframe to mount the seats and "W" members in lieu of and X brace...

oh and you can jack my car up on one corner and the doors still work fine :mobeer::):):)
 
Kid Vette said:
I've always enjoyed this quote from the 1963 Corvette SAE paper, "In the course of development, the point arose that if stiff was good, stiffer was better. Consequently, a frame was built with torsional stiffness considerably greater than the production design. Evaluation of a vehicle eqipped with the stiffer frame proved the ride to be unacceptable. This approach was quickly abandoned."

They probably shouldn't have abandoned it so quickly. :)


53538ff494c7c10.jpg
Click to expand...

Kind of confusing. They refer to a 1500lb load at "A" and with no "A" shown on the diagram. Also refer to a "comparison" and show no comparison. Im not aware of the bulkhead they added to the S bend on the frame rails either.

I'll bet they abandoned the stiffer frame for cost (not unacceptable ride).
 
theres a post some where here or on CF that has the work some guy did welding up the C3 frame at every seam and then adding gussets. it was really nice work. do you guys remember that post
 
bobs77vet said:
theres a post some where here or on CF that has the work some guy did welding up the C3 frame at every seam and then adding gussets. it was really nice work. do you guys remember that post
Click to expand...

See the tech section here for the Chevrolet Power Service manual and the section on Corvette Frame Preparation...
 
This winter I'll probably cut out most of my first try at a cage and redo with chrome-moly tubing.
 
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