The corvette has been Americas true sports car for the last 20 years, many advances in the art of suspension design and tuning have taken place since the 1963 corvette was introduced. Many of these advances can be applied to older corvettes to improve their handling.

Baseline
In stock form , our Corvette was pleasant to drive - as long as we motored along at the legal speed limit. Under any type of performance driving, however, our Corvette turned into a twitchy, uncontrollable monster. After determining that there was nothing wrong mechanically with out test vehicle, we set out to correct its handling deficiencies. Although our car had unpredictable handling characteristics when driven hard, it still produced .79 G's on the skidpad with new Eagle GT tires on factory rally wheels.

Test 1 - front and rear stabilizer bars
Our experience in chassis tuning has always shown that installing effective front and rear stabilizer bars improves a cars cornering power and handling characteristics. Installing VSE front and rear stabilizer bars increased cornering power to .85 G's and improved steering response, twichiness was still evident when driven hard.

Note: VSE bars are 1 1/8" dia front 1" dia. rear and are connected to the suspension with either rod end links or tie rod end links. At the time of printing front bars were $240 and REar $200

Test 2 - Steel suspension bushings and lower strut rods.
We determined that the erratic handling was caused by its rubber suspension bushings deflecting under high cornering loads. We quickly replaced the stock bushings with new steel and Nylon bushings. These new bushings eliminate most of the undesirable control arm deflections that cause the front and rear wheels to change camber and steeriong angle when subjected to bumps and high cornering forces. Because these bushings also control the camber angle, they increased the Corvettes cornering power to .88 G's. The steel bushings advantage was overall contrallability they offer. Driving response was so much better that you could hardly tell it was the same car.

Note: at the time of printing the front control arm bushings were $114 and the rear strut rods - including new mounting bracket were $175

Test 3 - Shaved gatorback tires and 16 inch wheels
For the next test we installed P255-50 Vr 16 Goodyear gatorback tires (as used on 1984 corvettes). These tires will fit an earlier corvette using an 8 1/2" wide wheel with 4" backspacing. Our goal was to beat the skidpad numbers of the new corvettes, so we shaved the tires to half depth. With these tires and wheels our 1973 Corvette test car recorded a .93 G's on the skidpad.

Summary:
Stock 1973 Corvette eagle GT on 8 x 15 rims 5.5 sec time 52 mph .79 G

VSE front & rear bars 5.3 sec time 54 mph .85 G

VSE steel bushings & strut rods 5.2 sec time 54mph .88 G

Shaved gatorbacks on 8.5 x 16 epsilon wheels 5.1 sec time 56 mph .92G

Goodyear 'S' compund tires, autocross rear bushings, lower front & rear springs, optimize camber for autocross
4.8 sec time 59 mph 1.02 G

My comments: In the report there is mention of the squeaking and groaning of Eurathane bushings and how they are not desirable. Since this report was written years ago, there have been significant changes in eurathane bushing technology. I know I wouldnt want to use steel bushings on the street.

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1978 L-82
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[This message has been edited by fauxrs (edited 05-25-2000).]

I have mentioned before on this forum that I do not feel safe driving in stock C-3's.

Your right on the Hemi-joint steel. I think Herb would have been impressed with new shocks and Smart struts. Years ago I bought the book on how to make your car handle. The basics are all the same the parts are better now.

I've tried various shocks and the ones that are on the car are the softest of all - Gabriel Pro Ryder Gas. The stiff VB glass spring that was on my car when I got it (360lbs) has been replaced by a 315. Why? Besides making the ride bad, the stiff spring induced oversteer when autocrossing.

I like the softest springs possible, and then control body lean with swaybars. I don't find the body lean of my '71 objectionable, so I'm still running the stock front & rear (big block) bars. I might go to larger bars to see what they do, though.

I know a successfull Trans Am roadrace driver (Lou Gigliotti). The way he selects springs for a track is the softest one possible that doesn't bottom out. A soft spring allows the wheel to maintain contact with the road. That's what you want. If bodylean is excessive, use larger swaybars, not stiffer springs.

Some old articles I found by Herb Adams agrees with this. He recommended people that had the gymkhana suspension to swap out the springs for standard springs.

I had poly bushing in he control arms and replaced them with stock rubber due to squeeks. I honestly could not tell a difference in handling, and I'm a VERY aggressive driver (the longest a tire has lasted me is 20k miles, and they were beyond bald!).

I have a accelerometer and a big parking lot that is empty on weekends. If I have some time this weekend, I'll go out and measure my lateral G and post.

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Markus Strobl

In the VB cat. They have suggested alignment settings. These are just close, every car is different. The only way to really tell is skid pad loop times in both directions and a tire temp gauge. I have found that front -.5-.75 neg camber and 4-4.5 castor and 0 toe works best. Any more castor and it would never hold the align specs. In fact when you start stacking more than 4-5 U alignment spacers in you can even loose them during hard cornering. For the rear I ran 0 toe and 0 camber and have kind of settled in on 0 toe -.25 camber.

70 & 60 series tires are a down fall. As for your front sway? It's small, but the big springs make up for it. If you feel that you have very little body roll your fine. As you get higher grip front tires the body roll will increase. So thats when you step up to 1 1/8. So just go with the alignment. Then when your tires are gone shift.

When I had 15 X 8.5 with 4.5 BS wheels. before shifting to 17 “ wheels. I had 245X50 front comp T/A Z-rated and 265X50 rear. The problem is they are very short tires, both under 25 inches in dia. My car was low due to the 1 1/8 550 one-inch shorter front and 360 mono and ¾ sway. I mounted the rear spring block to lower the car and keep it so it only was slight rake to the car. Lower make you corner, but you have to eat and get gas every once in awhile. I had to go at an angle even in and out of my drive way.

Then at some point you’ll have to make a decision on what you really doing. Then you’ll see like I did that its time to put the car on a diet, give it some power, give it OD tranny, body panels for a high top speed, and ……………………..

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79, daily driver, Daytona twin turbo front end,functional rear wing 3082 lbs full tank.
Full VB susp 550,7 leaf springs
hot 355 no-bottle with 1 3/4 hooker sc jethot coated
Speed demon mech second's on tall victor jr with cowl induction ram air
10.7 cr Manley steel six inch rods
roller cam 542 564 232 240 less @.022 lash deg @50 114 lc
New race ported dart iron eagle 230+ cc 2.05/1.6 manley proflow valves.
3800 stall 10 inch convert, full manual valve body, switched lockup 700r4 4.11 rear
17 x9 and 17 x11 wheels
135k 4th motor 5th tranny 5th rearend each time it gets better. Reno Nevada

I take it you are using stock upper A-arms. I thought you had to swap to VB tubular arms to get to 4.0 - 4.5 caster? I think my caster setting is at about 3.0 and it's got a pretty tall stack of shims in there on the back post. Only one shim on the front post on both sides.

Marcus: You said you had squeaks...

I went with PST polygraphite and have about 15K miles on them with no squeaks. I wanted to improve the handling as much as possible but I had also been told that the poly parts would last MUCH longer than OEM rubber so I decided to try the PST stuff. I'm really satisfied so far. The only drawback is their rear T/A bushing is a loose fit assembly. The mechanic that replaced my rear bearings and bushings recently recommended that I not use them. I needed it back quick so I let him put the OEM in rather than wait on something from VB. Figured I'd be replacing the rear bearings again long before the OEM rubber bushings wear out anyway.

David

Are you sure you got that right. It would seem to me that the short lever arm would equate to less mechanical advantage when trying to twist the bar and therefore a smaller diameter bar could be used when compared with one with a longer lever arm and hence greater mechanical advantage when trying to twist.

Maybe I've got it backwards. I dont think so though.

Ok cool, help me understand here then. Here is my thought process on this matter.

Assume two sway bars - 1 has a 12" swing arm - the other a 6" swing arm. For the sake of argument we hang 100 pounds on the end of each bar. therefore on the long arm the torque introduced into the bar is 100 ft-lb (1200 in-lb) the same load on the short bar introduces 50 ft-lb (600 in-lb) to the bar.

It would seem to me that the bar seeing 100 ft-lb would have to be larger than the one seeing 50 ft-lb if the resistance to the torque introduced remains equal.

Did any of that make sense?

I'll try to make a simple example, much like the one fauxrs did.

Lets assume the diameter of the spring is such that it requires a torque of 1 ft-lb to rotate it 1 degree (we're just twisting one end and the other is fixed).

Attach a lever arm to it that's 1 foot long and apply a force of 1 lb. The bar will rotate 1 degree (1 ft x 1 lb = 1 ft-lb)

Now shorten the lever arm to 1/5 foot without changing the bar. Now we have .5 ft x 1 lb = .5 ft-lb of torque. Since that torque is half of what we had before, therotation of the bar will be half as much (0.5 ft-lb x 1 deg/1 ft-lb = 0.5 deg)

The shorter the lever arm is, the stiffer that bar looks to the suspension.

Doesnt really matter in the long run - the idea is to have a stiff sway bar - how we get there is just details.

1) He's got TONS of roll stiffness coming out of those bars.

2) He's using HOLLOW bars. Cars today will use either hollow (tube with squished ends) or a solid bar. If a hollow bar fits the bill and can provide the stiffness, its a bonus of weight savings over a solid bar, and often cost, too. Hollow bars provide less torsional stiffness for a given diameter than a solid bar.

He also is a big proponant of solid bushings to eliminate (not reduce) bushing flex in corners. While I like the outcome - I would never do that to a street car - it would be fine for a car that spent all its time on the track, but that kind of harshness just dont cut it on the street. Keep in mind that this study was done years ago when urathane was new and the quality and quietness of the bushings was suspect.

So i would vote for 1) He's got TONS of roll stiffness coming out of those bars.
He may offer hollow bars now - I do not know.

For most driving, it would make for a much better ride.

phi (angle of rotation) = (TL)/(GJ)

Where:
T= Torque applied
L= Length of Bar
G = Modulus of Rigidity
J = Polar moment of inertia

Comparing properties of Steel:
Material G
Structural Steel 11.5 million psi
High Strength 11.5 million psi
Stainless Steel 10.6 million psi

The main difference in the steels is in their yield strengths of (36, 50 and 75 ksi respectively).

My manual says
G = Shear Modulus of Elasticity at (11,200 ksi) 11.2 million psi.

Fy or Minimum yield stress varies greatly from:

32 ksi (ASTM A36 mild carbon steel)
to as high as
100 ksi (ASTM A514 quenched and tempered alloy)

Since it seems to me that Sway bars do not corrode much and I am 90% certain they do not use mild carbon steel. I wonder if mfr's use an ASTM A242 or A588 atmospheric corrosion resistant high-strength low-alloy steel whose Fy vary between 42 & 50 ksi?

flyinhi:
I have never heard of a sway bar breaking during normal street operation and I would suspect it due to a bad componant. For the bar to fracture I would assume that the bar would have to have been hardened to a point of being brittle (compared at least to the correct level of hardening)

Q=(1000x TxT x KxK x dxdxdxd)/(RxR x L)
Where T = torsional rigidity = 58.75
This is given as a constant, I don't know the units
K = a Constant. I believe this would change with arm length but no other value is offered.
d= bar diameter
R = length of arm (the short portion of the swar bar that attaches to the suspension)
L = the long portion of the bar that attaches to the frame

My calculations for the rear bar with R=8.5 and L=21.25 are:
.5" diam = Q of 1586
.625" Diam = Q of 3873
.75" Diam = Q of 8030
1.0" Diam = Q of 25403

After running these numbers, it was apparent to me why I saw such a difference in feel between the 5/8" (.625") bar and a 3/4" bar. The 3/4" offers more than twice the resistance of a 5/8".

Comments, please.
Regards,Will

What was the source of the formula - I'd be interested in more information in a simialr vein.

I can't quite make out what's going into that formula of yours either. If you csn include the units of your inputs and outputs, that will help as well.

My front is so stiff that if I drive onto one car ramp both front tires are nearly the same hieght off of the ground. I never have tried that on the rear.

www.msu.edu/user/decarter/torsion.xls

I found an equation pretty similar to the one Will posted above. I'll have to look at his a little closer to see what's different between them.

I've got another equation I can throw into the mix here. It's from Puhn's book "How to Make your Car Handle" put out by HP Books.

He cites the following equation for anti-roll bar Siffness as:

K = 500,000* D^4
--------------------
.4244*A^2*B + .2264*C^3

....________B_________
....__________________
../...............................\..\....|
./.................................\..C...A
/...................................\..\..|

(Ascii art is rough. ignore the ... they're to keep the forum from scrunching the spaces together)

A = perpendicular distance
B = length of the bar
C = length of the offset piece
D = diameter of the bar (piece B)

The notes here indicated that this equation includes twisting of the bar plus bending of the driving arm. It is only valid for round bars as shown. The formula does NOT include flexing of rubber mounting parts, which can considerably lower the stiffness.

This means that as D increases stiffness goes up by D^4 or D to the fourth power.

Or as the denominater decreases (The arm length basically) the stiffness is affected C^3 & A^2*B or basically the third power.

The other thing to consider here is the movement of the wheel relative to sway bar.

Just thinking aloud here... if we have two setups with the same overall stiffness (K).

Setup 1 has a larger D and a larger arm.
Setup 2 has a smaller D and a smaller arm. (B is the same for both)

This would mean using the torsional rigidity equation Dave wrote out several pages up for 5 degrees of motion in torsion in the bar, B the for the same moment (ft-lbs) and the same stiffness Setup 1, with the larger arm would translate into an overall longer wheel travel. Setup 2 with the shorter arm, for that same 5 degrees of motion in the bar would have LESS wheel travel.

Now look at it from the other perspective for the case someone (sorry, forget whom) brought up earlier about one wheel hits a bump, but the other doesn't. Consider a fixed amount of wheel travel. What will the two angles be for the 2 cases? If we get the moment in the bar, we'll get amount of restorative force applied to the wheel at the other side of the car.

Setup 1 for a fixed distance, with it's larger arm will have a smaller angle. This will translate into less ft-lbs. Setup 2 will have more torque.

I think what we need to figure out next is the behavior of the wheel / spring / anti-roll bar for an impulsive bump on one side and not the other. Ok, brainstorming here... it's a system. The torque of the bar works two ways. The wheel which isn't bumping will counteract the bumping wheel in an equal and opposite manner that the bumping wheel will try to affect the smooth wheel. I think the amount of vertical travel over the bump will be controlled by the spring stiffness of the springs. Softer springs, more vertical travel, and more torque in the system. However the torque in the system is an effective mass (the other wheel) trying to resist that motion... So I think that the softer spring, stiffer sway bar will translate to a harsher ride... ie less vertical motion of the wheel and more absorbtion by the entire vehicle (ie feel it in your butt when you drive). ACK, this is getting confusing... Gotta read more in this book. Back in a few (assuming it's OK to butt in the middle like this).

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~Juliet ...overlooking Mill Creek on the Chesapeake Bay...
Loaded Bridgehampton Blue on Blue '70 350/300Hp TH400 with a White Ragtop

NEW!! 1970 Corvette Registry Online: http://www.annapolis.net/members/julepage/1970Registry.html

Click on my corvette picture to go there!

This is getting over my head so I'll just offer a couple of observations.

John Greenwood in an article in Corvette Fever several years back called Vette Improvement Program recommended that the rear spring rate should be 25% lighter than the front and that the rear sway bar rate should be 25% lower than front.

In 71, base suspension used 260 front spring with .81 sway bar and 140 rear spring with no sway bar. F41 used 550 front spring with .88 sway bar and 305 rear spring with .56 sway bar.

In 93, FE1 used 539 front spring with 1.02 sway bar and 294 rear spring with .94 inch sway bar. Z07 used 664 front spring with 1.18 sway bar and rear spring was 422 with .94 sway bar. I realize that the C4 suspension is so different that direct C3 to C4 comparisons are not valid but I think the relationship front to rear is informative.

Sway on, sharks.....
Regards,Will

I'm led to ask...."What was the original question?" At this point, I've forgotten what we're trying to answer!!

That not withstanding, Juliet is correct in her 'observations'. Of course she forgot to include the effects of friction and compliance in the bushings and sta-bar mounting points

As far as the ride being more harsh in one case vs the other, it depends alot on the road surface and impact event that we're talking about, too. Then we'll have to get into wheel frequencies, spring rates, damping rates and all sorts of other goodies that we probably are better off not thinking about right now

OK, now for the spring rates. As a general rule of thumb, the rear ride frequency should be about 1.1-1.2 times the front ride frequency. The ride frequency being determined by good old 'square root of K over m' for each axle. Where this parts from the whole spring rate observation is that the linkage ratios for the front and rear springs are not equal (I don't think). The linkage ratio is the movement at the wheel center relative to the movement of the spring's point of action.

The reasoning behind this is 1.1-1.2 ratio is so that since the rear wheel hits a bump after the front wheels, it sets up a pitching (porpoising) motion in the car. By increasing the frequency in the rear of the car, the rear bouncing effectively catches up to the front's bouncing. So you end up with a car that is boouncing up and down, rather than pitching fore and aft. This is much more pleasing to the people in the car.

Whether or not the C3's followed that rule of thumb, or if they were set up with different rates for different reasons is something I don't know. But it does look as though this rule of thumb has been followed fairly closely over the past 20 years.

In conclusion: All else being equal, larger diameter sta-bars are stiffer than smaller diameter ones.

Checking on the spring rate info Will just provided........looking at the spring rates and the approximate linkage ratios, I'm confused. The front has a linkage ratio of around .6 or so while the rear ratio is up over 0.8 or so. The spring rate * linkage ratio should equal the suspension rate, but in the cases I'm looking at, it doesn't. The advertised front spring rates are way too stiff for the suspension rates I see. (I'm ignoring the spring rate of the tire for simplicity here. Its much stiffer than the suspension. It will change the rates, but only slightly in relation to their magnitude.)

Dave

As to "what was the question?", I think we started this by discussing what the rear sta-bar size should be.

I found a table in an Addco catalog which is informative. It relates OE bar diameter to Addco bar diameter stiffness in terms of % increase when changing from an OE bar to an Addco bar - all other things being equal

Addco 3/4 7/8 1 1 1/8 1 1/4

OE
5/8 205% 382% 655% 1051% 1601%
3/4 0 186% 319% 511% 779%
7/8 0 171% 274% 418%
1 0 160% 224%
1 1/8 0 152%

This table tells me two things:
My math in the formula I posted above was correct in suggesting that the 3/4" rear bar is over twice as stiff as a 5/8" bar (whichI am now using) and
These increases are far from linear and care must be taken when jumping from size to size.
There is a note in the Addco catalog that bears repeating....
"Bigger is not always better. Bars that are too large detract from handlng, lead to "twitchy" handling, poor traction and can possibly damage the vehicle."
When I DECREASED the size of my rear bar from 3/4" to 5/8', I experienced better handling and traction (especially in turns).
Regards,Will

I've been following this to a certian extent.

My question to you is. Did your 3/4 rear that you removed have the spring adjustable end connections?

I'm willing to try something new if a 5/8's works better.

Mine do. My train of thought on these is small bumps and uneven road can then move a single wheel a short amount without affecting the other wheel Once movement is big then being linked by the sway bar the other wheel is now forced to move.

When putting any suspension together, balancing the suspension is important. Maintaing front to rear ride rates/frequencies that are well balanced is one part. Then, balancing the overall roll stiffness to get the desired body roll control AND balancing the roll stiffnesses front to back to get the proper roll couple distribution.

What we don't want is a car that has a rear bias of roll stiffness. The roll couple distribution essentially prescribes where the forces from the load transfer end up. If the rear is over-stiff, the outer rear tire is picking up more of the load transfer than it can properly handle. This basically makes the rear tires saturate and lose grip before the front.

The opposite case is a front bias for roll stiffness. This is the desired case where the front tires saturate and the front slides.

So, decreasing the size of your rear bar may in fact improve the overall handling of your car. There are other subtleties like bushings that influence not only the overall roll stiffness but also the rate at which the roll siffness is applied. The bushing needs to compress, so the initial roll rate is different from the rate after the bushing is fully compressed.

As Prof. Gumby used to say "My brain hurts!"