C2,C3 Corvette Roll Center

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66Vette

C2,C3 Corvette Roll Center

Post by 66Vette » Fri May 09, 2014 1:47 pm

I autocross a 1966 Corvette and am looking at altering roll centers to improve performance. The car uses stock suspension parts, but spring rates have been cranked up (800F, 475R) and the car is lowered. Tires are 315-35-17 Hoosier DOT race tires. I have a cage for stiffness and the motor is a 377 (4.125" x 3.5") SBC at a little over 500 HP.

Historically, guys racing these cars have pretty much used the above approach but run much higher spring rates. They basically remove movement from the suspension. I drive my car to and from events so I'm pretty much at the limit on spring rates. So I'm looking for any improvement without cranking up the stiffness. Lowering the car has dropped the roll center and some people have recommended using a longer ball joint on the upper arm to raise the roll center.

Crude measurements have the front roll center at around 4.5". CG is estimated at 14".

Any thoughts would be welcome. Just looking to tweak the car for better times.

Thanks, Cris
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Olefud
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Re: C2,C3 Corvette Roll Center

Post by Olefud » Fri May 09, 2014 6:32 pm

You don’t mention the handling symptom you want to address. Generally, altering roll stiffness with an antiroll bar is easier to adjust and install than dinkng with the roll couple. However, if you have a jacking problem –not likely- the bar won’t help.

66Vette

Re: C2,C3 Corvette Roll Center

Post by 66Vette » Fri May 09, 2014 7:58 pm

Olefud:

I've been fooling with bars and springs and feel like I'm getting close to exhausting what they can give me. So I've started to look at other pieces where I can start tweaking to see if it hints at any improvement.

If I had to try to explain what I am after is to get the car less twitchy. And I mean twitchy during transitions...braking, setting the car into a corner, and exiting a corner. On a big sweeper the car takes a nice set and feels solid and predictable.

It almost feels as if a big, loose weight is being thrown around during the transitions.....

So I'm trying to understand how roll centers might help or hurt my situation.

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Re: C2,C3 Corvette Roll Center

Post by clshore » Sat May 10, 2014 1:08 pm

66Vette wrote:Olefud:
...
If I had to try to explain what I am after is to get the car less twitchy. And I mean twitchy during transitions...braking, setting the car into a corner, and exiting a corner. On a big sweeper the car takes a nice set and feels solid and predictable.

It almost feels as if a big, loose weight is being thrown around during the transitions.....
...
Get ready to spend some time/money on shocks. ;)

But first take a look at where your suspension still has compliance.
Anything bushed in rubber is suspect.

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Re: C2,C3 Corvette Roll Center

Post by Brian P » Sat May 10, 2014 8:53 pm

Check rear toe behaviour as a function of suspension travel (and ride height).

On that rear suspension, the wheel is on the trailing arm, and when it is above or below nominal ride height, the lateral links (one of them being the halfshaft!) pull the rear alignment into toe-out, which is a recipe for getting pretty squirrelly. If you've altered nominal ride height, who knows what the toe behaviour is going to be. You might be getting some undesirable rear-wheel-steering effects.

With regards to the "usual" approach of using really stiff springs and dampers on those cars ... I forgot where this quote came from, but "you can make any bad suspension design work if you don't let it move!"

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Re: C2,C3 Corvette Roll Center

Post by clshore » Sun May 11, 2014 9:25 am

Brian P wrote: ...
With regards to the "usual" approach of using really stiff springs and dampers on those cars ... I forgot where this quote came from, but "you can make any bad suspension design work if you don't let it move!"
I recall something similar from Colin Chapman,

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Re: C2,C3 Corvette Roll Center

Post by Brian P » Sun May 11, 2014 5:38 pm

That would make sense for the origin of that quote ...

I thought about this some more. With that suspension design, the hub etc is solidly mounted to the trailing arm which is pivoted to the chassis at the front, the drive shaft guides the back end of the trailing arm (thus pulling the trailing arm and hub into toe-out in any movement above or below being horizontal), and the lower link provides the camber guidance. So if the rear suspension has been lowered without also raising the differential up into the chassis (and I don't know how practical that would be), it's going to be partially into its toe-out regime when it is at static (lowered) ride height. Bump motion will drive it more strongly into toe-out and rebound motion (up to the point of the shaft being horizontal) will drive it into toe-in - and that's assuming that the static alignment has been changed so that it's neutral (neither toed in or out) at the new ride height.

If no one has messed with the rear alignment after doing the lowering, it's basically going to be running toe-out at the rear all the time and this will get worse in bump. That's going to be squirrelly.

If the rear alignment was corrected at the new static ride height then it won't be toed-out all the time, but body roll will send the outside (more loaded) wheel into toe-out (roll oversteer) and the inside into slight toe-in (also leading towards roll oversteer).

Use brutally stiff springs so that the car doesn't deviate (much) from its new ride height and it won't display those problems as much, but of course it's going to be skittish on rough pavement just like any other overly-stiff suspension would be.

When the car is at its original design ride height the halfshaft is very close to horizontal, so although the toe-out tendency with suspension movement is still there, it's minimized because the amount of sideways pull in the arc that the shaft swings through is minimal in that situation.

And with that ... I don't know what else I can add. Modern multi-link rear suspensions no longer have the hub rigidly attached on that trailing arm, there's intentional "give" in it somewhere, and there is an additional "toe link" that allows the hub to be guided, so that the toe behavior with suspension movement can be controlled separately (generally, you want toe-IN with suspension moving away from nominal ride height if you want the car to have stable, benign handling characteristics).

So aside from a redesign ... either move the entire diff housing up in the chassis (if you can) to compensate for the lowering, or restore it to original ride height where it will at least work the way GM originally intended it to in 1963 (even if that's still not great by today's standards), or do what everyone else is doing and use brutally stiff springing so that it doesn't move and thus doesn't display the bad side effects ... The roll center isn't really the problem, the bad toe behaviour with suspension movement is ...

66Vette

Re: C2,C3 Corvette Roll Center

Post by 66Vette » Sun May 11, 2014 10:18 pm

Guys, thanks for the responses. Sounds like you have been under a C2,C3 chassis. Here's a little more info on the car. Front A-arms are bushed in Delrin. Rear strut rods are heim jointed. Diff cross member has solid mounts and diff front bracket is solid mounted. The only rubber left is the rear trailing arm bushings.

Brian, I aligned with the car lowered. The half shafts are tilted inwards, so as you point out, I do toe out on compression. I have no idea on how bad this effect is, haven't tried to take a measurement. The car is set slightly toed in at the lowered ride height. I have briefly thought about moving the diff up to re-establish the geometry, it is something I've seen on the vintage road race cars. I'll take some measurements just to see where the roll center is for the rear.

I think I have one obvious thing to try, the gas tank is the stock unbaffled tank. It could be my imagination, but when I run with more than 1/4 tank this "loose" weight feeling seems to be worse. Not sure if you have dealt with them, but Rick's Hot Rod shop makes baffled tanks. That's the next thing I'll try.

Thanks for all the inputs

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Re: C2,C3 Corvette Roll Center

Post by clshore » Mon May 12, 2014 7:07 am

66Vette wrote:Guys, thanks for the responses. Sounds like you have been under a C2,C3 chassis. Here's a little more info on the car. Front A-arms are bushed in Delrin. Rear strut rods are heim jointed. Diff cross member has solid mounts and diff front bracket is solid mounted. The only rubber left is the rear trailing arm bushings. (!!!)

Brian, I aligned with the car lowered. The half shafts are tilted inwards, so as you point out, I do toe out on compression. I have no idea on how bad this effect is, haven't tried to take a measurement. The car is set slightly toed in at the lowered ride height. I have briefly thought about moving the diff up to re-establish the geometry, it is something I've seen on the vintage road race cars. (Sounds like this issue has already been solved. What did they do with the rear trailing arm bushings) I'll take some measurements just to see where the roll center is for the rear.

I think I have one obvious thing to try, the gas tank is the stock unbaffled tank. It could be my imagination, but when I run with more than 1/4 tank this "loose" weight feeling seems to be worse. Not sure if you have dealt with them, but Rick's Hot Rod shop makes baffled tanks. That's the next thing I'll try.
Fuel cell foam?


Thanks for all the inputs

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Re: C2,C3 Corvette Roll Center

Post by Dan Timberlake » Mon May 12, 2014 8:21 pm

http://www.web-cars.com/corvette/1963_S ... php?page=0

Front and rear roll centers at stock ride height side view here.
http://www.web-cars.com/corvette/1963_S ... php?page=7
rear roll center of '63 at stock ride height rear view here
http://www.web-cars.com/corvette/1963_S ... php?page=8

John Greenwood had some pretty specific modifications published in Corvette news or somesuch.
As I recall he was pretty pleased with the front suspension in rubber bushings at least with race tires of the time.
The rear roll center was a little high (jacking) but toe in under acceleration called for some extra guidance of the trailing arm front bushings

66Vette

Re: C2,C3 Corvette Roll Center

Post by 66Vette » Mon May 12, 2014 11:25 pm

Dan:

Thanks for the link. I had dug that up doing a search on CF. Really good article.

Here's what I have for roll centers: 3.77" F and 2.51" R. I used the later year rear strut rod bracket that lowers the inside mount of the rear strut rod. Not sure to make of these numbers yet.

clshore: After taking a closer look at the rear, the half shafts are close to parallel, the strut rods point up as you move inwards. That will force camber gain on compression, but toe may not move out as much as I thought. I've bump steered the front, but never looked at the rear.

I'm looking for the low hanging fruit for mods. Was hoping the roll centers would say something obvious.

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Re: C2,C3 Corvette Roll Center

Post by clshore » Tue May 13, 2014 8:49 am

A quick Google reveals that the trailing arm rubber bushings on these cars are commonly replaced with spherical bearings for competition.

http://www.duntovmotors.com/vintage-rac ... ension.php

Poly doesn't work here because the arm must articulate in 2 planes, it's one reason factory chose the rubber, gives compliance.

I strongly suspect that's your culprit.

The arms in the link given above are not cheap, but also have additional work done (NFI, etc.)
I suspect that you could fit the spherical bearings yourself.
As it's a street driven car, I'd select the sealed ones, else take care to fit some kind of boots.

Carter

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Re: C2,C3 Corvette Roll Center

Post by mbrooks » Tue May 13, 2014 9:24 am

Usually the roll center is lower in the front to follow the usually lower Cg in the front, results in the close to same arm length.

66Vette

Re: C2,C3 Corvette Roll Center

Post by 66Vette » Tue May 13, 2014 6:13 pm

clshore:

Thanks, I had not seen the Duntov stuff. Guldstrand makes a spherical bearing replacement for a stock arm and Global West makes a custom tubular arm with a spherical bearing as well. Its on my list of mods to do.

Mbrooks:

Is there an easy way to get at CG? I am guess-timating it right now.

Cris

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Re: C2,C3 Corvette Roll Center

Post by clshore » Tue May 13, 2014 7:13 pm

66Vette wrote:clshore:

Thanks, I had not seen the Duntov stuff. Guldstrand makes a spherical bearing replacement for a stock arm and Global West makes a custom tubular arm with a spherical bearing as well. Its on my list of mods to do.

Mbrooks:

Is there an easy way to get at CG? I am guess-timating it right now.

Cris
Do you have access to corner weight scales?

Get the car (preferably with empty fuel tank) on a level surface (use a level or similar: iPhone app?) and record corner weights.
Get two ramps or blocks of equal height, maybe 6", although taller is better.
Get both tires on one side up on blocks, still on the scales, and record corner weights.
Record or calculate the angle (angle of the chassis)
Now get other side up on the blocks, record corner weights, angle.

Sum of corner weights will always be the same no matter what, flat, angled left, angled right.
Now use high school trig to calculate the CG based on weight changes and angles.
Basically, the higher the CG, the more weight will transfer for a given angle.
So knowing the angle and the total weight, you just work it backwards to find the CG.
(Sorry, I don't have time to look it up now)

Don't be surprised if left/right calculations show that CG is not exactly on centerline of the car.

Can use the same method to calculate front/rear CG location, but not quite as precise since the 'lift' over length of wheelbase vs track gives a smaller angle to measure.

Heck, maybe there's already an app for this (if not, maybe I should write one).

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