63 understeer revisited

[Steve D.]

63 convertible

About a year ago, I posted the following:

Among the many things on my shakedown list is what I perceive to be a significant understeer condition. By that I mean when going through a mild curve at 30 to 40 mph my perception is that it takes noticeable effort to pull the car through the curve. I don't have this perception when turning corners. I have not had a proper alignment done yet, only my efforts to get things as straight as possible. The car doesn't shimmy, nor does it pull to the right or left. Is this indicative of some known set up issue, or do I need to have the alignment done before assuming that there even is an issue?

In that thread I was advised to have the alignment done as a first step. The alignment was done, and there are still no issues while driving straight, but the posted observation still holds. I am just now getting back around to addressing the issue. My use of the term understeer was incorrect as I now understand that term implies a condition at the limit of traction. What I was trying to describe is a tendency of the car to stay on the tangent to the curve requiring more effort to pull it through the curve than I would expect at low speed. My issue is a low speed (30-40) one, and seems to me would have to be a very basic one. Responses to the earlier post mentioned 1) “failure of the PS system to keep up with your steering inputs”, 2) roll understeer 3) properly set up small blocks exhibit very mild understeer.
The power steering unit is one that I added during restoration and seems to work fine. Is there a known condition of the PS sytem that would contribute what I have described? Suspension wise, it is a 63 with no front stabilizer bar. The front springs were NOT replaced during restoration because there was no evidence of sagging as ride measurements checked out. Would weak springs contribute to this? The tires are radials and have lots tread, although they are about 10 years old.

Steve

[Wayne W.]

Re: 63 understeer revisited

I would be curious about the reasoning for no front stabilizer bar.

[Duke W.]

Re: 63 understeer revisited

...no front stabilizer (anti-roll) bar??? Maybe you mean rear!

The dynamic response of a vehicle - understeer or oversteer - can usually only be ascertained near or at the adhesion limit.

...saying 30-40 MPH doesn't mean much because that could be low lateral g in a wide radius turn or high g in a low radius turn. Pushing a car hard - near the limit of adhesion - through a tight freeway on/off ramp with a 180 degree or more azimuth change will usually reveal the car's dynamic response.

Maybe you are confusing steering effort with understeer. Steering effort increases with increasing caster, but this adds "feel" to the steering. Power steering from the sixties tended to be over-boosted, which reduced "feel" even with a lot of caster.

Bottom line is I'm not sure what your complaint is, but the '63 chassis setup yields very little understeer, In fact, it is so "neutral" that they can easily transition to oversteer at the limit, which is why Chevrolet modified the rear suspension to reduce the rear roll center on '68 models. This reduced relative rear roll stiffness to create more inherent understeer bias.

Early IRS Corvettes also exhibit what is known as "trailing throttle oversteer" - you can be in a steady state constant radius turn at the limit of adhesion and lifting off the throttle causes the car to go into oversteer. This is due to toe and camber changes in the rear suspension with suspension movement. When you back off the throttle, the car's pitch angle changes which causes the rear to rise resulting in camber change in the positive direction and less toe-in on the outside rear wheel. The camber change reduces grip and the toe change "steers" the outside rear tire opposite the direction of the turn.

I have found that replacing the front anti-roll bar link rubber bushings with hard urethane eliminates/mitigates the problem. Since the rubber bushings compress with roll, front roll stiffness decreases with increasing roll, but there is no such equivalent compliance at the rear.

The result is that relative rear roll stiffness increases with increasing roll, which drives the dynamic response toward oversteer. Overall roll stiffness distribution - percent front/rear - is a primary dynamic response parameter, and as rear percent roll stiffness increases, the tendency to oversteer is increased.

I had a similar concern with my '91 MR2 compared to my Cosworth Vega in Willow Spring's Turn 8 that I took flat out in fifth in both cars at about 105 MPH. The Cosworth was dead neutral, but the MR2 seemed to understeer heavily, which didn't jibe with its near neutral response at the limit in slower corners.

The MR2 has a variable boost power steering system that provides no boost above 80 MPH, and though my seat-of-the-pants told me it was understeering heavily, my intellect told me that my seat-of-the-pants impression was confusing understeer and steering effort.

Duke

[Stuart F.]

Re: 63 understeer revisited

Steve;

I too have a 63 convertible L-76 with radials. The phenomenon of which you speak is one I have as well. My alignment was done at the time I had the radials put on by a GM tech who adjusted it for the tires (he had experience with the change over and I will not get into the specs here). I did not notice this condition before the radials. If anything, the bias plys tended to oversteer. I have manual steering (not power) and the tie rods ends are adjusted to the quick ratio position, so mine tends to drive like a truck (hard to steer at slow speed). But, on the sweeping 30 to 40 mph curves, I too have to input a lot of effort to hold it into the turn. At higher speeds or more throttle input the effort is less.

I have had this car since new and, after I got over believing the hipe about what a good handler it was, I learned to respect the limits of it's envelope. In the process, I ran it off the road a couple of times including a spin off a freeway on ramp backwards. I broke the rear spring and cracked a strut rod on that one. I'm not a SCCA licensee by any means, but I've had several 55-57 Chevies that were a lot more predictable handlers than my Vette. I have a 3.36 rear ratio in it so using power to induce oversteer is not too easy at lower speeds.

Stu Fox

[Jack H.]

Re: 63 understeer revisited

"The power steering unit is one that I added during restoration and seems to work fine. Is there a known condition of the PS sytem that would contribute what I have described?"

Could be a key here... Corvette PS was different from Chevy passenger car PS in terms of certain component parts. Getting the recepie wrong can result in a 'twitchy' feel (car tends to dart either right or left from a straight line) or has a bias to left vs. right or right vs. left turns.

But, those symptoms do not appear to jive with the complaint in the thread.

Also, when the Corvette PS option was installed at the factory, tie rods were moved into the forward (quick steering) position and the standard attaching holes on the steering knuckle were plugged with aluminum rivets to safeguard against mechanics incorrectly setting up the front suspension during subsequent service.

[Stuart F.]

Re: 63 understeer revisited

I'd say the best way to describe my similar experience is to note that the corners where I notice this are posted 40 or 45 mile per hour. In these particular areas, I seldom exceed the limits as there are several entrances to sub-divisions in the immediate area with the potential for sudden traffic.

Out on the road at typical highway speeds (65-75+), mine runs as if on rails.

Stu Fox

[Steve D.]

Re: 63 understeer revisited

I meant to say no REAR stabilizer bar.
Steve

[Stuart F.]

Re: 63 understeer revisited

After re-reading Duke's post, I believe my off road experience was an OFF ramp and the description of the induced oversteer he describes fits the bill of what happened. It was many years ago and Independent Rear Suspension was new to me.

Stu Fox