JSibert

On 5/21/14 I wrote about a problem with the rear end of the car darting to the right when encountering a bump or dip in the road at highway speeds. Well, I finally have a resolution.

After waiting what semed like an eternity the dealer finally received the meter and tool to measure the rear caster. I took it in to the dealer and they checked alignment and said the rear toe was out of spec. I asked the mechanic about the rear caster and his response was "it was ok". I again asked what the rear caster measured and he gave me the same response. I immediately was suspicious that he never measured it or adjusted the rear caster. And that is specifically what I brought the car in to be checked. I waited for over a month for the dealer to obtain the digital angle gauge and adaptor. I drove the car home and it exhibited the same problem, the rear kicked to the right on encountering a dip or bump in the road.

So, I called my favorite Corvette mechanic. He was able to borrow a digital angle gauge and adaptor and I took it in to him this morning. He was able to measure the rear castor at +3.2 on the rear left and -1.9 on the rear right. Numerous adjustments of the rear camber, toe and caster and he was able to get the rear caster on both sides to -0.3 on both sides. The specification is 0.0 to -0.8. Took the beauty for a test drive and the problem was gone. No more rear kicking to the right. A shout out to Corvette Care in Columbus Ohio!!!!!!!!!

Sorry for being such a long post, but despite some of the earlier projections that the problem was related to bad suspension parts it really did come down to the rear caster being way off.

If anyone gets an alignment done on this car make sure they check the rear caster. If it is off it will cause many headaches. And my final assessment is that the dealer mechanic did not do his job even though that is what I specifically wanted checked.

*C7*

Very glad to hear you finally got it fixed.

Did the Corvette mechanic mention the toe or thrust angle settings when he received the car?

ErnieD

Happy to hear that the problem is fixed, Jim. Did you get the front alignment set a bit closer to optimum values as well?

Enjoy the car.

JSibert

Toe was 0.2 left and -0.3 right. Thrust angle was 0.03.

JSibert

Yes, the front alignment was set very close to optimum.

hankman

glad its fixed,,, i would hate not knowing

Trackaholic

I thought caster was the angle of the steering pivot relative to vertical. How does rear caster work? Does it have something to do with the axis about which the toe changes as the suspension compresses?

-T

mjw930

Caster is the angle between the pivot line (in a car an imaginary line that runs through the center of the upper ball joint to the center of the lower ball joint) and vertical.

For the front suspension machines use a method to measure that uses a gauge attached to the front wheels that measures the difference in camber as the wheels are rotated through left and right full lock. It eliminates any issues with proper verticle placement of the gauge on a suspension component and allows one measuring device to measure both camber and caster.

Because the rear suspension doesn't steer those conventional alignment adapters can't measure rear caster so a stand alone gauge is attached to the suspension knuckle to measure this angle.

Shaka

Unbeknownst the the OP, was that his shocks were replaced as per recall which fixed the problem. A broken shock/s caused the problem.
The caster angle as viewed from the side is a line drawn from the upper and lower ball joint. If viewd from the back it is called ball joint inclination.
In most cases, the wheel bearing is placed on this line toward the bottom ball joint. This minimizes caster trail or lead. In the case of ball joint inclination, the scrub radius is determined. Indy and LeMans cars place the wheel bearing ahead of this line.
Upper and lower wishbones or A arms comes with inherent problems with many load couples. Michelin came up with a suspension for F1 cars(OPT) which was promptly banned. CAD designed modern multilink suspensions are the result.
Due to the differing lengths of the upper and lower A arms, the steering arm will cause toe changes during the travel of the suspension. Each application will require specific geometry. Bump and droop toe is optimized for roll oversteer or understeer. Generally, the included radius keeps toe neutral or zero thru normal conditions. Porsche is masterful at attempting to defy physics in their 911 with clever suspension geometry.
The mere fact that the rear has an upper and lower ball joint, it will have a castor angle. Corvette has decided to make this zero. Since the rear wheels don't steer the car, caster stability is not necessary.
Should there be a discrepancy,(OP's data) the caster trail or lead is insufficient to cause a steering moment unlike the front.
The Corvette engineers made a very compliant suspension, IE: Lots of pitch and roll. Control that severely, and you will get adverse binding couples. Race cars are different, because of minimum suspension travel. Few manufacturers can optimize the friction circle at each corner like C7 does. A very complicated science.

JSibert

Shocks were not replaced as claimed by Shaka. The recall does not include my VIN. I have no idea where this came from. I was watching the entire time the rear alignment was done. And the caster was adjusted to nearly zero on both sides. Unbeknownst to me????

4GS7

You were there? I thought not.

JSibert

I was there and no shocks were replaced. Have no idea where that "unbeknownst to the OP" stuff came from.

Shaka

Oh, so you seem to know a lot about suspensions. Pray tell, oh wise one, with the specs our friend JS was presented and you have obviously studied, describe what turning forces there could possibly be at the back of his car to cause it to swerve on a straight road after a 'bump' on this said road? Describe how correcting the out of spec settings would solve his 'problem'. I thank you in advance.

JSibert

The problem was solved and no shocks were replaced. Is there some reason you said that "unbeknownst to OP" that my shocks were replaced? I am trying to understand what prompted you to proffer this statement.

LIONEL21

I have a 2014 non z51 with magnetic control and I feel this at highway speeds as well. I have spoken to a few dealerships in the area I hope showing them this will help fix the issue.

Higgs Boson

I get a squirrelly rear over big suspension compressing bumps and I am running coilovers and have had probably 4 alignments with angle gauges....

LIONEL21

Experiencing the same issue. took mine in today and was told alignment checked out. I asked for alignment and caster info hopefully they provide it as I will be picking up the vehicle tomorrow. I was told this vehicle comes with no rear sway bar making it feel like that? sounds like a joke to me.

meyerweb

http://www.vikingspeedshop.com/suspe...aster-and-toe/

Edit: Sorry, just realized this was a zombie thread raised from the dead.

Higgs Boson

There is no pull or noticeable issue with driving. This is after hitting a large enough bump to unload the rear suspension. When it happens, it wiggles side to side rather than just unloading straight up and down.

LIONEL21

I am experiencing the exact same thing. Feels a little loose on bumps and long turns. I will be taking mine in to a frame and alignment shop to have it checked. I hope that will fix the issue as others have reported. I will also be adding z51 sway bars and lowering the max the stock bolts allow.