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Got the track alignment done again by my chevy dealer and the numbers are a little off target from the manual. Wanted to see what you all thought of these settings.
Left and right camber and toe are different especially in the rear
Toe front and rear should be negative, but it's showing as positive
No mention of rear caster adjustment.
The calibration setting lists FE1-4FE1, but not FE6 or FE7
FE1 Suspension, 4-wheel independent
FE3 Suspension, Z51 performance
FE4 Magnetic Selective Ride Control
FE6 Suspension, Performance Ride and Handling
FE7 Suspension, Z07
I would go back. They are off mostly on your right side. I would want the car the same on both sides so it will react and turn the same. Wonder why they couldn't match both sides.
Just my $0.02 but I'd find another dealer, not a track alignment at all, just generic, not aggressive, and as you mentioned, no rear caster readings. I also don't like cross camber that steers to the right, would rather have it the other way to offset road crown, or just zeros.
Looks like your toe is positive as well, and no rear caster measurement. Your ranges for rear toe are also different from mine. Does it say FE6, FE7 at the top of your sheet?
Still amazes me that 99% of Chevy dealerships still do not have to proper equipment to do a full alignment for these cars where the manual strictly calls for rear caster to be at 0. This was the reason why the Motor Trend car was so messed up and lost to the GTR. Your numbers can be dead on but still have issues if rear caster is off. Unfortunately, a very select few can measure it.
Looks like your toe is positive as well, and no rear caster measurement. Your ranges for rear toe are also different from mine. Does it say FE6, FE7 at the top of your sheet?
rear caster doesn't do anything. the rear tires don't turn.
if your looking for a racing based alignment your not going to get it at a dealership, unless you pay extra. in reaction to OP's picture. your RR caber could be more equal. other then that setting everything looks fine. .03* of tow in the rear you wont notice. you should definitely notice the Front toe out vs your old toe in. that will give you a little better turn in.
rear caster doesn't do anything. the rear tires don't turn.
if your looking for a racing based alignment your not going to get it at a dealership, unless you pay extra. in reaction to OP's picture. your RR caber could be more equal. other then that the setting look fine. .03* of tow in the rear you wont notice. you should definitely notice the Front toe out vs your old toe in. that will give you a little better toe in.
Rear caster, when out of spec, definitely does something as seen in the MotorTrend test and subsequent re-test of the Z06. Without knowing what it is set at, we could in for the same "lack of confidence" feeling in the car. I don't want that.
For your second point, on getting the alignment done at a dealer. If it's in the manual, the dealer should be able to support it. Whether through new equipment or training, Corvette team should be making this a non-issue.
Rear caster, when out of spec, definitely does something as seen in the MotorTrend test and subsequent re-test of the Z06. Without knowing what it is set at, we could in for the same "lack of confidence" feeling in the car. I don't want that.
For your second point, on getting the alignment done at a dealer. If it's in the manual, the dealer should be able to support it. Whether through new equipment or training, Corvette team should be making this a non-issue.
Exactly. To have an experience racecar driver (Randy Pobst) praise the car at Rd Atl then basically say it's horrible in regards to handling during the MT test, says volumes when GM blamed it on rear caster. The Issue was fixed (rear caster changed back to 0) then the car goes out and beats the GTR time and most of all, Randy stated the car felt completely different from before.
If this causes such an ill handling affect, and the manual states to have caster at 0, then why doesn't a dearership who sells the car and services the car, have the ability to properly align it?
Exactly. To have an experience racecar driver (Randy Pobst) praise the car at Rd Atl then basically say it's horrible in regards to handling during the MT test, says volumes when GM blamed it on rear caster. The Issue was fixed (rear caster changed back to 0) then the car goes out and beats the GTR time and most of all, Randy stated the car felt completely different from before.
If this causes such an ill handling affect, and the manual states to have caster at 0, then why doesn't a dearership who sells the car and services the car, have the ability to properly align it?
ok, i'm willing to learn and can admit when i'm wrong..... tell me what handing characteristic does rear caster control?
The real reason the Z06 was slower was because Chevrolet gave us a car with the rear suspension out of alignment. It was supposed to be running 0 degrees of rear caster. Instead, it was running positive 2 degrees. This made the rear dampers less effective at controlling the rear suspension and as a result made it more difficult for the Z06 to put power down. This is exactly what Randy Pobst complained about during our test.
Mainly, it would change the ability of the rear shock to do its job in a given moment of suspension travel. More or less you could tweak the sweet spot of the shock to gain better weight transfer if needed. Also it would change the ability of the rear suspension to absorb bumps and maintain traction effectively. It would effect things like wheel hop and putting power down over the range of motion. ie mid corner, corner exit, etc.
Think 3D for a while and let's take the example of a double wishbone.
1. In pure kinematics (no compliance), the outboard (upright) pickup point moves on a plan perpendicular to the inboard (chassis) axis, right? If the inboard (chassis) axis is not parallel to the ground, the outboard point will move on plane which is not perpendicular to the ground, still follow me?
2. If you have some sort of anti and if the top or bottom axis of the inboard pickup points are not parallel to each other, the top and bottom outboard pickup point (on the upright) will have a different longitudinal displacement and therefore a caster variation.
3. If there is a caster variation when the wheel goes up and down Vs the chassis there will be a vertical movement of the toe link outboard point that will be different than the vertical movement of the wheel itself.
4. But at the same time there is that toe link. The outboard pickup point of the toe link will move Vs the chassis on a sphere of which the center is the toe link inboard point and the radius is the toe link red end to rod end center.
5. Put 3 and 4 together and you will see that there is a great chance that while the outboard toe link point goes up and down and also rotates around the wheel center, there will be not only vertical but also possible lateral and longitudinal movement of this toe link... and there fore...bump steer.
On a car with two eccentric adjusters one the bottom of the A-arm such as the Corvette, it is very easy to get the camber set correctly and totally have the incorrect caster. On the front where it is normal to check the caster, this gets taken car of.
For the rear suspension where virtually no one checks the caster because you can't steer the rear wheels on the alignment rack as is done to check front caster, it is possible to adjust camber by moving either just the front eccentric, the rear eccentric or some combination of both. This means you can have totally different caster on each side of the car, and yet have the correct camber and toe. So you could potentially have -2.0 degrees caster on the left rear and +2.0 degrees caster on the right rear or vice-versa. When one wheel compresses, it will move backward while when the other wheel moves upward it will move forward. This will change the response of the car for left and right hand turns, when the design intent is for no front or back motion of the wheel as they are set to 0.0 degrees caster as per the spec.
All you have to do to see just how much the rear wheels move around is to put the car up on a lift and tweak just one of the rear A-arm eccentrics and see the motion of the wheel as I did on the weekend. Very easy to set caster wrong and hard to set it correctly. One needs a digital angle gauge attached to a GM tool that fits into two pre-made holes in the upright to measure rear caster. The reading will not come up on the alignment print-out, and it has to be written in by hand from the digital gauge.
When I checked my car, the alignment guy had set both rear sides differently to set the track camber, with the front eccentric all the way out on one side and the rear eccentric all the way out on the other side. Needless to say he did not measure or adjust rear caster. My GM dealer said they would not align my Corvette because they don't have a good alignment rack. Still looking for a shop that has a capability to measure rear caster.
I am by far no expert so I will post a pic of what the MT article stated.
mmmm i consider this a stretch. but i will consider myself more educated as to why caster could influence a car's behavior. i still am having a hrd time believing that rear caster could effect a shock. just because you have 2* of caster doesn't mean you have 2* of side load on a shock. that's why i'm calling this a stretch.
mmmm i consider this a stretch. but i will consider myself more educated as to why caster could influence a car's behavior. i still am having a hrd time believing that rear caster could effect a shock. just because you have 2* of caster doesn't mean you have 2* of side load on a shock. that's why i'm calling this a stretch.
Yeah, I spoke to a few experts on this as well and they also question the validity of this as they too feel it's a stretch.
Rear caster is normally set to zero, as you don't want the wheel to move backward or forward as it moves up and down. Rear caster adjustment capability is typically used to compensate for any bump steer in the rear suspension. If the suspension was properly designed and there is no change in toe as the wheel goes up and down (bump steer), then one just leaves the rear caster at zero degrees. If the caster is incorrectly set, then it is certainly possible that the car will exhibit rear bump steer which could be a nasty thing if the toe goes outward, making the rear of the car steer outward , causing some unpredictable overseer. The is the car has the opposite setting on the opposite side causing the rear tire to toe in on bump, that does not help driver confidence with a car that behaves differently on left and right turns.
How much rear caster affects the bump steer and thus how much of an issue it is would require measuring the bump steer, which would mean taking out the spring.
Rear caster is normally set to zero, as you don't want the wheel to move backward or forward as it moves up and down. Rear caster adjustment capability is typically used to compensate for any bump steer in the rear suspension. If the suspension was properly designed and there is no change in toe as the wheel goes up and down (bump steer), then one just leaves the rear caster at zero degrees. If the caster is incorrectly set, then it is certainly possible that the car will exhibit rear bump steer which could be a nasty thing if the toe goes outward, making the rear of the car steer outward , causing some unpredictable overseer. The is the car has the opposite setting on the opposite side causing the rear tire to toe in on bump, that does not help driver confidence with a car that behaves differently on left and right turns.
How much rear caster affects the bump steer and thus how much of an issue it is would require measuring the bump steer, which would mean taking out the spring.
I don't think this is 100% correct. Just because you have positive caster, for example in the front, doesn't mean the wheel moves backward and forward as it compresses/rebounds.
I agree that for the rear A arm, one eccentric in and one out causes caster, and would even change the wheelbase slightly, just not under compression/rebound. The A arm whether the eccentrics put it forward or back; is still going to move perpendicular up and down.
The reason making 0' caster essential is so your camber doesn't change when making rear toe adjustments. The rear toe is where the sensitivity to oversteer figures in.
I don't think this is 100% correct. Just because you have positive caster, for example in the front, doesn't mean the wheel moves backward and forward as it compresses/rebounds.
I agree that for the rear A arm, one eccentric in and one out causes caster, and would even change the wheelbase slightly, just not under compression/rebound. The A arm whether the eccentrics put it forward or back; is still going to move perpendicular up and down.
The reason making 0' caster essential is so your camber doesn't change when making rear toe adjustments. The rear toe is where the sensitivity to oversteer figures in.
You are correct, its' not about wheel moving forward or backward as it goes up and down. While zero rear caster does mean you can adjust toe without affecting camber, that is only of use while on the alignment rack. The potential problem is bumpsteer with caster off the design value causing instability due to toe change.
Suppose the rear caster was from the factory correctly at 0. What exact adjustments would cause the caster to be affected/changed? I understand changing the camber would affect it but what part of changing the camber (like removing a washer etc) will cause the caster to be changed?
06-02-2015, 09:31 AM
Track Alignment targets and actuals
