I have a 2017 GS 2LT A8 that I track 90% of the time-trailer. Ordered DSC controller. Looking at the alignment specs for track/street. Instructions say to remove 1 washer behind both upper control arms front and rear Total of 16. In my manual page 166 for grand sport states do not remove washers. What is everyone doing here? Im a directions guy, want to get the most out of my controller. Is a good garage able to adjust rear caster? or do i have to road trip to DSC?

 

Taking out the washers on the rear suspension (one per each upper control arm mount bolt) helps to provide a bit more toe in adjustment when you increase negative camber. Unless you are using an aftermarket adjustable toe link, this is important.

Not sure why your manual would say not to remove one washer.

 

Thanks for info. The do not remove washers is for grand sport only. z51 and z06 you can following the manual? If i am able to to get .02 toe at all corners with - 2.0 camber at all corners is the removal of washers still necessary?

 

I think anything above -1.5 camber in the rear (especially if you don't take out the washers) is going to be a problem. As you increase camber, the toe link runs out of adjustment such that it is too short. So you end up with toe out that cannot be adjusted out.

Caster adjustment also factors in the toe link range problem. When you set caster more positive, the rear adjustment cam is set farther out and the front cam is set closer in. Since the toe link is at the rear, more positive caster makes it too short.

Why do you want -2.0 deg camber at all four corners? That sounds like too much in the rear and not enough in the front.

 

I now realize the -2.0 rear is too much as i have reviewed the DSC recommended specs. My last alignment doesn't show rear caster that i can see is this part of the problem? Do shops normally adjust that? I have downloaded to different specs from DSC for C7's i am calling Monday to check witch specs and try to set up alignment by them, Running SCCA time trials at New Jersey Motorsports park Lightning coarse august 28th DSC an hour away.

 

Almost no shops (including most GM dealers) have the ability to measure or properly adjust the rear caster. And it is important to get rear caster right to avoid rear end stability problems.

To measure and adjust the rear caster requires special tools. Alignment racks do not have this capability. And when you adjust rear camber, caster will change. I built my own tools and do my own alignments.

Rear caster is extremely sensitive. By that I mean small adjustments to camber can really throw off the caster. For instance, when adjusting camber, if you put in 0.2 deg of camber adjustment ( and say you do that by adjusting only one of the lower control arm cams) that will throw off the caster by about 0.6 deg. The manual is not correct in saying that one cam is for camber and the other is for caster. Both cams affect camber and caster pretty much equally from what I have experienced.

To set camber properly requires adjusting each of the two lower control arm cams and going back and forth between while also getting caster correct. It is a tremendous pain in the back side to get it right.

If you can have DSC do your alignment, they will get it right.

 

Just finished my alignment today to DSC middle specs camber 2.0 and 1.7. I was leery about the 0.5 toe OUT both ends but I did it. I have only driven it about 30 miles on the street but the handling is awesome. Just really tight and feels like it's digging into the pavement on corners. Impressive.

We did this in combination with installing Granatelli rear toe links so we could adjust the rear toe quite precisely versus the stock setup. I do not have equipment to measure rear caster so it is what it is.

Looking forward to a track day on slicks in a few weeks. That's why I wanted the extra camber.

 

You do know the DSC recommendations for rear toe have changed considerably? Check their Dec 2019 recommendation on their website.

As for shims GM's recommendations on shim removal have also evolved. Originally in their Feb 2016 TSB on how to do the track alignment they recommended removing one shim from behind the UCAs at the front of both the Z06 and the Z51 in the rear they only recommended removing a shim from behind the UCAs on the Z51. Once I got in there I noticed the Z06 only came with one shim in the rear. I and others have wondered why GM didn't recommend removing all shims and we suspect they don't want the UCA dog bones directly mounted against the aluminum frame. Recent info from GM indicates they don't want any shims removed when setting the track alignment.

Bill

 

Do you realize, your rear caster is probably way off? I did a test and by adjusting the rear lower control arm cam only, there is a 3 to 1 ratio of caster change to camber change. So if you adjust camber say 0.2 degrees, your caster changes by 0.6 degrees.

 

Hmmm, no I did not realize that. I went to their site directly now and found the new document which is 0.5mm and 1.7mm positive toe, which is a HECK of a lot different than 0.5mm negative! Curious thing is that if you do a google search for "corvette c7 alignment specs", the old DSC document comes up. They still have the document on their server for google to pick it up. If their opinion has changed so much, they at least should take down the old document.

Apparently they thought 0.5mm negative toe was a good thing at one time. It actually felt surprisingly good to drive, but since I have access to the equipment I'll change it next week.

 

Yes Rik.... I think I said as much in the original post. I may get a measuring tool and set that, probably the same time I take out the now-obsolete toe setting!

 

All you need are two 1/4" diameter pins plus a digital torpedo level and you can measure your rear caster angle. You can see in the picture that I am measuring 89.9 degrees which means I have 0.1 degree positive caster in this photo,



2" long x 1/4" diameter steel pins inserted into suspension knuckle arm

Digital Torpedo Level

 

Looks easy enough. How long should the pins be? How far apart are they, in terms of how big the torpedo level should be?

 

The pins need to be 2" long. Any longer and you can't get the lower one in the hole because the shock gets in the way. My torpedo level is 8" long. The pins are about 4" apart in the suspension arm.

Measuring is easy, just make sure the car is on level ground. Keep in mind you are measuring precise angles.

My garage floor slopes 0.6 degrees towards the door, so when I measure my caster, I have to account for that slope. For example, to get positive 0.7 degrees caster angle, I need to actually set my caster angle to positive 0.13 degrees when measured on that garage floor.

Adjusting the caster is the opposite of easy!

 

Another suggestion. Do yourself a huge favor and set your toe to zero on front and rear tires. Toe in or out will eat your tires alive. And its effect on handling is so slight there is no way it is worth cutting your tire life in half or worse. Seriously - toe affects wear that much.

 

Was there ever any discussion about why DSC originally recommended to out, and then later changed to a more conventional toe in?

 

I got something like this.
http://vi.raptor.ebaydesc.com/ws/eBa...=1596976386964
Cut the heads off of two 1/4 inch bolts. I drilled two holes in the one side = to the distance the holes in the upright are apart. Put the bolts in this plate using a nut on each side so I have the solid shank portion of the bolts sticking out. I stick them in the holes on the upright and use a magnetic angle finder like this.
https://www.lewiscontractorsales.com...8697b248804b0f

 

There is another way to adjust for a floor that isn't level. I place a 6 ft long carpenter's level running fore and aft under the car next to the rear wheels. Then I put my digital angle gauge on top of the carpenter's level and zero it to the angle of the floor. I use a saw blade angle gauge that I can orient to the GM adapter so I can read caster angle without any arithmetic. If you are doing this measurement on a floor let me tell you that is not easy squeezing under the rear fascia and getting your head and shoulders in place so you can even put the gauge in place Vs hold it and read it. I prefer using the GM adapter or several different versions of home made adapters that let a person magnetically attach a gauge to the adapter and then rolling back to get their hands out of the way so they can read the gauge.


Bill

 

Bill, I decided to find out why this has changed. Was it just a misprint on the earlier recommendation? I talked to DSC at length today. Their rep Tom is quite knowledgeable and explained it all to me. Per that discussion.... When the suspension compresses toe-in increases. If you start with toe IN, then under compression you have too much. The original recommendation for 0.5mm toe OUT was selected to allow for that toe change when cornering. DSC's opinion has not changed. its just more complicated than they can state on a single page. For those who would be driving "enthusiastically" especially on the track that is still their recommendation.

The later change to 0.5mm toe IN was really based on the ZR1 with much firmer shocks and springs. There is less compression so that extra 1.0mm is not needed. They ultimately decided to use toe IN as their published setting because toe-in adds stability. For most drivers on the street that stability is desirable. DSC's position has not changed, they like the slight toe OUT on cars like mine with softer springs because there will be more compression. FYI I have a Z51 that we did a full GS/Z06 body conversion, but I still have the softer Z51 springs.

Those of you with the Z07 package probably would not want to use toe out on the street. Z07 has a much higher spring rate.

My car doesn't get more than 1,000 miles a year on the street. Tire life is not really an issue. My desires are optimum handling on the track with Pirelli slicks, hence the large camber I selected. I find the car extremely stable on the street even with the toe OUT. When you turn into a corner it really grabs. I like it. After discussing my needs he recommended I stay with the toe OUT. It isn't much remember we're only talking about 0.5mm. Most tire shops doing an alignment aren't adjusting to that level of precision anyway.

By the way my digital level and 1/4" diameter pins arrived from amazon today. Gonna tackle the rear caster tomorrow. I understand how to measure it. How is rear caster actually adjusted?

The bottom line is that there is not just one optimum spec. Everything is a compromise.

 

Rear camber is set by adjusting the two lower control arm cams. If you need more positive caster (angle is leaning back at the top) the front cam needs to move the control arm inwards and the back needs to move the control arm outwards. Unless you move both cams exactly the same amount (which is near impossible) you will change the camber when you do this. So get it close by adjusting both cams and then start adjusting each cam one at a time.
You need to tighten the cam bolts to say about 20 ft-lbs to prevent them from slipping while you are adjusting things.

Don't forget to tighten them fully when you have it just right. And then double check your final angles after torquing the bolts to 125 ft-lbs to make sure they did not slip while final tightening.

So lets say you need 0.2 degrees more positive caster and you need less negative camber, that means the control arm needs to go inwards, so you would adjust the front cam inwards. The ratio from my experience is that you get about three times as much change in caster as you get in camber. So if you go 0.3 degrees more caster moving one cam, you get about 0.1 degree of camber change. It is a bear to get both angles where you want them, but you can get there if you don't get frustrated and call it quits.

There are four cases so I will describe each case:

1) If you need more pos caster and less neg camber - adjust the front lower control arm cam inwards.
2) If you need more pos caster and more neg camber - adjust the rear lower control arm cam outwards.
3) If you need less pos caster and less neg camber - adjust the rear lower control arm cam inwards.
4) If you need less pos caster and more neg camber - adjust the front lower control arm cam outwards.

Which digital level did you get? I just ordered a new one because my Craftsman Digital Torpedo level was not really accurate enough. I ordered a Digi-Pas DWL280PRO. I hope that one is more accurate. It has 0.05 degree resolution which should be sufficient and the accuracy is specified at 0.05 degree near zero / 90 and 0.1 deg for other angles. I built a test fixture and found my Craftsman level was only good to about 0.3 degrees accuracy and it is not very repeatable.