hisvett

If this post sounds like I'm a little PI$$ED OFF, well I am It has been almost a year that I have been putting up with what I would reefer to as "Rear Bump Steer". The car has been to two different Firestone dealers and one (Very Large) Chevy dealership in the DFW area to sort out the issue. The first thing everyone whats to blame it on is bad shocks; so just to confirm that they were wrong, I bought two new ones from GM Direct. I now have two slightly used rear C7 MRC shocks for sale.

So that the next person doesn't have to go through the same hair pulling exorcise as I have, here are the symptoms;
1. The faster you go, the more noticeable it is.
2. When hitting a dip or bump on the freeway at 50MPH or more, it feels like you are in a row boat and just got hit by a swell from the side.
3. The rear of the car rocks/sloshes from side to side rather than bouncing up and down....just like being in a boat.

Before going any farther, I have to give a big shout out and thanks to Corvette Forum posters Bill Dearborn, DSC Sports, Rikhik and Thebishman; I spent hours going through their threads and found that these guys speak from experience rather than many people on here "well my friend said or I think it should be". I got lots of good information from Bill on how to build my own Rear Caster Gage to double check the alignments shops work.....and guess what, the drivers side was right on the money at .7 degrees while the passenger side was close to 5 degrees out, and that's NOT POINT 5, that's 5 whole degrees out of spec. No wonder I was getting seasick on the interstate!

So here's what I did; first of all, the rear needs to be a few inches higher off the ground (with full weight on the tires) to access the inside of the rear spindles. I went through my old machinist tool chest and found a few 1/4" harden & ground dowel pins to push into the alignment holes provided by GM.



I then found a piece of 1/2" Key-stock in my box and with a Digital Angle Gage I purchased off of Amazon, I could lay the Key-stock between the two dowel pins and read the angle. Sorry for no pic, didn't have enough hands to work the camera.

Here's my Digital Gage stuck on my piece of key-stock;



This is how I found the passenger side to be off by a mile. Then I noticed the adjusting cams on the lower "A" arms (not the cams to the rear of the car, I'm looking at the ones inside or toward the front of the car. This is a photo of the Driver side adjusting cam;



This is a photo of the passenger side adjusting cam;



NOTE; the two cams have scribe lines from the factory, the driver side lines point down and the passenger side points almost 90 degrees to the right. This is something a blind man could have noticed!

Something you all should know is there are Very, Very few cars on the road today that have rear Caster adjustments (less than 5); this is why very few alignment shops have the equipment or even proper knowledge to deal with a C7 rear alignment correctly. So take this shade-tree mechanics pain and suffering to educate yourself and whoever does your alignment; tell them "NOT TO F$#K with the front two cam adjustments"!

taz2016

Thanks for the pictures and explanation. Were you able to correct the problem with your homemade setup? A more complete explanation on how to do it would be helpful.

Dif

WelderGuy

I think the marking pen lines (witness marks) on the cams are supposed to line up with the marks on the nuts to verify they haven't been disturbed since being installed at the factory. They have nothing to do with proper position of the cams.

Do you know for sure the car was properly level when you did your check/adjustments? Where/how did you verify this?

Foosh

Rear caster setting arguably has as much, or more, to do with handling quality than most other alignment settings on this car. It is amazing that most dealers don't bother checking it, don't have the tool to do it with, and don't know how.

I've seen reports where several cars straight out of BG had positive rear caster on one side and negative on the other. That's would be a complete mess in HPDE or spirited driving.

Bikeman

Good post! Ingenuity. Is it just me or is there a fracture chunk off on the lower edge of the passenger side frame support? That part is riding a lot higher on the bolt than drivers side.

rmorin1249

Another example that supports only well qualified and equipped dealers should be allowed to sell and service Corvettes. Shame on GM. Sad, very sad.

hisvett

Yes, I could adjust the one side back to or at least closer to spec but in doing so, it will throw the toe out and probably the camber. I have a lifetime alignment policy with Firestone and an appointment for tomorrow to put it back on the rack. I have one guy that I have always gone to because he has lots of experience with earlier Corvettes. As I mentioned before, there are only a hand full of cars that have been put on the road with adjustable rear caster and none of the current popular alignment racks are setup to measure it. We as Corvette drivers, need to take it upon ourselves to educate others in the field....because no one else will.

I'm taking all my makeshift tools with me tomorrow and will have a class with my alignment guy and his boss; I'll let ya know how it goes.

davepl

As Rik knows I was originally dubious about assigning too much responsibility to the rear caster, but he's the expert on getting set right, and has worked diligently to document it for the forum.

It seems to me that no two Z06s are ever the same on the track. I swapped with a friend and his was "different", and then at Spring Mounting I drove I think 3 different cars and no two were the same.

I'm a noob, so bear with my description, but I remember having to "learn" each car for a lap or two to see how loose it was, and it didn't feel like tires. Each would take a different amount of throttle steering on the wide high speed turns, for example.

I'm wondering now if the reason every car "feels" different is that the rear caster is effectively "random" coming out of Bowling Green?

Or do they make any attempt beyond assembly to get it correct off the line?

It'd be an interesting opportunity to drive two cars that were aligned the same to see if they then drove the same. All I know is mine drives better than most and a -lot- better than some. Yet other than rubber you'd think they'd all have the same dynamics. They don't!

Or is it something completely different that makes each car "feel" different from the one before it in terms of turn in and rotation?

Landru

Beautiful job describing and documenting this matter.
As others stated most shops of any kind are equipped w/ required tooling and/or knowledge to at least check this setting. Many 'techs' aren't aware it exists, nevermind its importance.

Nominating this thread be a sticky.

ILXgirl

Good thread. How would a novice like me even begin to deal with something like that.
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hisvett

Good guess on the marks but not entirely correct. There are no marks on the nut(s) and all 4 cams have the same marks AND the only cam that is WAY different then the other 3 is the problem child....passenger side caster.

And yes the car was re-leveled after rolling up on blocks. Even if I hadn't, the Delta between the two caster readings would still have been way off the chart.

hisvett

GOOD EYE! I had to go back out to the shop for another look. You are correct, it does look like a chunk has been taken out due to a hard blow or something. The truth is, the other side looks just like the photo so I'm not worried about.....sneak a peek at your car.

hisvett

First; if you are feeling like you are in a row boat that is getting hit by a side swell when on a lumpy freeway.

Second; grab a look under the rear and check the marks on the 4 cams. If one of the inner two (toward the front of the car) look way different than the other 3, you probably have a caster issue.

The rear two cams adjust the camber and the front two are used to adjust the caster. Sometimes your handy alignment tech doesn't realize this and tweaks front cam to find tune the camber without knowing he's moving the caster. This is what happened to my car.

hisvett

Please note guys; the GM spec for rear caster is "0" degrees, Plus or Minus 7/10's or point 7 of a degree. This is how or why you hear story's of one side having a negative reading and the other positive. This can happen and the car will still be within GM spec.

This is a link to a really long thread that has tons of great info from many people in the know, I suggest taking some time to read through it.

https://www.corvetteforum.com/forums...-caster-3.html

Foosh

Yes, it is a a great thread. There's a lot of discussion of Mike Levitas (former 24 Hrs of Daytona winner), his friend Randy Pobst, and the TPC Racing/DSC Performance alignment specs, which were developed based upon a lot of C7 track time. I had my car set up at that shop w/ Mike's specs, and their positive rear caster specs (+.7) make a world of difference.

BrunoTheMellow

Eh...I may be the only other one that has experienced this as no one else on the DSC thread had.

I had the same feeling on my z51. Similar to what you see trailers do in strong winds, but just 1 sway every big dip or crest. It only started after I had it aligned to DSC specs at an unnamed "track" shop in North Texas and DSC sport controller (softer shocks + bad alignment made it obvious). They did not do rear caster after telling me they could. Note that the car was also full up on bolts at the rear.

I got the problem to go away by lowering the rear to level with the front, got realigned to GM track specs at a Porsche shop (gets rid of the toe out), and adjusting the rear caster to 0.7. my before settings were 0.65 and -1.4. The car is MUCH more stable at speed over hills and dips right now (helps that I run stage 2/3 aero now too). I can't say that it was only rear caster that fixed it unless the first aligner severely threw out the caster by putting a mild track alignment (-1 camber from -.5). The car did not have any stability issues whatsoever from factory before the first alignment and addition of DSC sport controller. Even with the suspension raised all the way up in the rear. I think the softer z51 doesn't like toe out in the rear and that did it more than the caster.

hisvett

Pos .7 caster seems to be the consensuses of all the guys in the know (that I trust) and have spoken to or have read on CF. I will be trying to hit that magic number once I have it on the rack.

Foosh

Im lucky as hell to live 40 miles from Mike's shop, and Mike personally supervised my alignment. He's a lot of fun to hang out with, and like the mad scientist of suspension technology. He's also incredibly OC about the precision of his state-of-the-art alignment rack. He frets about fractions of degrees.

BrunoTheMellow

Oh I just saw you're in DFW. Duh. Take your little assembly for rear caster and go to RAC performance. They are the ones who got mine right. See my previous post. That is exactly what I did. At first they said they couldn't but I told them to use my angle gauge and pins and what not and he did it. Got 0.8 and 0.7.

At first I measured the caster myself at home and get 0.65 right rear and -1.4 left rear. Not as wild as yours. But note that I also got rid of my toe out in the rear.

Btw You cant measure it without weight on the car. If you raise it, the car needs to settle by driving around. Suspension tends to hang after being unloaded. I was able to fit the two dowels just first without raising the rear, get shorter dowels. Remember that the car has to be perfectly flat. If not , measure each side, turn the car around and do it again and get averages.