savage

I looked at adjusting rear camber 11 months ago and stopped because I had not enough information on it at the time - I just found this on the shelf!!!

MSR

Caster and camber in sequence isn't necessarily correct. Caster and camber changes both result from turning the lower control arm eccentrics. It's tricky to set camber without affecting caster, and vice-versa, you sort of have to work both at the same time. In other words, if one person with the angle gauge sets caster and then you start fiddling with camber, your set caster will be gone.

-michael

savage

I could not have said it better , look at how small .8 degrees is - this is 3 sheets of paper folded over to make 6- table was at zero before test

NTMD8R

Thanks guys, for that info.

OK, I still think we can do it with the "parts" I have,
but we will do Caster and Camber together.

And yes, .8 of a degree certainly isn't much.
So I do question whether or not this will be enough.... but I can find no-one to say.

MSR

Just a little more info on that, since I've been through the process 3 times in the past month...

I have a SmartLevel that's built into a camber gauge. The first time I measured caster I used that SmartLevel. I didn't like having to take the SmartLevel apart to separate it from the camber frame, so I bought an additional gauge that cost $50 and had decent reviews.

The new gauge I bought reads differently upside down, meaning the top edge and bottom edge aren't perfectly square to each other. I had to go back to using the SmartLevel in order to ensure my measurements were equivalent on each corner of the car. The SmartLevel is pretty expensive, but given my experience, I'd question that gauge you have in your picture. Make sure it reads the same regardless of which edge is facing the measured surface.

I autocrossed this past Sunday with +0.9 caster on both sides. I was initially disappointed because the rear still seemed to step out, even though it did feel better than before. It was good enough that I could drive around it by concentrating on momentum and using less throttle. I wasn't paying attention to overall times and didn't realize until later that I went radically faster compared to the usual people. This in spite of multiple mistakes in driving (as you'll see in the video). I'd have to call the caster change a big win for overall speed, if not a win for what my idea of "good handling" is.

5th in PAX, out of 132 people, completely stock car with just the alignment and RE71Rs on SS-legal wheels:


-michael

NTMD8R

Michael,
thanks for the info and video.

I am making my own "tool".
I bought a digital gauge from Amazon ($40),
and a piece of angle bracket and some 1/4" bolts from home depot.
I'll cut the "ears" off the bracket.
Then drill 2 holes in the bracket EXACTLY to conform to the holes in the spindle.
Use the 2 bolts (cut off as necessary).... fasten them to the bracket with nuts,
so the bolts stick out enough to be placed into the holes in the spindle.
Place the gauge on the other side of the bracket.... tada.... it should work.
I'll put the bottom edge of the gauge on the angle bracket, so it is always
measuring from the same position. Of course, it will now read somewhere in the
plus or minus 90 degrees. But we can use this.... just make sure our measurements
are 89.20 or 90.80.



Now... another question... what harm can we do if we EXCEED 1.0 degrees ?
In other words, if .8 degrees is good, would 2.0 degrees be better ?

MSR

As someone pointed out earlier, changing caster is effectively changing the bumpsteer curve. "Bumpsteer" as defined by toe change with compression or rebound. That's really the only way it's affecting handling, I can't see how any other consequence is the same order of magnitude. Within a ballpark range, more positive caster will result in more toe-in per amount of suspension compression. The flipside is it will also result is more toe-out with suspension extension. This may actually help!

There's a few complications in the overall equation, I'm not sure I can do them justice, but here's my analysis based on what I read in that Z06 thread that identified +0.8 as "optimal":

With positive rear caster, the net effect is a higher rate of toe-in increase with compression. Again, when adding more positive caster, this is what happens when a car rolls into a corner: The outside rear toes in, the inside rear toes out. This effectively turns the car into having dynamic rear steer, where both rear tires point inward to the corner. I believe this is why they settled on very slight toe-out in that Z06 thread. If the outside rear toes in a lot, and the inside rear is already toed out, then the effect of "dynamic" rear steer inward is more pronounced. This should settle the car down, especially in transitions. It's so counter-intuitive and contrary to the usual experience, that I'm having a difficult time believing I should try rear toe-out.

The risk you run with adding more and more caster is that, geometrically, there isn't a linear relationship in the suspension. You may end up with bumpsteer that does really odd things, and changes the rear steer effect. That said, though, I don't think adding another half a degree will put you into an ugly range. If you're really concerned you could measure bumspteer at different caster levels and see what happens.

I'm going to try more positive caster, by the way. I have a practice event this weekend. I'm trying to talk a friend of mine to drive the car while I change alignment settings. He's a national champion and I trust his evaluation better than mine. (Not to mention how hot, sweaty, and dirty I'll be from doing all the alignment changes.)

-michael

sccaGT1racer

Instead of all the guessing why don't you measure the bumpsteer? This will tell you exactly whats going on back there.

MSR

You can measure and plot bumpsteer, but how do you map that to what the car does when on the course? You'd need to know a lot more in order to understand what dynamic toe is. Hence the "guessing" or trial and error.

If you had wheel travel sensors and had something like 10Hz telemetry, then the bumpsteer plot would do a great job of painting the whole picture. You could write a little tool and plot toe relative to time as you're racing.

-michael

sccaGT1racer

You measure the bump and rebound at 1 inch increments. starting at ride height. then adjust to get as little toe change as possible throughout the travel. This is basic racecar setup procedures. There is no guessing on what the numbers are.

MSR

Apologies, I didn't understand that's what you were trying to say. I thought the discussion was around understanding why the caster changes in the Z06 thread "work."

From what I measured, you need negative caster to get close to zero toe change. I think people in C7s are deliberately attempting to dial in bumpsteer (without realizing it).

-michael

AUTO_X_AL

Not on a C7. The toe is now on a cam and will change the camber when adjusted. Worst. Design. Ever. I'd kill for a regular toe link on the back.

NTMD8R

I have mentioned all the alignment settings I have tried, along with various sway bars and combinations and tires.

I had my dealer align the car Friday ($335 for an alignment !!!!, plus "shop supplies", plus taxes .... $413).
Supposedly I now have
Front -1.9 camber, 7.0 caster, 1/8" toe out
Rear -1.5 camber, .8 caster, 1/16" toe out
I am running 8.5x18 front wheels, 10x19 rear wheels
and RE71 275/35/18, 285/35/19 tires.

On the weekend, we had 2 days of autocross; hot (30 deg C), and dry.
With these tires we have to cool them down (with a water bottle sprayer) between runs.

No matter what we tried we could NOT get the rear to "hook up" going out of a corner. It just continues to slide out on us.

Here's a video of my wife on one run. Note her comments.
(I'm not worried about anyone taking this, and making comments.... it is on an autocross course, and it is video only.... not from a PDR).


One other aspect I had done to the the car quite early on... I removed ALL the washers from the upper A-arms (32 washers in all). Yes, I know GM says to remove only 1.
I am wondering if in some misguided way, this could be affecting our handling? Maybe the MRC system needs some electronic connection which we have broken, or it was not connected, and we have now connected it, or ????????

Next Saturday we are doing another Autocross, and I am going to re-do my alignment (I'll have my rear caster tool ready then).
I plan to put in more camber for both front and back, and I'll try more caster in the rear.
As for rear toe... all conventional wisdom says to put toe IN at the rear. I am going back to that.

I welcome all comments and suggestions.

blackozvet

I'm still following this one, dying to know what the final answer will be !

Im no expert on the c7 , is adjustable castor in the rear new to the c7 ?

I had an alignment before last weeks hillclimb round, I went to a different place than I normally go (but only had to pay $85 ) the tech who did it is a dirt track racer so who knows if that theory translates to the hard stuff ?
Anyway, a couple of things he said to me might be of interest to you, firstly he reduced the negative camber in the rear from 1.1 to 0.5, said that too much negative camber will not let the car drive hard out of a corner.
He also put TOE OUT on the front and TOE OUT on the rear +.7mm and +.3mm, saying that a small amount of toe out on the front and a lesser amount of toe out on the rear will allow the rear to "follow" the front better coming out of a corner.

I have no idea if he is right, but as they say the proof is in the pudding.
Ok, raceday, cold and damp track, then when the sun came out the car was really hooking up hard and driving out of corners, in fact it was driving out of the corners so hard I was getting knock sensor activations from fuel surge. turns out the fuel pump shield was failing, but thats another story.
Less negative camber worked and so did toe out/toe out.

mine is a c4 but suspensions are what they are (except for adjustable rear castor in a c7),
so maybe more negative camber in the rear wont help ?

its seems funny to me that a number of people on here are having issues with the c7 over-steering, and the difference in the car from previous models is adjustable rear castor ??