RBbugBITme

UPDATED 2/22/15

I'm working on modeling the C5 suspension geometry and I'm willing to share some of it. Let me know what you'd like to see. My data isn't 100% accurate yet so this is subject to change...

C5 Front @ stock ride height
RC height - 2.595" (very linear with bump)
RC height (in roll) with +/-3deg of roll = - .025" in RC height change
RC width (in roll) with +/- 3deg of roll = +/-.733" in RC width change
Camber gain (ride) - about -0.465 deg per inch of bump travel
Camber gain (roll) - about +0.774 deg per deg for outside tire
Motion ratio - .754 (very stable through travel)
Caster - 6.358 deg (varies quite a bit in roll and bump but it looks like it does so in a good way)
Caster trail - 1.412"
CG to RC moment arm is very controlled throughout the whole suspension travel in combined bump and roll. Gets worse when lowered but its still so controlled it doesn't matter.

C5 Rear @ stock ride height
RC height - 3.425" (very linear with bump)
RC height (in roll) with +/-3deg of roll = - .035" in RC height change
RC width (in roll) with +/- 3deg of roll = +/-.364 " in RC width change
Camber gain (ride) - about -0.271 deg per inch of bump travel
Camber gain (roll) - about +.880 deg per deg for outside tire
Motion ratio - .680 (very stable through travel)
CG to RC moment arm is fairly well controlled throughout the whole suspension travel in combined bump and roll at stock ride height. Improves when lowered.

See page 3 post 46 for more detail

Solofast

What you are seeing is exactly what good folks at GM intended. By the time the C5 was being designed the design of proper geometry was pretty well known and the guys doing that work were very savvy engineers.

You are also correct in that the amount of lowering for best handling isn't slamming the car on the ground.

All of this is pretty well known and has been discussed at length in the Autocross and Road Racing page of this forum..

RBbugBITme

I've asked to move this thread to that page since its more appropriate there. Probably before the end of July I can get the C5/6/7 suspensions analyzed and post up screen shots at stock, -0.5", and -1.0".

RX-Ben

Is that motion ratio to the leaf spring pad or the shock mount?
I assume these are front measurements?

RBbugBITme

That is shock/coilover MR. Front only for now.

Scooter70

Send me your email address and I'll send you a spreadsheet with every data point - front and rear.

Never mind... here's a link to the file in my Dropbox. https://db.tt/n1yNNf0T

RBbugBITme

Thanks. I'll see how that looks compared to my numbers.

RX-Ben

Did both of you measure the various geometric points on a flat flat floor with plumb bobs/lasers etc?

RBbugBITme

I have suspension CAD files from GM. The C5 stuff is quite old though and missing the upright model so I'm going to be very close but not perfect on the upper ball joints. It also doesn't have the steering rack assembly. I'll probably end up using Scooter's tie rod points just to have something in there.

I think the C6 models have steering components and I know the C7 models do.

edit: Looks like I can get the upright models.

NemesisC5

In for the data

Scooter70

My info came from Performance Trends Suspension Analyzer v1.1 software package.

RBbugBITme

Alrighty, I've confirmed the C6 and C5 spindles are pretty much identical and I just received the C6 model with ball joints that have a sphere I can pull a center point from so I will put that in when I can to correct my upper ball joint for the C5 suspension.

Scooter, for the most part your points are all within a half inch and many within .1".

I need to buy more RAM, these models are killing me.

TypeIsCheap

Interesting, subscribed. It would be good to know toe change and other variables that change with height changes based on a decent model.

jtmck

Have you been able to plot the roll center movement vertically and side to side through a corner with ride, roll and steer?

RBbugBITme

Yeah I'll post graphs by Monday probably.

In other news I did my baseline track runs tonight. I ran a 13.6 @ 107. In a couple weeks I'll go back with the only change being a set of Penske 2-ways.

RBbugBITme

Maybe this will be useful for some of you. The C5 and C6 front suspensions are identical as far as I can tell. The rears are a little different and I'll get to that over the coming weeks.

This is a front roll iteration only, no steering/pitch inputs and I set static camber to -0.8 degrees. I'm comparing stock ride height to -0.65" through +/- 3 degrees of roll. From this and some of the steering inputs I looked at I'm definitely going to start something more like twice that static camber. I still have plenty of calcs to do on roll stiffness etc. and I haven't chosen my spring rates.


PLEASE NOTE: I'M NOT SHARING MY CAD FILES WITH YOU AND I DON'T HAVE THE TIME TO PUT YOUR EXACT CAR SETUP THROUGH SIMULATION RIGHT NOW SO DON'T BOTHER ASKING.

Just saw the 2nd graph has a poorly labeled Y axis. Its really supposed to say something like "Inches for RC Width, Degrees for Camber"

Apocolipse

I found an inch lower was just about perfect for camber gains. Toe stays linear through the range of motion which is nice. Too low causes the roll center to hunt around more during a turn and roll motion.

theseal

Im curious about the c6z differences from c5z. My c5z was an absolutely brilliant handling car. All c5z's i have driven handle great almost regardless of alignment or other setup-and can be worked at the appropriate slip angles into and out of any radius corner.

Both my c6z's are garbage. The difference is shocking. They cannot be worked into a corner appropriately except extremely large radius turns where you dont want much yaw in.

Part of the problem is definitely the abs, but there is more to it. Something is going on with the geometry. I have even tried putting all c5z springs shocks and bars on the c6z, and disabling the abs, and it still does not even come close.

theseal

That probably doesnt read well - bottom line the c5 can come in neutral and gripping hard with some yaw on the brakes. Most people dont lean on the cars that hard to really get into that zone, but its how you dominate national solo and tight corners on road courses. The front and rear are both working hard at the same slip angles.

The c6 abs forces a very bad push if you try to come in that way, but even if disabled, the car is either pushing like a pig or skittering in sideways, both if which are slow as ****. You can of course do the 'roll in on the front tires without getting into abs' bit and go fairly quick, but you are losing seconds compared to someone coming in hot. For some reason something has changed with the c6 and you cant get both outside tires working hard on corner entry. It feels like the roll couple is off or something. Its like the rear loads up too slowly so the front is more likely to go into push: but if you start loading the rear it tends to overshoot and get loose.

I'll be testing again soon and will post whatever i figure out but if you have any ideas i'm all ears.

I'll probably try to measure some differences, then go back to c5z bars shocks springs, then play with rake, rear toe, maybe rear caster?

Apocolipse

They changed wheel base but I don't see it making that drastic of a change.