Does anyone know, or has anyone used a program to find the front roll center height for a stock Third Gen (with the GTA 16" or IROC wheels)? I know it will be different from car to car, option to option, etc. I know how to calculate it, and I'll proably do it eventually for my car. I'm just thinking through some possible updates and wanted to know roughly where it is on the front. I only want to know what it is at regular curb height. I did a search and couldn't find anyone who mentioned a number. Thanks

 

I don't know that we have an actualy number. and as you said it will vary from car to car, even alignment specs will have an effect on the front roll height. now you have me thinking, i might have to go start measuring my car. I do know that at least on anything with any lowering, the front is much lower than the rear (which is VERY easy to find). i would not be suprised to see some cars on this board, end up with a roll height in the dirt.

 

aren't the old beatles with the roll center up above the car?


where is a good spot to have the roll center?
up high or down low or right in the middle?

 

idealy it would be the same height front and rear as the cars center of gravity. and it wouldn't change under roll, but that is basicly undoable. really what we would be aiming for is to have the front and rear centers match when the car is sitting.

 

Thats me. I have no idea either exactly where it is but my estimated guess is my front end is about ground level.

here's just a quick"Estimate" of how this is measured. None of my angles are correct, they are just an estimated guess in the picture. The a-arms are not as inverterd as they look in the picture because the shot was not level and taken from lower- but this gives the general idea. Red "X" is what I suspect it to be aprox. Green "X" shows where it will stand when I correct the a-arm angle better with an adj balljoint and a spring spacer. Only way I get away with this is with such a lightweight frontend and 800lb springs + some other goodies I have proprtioned in the rear and tweaked the adjustments. I still need to get the front up 1", and the rear down about 2".

 

what we really need is someone to build a k-member that will fix the a-arm geometry for lowered cars, and give it some more anti-drive properties.

 

I agree, However the more camber given for racing specs, the more it raises

 

Thanks to everyone for the info. Vsixtoy- it looks like yours is at about 1" or so above the ground. I know your car is lowered, about how high are the lower control arm bolt centers off the ground? Thanks

 

I tried to get a little ruler under there but its very hard to read unless the car is jack off the ground- and of course that would not give the results you want.

My "guestimate" is it looks to be around 6 5/8", but again that is with two different measurements aded together and could be + or - 1/4" (I can get the height to the bottom of the a-arm just below the ear bolt, but I can't see the bolt and am "feeling" to get that measurement (I can't see it)

I also want to reiterate that my above example of my car was just a guess off of the picture. Note that the strut anngle was a guess, the ground level in not accurate because of the picture angle, the center line of the car is just a guess, The a-arm angle isn't corectly viewed in the picture (viewed from lower making it look more angled than it is) That example above is now way accurate enough to correctly estimate my roll center to be 1" off the ground- again, its just a guess. One of these days when I get a little time to mess with the car, I will accurately measure these points.

EDIT: Personally, it doesn't matter to me to know exactly where the rollcenters are on paper. I just know what I need to do with any corrections by how the car feels. This car feels great for what its used for (wifes daily driver) and I will get to tweaking things more in due time when and only when I feel a need to do maintenance or upgrading ar problem area. This area will be finally addressed when I install my Wilwood front brake setup because I am sick and tired of warpped front rotors. Otherwise, It would probably be years before I ever messed with it- its good enough now.

 

I dont have stock measurements, but here are a couple of reference points from my car.

config 1:
inner a-arm pivot 7.5" above ground
ball joint pivot: 9" above ground
RC height = 1.5" below ground
In this configuration the car was probably lowered about 1" vs a stock IROC

config 2:
inner a-arm pivot 8" above ground
ball joint pivot: 8.5" above ground
RC height = 2.25" above ground

These were both done with 17x8.5" wide wheels, 4.5BS and a 245/45-17 tire.

To get to the second configuration, a slightly stiffer spring with the same free length was used (minus the insulator), and an adjustable mono-ball was substituted for the ball joint which effectively increases the spindle length. The ride height increased 1/2" over config 1 (which was my intent).

I'll post some pictures later today.

 

DSM, What adjustable balljoint are you using to get a 1" drop from spindle to a-arm? The most I have ever seen is 1/2" max adjustment?

 

I would also be interested, the largest i have seen are 1/2. like the rebuildable ball joints sold by Coleman.

 

I will measure my car tonight. Mine is lowered about 2", with stock (Moog) ball joints, and stock spindles. DSM- how did you figure your lower ball joint centers (I think this is the hardest part to measure for determining the true roll center). Thanks

 

Sorry for the incorrect info....had a typo above....thats what I get for posting from work while I'm in a hurry. Info has been corrected in the original post. The mono-***** I am using are Allstar #56274 which is a direct replacement for the stock ball joint (press fit on the stock control arm) and raises the effective spindle height 0.5".

Also, should mention a couple things...first, there is a more "streetable" alternative to the monoball lower balljoint. Its available from Howe Racing under their precision ball joint line. It increases the spindle length by the same 1/2" but is a lot more durable than a monoball. http://www.howeracing.com/Suspension...ints-Lower.htm
For the F-body/G-body they are special order, but you can get them ready to press in, with the longer stud in place and a dust boot from Savitske Classic and Custom http://www.scandc.com/trial5_content.html Just ask for Marcus and tell him I sent you.

The second thing is bump steer. Increasing the effective spindle height with monoballs/Howe ball joints will cause massive bump steer problems. One way to fix it is with Allstar tie rod studs and heim joints as I did.

 

DSM, you and I are still about the same angle relation Before you fitted the adj balljoint. I am already setup with bumpsteer provisions and am currently spaced pretty much identical to yours in picture (same distance frome spindle, just bushed different with spacers rather than threaded nut). What I am looking to do is exactly what you're showing with the A-arm droped about parallel to the tierod. You'll notice in my picture that my A-arm angle to tierod angle varies where yours now sits more parallel after the monoball drop adjustment. I will be spacing the springs to retain the exact ride height so the only thing changing is the a-arm angle will drop to a better radius swing and slightly raise the roll center. I only utilize about 1 1/2" max travel on the front end under the harshest conditions- its very stiff. Most slow speed damping is the cushion of tire flex, thats why I stay with 50 series 16" tires to keep some rubber under there.

I will be using the same Coleman adj GM6145 joint John (Dewey) mentioned.

Ps, I just noticed the g()d damn cricket under the front tire. Gotta go catch that bastard before he gets into the house!

 

Dean,

Your looking at this part correct?

21315 Stud, Lower, GM 6141, +.500

that along with new springs are part of my master plan for this winter.... when i get them in, I'll let you know.

 

OK- here's what I've got:

Current set up:

I run about -2 deg camber on the front

LCA inner pivot = 7.6"
Balljoint ctr= 9" (I didn't measure this- DSM's was @ 9" w/245/45-17 and I've got 245/50-16, both are 25.7" tires- correct me if I'm wrong).
Estimated roll center= 1" below ground (similar to DSM's original number)

Proposed set up- I've got a set of 2" drop spindles I'm thinking about using (I'll need to cut them to mount the calipers). With this I would have:

LCA inner pivot = 9.6"
Balljoint ctr= 7"
Estimated roll center = at least 5" above ground

I think this would work better, my rear roll center is at about 11.5"- the centers won't conflict, and I'd have a better camber curve. But I'm worried about road feel with this set up. I really like the way my car steers and turns in right now, I'd hate to mess that up.

 

Thats a "large" (GM K-6141) lower ball. We run "Midsize" ( GM K-6145) style on 3rd gens. The part # you want is 835-450. The noice part of these is the replacement bearing they run is a "Com-12" size which is the same as the billet strut mounts. Keeps sevice parts common on the front end.

Dale (Jay) informed me awhile back that QA1 offers a much higher load rated bearing in the COM-12 style than what I am running with the Aurora PNB-12T bearings-

Aurora COM-12: Radial Static Load of 31,920lbs (lasted 2 months)
Aurora PNB12T:RSL 46,400lbs/Axial load 6,750lbs (1 year)
QA1 PSB-12T: RSL 78,000/ No axial load rating listed

The "Axial" load is what the upper strut mount suffer from. This is the up and down thrust on the bearing trying to force the ball out tof the housing ends. I plan to try the QA1 upper end bearings next. The axial ratings in proportion to the radial ratings would put the PSB-12T bearing at aprox 11,350lbs.- almost twice what the best Aurora bearing will yeild.

 

edited to nothing, because I didn't read very well.

 

I probably threw you for a loop thinking I was refering to the strut mounts, and not the Balljoints.

The original balljoint you asked about is a Percision balljoint with a longer tapered shaft, but are not offered in the K-6145. What I recommended is a Mono-ball that utilizes a spherical bearing and is adjustable (You most likely had that figured by now hence your edit). The Mono-***** wear quicker with the standard COM-12 bearing, thats why I listed better alternatives. They are also mcuh much easier to service than having the standard one pressed out and a new one pressed in, but of course the standard type do not wear as often so replacement is almost nil. The mono-***** however do have much more freedom of movement producing less suspension bind.

 

yeah, I didn't notice that it was a mono-ball style, and was asking how you knew it was .5" longer. then realised what I was looking at.