Dean and Norm or anyone else that might have a solution,

OK, I have some questions regarding my car, first I will give some quick specs. It is a CP autocross car. Front suspension is stock with Polyurethane bushings, 1000lb springs and replacement struts(I know I need better struts) The sway bar is the factory bar that came on the Base V6 1983 camaro(sorry I didn't measure it). Rear suspension is a 3 link with a torque absorber upper link(biscuit bar) the lower a-arms are boxed stock with polyurethane in the front and rod ends on the rear, LCA's lowered 1.5 inches at the rear end. Converted a 19 MM hollow sway bar from a C5 Corvette to work on this car. 425 lb springs, Bilstein circle track shocks I believe 4 valved with 50/50 valving. Weight jacks on all 4 corners. Lowered so that I only have about 3/4 inch clearance between the fenders and tires, Road race slicks, 25X12X16 fronts and 27X14X16 rears on 16X12 rims with 4 inch backspacing(Yes, they stick out of the fenders a couple inches)

I am very happy with the way the rear suspension is working. good traction in allmost all situations forward and laterally, puts down the 450 HP I have very well. the problem is The car pushes and I can't get on the throttle as soon as I want to, I have tried springs rates from 750lbs up to 1300lbs in the front and the rear has had either 375's or most often the 425's. I have played with alignment, tire pressures ranging from the low 20's up to 30(seems like 25-26lbs has been the best) Running with and without the rear sway bar, even tried without the front sway bar as an experment that failed. I have tried changing my racing line and even tried trail braking to help loosen the car up with no results. The best combo so far has been the 1300 front 425 rear, sway bars connected and 26 lbs in the tires but it still pushed. To me it still feels like I need more rear spring because The car got much better when I first installed the rear sway bar so I will be putting in some 500 lb plus springs for next season. BTW the car works better and is more balanced on slick sealed asphalt then it is on grippier surfaces, seems like the grippier the surface the worse the car pushes. I will usually finish in the top 5 overall on slick courses but will be about 15th overall(out of 30 cars) on grippier courses. Regardless of surface I don't think I could spin the car out, it only gets a touch loose on corner entry when I am off of the throttle, As soon as I feed it some throttle it will straighten out then go into a push mid corner to exit.

It is my thinking that the RC in the front is way to low so it needs stiff sorings to keep body roll under control, once I get body roll under control with springs then I am too stiff in the front so it still pushes and I would get the biggest benefit from dropped spindles and raising the RC. I am thinking that I could also use some IA/SAI angle changes. Anyone have any thought on this or any direction to go to get rid of the push.

Dean, How much did extended ball joints help with changing the RC? I know it's only about a 1/2" longer stud a most but I have thought about doing that and also getting the adjustable tie rod adapter stud so I can adjust bump steer as a lower cost alternative to dropped spindles.

 

To answear some of the questions in the other post.
Front RC is at least an inch under ground, Rear is about 10 inches above.

The rear LCA's are close to parallel, slightly pointing up towards the front.

On the upper link, I did cut out a section of the rear floor and the front is mounted to a bracket on the main hoop of the roll bar. It is 15 inches long and I initially set the angle at 10 degrees. I have since moved the angle to between 15-20 degrees for more forward bite. Sorry I have no pictures

 

On alot of chassis the baseline setup on a 3rd link is usually in the 15* area.

I basically sugested quickly in the other thread that by raising you front roll center will lower the roll couple (lower the leverage) and in essence stiffen the lateral spring rate making the outside front tire load more and push worse.

The higher rear of the roll axis makes the inside rar tire want to lift and the outside front roll over which makes the car loose- but you say you are still too tight. I am wondering if your front roll center is as low as you think it is.

The little sig pic you show in a right hand sweeper lloks to have the right front lifing as if the roll center is level and should be lower in the front. You are getting tremedous lateral bite wit those 12 and 14" slicks so obvoiusly you need higher spring rates to compensate for excessive roll- thus the 425 rear. What puzzles me is that is high already compared to the front and the car should in my opinion be loose at steady state.

Average struts means average rebond dampering- which in turn means the nose releasing upon throttle bursts and the inside frront remaining contact to steer.

You should be running at least 1/4" toe out and about 5 caster and -2 1/2 camber. With the scrub radius out as far as you have it, you need to have that toe increased.

Tire pressure on that large and type of competiton tire really should never be run higher than 25-26 cold max otherwise they will crown when hot- you seem fine there. I would lower the fronts to 23, and the rear leave at about 25 to see if that will loosen you slightly- but not anywhere near as much as you describe you need. (However, little tire changes with race skins make big differences- radial tire show hardly nothing in change, they do not grow like race rubber.)

Are you getting pyro temps (do you have the man power and equip)? I would be curious just how hot and where the heat it those front are reading comoared to the rears.

 

Also, what are your track widths? A bandaid fix would be to quickly run some wheel spacers on the rear, That will help tighten you under braking on corner entrance and loosen you on exit.

The more you space, the more this effect.

i was just playing around with Grep Pickets old Trans Am seires car this weekend in SanDiego. but it is not a factory platform. Tis is a complete tube chassis car withOUT struts and an independant rearend.. Howeever, the car is still ran as it was engineered back in 1983 with major rear offset of the 16x12's and 16x14 on it

 

The front geometry on these old TransAm Series cars is handmade magnesium risers and spindles rear and front. The scrub raius is 0 on front and is a double A-arm setup with the upper rear leg of the arm attached to the firewall via an aluminum spacefrace plate (looks kindof like a motorplate) that is rivted to the fiberglass firewall. The aluminum rear upper stryt rod attaches back at about a 75* leg- very trick setup for back then. Not alot of adjustments on these chassis, but the guy that engineered them back then did alot of prototypes and new fixed suspension points that worked in the protos and carried it to this chassis.

I am only showing these pics so you see what the old race cars looked like. Many people thought they were porduction cars like ours with body panel flares, they were not. It took alot to get those big slicks under the car correctly. When you run that big a tire and grip increases- sesitvity increases as well as you have clearly stated from slick courses to grippy courses showing the problems worse.

Alan (AIM91Camaro) actually had the best suggestion for you to go beck to the same size front and rears in your next tire purchase if you have the rims for it. Getting the contact patch outward and smaller will help loosen the car. You have just TOO much inner rear grip on both sides lifting the nose with lots of power and a tight carrier locking it up also it sounds.

I think just too much rear tire for starters/ or not enough track width to get away with it.
Note though- More track width will equal higher autox lap times. not only do you have to get the car laterally another ich lets say on transistion side to side, but after you pass the cone your attack angle on the transition cone will be steeper. Higher corner attack angles means slower speeds. Its why they PAX index cars in autox, its why the little short and narrow cars are so fast. They go straight through where we have to slow down and turn through.

 

Dean, I am a one man show when it comes to autocrossing the car so most of the time I just check tire pressure and then it is time to get ready to go again. I have tried 23F/25R and tire pressure just don't seem to make a difference. never checked temps but the tires are only warm to the touch after a run. I can borrow a temp gauge but the next event won't be until next year.

The car is lowered a good amount only 3/4 of an inch between the top of the tire and the fender lip, and After looking at the car again today that might be part of the problem as I just noticed that the front tires are rubbing the fender in turns and that would pretty much make the wheel rate go thru the roof and that could be a big cause of the push. I am probably wrong about the front RC. The Ball joint pivot looks allmost level with the a-arm pivots so that should put my RC above ground level, maybe an inch or 2 but I have never really sat down and and figured it out for sure. I am wondering if I should lower the rear roll center an inch or 2.

I have been told by other people that my car should be pretty tail happy with the spring rates I have. And then I had one very knowledable autocrosser tell me I will need 500-600 lb wheel rates all the way around to be in the ballpark so I would be a bit low with my current springs and about right with the 1300F/ 550R I am going to put in next season, All I can really say is the car seems to be faster with the stiffer springs as I was consistently in the top 5 overall when I was running the 1300 fronts and we do have a few drivers in our region that have trophied at nationals or even won their class in Divisionals, so there is some pretty good competition that I can gauge myself aganst.

I have 1/8" toe out so it looks like I need more there. Camber and caster I play with at events so I really don't know what they are. With the 1000lb springs both caster and Camber are maxed out with the factory adjustment and with the 1300lb spring I could back off on the Camber a little but still kept the castor maxed. I know not very scientific but with the racing budget I have I save money where I can. I have less then $6000 into this car as it sits so I am working on a very strict budget, A wife and kid tend to do that.

Still going over all of the info in your ultimate suspension posts and feel raising the front RC would be a good idea. I also think modifieing the IA/SAI might also benefit but it will reduce the total amount of camber I can get. Just something else to play with that I never thought could be changed on this car.

 

I really don't want to widen the rear any more as most of the cones I hit are with the rear tires.

Would need to measure the track width, I am heading back to the shop in a few minutes so I can check that.

 

If I measured right, the track width is 78" in front and 79" in the rear. I just measured to the end of the tread and not the buldge. If I add the buldge then I am looking at about 80" in front and 83" in the rear. I also measured at the bottom of the tire so the caster in the front made the measurment wider then if the tire was straight up and down.

I also measured the front A-arms and it is about as parellel to the ground as it gets. Exactly 8" from the ground at both the balljoint pivot and the A-arm pivot, so my RC should be well above ground. just an eyeballed guestimate would say my RC is more then 5", but my eyeball is not precision calibrated.

 

The wider the track width of the suspension the higjer the spring rates will need be in the factory locations. They too are effected by leverage. The longer the tire edge from the pivot point the more pressure exerted to compress the spring.

Track width really is footprint outer edge to footprint outer edge- EVEN THOUGH racing orginizations will gauge it wheel lip to wheel lip for sake of complying with rules (tire bulge can varry with pressure)

RC- If you can give me measurements I can help try and get you a close measurement of the actual RC. You gave me the tire width and track, the ball joint height and the pivot height of the A-arm. I would need the A-arm pivots spread (distance betwwen the left and right pivots), the strut bolt spread (strut tower bolts distance between left and right, and the strut bolt height (I will mnus about 1/2" from the bolts to the center bushing.)
I also need the distance from the pivot to the balljoint, I do not have that measurement either (in other words, I do not know the length of our a-arms, I have never measured them.)
I do know my arms were just slightly inverted to start with, so I had camber loss in articulation. However, my car did not hardly move more than 1" of normal travel it was so low and stiffly sprung. I started with -.8 camber and 5.5 caster so any turn of the wheel gave me good camber equalization through SAI of caster. I had to bump the car and reset the tierods though becasue I was loosing 3/16" through that 1" travel. I never had access to drop spindles (Beltechs were discontinued- I tried hard to find some to no avail). So I ended up using the extended ball joints to drop the nose 1/2" and keep the RC from going lower. I basically got the car as low as I could without suffering tire contact on any part of the wheel wells. My car never rubbed anywhere. The 1/2" balljoints dropped me another 1/2" so I used a urethane 1/4" spacer from Coleman Racing and tied it up top the coil with electrical tape in the top of the can. That brought me back up just a tad more than a 1/2" I lost from the ball joint drop. result? the inside rear stopped jacking as much, it stopped rolling over as much on the outside front and tightened the car. I now could give a good consistant pull on the steering wheel witout it being so light. It had great feel and control, not heavy, just weighted well on turn ins.

I am only telling you about my car because you are asking about my extended balljoints so I am describing to you my results. My before and after and why I did it. Basically I was lower than you are with probably a much lower front RC. My rear was only 1 1/4" lower than your rear. Even though the weights are different, you still need to keep the inclination of the roll axis in check within reason of parallel rather than dumping the front RC side REAL LOW and making up for it in high spring rates. Mine roll axis rear to front was probably very close to yours now and mine was still on the loose side if I say it favored any side. When a car lacks power you keep it on the loose side, mine lacked power. I kept it tight going in, progressing to loose in center and off than I would have if I had more power- in other words, this car would not have been balanced trough and off it I had more power to lay down.

i just can't see your need for the roll center to go up in the front. I will say again it looks like you are lifting the inside front fender. I would be more apt to decrease the front spring rate and increase the front bar to compensate for the same lateral roll rate and use inside leverage of the car weight to hold the inside fender down- You did say ift was a small V6 bar on front. That will keep the front RC from migrating as much dynamically and maybe at the very least give you an easier car to drive. just giving some brainstorming feedback. You have a different animal with those wide track slicks on that massive offset and leverage.

I would like to drive it, I could feel it and tell whats going on complexly. I test drove a AMG E63 this weekend on a little autox run for the general public down at the Speed Festival weekend. by the 3rd corner I was tricking the "traction control" and was getting wheel spin through the corners through pedal work and chassis set. The guy running the test drives said no question I gave him the fastest lap all day. It was actally quite commical, my brother was in the rear seat watching the guy clinch anything he could. They did not know their own Mercedes traction control could fail so badly....
..ps- it wasn't my first rodeo.

Can you tell me what you are doing to get it to rotate driving habits wise. Rather than me describe scenerios to you, I would like to hear what you can tell me first un jaded by my possible lead. Pedal and steering imputs- close your eyes and re-run a lap in your head. Tell me your trouble spots and what you are doing pedal and steering wise to compensate and were in the corner you do this. Break it down to a pie chart 180* with 6 parts if need(thats what I do with the circle track guys. Give me 3 parts of a 90* corner for example.)

 

Let me try and sketch your aprox front RC off of what I do knidof know and show how it is raise from what I have. keep in mind that you CG is slightly higher with the V8, but I can not see more than 1/2 higher cg even though it is alot more cg weight on the roll couple- but you are saying you are using 1300lb coils for best results on a light (unknown) V6 bar/ I have aprox 825 rate and a solid 34mm bar. But my geometry is far different and I will show just for comparison how things affect the roll center.What I can't really show is how yours is changing drastically more in migration of body roll. But that is the point why I suggest lowering the coil rate and bringing it back up with a larger swaybar so the outside wheel rate stays the same, but the inside corner wheel rate changes.

Give me a minute to sketch yours

 

Here is an estimated value of what you have told me. I over laid yours on top of mine.
Note that you cat is slightly higher stagnant than mine you I raised your CG and your Arm angle straight as if your car sits higher.

You have more positive offset with a wider tire so your centerline is outward.

You are most likely running slightly more negative camber. My distance of strut rod length should be lower other than the fact I have raised sturt monts making them back the same length of extention aprox.

Side Note- notice your scrub radius(I did not bother trying to show your tire more negative camber- just pretend it sits slightly angled.)

Look at your CG from mine and then your roll center to mine. Note the arching links as the distances between each. This distance is the roll couple (or your leverage)

Conclusion- You roll center has to be much higher than mine as stated prior to this when I guess just off the look of your sig pic. What happens when you go the higher 1300 spring rate from the 1000 is your RC MIGRATION is less and the car is easier in feel or control to handle because of less transition roll and less RC change, BUT to the price of loading the outside front more. Try the same with a bar increase rather than a spring rate increase. I would not raise the front RC.

 

Would you lower it? Do you think 2 inch drop spindles would benefit me? I am not opposed to lowering the car a little more but I am worred about making the camber curve worse. Drop spindles will raise my RC. I guess I just don't know where to go from here. I would say that the overlay is a pretty good approximation of my RC.

 

Part of my problem is driving technique, I know I can use help there. I started autocrossing with a 350AWHP Dodge Stealth and being able to stand on the throttle shortly after entering a sweeper ruined me, I think that car gave me many bad habits that I haven't been able to break.

As far as what I am doing in the car. On higher speed 90 degree sweepers the car will be slightly loose when braking on corner entry, not bad and will straighten out when I pick up the throttle mid turn. Mid corner to exit the car will start to push when I start feeding in more throttle, Really when I really want to be hard on the throttle on corner exit I can't be because the car just is not turning. Transistions like Slaloms the car does pretty good. I would say just a touch tight. 180's absoulutly kill me as I pretty much need to idle around them or the car will push. The last event was the first time that I had to really slow down to go faster, The first time that I felt I was overdriving the car. I have tried to slightly brake going thru some of the sweepers and at the same time give it throttle and that has seemed to help slightly with the push but also helps the torsen differential bias so it dosen't act like it's open. I would say that I have most of my problems on corner exit.

 

Your 3rd link has changed the characteristics of the chassis and how it loads under throttle and braking from that of a TQarm factory style setup. That absorber, Torsen diff and massive racing skins combined are all giving you what you wanted in hammering the throttle without wheelspin. Unfortunately with the setup you have you have adjusted yourself into a great straightline drag car that fights to go straight in those dynamic throttle bursts.

Start backing down the adjustments. I would personally take the Tq absorber off and use a straight bar- That alone will give you the ability to light up the rear end around a tight corner and throttle induce rotation. Then you back down the angle gradually and apply the throttle more smoothly, Learn to pedal it, not stomp it in bursts to get it to turn.

I will stand by my statement that I would not raise your front RC. Try and find an inexpensive heavier front swaybar to go with about 1000-1100 lb springs and get results. Still tight after that? raise the rear roll center preferably, or up the rear spring rate until it loosens. If you up the rear spring rate first and only, you will be way loose on corner entry under braking. Please try and follow my steps, I put them in order accordingly.

IMPORTANT NOTE: 3rd link changes are major! one inch up or down can and will wreak havoc in one direction. You will know it when you go too much. It will surprise and scare the crap out of you as the car highsides, etc. Little changes at a time I generally go 1/4-1/2" changes max on 3rd links at a time. get a feel for what it does under braking and throttle bursts. Notice any improvements here first the way the chassis loads. If it feels better on entry and exit then try and adjust the sway bars only from there. If you cant get the feel, change the coil rates and bars I suggest and try again. It will take some time and testing.

3rd links adjustments will do funny things on corner entrance. It all depends on the car and driving habits. A 3rd link will determine how the car set or loads the front and rear axles under braking- combine this with brake bias variances and it is hard to point one all around good angle- so many factors that you will need to research your individual need based on braking characteristic.

Do you get the rears brake hopping at all? ...or have you had to do anything like turn the rear bias way down in the past so you are not so loose coming in?

You can now see why I asked you to move this question yo a new post. I knew it would take up alot of space about a 3rd link setup

 

Moved the questions here.


Just where are your roll centers, and what is your camber situation mid-corner?

As you expect - the stiffer front springs and bars become too much relative to the rear and you get a push. This does get worse as grip gets better, because the percentage of load transferred through the springs and bars is more heavily biased toward the front than that transferred through the roll centers. More grip = more roll = relatively bigger spring and bar effect. More grip = more roll = greater camber loss as well, but that's a separate ↓ issue.

Your rate of front camber gain drops as the front is lowered via short springs. This might be good for braking, perhaps, but is not so good for cornering. The first "fix" might be to set camber more negative, caster more positive, or some of each.

Rough numbers, I think a half an inch taller ball joint would be worth about a 1" higher roll center.


Are your rear LCAs close to horizontal?


Your differential may have something to do with this as well.

 

Agreed.

Since the front RC is already above the ground rather than (as feared) below, I'd expect that making it higher would add a little transitional push to corner entry without helping anything on corner exit.


Norm

 

Norm, RC are approximatly 5 inches in front and if my memory dosen't fail me 10 inches in the rear. It been a while since it was measured last

 

No rear brake hop at all. I kinda feared that and it was part of the reason for the biscuit bar as it has both a braking and acceleration bushing. The bar is adjustable and I do have it set as stiff as it will go. I can easily make the bar solid and readjust the front a little higher and it sounds like that is where I should start.

I will probably drill some holes so I can easily change the rear RC during events. Maybe even make it adjustable like NASCAR. Probably won't make any spring changes because it sounds like I might be in the ball park with a bigger front sway bar. I kinda subcribe to the Guldstrand theory of big springs and small bars but so far it isen't working for me so time to try something different.

BTW, here is a video of one of my runs when I had the 1300lb front springs from last year. The car ws pushing but not nearly as bad as it did on this lot this year.
http://www.youtube.com/watch?v=MwhlLbnPCjM

 

Guldstrand had that theorey mainly for the roadster C2 Vettes that were light roll weight. Even my C3 t-top does not need large bars. D*ck has never touched my Vette, I used to prep it myself when I race for him. I was the first person to ever clamp the rear leaf so it would not recoil release and highside the old vettes. I learned alot though from watching him as a kid hanging out in his shop on occations in Thunder Alley. I was a major Mark Donahue fan also (A driving engineer- I grew up with that inspiration)

I watched the video several times. You are having to turn the stering wheel over a few different times showing the push, but not real bad. Your throttle control appears good. I think you just have to much roll weight still up front and that heavier inside spring is lifting the inside frnt rather than a stiffer swaybar holding it down.

Yes, see if you can staight bar the absorber and raise the front mount 1/2" max for now, 1/2" is alot. Then if you could also have rear panhard adjustment that would be great for corner balance of spring rates. That gives you alot to shoot for and a good solid foundation to adjust from.

I have note seen Guldstrand for about a year, I hear his health is fading. Here is a shot of the two of us last Nov 2008. My father has known him for over 45 years.
https://www.thirdgen.org/forums/albu...pictureid=5599

 

That video was from last October, last event of the year and it was downright cold in the high 30's lower 40's. Most everyone was haveing traction problems. Being that it was slick my car did allright but it did spin the tires pretty hard going through the finish on that lap. At the event earlier that Summer and both events at that location this year the temp was in the 90's and my finishes have been much worse. That video is not represenetive as to how bad the push really is. The loop right after the start is where I lost 1-2 seconds over most other cars there. At the 2 events this year I was going even slower through that area,only about 1 way to set up a course in that lot, so it is similer at every event.

Anyway I thank you for your patience in answearing my questions as I was going in the complete wrong direction. You have given me the answears I was looking for even though not really the answears I wanted to hear and also gave me reasoning as to why you would change this or that so I believe it will help me out a bunch especially when I start combining it with your ultimate suspension threads. Now I have just enough info to be dangerous with

 

Glad to be of help. I always try to back my opinion with a solid reason or facts as to why I say or suggest something.

It leaves me wide open for criticism- I make mistakes sometimes, but when I list things openly in discussion it allows other giving suggestions to keep me in check.

With that said though, to each his own. It is merely my opinion on what I would do. Lots of combination's that can work, its all about what you ultimately like. We all make different purchase decisions in building cars. You would not do as I, nor would I as lets say Norm, or would Norm as you. Good luck to you.
I have a guy right now INSISTING on running a 250 LF and 275 RF spring rates on a chassis designed for 350/350. He is limited to tire size so the roll ratio is pretty much known. He will roll too much and loose tire contact patch through the greater articulation. I just talked to him last night and told him he had better start his static camber -1* more than he had. It goes much deeper, but he will not understand. unfortunately Saturday is my birthday and I will not be at the track with him (ps-its my Richard Petty birthday! #43) I will have to trust his feedback over the phone and try and tell him what to set based on what he tells me. Much better when I can see the truck in action. I am just shaking my head with this guy. I have worked with him on 3 races bringing him up from 20.8's to 20.4's ON THE SAME WEARING TIRES. If he could afford new rubber on each race like all the front runners I would have in in the top 5 running with 20.1-2's. I find out he pulled the front end apart to go these springs BEFORE he tells me. I had nothing recorded on good scales- there goes three races of lost info. I told him several times to get the truck over to Supertruck Inc and get readings where we are for a baseline- the track scales suck- Oh well, you can lead a horse to water,........I pull my hair out sometimes. I xcan get the truck back to where I had it based of of tape measurements I charted, but that takes forever and it needs to be rolled back and forth many many times into the same exact spot of the shop floor I initially set it at.

On that note, You have so many variances on your chassis where you do not have somewhat fixed lengths of control arms and tire width, track width, weights, percentages,etc that you can alter many thing around to get many different spring packages to work. There no "one way that is correct and thats it" in what you are doing with a custom build unregulated to restrictions like I posted above.

you'll just have to test things and find your niche. You have alot of different things going on leverage wise from a normal 3rd gen platform.

Dean

 

Dean, ultimatly I am the one that decides how to set up my car. I have tried a few different combos but allways try to give it a few events before I decide my next step. It is time consuming and I have been struggling with this car for 4 years now, The best I had it was with the 1300 lb springs but I know it can be much better. I just need to face a fact that with my limited knowledge on the subject, what I have been trying just hasen't been working and it's time to take a new direction.

Will the 1300 lb springs end up back in the car? chances are I will at least give them a shot at some point next year. I also have some stiffer rears that should complinent the fronts pretty well so it's something that won't hurt. I like a stiffer car, probably because I get more feedback into what the car is actually doing.
The first step will be the third link and PHB changes along with a larger front sway bar. I will also need new skins next year so I may try the smaller 25X12's or 25X13's on the rear again and see what happens. It will allow me to lower the rear about an inch and that is going to angle the upper link even more so I will need to take that into account.

 

Is the 3rd link front mount higher than the rear?

 

No. It is angled down towards the front of the car, lowering the car will angle it more

 

27x14's to 25x13's in the rear will lower the rear and yeild less angle slightly on the 3rd link. Parallel to the ground is 0*

 

I meant actually lowering the car with the weight jacks. That should angle the bar more.

 

So you're going to lower the rear suspension as well as install shorter tires?

Upper and lower link inclinations will change, the side view instant center will move a bit fore/aft and vertically, and the sprung mass CG will drop a little. Too much going on to just guess at, and not enough numbers to throw at a spreadsheet.


BTW, if your front LCAs really are horizontal (between the pivot and balljoint centers), I think the front roll center will be somewhere around 1/6 of that height.


Norm

 

Norm, I think the drawing Dean did compareing his front suspension with what he thought mine would be is going to be pretty close to accurate. Of course it is also just a guestimate and I would need to measure it and draw it out for myself to really find out what the roll center really is but by looking at Deans drawing it appears to be about 5 inches.

 

I found some more pictures of one of or autocrosses. Lots of pictures of my car on page 13,19,26 and 27.
http://s285.photobucket.com/albums/l...U17August2008/

Not very high quality but maybe you can see what is going on with my car

 

Wow, I did not realise your front track was that much greater than factory. I know you saidthe tires skick out, but I did not realise ALL of the extra width for the most part stick out raising a massive positive scrub radius.

Here is what I have first come up with. Took me a while to draw this thought. I need to go, I will have to ponder this more later to think of what other ill effects it is causing you.

1st off you are gaining x-weight in the direction you turn massively. This alone will cause the care to tighten. Your diagonals from wheel to wheel increase on the outside front to inside rear, and they decrease in distance from inside front to outside rear...fairly massively. Just look at the normal center of tire pivot point (Gray dot) on the normal 8" tire (Grey tire 245-50-16) compared to a guestimate of your 25x12 (12" black) tire and the scrub radius (Black dot) and how it migrates. Look at the extremities of the tire edges (Verticl lines both green and purple. you see how the greem lines stay vertical for the most part where as the purple lines canter fore and aft front tip of the left tire to tip of the right tire. Same canter of the scrub radius dot (black migration dots).
X-distance increase like this is the same as adding x-weight which will tighten the car and make it push.

The cars balance looks decent spring rate wise. Nothing odd in the pics.

This does go to a need to correct scrub radius, but there is no way you can do that through SAI and IA on a strut car without the SAI being so extreme an angle due to the strut tower. Double a-arm yes, strut no.
Sorry for the bad news. You will just have to bandaid fix the poor wheel offset with other chassis loosening characteristics.

The chassis platform was not designed for those offsets. You can correct minor scrub radius increases through fudging the IA which slightly increases SAI, but were talking maybe 1/2", not 2-3 inches

 

Please do measure it. I'm not saying that 5" absolutely has to be wrong, only that it looks suspicious and doesn't hold up well against the following sanity check.

A 5" high RC with level LCAs at 8" suggests an unreasonably ahort front view virtual swing arm (distance from tire to intersection of LCA axis and perpendicular construction line through the strut top mount). I really doubt that any OE-based strut suspension has a FVSA at static ride height that's quite as short as the Twin I-Beam that Ford used to use on its pickup trucks.


I'll have to look at your pictures later; for some odd reason or other the office internet filters don't approve of photobucket.


Norm

 

Norm, I should really measure it. 2-3 inches is closer to what I was thinking but I do have lots of camber plus the center of the tire is probably 3 inches farther out then your typical 3rd gen. I think those 2 factors combined are what is causeing the abnormally high roll center.

Dean, the scrub radius just might be a problem that is going to be hard to overcome and if I can't come up with a solution then it may be time for some smaller slicks on either 16X10 or 15X10 rims. I thought about getting rims with more back space put I am all ready rubbing the inside of the fender at full lock so that is out. Another option is 17 incher but right now the weight penalty still stand for larger the 16 inch rims so not sure if that is a good option or not.

 

If you aren't currently within 100 or 150 lbs of minimum depending on wheel width, you're already carrying the >16" tire size weight penalty without utilizing what it would permit. The question then becomes whether any of the rubber in 17" or larger sizes is better than the 16's. Either that or the car needs to go on a diet.


Norm

 

On the surfaces I run on the large 17 inch DOT tires are just as good as slicks. The car is about as gutted as it gets and I am still carrying nearly 200 lb in ballast so I would need to add more. I have allways run my fastests laps of the day with a passenger so maybe a combination of 17" DOT's and another 150 lbs of ballast under the Passanger seat would help the car out. I am just trying to get back to running with the fast guys in our region. The other advantage is wide 17 inch rims that will fit my car and are fairly light are pretty inexpensive compared to the 16 inch rims. the best option for 16X12's is Real racing wheels for about $1600 a set. I would still like to make the big tired combo work and will start with Deans suggestions and see how it works before I start messing it up with my thoughts.

 

This is a cool thread! I understand the basics of what you guys are talking about, but I have never calculated or changed any of my instant centers or suspension points front or rear. But I'm very interested in the dynamics involved, you guys are just way over my head with it all.

For now I'm satisfied with my stock front end, but I want to put a 9" ford rear end in my Camaro, and I'm looking for advice on how to change it over to a three link rear end mounting bracket system at the same time. I'd like to free up room in the floorboard/tunnel area for exhaust pipes by eliminating the torque arm and also not have to rig up torque arm bracketry on the 9" axle housing.
I'm willing to put in a roll cage and supports and I don't mind losing my back seat area, even though this will be a street car that I want to handle like an autocrosser, or at least as good as I can get it on a budget.

So I have questions about building and mounting a top/third link on the 9" rearend. I'm assuming that using the stock LCA's and brackets will work out OK, and that they should remain pretty much parallel to the ground.
But I wonder how long the new top link should be, and at what angle should it run at static ride height, uphill, straight forward, or downhill?
Is 2/3'ds of the length of the LCA's a good top link length? Should the height from the centerline of the axles to the top pivot point on the rear end be the same distance as the LCA's pivot point is below the axles center line?

And isn't the stock torque arm setup with the panhard bar kind of the same as a three link rear anyway, except for the lower, ridgid mounting on the axle housing, and a forward pivot point that is just a lot farther forward than on a normal three link?
How does the stock torque arm compare or perform compared to a well built three link setup?

Thanks!

 

Not exactly, at least not in the OE configuration as compared to the usual 3-link.

The geometry is a bit different, which makes for subtle differences in the way anti-squat varies with ride height. Instant centers and 'anti' effects do not remain constant (and 'instant' is in this sense an abbreviation for 'instantaneous', or what you get with the various links in whatever specific positions).

A torque arm ends up being heavier, as it must be able to resist bending, and more of that weight is toward the axle end. A single upper link is shorter and need only resist tension/compression. Offhand, I'd say that the weight issue makes the 3-link a better choice as overall car weight goes down (involves the ratio of sprung to unsprung mass). This is more of a ride quality issue for most folks.


Norm

 

First, I want to say that torque arms work great when sorted out. I didn't give the torque arm very much time before I went with a 3 link because it was recomended by allmost every CP driver I asked.

You should mount the upper link higher then the lower links in relation to the centerline of the axle. How much I don't remember as it has been a few years. The stock mounting points for the lower arms are fine as long as the lower arms are parallel as you said. 2/3 the length of the lower arm is a good rule of thumb, mine is a little over 3/4 at 15 inches long. The length of the upper arm will affect how hard it will initially plant the tires, If I remember a shorter bar will plant the tires harder. A longer bar won't hit as hard but will get better traction for a longer period when on the throttle. Again this is all from memory so someone in the know can correct me if needed.

 

Ok, So after going over Deans Ultimate third gen suspension thread again I have some questions again. First, Dean said that drop spindles are very important. Honestly the only reason I would use drop spindle is to raise my RC and Dean has said that he thinks the RC on my car is allright where it sits. I am just a little confused. Do I need dropped spindles or would extended ball joints be enough? Personally I feel raising the RC will allow me to maybe lower my front spring rates a little bit instead of raising the roll rate like I am planning on.

Next concerns scrub radius. I know with my wheel tire combo my scrub radius is pretty bad. I am planning on modifieing the spindle so I can get more SAI therefore reducing my scrub radius. Most of the competitive CP third gens I have seen have the tires sticking out just as far as mine so not sure how much of an issue it really is but I believe that changeing my SAI will only help. I know I won't be able to get it perfect but any reduction in Scrub Radius should be beneficial.

Thoughts???

 

I don't know if the same is true for your car but in the past I ran 23.5 10.5 16 goodyear g19 bias ply slicks. All the cp cars in my club recently switched to the g19 radial in the same size and compound. We all now have the problem with massive understeer and none of us have been able to alleviate it. Even in grid you can feel the front tires being scrubbed and the car just wanting to go straight. Put a set of old bias plys on the front and it just plain sticks. we tried every different spring shock and alignment combo we could but could never get it to drive right with radials. I checked tire temps and we had the right temps I think..... we gained 15* from inside to middle and another 15* from mid to outside

 

Still useing the Bias Hoosiers R35 compound in front(25X12X16). The tires are pretty old and well past their usefull life(Allmost down to cord) but I had the same push even when they where fresh, I will at a very minimum need some new fronts for next year. The rear are the harder R45 compound(27X14X16) and even after 2 1/2 years of events they have very little wear on them. They are getting hard but it seems like a few liberal applications of Formula V between events brings them back to life.

I still might go with the 25X12's or smaller all the way around. Somewhat concerned about forward bite but it just might be better to have equal tires all the way around so I can make gains elsewhere at the cost of a little forward bite. Plus I can make changes in the rear suspension to increase bite.

 

we run the 430 compound and I go through about 8 tires a season. we basically use em till there are patches of cords on every tire. I wish I could get away with one set for three years It would save me from having to sell my kidney

 

I only make about 7 events(Approximatly 40 laps) per season so that is part of it. Then again maybe I just don't drive the car hard enough, I am told at allmost every event about how smooth my driving looks.

I feel that considering the push the car has, 100 or so laps on the front tires is pretty darn good. The rears I honestly feel I could get another 100 laps out of without a problem all though by that time they will probably be so hard that I might as well run rocks for tires. Formula V is the only reason I was able to get this much life out of them but I feel that it didn't work quite as well as it did the first year I used it. It still seemed to make the tires soft but they just didn't seem as sticky as they used to be. I am still a firm believer in Formula V because I ran my first set of tires untreated and corded them withen 60 laps.

 

ya my season is about 140 runs per year with all the clubs we run with so I guess that explains it