FireDemonSiC

Me and a friend installed a 1.25" Tokico drop kit on my car. Before the alignment was great but obviously this threw it way off. The problem I was having was a huge dead zone at the center of the steering. It was difficult to keep the car straight and looked like I was drunk going down the road.

Now after the alignment, center feels much more defined but something still doesn't feel "right". If you are going down a perfectly flat road with the wheel perfectly straight and slightly move it in either direction, the car will follow your input except it doesn't straighten itself back out after you let go of the wheel. It will continue to wander and veer off in the direction you pointed it until you grab the wheel and "bump" it the other way.

So, what did they mess up? I'll post a photo of the before and after specs later today.

//<86TA>\\

iTrader: (14)

Who did the alignment? What are the specs it was aligned to?

FireDemonSiC

Check back about 7pm I'll post it up.

FireDemonSiC

Hit the burners to my house on lunch break and grabbed the sheet. Here are the specs they aligned it to.

On my way back I also noted that centering the wheel after steering does not center the car. The wheel must be turned SLIGHTLY in the direction opposite of where you last steered. This occurs to both the left and right equally.

SlickTrackGod

WOW


I can't believe shops get away with this ****. Where's the caster split for christ sakes.


Take it back and tell them to set it here becasue what they have is not correct even though their "machine" says they are in the green...such bullsh*t


Camber should be negative, not this positive over .5 crap. If its going to be positive then keep it under +0.2. you really want them more like -0.3 to -0.5 range

Caster should have a +0.5* split higher on the right side for the angle of the roads 9due to rain runoff, road are built with a crown that if caster is equal the car will drift down off the right side of the road. A caster split will give just a tad more lead to the right side fronttire keeping the car straight on a crowned road)
Caster should be +4.0 left, and +4.5 right for decent wheel return and high speed tracking. You are figting the wheel and it makes a very unstable feel at freeway speeds always having to correct the wheel.

Toe? I do not know degrees in toe, I do old school 3/32" in measurements for average street use.

Where the hell are the SAI measurements making sure the suspension parts are straight. Tell these guys you want SAI readings (steering angle inclination). That is what tells you the strut and spindle joints are even from left side to right side or else the car will pull for that one way or the other based on which is higher.

Jez, Im glad I do my own alignments these days

SlickTrackGod

Just print what I wrote and take it to them^^^^


edit- you have a very nice rearend specs on that car- good thing they can;t change those and screw them up. you have good camber on the rear and good toe...and a 0.00 thrust angle which is perfect. Most are within about 0.02 tolerances

Just a little side note: The passenger side should always be set first because it has the higher readings, then you set the drivers side. I run alot of Caster for racing. Often over 5.5 on the right. If I set someones car and its is not lowered, I know that I do not have that much adjustment. Lets say if I started witht he driver side and set caster to 5.0, then I find out I can only get 5.1 out of the pass side, I have to go back and reset the driver side lower to 4.6- that's why you always start with the pass side of the car first, unless you know the car from workingon it previously

FireDemonSiC

Yeah I had a feeling something was seriously not correct. The guy I spoke to on the phone said that "was the best alignment he could give me". I told him if he could not correct it any further that was acceptable as long as they refunded me the full amount. I'm going there tomorrow after work so we'll see.

For reference, here is my full suspension mod list:

- Tokico HP shock/strut and matching lowering spring package
- UMI subframe connectors
- UMI STB
- UMI LCARBs
- UMI solid motor mounts
- Lakewood control arms
- Spohn panhard
- Stock IROC wonderbar
- 275/50 NT555R on 15x8 in back
- 225/60 Ecsta ASX on 15x7 front
- Every polyurethane component possible.


It pisses me off that I spent all this time and money on these components and It's being hindered by a crappy alignment. I almost feel as if the car handled better before the alignment was done even with the massive center deadzone. Maybe I'm just imagining that...

Here were the before specs:

FireDemonSiC

I just noticed something.

Does anyone find it odd that the rearend specs changed significantly between the before and after readings?

AMAXRX

With stock suspension, I just had les schwab give me -1.0 camber on both sides, 5 deg caster on pass, 4.8 deg caster on drivers, and .02 or so toe in. Car feels great.

The tech did say that is was very difficult to get to those settings without "proper GM tooling" but he did get it there.

I think your tech was lazy.

and yea the rear settings shouldn't be changing... the only way to correct for thrust angle is with adjustable lower control arms...

Marc 85Z28

iTrader: (3)

The rear plate pins were probably never pulled on the rack.

blackbmagic

I wouldnt ask for you money back. If you do wind up getting it back be prepared for the mechanic to go out of his way to put your car back to the original specs. I have done it before to customers who didnt want to pay. Or came back with an issue that could not be fixed by an alignment.

The caster is correct. There is a lot that can be done with the hunter hawk eye in order to make it display the correct measurements even though they may be false.

Make sure the tech had in fact moved the camber/caster plates.

If the car seems to wander what the car needs is more positive caster, this will negate the wandering effect and give you a more responsive steer. It will also give you a slightly harsher ride as the angle of the shock will be more toward you instead of further away from you.

Those are not bad numbers they just need some actual tweeking. When you do 5-10 alignments a day you cant stress making every car 100% perfect. They dont even come out of the factory 100% perfect. The tech is also not going to know how every car reacts to certain measurements, every car is different.

So cut the tech some slack, go back with a nice attitude and ask if he can adjust the caster a bit more positive because you are getting too much of a wander.

And the rear specs are only 2 hundredths of an inch different. That is not significant a simply moving the car up and down on the lift will cause more differnce than that.

FireDemonSiC

You are getting the wrong message and taking things out of context. I don't need to cut anyone any slack or go "back in" with a nice attitude because I never never went in with a crappy one in the first place.

I told them what I felt was wrong and they tried to blame a part on my car and said that was "the best" they could give me so I gave them the money back as an alternative if they weren't willing to work with me.

From the get go in so many words I asked if they felt that they were knowledgeable enough to tune the alignment on my car and they said yes. Now if they are just now realizing they aren't, they need to give me my money back so I can find someone who is (Is willing to dial in the correct numbers).

If they want to put everything back to the way it was before that's fine. It's honestly not like it was much worse.

blackbmagic

Hmm, dont take what I am saying the wrong way. I was just saying have a nice attitude, I don't know what attitude you had previously. Being nice gets you a lot further than demanding. Also having knowledge on what you are asking for helps alot, so good thing you came here.

What part did they try and blame on your car?

Do you want the specs back to original? Because that is most likely what will happen if you ask for your money back. Its an old car, unless you have a 100% new front end its highly unlikely it will be "Perfect". But what you can do is what I said have them adjust the caster slightly more positive but still "green", and if you are getting a true wander it will go away.

FireDemonSiC

First they tried to blame the lowering kit then they jumped to the PSI of my tires.

In case I never mentioned it the steering in this car was SO responsive and extremely pleasant at high speeds (Triple digits) just before the lowering kit. Obviously this changes the suspension geometry so it will need a new alignment. But right after the lowering kit and still even after the "alignment" it feels so loose and unresponsive and wanders in either direction (Centering the steering wheel after turning does not straighten the car it will continue to wander in whatever direction you just steered in until you "bump" the steering wheel in the opposite direction).

rsmith085

iTrader: (1)

Tire Kingdom totally jacked up the alignment on mine after I went to caster camber plates and Eibach lowering springs. The car was all over the place and was doing whats known as "tram lining" where it'll follow any bumps/dips in the road because the camber was +4 positive on the passenger side with more than a 1" of toe out. They couldn't figure it out and just played stupid. With that much positive camber you could sight my car down the side and see the tire leaning out big time. I ended up reading A LOT about the theories of car alignment and ended up doing it myself and it drives like it should now. If you don't want to do it yourself I recommend finding a heavy collision body shop. They deal with seriously twisted frames and jacked up suspensions on a daily basis and usually do a real good job on alignments. I think the problem lies with the majority of newer fwd cars the only adjustment is toe and they have no idea what their doing with a fully adjustable front suspension. Alignment machines also have to be calibrated every so often and I doubt they hardly ever if at all get that done.

Reid Fleming

iTrader: (2)

There are two issues that we all face.

1) Stock factory spec alignment is positive camber. This guarantees wearing out your outer tires.
2) Red and green colours on the machine are corresponding to GM's weird specs.

So in order to get a good alignment, you'll need to get a tech who will give you a "red" alignment based on your settings, not what the book tells him.



For camber, the -1.0 is a bit extreme for people who drive on the highway all the time. Great for cornering though. -0.8 is about the sweet spot for performance and tire longevity. And -0.5 is good for anybody. (Giving credit here, this is straight out of Vetruck's advice on alignment)

Caster depends on spring height. For people like me (Stock height), it's harder to get the 5 and 5.5 split. For lowered cars, it's easier. Stock GM settings are 4.7 on both sides. Decent, but not great.......Go for the -0.8 camber and then as much caster as you can get while still retaining the half a degree split between left and right.

SlickTrackGod

Reid, In case you didn't know, I am Vetruck

Dean

ps- even if the printout is showing green range, the alignment tech should pull his head out of his *** knowing he needs to give an adequate caster split. Like someone stated, the guy WAS LAZY. I don;t care how many cars you do a day, you do every one of them correct oer you will have alot of pissed off people like what just happened. its the tech job to give the customer a proper feeling car, that is what test drives are all about before you hand the car back to a client. This tech sucks.

FireDemonSiC

What would the 3/32" toe be in degrees?

FireDemonSiC

One thing I cannot blame the tech for is the test drive thing. I am very conservative about who I let drive my car due to no one knowing how to operate a ratchet shifter much less a manual valvebody and you can damage the clutches if you try to start in 3rd or 4th gear. I tried explaining to him how to operate it so he could pull it into the bay and he was afraid to mess with it for even that and I had to drive it on the rack. No way I'm letting him test drive it.

hellz_wings

iTrader: (2)

3/32 = 0.09* ... Your right front is spot on, but the other wheel could be adjusted better. They should be equal.

Everyone is saying the right things about camber and caster here, about negative camber and as much caster as possible while retaining the 0.5* split (+0.5* on passenger side).

It is important, as Dean noted, to pay attention to SAI (again, learned this from him, as well as personal experience dealing with crappy SAI issues). SAI is important just as scrub radius is when doing low speed turns and other things. SAI should be the same on both sides, or really close. It should be also be in the low to mid 20's. If not, you could have bent parts, or parts that you installed that are not adjusted properly (like strut to spindle adjustable bolts for example).

Oddly enough, because you lowered your car at the springs, you should have an easier time getting negative camber now (before you had more than enough negative camber on the passenger side.. only need -0.5 to -1 for street cars), as well as more positive caster than what you currently have. He followed the GM 3rd gen alignment text book settings. A big boo-boo that everyone who is not in-the-know does (including myself the first time I had it done).

If this doesn't cure steering, then it's time to look into the steering box. Alot of us have had problems with off center steering boxes.. Even with new boxes. It's a common issue and you can check to see if your box is faulty by seeing if your steering shaft inputs move the pitman arm the same amount. If not, then there is play in the box and that sucks let me tell you lol.

And for what it's worth, I HATE having to let the tech drive my car on the racks.. They always turn it on and put it in Drive when the car is idling at 1500RPM still. I cringe at the sound it makes when it jumps into gear

FireDemonSiC

When I took it back today the SAI split was -4.3 IIRC. That has me a bit concerned but not much I can do at the moment.

My next plan is to grab a quick ratio box from the yard and a rebuild kit from work. This way I can continue driving the car while I am building the "new" box.

I was thinking about a rag joint eliminator but after reading all the problems people have with them I might just go with a new rag joint.

hellz_wings

iTrader: (2)

I bought a flaming river steering shaft with u-joints. One joint replaces the rag joint and the other connects to the steering column. It's solid and I don't mind it for street driving contrary to what some people say about it.

I would say the most important thing is a functioning box though.

The SAI split isn't a huge deal as mine is not 100% either. Our cars are old and not perfect. When you put the camber to -0.8* the SAI might change and be more close together because changing camber changes the SAI. You can also get adjustable strut to spindle bolts and adjust from there. There is a thread on this explaining what to do if you really want to get it perfect, including helping scrub radius: https://www.thirdgen.org/forums/susp...rub-other.html

JamesC

iTrader: (5)

You might consider the Jeep or Astro van conversion:

https://www.thirdgen.org/forums/susp...ing-shaft.html

JamesC

SlickTrackGod

THis quote is wrong. It does NOT change the SAI at all.

The SAI is a fixed setting. Lets state basic here- SAI is Steering Axis Inclination. It is the angle of the spindle shaft in relation to the imaginary line from the balljoint through the top strut shaft nut. When you bolt on the strut to the spindle, there is a little bit of "fudge room" you can do with the angle but not much when tightening the two large bolts that hold the two together. SAI does NOT change when the camber changes, it only changes if you fudge these bolts and try and kink the SAI in one direction or the other when tightening.

You can build a little jig at home very easy to level the spindle shaft and then gauge the strut shaft abgle with a pinnion angle gauge to check if both sides are equal prior to installing the spindle/strut assembly back onto the car.

I guess a sketch is in order, Ill do one for explination.
Edit- here, I just stole this from a website- Ill do another sketch below to show how to fudge them

hellz_wings

iTrader: (2)

Hmm correct me if I am wrong, but when you move the strut towers in to -0.8* camber from +0.5* camber wouldn't that move the strut nut more in towards the center of the car, thus increasing the SAI angle? If the angle is based on two points on two imaginary lines, one is that of the ball joint to strut nut line vs. the imaginary verticle centerline of the wheel, then moving the ball joint to strut nut line farther away increases this angle. (refer to following pic as reference)

hellz_wings

iTrader: (2)

To further this, by increasing NEGATIVE camber, this would reduce the IA (included angle) and if you have equal camber on both sides the IA should also read equal. (see following pic for reference)

And also to go even further and talk about scrub radius since it's somewhat related: When we move towards more NEGATIVE camber, we are reducing POSITIVE scrub radius (as seen in the pic below) and bringing it closer to neutral, which is a good thing on our cars since we have wide wheels (16" IROCS and up are wider than what the factory designed for, which were the 15" diameter, 7" wide wheels). Most of us also have bigger brakes and that pushes the front wheels out 0.3" more, which increases positive scrub radius, which is a bad thing on our already over-positive scrub radius cars.

I know that you (Dean) have mentioned this before and that a slight positive scrub radius is good, but closer to neutral (zero) is best but never be at zero or negative on our cars. Anyways this is my understanding of it all.

SlickTrackGod

Too funny, as I editted my post up top to put in a quick picture, I see you posted the same picture. I just read qhat you wrote and technicaally you are correct with what you are saying, but I am speaking of something else so let me expain why you are partially right, but also partiially wrong. I am also partially wrong in my typed explination- I see where I am being confusing and misleading.

I am speaking right now ONLY in terms of the relation of the steering angle in relation to the spindle being level (0* camber of the wheel) I am taliking about what the SAI would be if you completely take camber out of the equation for right now. The angle between the strut and the spindle as stated above
...i willcontinue, just submitting this for now as I type more and do the sketch

SlickTrackGod

Ill make this easy. Just look at your last two pictures you posted together. The SAI and the IA. Take the left one, the SAI where it shows the true vertical. if you were to fudge the joint of the spindle to make the tire equal to the true vertical WITHOUT moving the SAI is what I am talking about. That would increase scrub. Now ig you put the tire at true vertical and you lay over (increase the SAI angle) so the line on the ground meets the vertical centerline of the tire, that would increase SAI bu manipulating the spindle to strut assembly.

edit-
whoops, your last two were not the SAI and Ia, they are IA and scrub. Look at the first one you posted, the same as the one I did

SlickTrackGod

Bassically, the reason why I stated your comment is wrong is because the SAI needs to be considered when the camber is at Zero.

If course the SAI will change if you inclide positive or negative camber- but the vertical line changes with it so it technically does not change the relation,

The alignment printout should show camber AND SAI.
why?
becuase SAI could both be 18* left and right but the camber could be 0 on left and -2.5 on right

Thus, changing camber will not change the SAI to correct reading of equal on both sides- it WILL change it, but it will not CORRECT it


Our confussion between us was the difference of "change" and "correct" in our discriptions and interpetations. This is the crap that is hard about learning and teaching things on the internet

hellz_wings

iTrader: (2)

I think I get what you are saying.. If the ball joint was directly UNDER the strut mount nut SAI would be ZERO because the imaginary line would run parallel to that of the imaginary wheel centerline line (although I think this is theoretically impossible on ours cars without massive fabrication?) I am confused as to how this affects the strut mount though.. Is it correct to say that regardless of outside elements, the fact remains that moving the strut mounts in will increase SAI (albeit by a small amount)?

EDIT: Just read your last post.. the RELATION. I see! lol.. The relation between SAI and CAMBER remains the same because when camber is adjusted in one direction the SAI moves in unison. Finally.. LOL.. my head.. haha

EDIT2: I mis-typed my original quote. I should have specified! The reason why I made the statement that it might help correct SAI to become closer together (and higher) is because:

a) By increasing negative camber you increase the SAI (although the relation stays the same, leading to my next point..)

aaand *drum roll please*

b) His camber was not equal to start with. It was at 0.8 driver and 0.6 passenger. So by bringing them at the same level (whether he puts it to -0.8* or +0.8) will help SAI be more closer towards equal. That's what I should have wrote earlier

And to add to what you were saying about putting camber at zero and then getting SAI straight, this is an interesting way to do it that I did not think of. THAT is why you need to have an alignment buddy to get it right because setting camber to zero is easy via strut mounts, but it's when you try to adjust strut to spindle bolts that it gets funky because when you do that you change camber reading too and that you have to change strut mounts at the top again and then I guess this repeats several times until you get it right or the tech tells you to "F" off lol.. I am going to try this next go around at the alignment shop

SlickTrackGod

YES!!!! on all points

On your b): Technically we do not know what his SAI was. So yes putting -0,6 and -.08 to both the same "COULD" bring the SAI to a closer reading, but then again it coud NOT either based on what the readings actually are when both are set to zero to check them.

Yes it pays to have an alignment friend. I resently lost that when Jon sold Accurate ALignment and retired to LaPaz Mexico with his fishing boat.

I have resently bought alot of my own stuff to do things in my garage since I also lost the NASCAR race team equipment since the local Toyota Speedway closed up and mo more local racing. My driver moved to Washington.

I bought one of these trick digital gauges so I can do alot of angles myself very accurately. This is what you can use to setup your own jig assembly in your garage at home to try and set your own SAI.

THink of SAI like golf clubs. They do the exact same thing on club face angle in relation to a persons swing angle. Everyone is different and has to be fitted properly to the correct club face angle SAI (Swing Angle inclination) Its basically the same thing,

On that note though- more SAI will cause more lift on the inside wheel- thus more body roll as a by product. We have no dive and anitdive adjustments built into the McPherson strut car. The lower the car and more Caster, the more anti-lift of the rear, but does not really affect antidive.

In NASCAR circletrack type racing- we use dive in the left front, and antidive in the right front to reduce body roll under braking and turn-in.

Here is those sketches showing how the two bolts (Red dots) can be fudged a little upon tighting to gain or loose a little SAI. Note the first is what appears to be "normal" (I do not have any old data from my car to reference so I can not tell you the SAI readings I had, I did this 15 years ago and have touched probably 100 cars since so I have no idea trying to remeber something like that- lets say average is 18* just for EXAMPLE.

Then the next would be aprox 20* (note the decrease in scrub, but the increase in inside chassis lift on sharp turns- this makes the car rotate easier like a gokart does in lifting the inside rear wheel, otherwise a gokart would never turn because it has no suspension). Lastly the 3rd would show aprox 16*. You would gain negative camber adjustment range, but would also gain scrub radius bigtime and loose straight line tracking (remeber, SAI is alot like caster, that lift to turn will make the tire recenter on its own to go stright)

SlickTrackGod

A little shop photo of Black Mamba's car in my shop being setup for a local timeattack day we all did. THis gauge is a little pain in the acss when you need to bang the strut tower with a rubber mallet to set the camber/caster on a thrid gen. It will fall off if noone is holding it while someone else bangs, so its a two person job but so is setting tow with toeplates.

This gauge works great on all my other cars where no banging is involved. I am going to fabricate a trick adjustable STB for Valentin (Black Mamba) so we can push and pull the strutmounts into adjustment with the 3pt braces one at a time while the other side stays locked in place.

hellz_wings

iTrader: (2)

In your example it seems 20* would be the best of the three because it scrubs the least, actually even 22* looks like it would give only the slightest positive scrub. The problem is now with extra lift caused by more SAI which increases front roll. Couldn't this be fixed by adding a stiffer front sway bar, or stiffer springs, or in my case with my extended ball joints that raised the front roll center by 0.75" would be enough to counteract this extra lifting of the inner front tire?

SlickTrackGod

On the contrary, you want a smaller swaybar or less stiff springs/ or both in combination.
Why? because when the tire lifts, it lifts the A-arm/balljoint...thus...the a-arm is linked to the swaybar which lifts the chassis.

I see your thought process was to go to what might reduce body roll, but in this case those parts are what CAUSE the body roll when the geometry lifts the balljoint.

So now going to the roll center comment- yes the higher roll center will help reduce positive body roll in general by putting the roll center closer to the cg(center of gravity in lateral terms)- so.... as a byproduct of that, you can run a smaller swaybar which allows for more independant wheel movement from the chassis, rather than locking up whell movement with chasiss roll.

What holds a chassis down with swaybars will also lift a chassis up with swaybars. A swaybar is a better reduction choice then spring rate because the swaybar connects to both sides and limits independant movement of each wheel.

A better choice then reducing spring rate is to reduce compression valving to aloow the whell to move upward faster independantly without as much slowing damper- this will not throw the body over to the other side as much. Reducing spring rate will cause the loaded side of the chassis to roll to a higher roll angle at mid corner steadystate- don't want that.

lastly. What size extended ball joint did you use- is it 3/4" tall? if so then your roll center does not go 3/4" higher, it probably goes about 2" higher.
A 1/2" balljoint will raise the roll center aprox 1 1/4 to 1 1/2" if I remeber my calculations on my car. It is based on camber setting though.

SlickTrackGod

McPherson strut roll front roll center height aprox on a 3rd gen lowered to about a 26" fender height with a standard 25.7" tall tire.

1st example is with standard balljoints and about -1.0* camber.
Note: the more negative the camber setting, the lower the purple roll center calculation.

2nd example shows extended ball joints and how the roll center raises. This allows more of the chassis weight to remain under the roll center so in lateral force (cornering) the car body will not roll over the top of the roll center as much. The lower example shows the car with more body weight under the roll center than the first example does. Having equal weght above and below the roll center will create a neutral roll. More weight above is postive roll, and more weight beloww is negative roll. Most all cars have a very positive roll weight thus they need a large swaybar to counter act that positive body roll.

pg29

Awesome advice SlickTrackGod, the diagrams really help when it comes to getting your ideas across on the internet. You really know how to get the most out of these cars in terms of handling, and once I put my car back together for auto-x and track days I'm going to be using lots of what you have taught us on thirdgen.org

hellz_wings

iTrader: (2)

That's a very nice diagram. Could you write in some labels as I'm trying to figure out what points are what. Pretty sure the purple blot in the middle is the roll center?

SlickTrackGod

Yes the purple dot is the roll center. this, for those that do not know, is the imaginary calculated point as to which the front of the car rotates around. Think if it as a rotisserie point like on a BBG grill. Everything from front veiw rotates around this point. THis is STATIC rc- meaning it does move when the car moves. As the suspension moves differently over bumps, use your imagination on how the rc will migrate in dynamic motion. It is always where both sides intersect so it does move off centerline in dynamic motion.

i will do a sketch of a car in dymnamic motion in roll of a left turncoming towards you and the right/outside wheel alo hits a bump.

The two red dots are instant centers for each side of the car. Its where the A-arm link angle (from balljoint pivot center of the ball)- through the a-arm to chassis mount bolts (the ears) and goes into infinity until it meets the imaginary line that is 90* off the top strut mount shaft bearing.

edit- oh yeah- blue shows the ball joints. The second pic has extended ball joints, hence the lower angle of the A-arm from chassis out.

SlickTrackGod

Car coming at you in a left corner hiting a bump on the right side front tire.

Notice how the roll center migrates over to the left with this type of a suspension and the body roll weight goes mainly about 80% load on the right tire and 20% load on the left (just aproximations for example). The roll cause most of the weight to now rest on the right spring of the car [on the front axle, we are not showing the weight on the rear axle]

THis is very extreme. It would generally never migrate that far off center in normal conditions.

JamesC

iTrader: (5)

Please keep to topic.

JamesC

hellz_wings

iTrader: (2)

FireDemon (the OP): Any news on the new alignment? Please let us know what happened with your SAI once you got camber to be equal (and hopefully negative camber).

Dean: Interesting example of how it all works. As you experience body roll the roll center lowers. So if you turn to the left, you body rolls to the right side, and but instant center changes. When right side compresses it increases negative camber so the 90* line from the top of the right side strut mount is aiming at a lower point because the a-arm is also angled lower and that point where the 90* line and the a-arm line intersect is the instant center. Same goes for the opposite side. So the roll center is the point where the imaginary line between instant center on right side to tire contact patch on left side, and the imaginary line between the instant center on left side to tire contact patch on right side.

So to associate this with alignment and to explain why we need negative camber, can it be said that if there was positive camber the car would roll more? And with more negative camber the car rolls less because as you move the strut to be more negative it moves the instant center more towards the center which means that the intersection points between the instant center and the opposing tire center contact patch will be higher (that point is the roll center.. so the front of the car rolls less in a turn.. so this is a good thing since our cars are so front heavy and have alot of roll already and reducing roll without adding stiffer springs or stiffer sway bars is a added benefit!)

I must say I did not know this before! This is very good information to have for aligning a 3rd gen.

SlickTrackGod

What the hell does that suppose to mean. It is on topic. The roll center has an efect on the alignment SAI

James, I like you as a moderator, I think you are the best they have ever had. Maybe Im mistaken and there was something else that came up on here the you erased and I did not see?

I was showing Hellzwings how the inside tire can actually unload weight via how the car rolls about the roll center- this helps calm the chassis and leverages the inside whee in reverse fashion to keep the chassis flat from the angled SAI thrust whent he wheel is turned. If the roll center were over on the other side, it would not help hold the inside wheel up with the swaybar keeping that suspension lifted so the downward thrust is not a previlant

JamesC

iTrader: (5)

Correct.

JamesC

hellz_wings

iTrader: (2)

Phew that was a close one lol

SlickTrackGod

Yes where they intersect. The purple dot. It is not where they interset the centerline of the chassis. However, on a symetrical chassis setup (no circletrack where we use different spring rates and a-arm angles on either side, road race type setups are generally symetrical left and right) the roll center is just about always ont he centerline of the chassis at static position.


Yes and No 1)The positive camber keeps the roll center lower, so the swaybar is less effective on the lateral cg because the rc is closer to the cg making more positive roll weight. BUT- 2) the positive camber yield less tire footprint and thus less grip so the car will slide easier and roll less.


Basically yes on all aspects.

The roll axis, as you amy know, is the imaginary line that connects the front rc and the rear rc upon which the entire vehicle chassis rotates on like a rotisserie- think of this like shoving a rod all the way from the front bumper through the front wheels rc, then trough the rear wheels rc, and then out the back bumper and kingkong grabs those two ends like and ear of corn and spins the car. The car spins kindof oblong when the front rc is slightly lower than the rear rc(this stuff gets very complex= put a pencil through a retangular kleenes box and spin it and watch how the chassis (kleenex box) reacts as well as the upper front weight migrates laterally more than the rupper rear weight in a corner. The centerline of the cg actually moves against the direction of corner the lower the rc is.(this one will make you head hurt.I see this stuff in thin air- it takes years to train your brain to see it adn all the complexities going on everywhere as a result).
Let me show you an example where a front roll center is higher than the rear (very rare for a vehicle to have this). This is a GREAT lesson and very important in understanding rc's and roll axis. My Vetruck is so tall and heavy in the front half and low and light in the rear half that the roll axis on it is actually inverted downward from front to rear. The front RC still needs to be higher for all the cage weight and frame addition weight Ive welded in about 700lb on it over the years for towing capacity and payload carrying- 300 on the frame rails, and about 400 with the cage.

you can see in this still shot of it where the rear is fine but the front half of the truck looks like its twisting because the roll center is high but not high enough. What happens here is the truck rolls under braking and wants to lift the inside 'front tire", not inside rear tire as normal. you do not feel it inside the drivers seat, but you can clearly see it in the photo. I am doing something with a truck that it's not suppose to do. It is just too tall with too much upper positive roll weight on the front half of the vehicle. It handles great- beat every Corvette on street tires also that was there that day, but I also know I am limitied from making it any better due to physics. There is no way to keep that front from outsie dipping and inside lifting as the rear stays flat- and yes the truck rotates amazingly well even though it looks this way(its a marraige of settings that is complex, I have massive camber gain) Most cars would be WAY tight with a roll axis like this- I am actually still loose but I do loose overall grip form what I could have it the truck were 2 feet shorter in height (albiet the same weight and tire size)

ps- I hate this shot, but it shows alot for learning RC's adn how a chassis rolls uneven front and rear

SlickTrackGod

I can't stop laughing. Thank you James. I was like wth did I do now?

hellz_wings

iTrader: (2)

My front RC is higher than my rear. I am running stock MOOG IROC springs with 4th gen isolators front and rear, so my front control arm angles are higher at the frame mounts than at the ball joint (I don't know exactly how much higher but I know it's higher.). I also run 1.2* negative camber and have 0.75" extended ball joints. I have installed unbalanced engineering's rear PHB relocation brackets and have lowered my bar to the lowest hole (the only hole on their bracket). Believe me, the car is VERY neutral handling wise and grips like a ****. Even with the heavy front V8 engine i've done tons of weight reduction and with cut springs in the rear as well making the rear stiffer, the rear grips in the corners even at full throttle it's nearly impossible to break the tires loose in the corners and when they do it's a very controlled looseness not at all close to spinning. The front is very precise and braking has improved as well (alot less brake dive but I have only LS1 front brakes and aluminum drum rear brakes). The massive negative camber, stiff MOOG springs, and extended ball joints (raising roll center in front) with 34mm front and 24mm rear sway bars make the ride very neutral handling (very very slight oversteer which is what I wanted)

TEDSgrad

iTrader: (4)

Hehehe I didn't see the corner worker, either . Crash debris ahead.
CG and RC are getting back to normal, now.

SlickTrackGod

I promise you you are still not higher on the front than rear. you are probably very close to parallel, but ddefinately not higher. It would take ALOT on a thoird gen to get the rear rc lower than the front. they are designed from the factory way high to start with compared to the front. If your rear rc were lower your car would never turn. You do not have enough upper roll weight to loosen the rearend and make the car rotate if it was.

I custom built the entire front suspension on this truck to get it that way- I literally cut the suspesnion mounts off the frame rails and K and made my own suspension geometry on the front of this truck. I can not go to a parts store and buy factory parts anymore to fit it.

Trust me when I tell you I do not want it that way, but it is the only way this truck works because its so heavy and tall. To do this on a lower sleek 3rd gen would be handling suicide. I run extended upper AND lower ball joints with modified length control arms that induce -3* camber in 3 " travel. I start with a -2.6* static camber on this truck (daily street setup) and with a 4* roll I still have aprox -1.5" camber in full corner roll on the tire footprint. There are alot of bandaid fixes I've done to make a truck hancle somewhat like a sportscar but its not optimum for tire wear- fact is, I chew the crap out of tires. But enough about that. I was just using this as an example of what not to do and what it looks like when it happens.
last note and thats it- If I did not have such a long wheelbase I would be lifting the inside front tires off the ground like you see on alot of Porcshe 911's and such

SlickTrackGod

Hellzwings- I took some time to research your car a little this morning looking at your picture link. I know those are old photos, but based off what you told so far on modifications and parts I know these cars like the back of my hand and can aprox figure where your rc's are for your roll axis.

Take into account that the rear rc on a 28" fender lip car is 12.5" with the PHB parallel. Ill use you are about that. If your link pic specs are correct you are average 28.5" but I put you in the car also for drivers weight. (lowering this helps support your thought, not hurts). Now- knowing that the Jegs PHBRB will lower just one side of the bar and reduce the rear RC a total of 2.5", I'll grant you a total of 5" since you state the UE setup(even though I think its more like 4" max drop). This puts your rear rc at aprox 7.5"(Your car would be dead tight and steering would be massivinly heavy, but Ill go with that figure- like I said, I know these cars and do setups on them regularly)

now the front. I figure if you are at the heights of you pics this witht he extended ball joints puts you A-arm angles witht he balljoints about 2" lower than the bolt socket at most. Puts it back to about fatory settings WHILE the car is lower cg than factory. Taking into acount for your -1* camber and ride height, plus extended travel strut mounts re raises the strut length to factory also, this puts you right near the factory rc height of about 5-6" off the ground. Lets give you 6 for levelist results (if levelist is an actual word- haha)

So your front is 6" at highest and your rear is at 7.5 at lowest. Wider rear tires tighten you so you need pretty stiff rear springs to get the car to rotate. Problem with stiff rear spring sis they unload upon braking, but if the rear rc is down like that it will help reduce the rear jacking effect these car suffer from. The way I got my car to rotate was to induce progressive spring rates rapidly upon outside rear suspension compression. This also allowed me to migrate my rear rc outside very rapidly to leverage weight onto the inside rear and kepp the inside rear from jacking up in the air with both a 25MM bar AND a softer release progressive spring (and fairly high rebound shock settings). I could get away with the 25mm bar because I had a very light nosed V6 car. I am sure it is interesting for you to learn how other cars work and why it only works for that car based on weights. I can see you are one to have great interest in this stuff and thats why I am sharing multiple info you you see real world senerios with your car and my two cars and how they all differ, but work.

Your roll axis still angles aprox 1.5 to 4" range. The exact really does not matter, but I can promise you its not inverted. We can do rc's onpaper all day long but it really needs to be adjusted off of driving the car and feeling it, not penciling it. penciling it gives an idea on how to further design something if you lack the peramiters, or to outright enigineer a platform from scratch. The best engineers in the world still need guys like me to take it off papar theory and take the actual product to the test track and tweak it in a real word enviroment.

Last note- the 3rd gen front rc if not corrected will go into the dirt real fast with basic lowering and no camber. the front RC drops at a fast progressive rate and goes underground in no time. Its a flaw with these cars from the factory- the rear RC too high. Also, the front going low fast compared to the rear if lowered. The notorious push/oversteer these cars suffer from is from bad geometry combined with bad spring rate choices to try and make the car safe and road worthy for consumers. They do very decent (stock IROC) when driven hard but in reasonable control, but when overdriven they go right to plowing becasue of a few things that give way- one is the rear jacking fromt he bad roll axis, another is the chassis flex, and the last big one is the strut mounts giving in to absorbtion and then reacting. All create snap induced oversteer.