Discussion of Chassis Setup for 8 Sec. Assault

[Alky V6]

With a driver in the car, the left front spring was adjusted to give equal lengths to both rear springs. The roll bar slack was taken up to favor pushing down on the right rear.

The location of the spring seat of the left front shock ended up .900" higher on the shock body than the right side. An indication of mainly the bent frame and a little extra to compensate for driver weight. About .250" of that .900" was to compensate for the driver weight.

Camber was adjusted to 0 degrees.
Caster at +4.1-4.4 degrees.
Total Toe-in at +1/16-3/32"

 

[charlief1]

Just to prove you're right, scale the car when you're done. It will show another way to set the car up. Good reading so far.

 

[scot w.]

This might explain why the rear right side is a 1/4'' higher than the drivers rear. Never once thought about the front springs as a cause. Something else to look at..

Scot W.

 

[Alky V6]

The last alignment done on the car was before I did all the suspension work, front and rear, on the car. I didn't have the coilover shocks in the front and didn't have any way of adjusting corner heights, so the last alignment was done with the front end heights whacked out due to the bent frame.
After all the suspension upgrading and with all the latest spring adjusting, the next step was to look at the alignment. I used a simple Home depot digital level against the flat bearing cap of the front wheels to check the camber on each side. I found +2.0 degrees camber on the right side. Here is where it gets interesting. There was also more positive caster on the right side according to my suspension point mapping and the suspension analyzer. The software has a calculation window that allows you to play with adding and taking away shims to adjust the caster and camber with standard size shims or custom size shims. I used the calculator, installed the shim packages I came up with, and on the first go, the camber now is an equal zero on both sides. I still have to confirm if the caster is where I'm hoping it is.
I think I have a way of checking the caster without buying any extra equipment. Steering angle camber gain?

 

[Alky V6]

Just to prove you're right, scale the car when you're done. It will show another way to set the car up. Good reading so far.

That is the plan. I will pass on what I find.

 

[Alky V6]

This first chapter of my chassis tuning adventure is an attempt to get the car to stay hooked to the track on the top end. Above a certain boost level, which is relatively low, the car feels like it's on ice. As long as I stay just 1 psi below that boost level, the car stays hooked and the car has been very stable and a dream to drive.
Constructive critics of my project have let on that they believe my problem can't possibly be engine performance related and that my problem must be chassis related. Hence this first chapter of my chassis tuning adventure. I have to say, so far I don't believe I've come across any detail in the recent past chassis setup that was way off, and certainly nothing that screams out that the chassis was the cause of the tire slip on the top end.

 

[Alky V6]

Caster Check

I got the opportunity to use a pair of Snap-On slip plates that were kindly loaned to me. Pretty neat how they work.
As I've already covered, the camber has been adjusted, and I needed to double check the caster value for each side. The slip plates have provisions that lock the plates once 20 degrees of turn has been reached. I turned the wheels to the left, and then right to these 20 degree locks and recorded the camber gains. They were as follows:

Driver in the car
Wheels pointed / Left wheel camber degrees / Right wheel camber degrees
Straight / 0 degrees / 0 degrees
20 degrees left turn / +1.4 / -.84
20 degrees right turn / -.41 / +1.0

These were the values I recorded after making the one camber/caster adjustment to the right wheel yesterday. No other changes were made except for toe in.
Using these measured values, I went back to the suspension analyzer to fine tune the right side caster figure until I was able to get matching camber gain figures to the measured values. It took less than a 1/16" shim adjustment to the front and rear A-arm mounting points for the right side upper A-arm to get the software to correlate with the measured values. Very impressive.
The caster value for the right side turned out to be off by less than .2 degree before the adjustment to the software to get the figures to match.

Final confirmed caster adjustment:
Left side: 4.10
Right side: 4.37

The OEM tolerance for the G-body is +/- .5 degrees for caster and camber during a setting procedure. Up to 1.0 degrees caster, .8 degrees camber during a simple checking procedure.
The side to side variance tolerance for caster and camber is no more than .5 degrees during a setting procedure. Up to 1.0 degrees for both caster and camber during a simple checking procedure.
Toe in was set at 3/32" while the front of both tires were loaded inwards towards each other.

 

[Alky V6]

The shim packs used to correct the right side caster/camber were:
3-1/8" shims at the front location.
2-1/8", 1-1/16" shims at the rear location.

I didn't record it, but I believe it was about one turn on the right side to bring the toe-in back in to where it needed to be.

 

[Alky V6]

The tools that were used were:
Tape measurer
90 degree ruler
Simple digital level
Slip plates
Plumb bob and string
Felt marker
Suspension software
Height guage with bent rod to locate upper A arm suspension points.

 

[charlief1]

Well from what you posted it sounds like you SAI is off Donnie. Did you remeasure everything after you made the corrections?

 

[Alky V6]

Well from what you posted it sounds like you SAI is off Donnie. Did you remeasure everything after you made the corrections?

Mapping out the suspension points is not an exact science. I figured from the beginning I would have a range of error of +/- 1/8" for all the suspension points I would be attempting to map out. We all know that when you deal with computations, garbage in, garbage out. Regardless, I was very surprised at how close the software was, even considering the +/- 1/8" range of error on the suspension point mapping input data. I guess that means I did much better at mapping out the suspension points than the 1/8" range of error that I gave myself.
All the specifications were checked on the car after all adjustments were made. Once I had the actual figures off the car, I used them to fine tune the software. As I explained earlier, it didn't require much to make the software match the real world measurements.

The measurements in post #47 are actual measurements off the car, except for the caster numbers which were calculated using the camber gain numbers I measured on the car.

 

[Alky V6]

A problem has been identified. With more than 2.5" rise in front end suspension travel, the caster goes negative. Presently, I have bumpers that restrict front end suspension rise travel to 3.0". That will be changed to 2.5". THANKS, Chuck.

The toe-in at 2.0" front end rise goes to about 1/4" total. I may set the toe-in at static ride height to 0.0". That will give me a toe-in of .050" total at 1 inch rise and .140" total at 2.0" rise. This will mean a toe-out condition with any amount of front end dive. With a 2.0" dive of the front end, I will have -.140 toe-in (toe-out).

 

[Alky V6]

After modifying the bumpers, I came up with the front end suspension rise travel being limited to 2.0".
Now, I'm going to look at limiting the dive travel. I'm thinking of modifying the bumpers there to limit dive travel to 1.5" to 2.0" there.

 

[cdsttype]

I run 7 degrees of caster on the slip plates.I jack
the front end and adust toe and camber to simulate
down track performance. The factor i can not measure
is how much front end lift i have at the big end.Toe
settings are positive not only for ride height change
but also for tire scrub.Shock travel will be a limiting factor
on lowerering the car with the coil overs.Travel limiters are
a great way to stop bump steer.Jounce bumpers on the shock shaft
are a must.I would rather loose traction than put it on the bumper,
and gain more travel with track testing.I am working on getting my
car in the teens on a 275 drag radial .15 to go!I think when
i measured the lift at the wheel it was 1.5 inches max.I run
a 2 degree pinnion angle.Things i think are important big caster to keep you straight. Slight negative camber to keep you straight and zero toe
with the height change and scrub applied thats the tricky one!
When i aligned tony gomes car i told him he was crazy to go the
speeds he was going with the adjustments i saw. He made it easy for me
his chassis guy scaled and did the ride height before i got it
I just put it to the specs i like.Hope this helps

 

[baadgn]

I run 7 degrees of caster on the slip plates.I jack
the front end and adust toe and camber to simulate
down track performance. The factor i can not measure
is how much front end lift i have at the big end.Toe
settings are positive not only for ride height change
but also for tire scrub.Shock travel will be a limiting factor
on lowerering the car with the coil overs.Travel limiters are
a great way to stop bump steer.Jounce bumpers on the shock shaft
are a must.I would rather loose traction than put it on the bumper,
and gain more travel with track testing.I am working on getting my
car in the teens on a 275 drag radial .15 to go!I think when
i measured the lift at the wheel it was 1.5 inches max.I run
a 2 degree pinnion angle.Things i think are important big caster to keep you straight. Slight negative camber to keep you straight and zero toe
with the height change and scrub applied thats the tricky one!
When i aligned tony gomes car i told him he was crazy to go the
speeds he was going with the adjustments i saw. He made it easy for me
his chassis guy scaled and did the ride height before i got it
I just put it to the specs i like.Hope this helps

Bill,
You Nailed it on my car, I was fighting the sumbit*h before you put your hands on it Awesome job !!!

 

[johnplogii]

This will mean a toe-out condition with any amount of front end dive. With a 2.0" dive of the front end, I will have -.140 toe-in (toe-out).

Won't this be very hard to control with it wanting to dart one way then the other? At the speeds you are going to be at it just sounds scary. Maybe I am wrong & it takes more tow-out to do that?

John

 

[Alky V6]

Thank you very much for the input, cdsttype. It is very helpful information.

There are a few things I've come to realize with the studying I've been doing on drag alignment settings.

Caster has much to do with how self-centering the steering is. The more caster, the more the steering wheel wants to steer straight. Also, the more caster, the harder the steering is when cornering. This is why you commonly see more caster used in drag racing. Cornering is not real important. Keeping the car steering straight is.

With the stock upper control arms, you are limited to how much positive caster you can get out of them by adjusting them with shims. You end up with too great of a difference in shim pack thickness between the front and rear mounting points when you try to go too positive. The upper OEM specification limit for the caster adjusment is 4 degrees positive, and to get more than that, and keep the shim pack thickness difference between the front and rear mounting points reasonable, requires special upper A-arms that are manufactured to provide the higher positive caster angle preferred for drag racing. TRZ upper control arms are one example of a control arm designed to provide the more preferred 5-7 degrees caster for drag racing.

An OEM limit for the difference in thickness of the shim packs between the front and rear mounting locations is .500". It's considered unsafe to go beyond this limit.

Attempting to adjust to a high caster angle with the stock upper control arms will cause higher caster gain numbers.

With the stock upper control arms, as the front suspension rises, the caster angle moves towards zero and then negative. Negative caster can provide unstable steering. It's preferred to have the caster stay on the positive side.
If suspension travel can be limited before the caster can go negative, that can certainly help. Another thing that can be done is to start out with more positive caster angle so that it takes more suspension rise before the caster will turn negative. So if you had to have a lot of suspension rise travel, it would certainly be preferred to adjust in a lot of positive caster so that you wouldn't go negative by the time you reached the limit of the front end rise.
If you can get away with limiting your front end rise, then you can get away with less caster as long as you don't go negative at the top of the front end rise travel. That's kind of where I'm at. I've limited the suspension rise so that I wouldn't run into negative caster when the car reached full front end rise. If it turns out that I need more front end rise travel, then I will have to install aftermarket upper A-arms so that I won't run out of positive caster as the front end rises. Also, camber and caster gain will be minimized.

 

[Alky V6]

Won't this be very hard to control with it wanting to dart one way then the other? At the speeds you are going to be at it just sounds scary. Maybe I am wrong & it takes more tow-out to do that?

John

How much toe-out before the car starts to feel unstable is a very good question.

The problem is, as the suspension rises or dives, the toe-in changes. As the car rises you get more positive toe-in which causes tire scrubbing, but maintains good steering control. As the car dives, you get more negative toe-in (toe-out). Toe-out also causes tire scrubbing, but worse steering control.
The dilemma for the racer is that the car runs at a higher front end ride height accelerating down the track. As the car rises, the tires toe-in more and drag is created slowing the car down. Steering staying stable though. At the end of the run, and during braking the front end dives. At a certain amount of dive, the toe-in will go negative. Negative is never preferred, but with our style of suspension, it can't be avoided. The more positive toe-in that is adjusted in, the more dive it takes before the toe-in will go negative. But the more positive toe-in you adjust in to prevent from going negative on a dive, the more positive the toe-in will go at that certain amount of front end rise that your car runs down the track at.
The toe-in adjustment is a compromise between the amount of drag due to tire scrubbing during acceleration, and the amount of negative toe-in that you will end up with during braking and front end dive. When you pick to adjust toe-in for the least amount of drag for the run at a certain amount of front end rise, it means you will have more negative toe-in during braking.
One way to prevent the toe-in from going negative is to restrict or limit the amount of dive travel that the front suspension has. That way you can get away with adjusting the toe-in to zero at a higher front end rise figure.

It could very well be that a small amount of negative toe-in can be tolerated. I don't know. Maybe someone with experience can let us know.

 

[Alky V6]

On the subject of negative toe-in (toe-out), I'm finding that in a bump steer situation, it's preferred to have the tire toe-out rather than toe-in.
Bump steer is the amount of toe-in change when one tire or the other hits a bump in the road.
If you're cornering and the outside tire hits a bump, the outside suspension will compress and that tire will toe-out a slight amount. This ends up giving the driver an understeer sensation. Understeer is when the driver feels he has to put more steering input into the car to make it turn. This is considered a safer condition than the alternative where the tire would toe-in on a bump and cause an oversteer situation. Oversteer is when steering becomes more sensitive to the drivers input. If one were in a corner and the outside tire hit a bump and the suspension travel were to cause an increased toe-in on that tire, the car would feel as though it suddenly wanted to turn tighter even though the driver did not change steering position. The driver would then have to quickly correct his steering.

If one were to increase caster using the stock control arms to a value past 4 degrees positive, what would that do to the toe-in gain curve? Would it increase the amount of +/- toe-in change per inch of suspension travel (rise/dive)? What if aftermarket control arms were used that had the extra positive caster built into them with the more reasonable mounting point locations? I know I have the software where I can punch the numbers in, do some experimenting, and quickly come up with some answers. I'm just throwing those questions out there. Maybe someone has been through this scenario already and knows the answer off the top of their heads.

 

[Alky V6]

I'm going to go out on a limb here and suggest that small amounts of toe-out during braking or suspension dive is not detrimental. Look at the stock suspension. It has 4 or more inches of dive travel, and the way it's designed, and using the factory alignment specifications, you will have toe-out during hard braking. This is how this style of suspension has been used for decades. Toe-out during hard braking doesn't appear to be that big of a problem. If it were, I would think the manufacturers would have changed the design a very long time ago.

I'm going to suggest that during an emergency braking situation, if a person were to input steering, one direction or the other, he would most certainly prefer an understeer situation rather than an oversteer situation. I'm talking about you're typical everyday driver. An oversteer situation during braking could get someone in a situation where the car might get too sideways and would want to flip over. This could be one reason why the manufacturers built in toe-out during braking.