intex1982

iTrader: (2)

what is instant center and how does it work and what the best way to have it set on the around 50"? would someone explain to me?

BobItzaboy

To make a long story short and easy, its basically the length of your tq arm. The only way to adjust the fore and aft position is to buy a different tq arm of the disired length- Basically, if one of the few offered by aftermarket does not suffice, then you will have to have one custom made.

IC vertically is where the LCA angle crosses the vertical line of the tqarm. The length and the imaginary line from the LCA's make up the percise IC to each individual 3rd gen.

There is no solid answer for what you ask and are thinking on setting to. It has to be married with the reat of the car. If you were told 50 inches in length is best for some reason or maybe you saw that on a drag car or such, it may be best for them but may be crap for you unless your car is 100% identical in specs and modifications.

IC on the front end is a whole lot more diffecult to explain., There are chassis books with good diagrams the show it far better than any explination.

sofakingdom

iTrader: (1)

IC is the imaginary point at which the suspension ACTS LIKE it applies accelerating or decelerating forces to the car.

It is not necessarily a "physical" location on the car; it could be in the center of the passenger compartment, below the ground, or in front or behind the car. It is determined by the suspension design and geometry, in the manner that Dean described. Much as the car's center of gravity may or may not be a physical object of the car, but instead might be somewhere in the middle of the cabin.

For "estimation's" sake, the CG is usually assumed to be approximately at the shifter ****. Crude and inexact; but works well enough to serve as a useable "rule of thumb".

The IC changes depending on what the car is doing at the moment. That is, as the suspension members move through their travel (jounce and rebound), the IC moves around accordingly. Different designs of suspensions produce different changes in teh IC location during their operation.

For optimum stability under all conditions, you'd want it to correspond as closely as possible to the car's center of gravity. That is, the forces tending to change the car's motion (acceleration and braking), need to be applied at the same point at which the car resists changes in motion (center of mass, or center of gravity). Sounds pretty elementary, eh?? Well it isn't as easy as it sounds.

When that condition is obtained, the act of applying power or braking to the car doesn't cause the car to squat, nose-dive, wheel-hop, etc. etc. etc. that ALL derive from those 2 points being significantly different. Especially and above all, the designed-in defect that comes with these cars from the factory, wheel-hop; which is created by the angle of the rear LCAs.

So for example, "squat" is BAD. If a car has it AT ALL, it indicates that the IC is BELOW the CG (the most common condition in anything resembling a stock suspension). Likewise, "anti-squat" (where the rear of the car RAISES on accel) is bad too. If you could get the suspension PERFECT, the car would neither squat nor whatever the opposite is, when it launches.

intex1982

iTrader: (2)

do shocks and springs play a part in squating

AlkyIROC

Just about everything plays a part in squatting including front suspension.

As for moving your IC, unless you have a 4-link like in my car, you have very few options on an IC point. I'm not really sure where the IC is with a torque arm/LCA system because both are used to calculate the IC but not in the same way as other suspensions do. Theoretically, the IC is the front mount point of the torque arm.

With ladder bars, the IC is the front mount point of the bars. The length can't change however the front mount can be raised and lowered a few positions to change the IC height.

With a 4-link there are literally hundreds of IC points but only a small handful are useful.

For most cars, having the IC roughly on the 100% anti-squat line is the best. That way the back end doesn't drop or rise up when launching.

When the back end rises up the diff is being forced down for better bite. This can bog the car down and cause tire shake and wheel hop.

When the back end drops down, the diff is moving up. This can cause wheel spin or unloading the suspension.

It depends of the car's power level, how it's launched, track conditions etc as to what IC works best. Slower cars (9 seconds and slower) do better with the IC slightly back from the CG and a few inches off the ground while cars like ProStock want it long and very low.

BobItzaboy

I perfer to have the IC lower than the CG so the rear digs and the front lifts. All the while controlling and limiting the rear squat with spring rate. It loads, but it does not squat. Too much lower though and thaing go worng, they go wrong in a big way, but when dialed, its on and boogying.

I do not like 100% anti squat because you get no weight transfer from front up over to rear. A teeter totter launch is good and requires less than 100% antisquat but higher rate coils and rebound damper. what percentage? it would depend on HP and traction but I like playing around the 85% range so when a mistake is made it is not as detrimental as say 70%.

Since when is squat not good, could someone explain to me how you would otherwise load the rear tires on launch?

Sofa, the opposite of squat would be duh......Lift maybe.

Norm Peterson

That's the whole point - loading the rear tires. And loading them as quickly as possible. Consider that you have three load paths that contribute to tire loading. Geometric (aka anti-squat), elastic (springs), and damping (shocks). They do not act the same if you look at their effects over small time steps. And if the instantaneous total is less than what you need at that time, wheelspin starts (and for at least a couple of reasons it's harder to stop wheelspin once it does start).

Anti-squat is the fastest-reacting load path, as it occurs as a direct and immediate response to acceleration traction forces at the contact patches. Any 'delay' is due to natural frequencies in the linkage (which are pretty high and happen very quickly).

The damping load path is the next quickest to respond, since it is related to the speed at which the rear end squats. Obviously, this means that the rear end must be on the way down, but the damping force will die out as the car approaches its final position.

The slowest path is the springs, because that's based on the actual ride height. Max spring force happens at the final suspension position, assuming that there's enough damping to prevent 'overshoot'. It is true that stiffer springs will help here, but for a given amount of squat it's still better to use less spring and more A/S simply because the A/S 'help' gets going quicker than that from any spring short of an infinitely rigid one.

It is possible to overdo A/S - if you get too much 'separation', the rise of the rear must eventually reverse itself. And as it comes down, that will remove load (and traction) from the tires. If nothing else, the elastic and damping paths cannot make up for the loss of rear tire load as fast as the geometric load goes away and you get a temporary 'deficit'. IOW, possible late traction loss.

All relatively small effects, but they can matter when you're being timed to the thousandth of a second.



Norm

AlkyIROC

Here is a picture commonly used to explain anti-squat. The article accompanying it can be found here http://www.raceglides.com.au/TechInfo.htm

Dewey316

The other thing to take into account is usage. 100% A/S on a drag strip launch pad is one thing, real world is a diffrent one. When you have 100% AS, 100% of the wieght transfer is going through the suspension links, this are non-dampened VERY high rate. On a smoth race track this is OK'ish (i don't like 100% A/S even there), but on the streets where there are bumps, and man-hole covers, ruts, etc. You are taking the springs out of the equation, and one small bump causes the car to lose traction, then you get into the condition of losing traction that Norm describes in his last paragraph.

IIRC, Carrol Smith (who I don't agree with on much) said, no more than 20% A/S, other people put out diffrent numbers. But for almost any car other than a dedicated drag stip beast, you probably don't want a high level of A/S. Especialy on our cars, where the only way we really change our IC, is by the LCA angle, you also get into some other issues with the geometry when you try to make the IC change. (you can read the novel that is the wheel hop thread for more info). You also have to move the IC on a vertical plane, we can't easly adjust the IC, so when you can't move the IC forward to increase AS, we have to move it up to increase AS. You can liken this to raising your roll center, and how it effects the way the car rolls in turning. As the IC gets closer to the CG (A/S asside, as AS is also a function of wheel base). You can use the IC location tune how the weight is transfered and get the most of your car, but giving it too much, and you have a car that does funny things.

racing geek

iTrader: (5)

Thanks for the picture Stephen. It really helped me understand what all of you are saying.

AlkyIROC

The most efficient setting is to push the refrigerator forward without it tipping forward or backward. If you've ever tried pushing a fridge, you know you need to push it below the mid point so it doesn't tip forward. If you push too low, you'll never push the bottom out like in the example but you'll lose your footing. Once you find the sweet spot, it will push straight without tipping or lost footing. Finding the best IC on a car is the same thing. It all depends on the car itself and since no 2 cars are exactly the same, what works well on one car may not be best for another.

The example in the picture shows the extremes as to what can happen. Depending on the car, the example IC settings may still work and they would have to move even more to be extreme.

You can see the neutral line from the bottom of the rear tires to the CG height at the front axle. The IC anywhere along that line is still 100% anti-squat. Because of the angle of the bars on a 4-link, there are only a few positions along that line that are effective. Normally the IC is slightly below the neutral line. Below the line is less than 100%, above the line is over 100% anti-squat.

Wheel Spin

Good discussion of F-Body TQ Arm suspension here with Billy Shope, ex GM and Chrysler suspension engineer:

http://speedtalk.com/forum/viewtopic...er=asc&start=0

And a direct link to Mr Shope's web page with various calculators. The "Intro Section" at the bottom of the first page is very informative: http://home.earthlink.net/~whshope/

rx7speed

I always thought for drag racing anti squat can be helpfull depending on the situation.
or is it more of a bandaid fix then anything?

Shagwell

as sofa said, PERFECT. Since actual perfect is basically unachievable, we tend to use some anti-squat. The problem is that anti-squat cannot be dialed in only for launch, it's there all the way down the track. The rear tries to lift at every gear change and it may also create roll steer when cornering. That lift at gear changes creates excessive drag and generally upsets the suspension on the car, thus can easily create issues at speed.
As a general rule, slight anit-squat will not be noticable at speed for most street type cars. It becomes less neccessary the better your weight ratio and the better your traction. Remeber, anti squat is basically intended to increase the initial "plant" on the tires. The more traction you have, the less you need the suspension to increase the tire load.

rx7speed

what is roll steer or rather how does it work? does it effect both IRS and SA designs?
I know with the anti squat it can also cause problems with traction after the intitial launch when the suspension tries to resettle it can take too much force off the tires. but other then that I didn't know of the other problems it could cause. I'm assuming antidive can cause the same problems as well?

atc3434

iTrader: (4)

You need this thread here - https://www.thirdgen.org/forums/susp...nderstand.html

Cut through the BS, and there is some good roll-steer discuss. It only applies to the solid rear axle design.

Shagwell

rear roll steer related to our cars -
Roll steer is when the suspension geometry causes a steering effect during body roll. The suspension will move for/aft through its range of travel, the amount of this movement is dependent on the length of the arms used and the amount of travel. If you lower the rear mounting point of the control arm, you create anti-squat, but if you lower it past parrallel at ride height, it can/will create an excessive roll steer around a corner. When you corner, one side's suspension is at droop, the other compressed. If the arms are low towards the back, then the compressed side becomes parrallel, while the droop side swings even lower towards the rear. Thus, the droop side is pulled forward, the compressed side pushes back and the rear is now crooked under the car, thus you have roll steer.

As for anti-squat down track, click high gear at 130mph and the suspension tries to transfer weight back, thus lightening the front tires load and increasing the rear. Not a very safe condition, and not a good feeling.

rx7speed

kinda wish we had a rep system here so I could rep+ you guys. now still being somewhat related to the topics discussed what about anti-dive? can that cause any problems also?
sorry about my lack of understanding here. I'm don't know a lot about the details of the suspension on your cars, and only vauge on even whats on mine. have antisquat and antidive new but with sagging springs doubt it has much of an effect anymore. other then that what they do how they work and what problems/pro's come from it I'm hazy on as well.

Dewey316

Anti-Dive (I assume you are talking about the front suspension here) has to do with the SVSA IC location of the front suspesion. Basicly, on our cars, unless you start doing some major modifications, it is not adjustable at all. Basicly you would have to change the front a-arms rear/aft mounting mounts.

All of these changes, all effect something out. If it was as easy as making once change, this wouldn't be so hard. ;-) A/D can effect steering, and other camber/caster/toe changes in bump. Again, unless you are going to start modifing the pick-up points for the front a-arms, A/D is not something you can really adjust at all on our cars.