I'm curious how you start out selecting an IC and what trends do you see as far as IC, power, RPM, shock/strut settings and clutch/converter affect IC?

I personally like the longest lowest IC I can use and not have the car squat and not control the squat with shock settings.

Where are you trending. This would be 4 links cars only as ladder bars can't achieve IC's like a 4 link.

Personally, I don't pay much attention to it. In other words, I don't pick a length and height, then look at a map of intersect points and arrange the bars to get there.
Rather, I pay attention to angle of bars, spread of front and rear mounting points, percentage above and below axle C/L for the axle mounting points, height of the whole system from the ground, etc. All of which are determined by power level, stroke, weight distribution, center of gravity height, gear ratios, clutch or converter, intended use and how you want the car to react, etc. The geometry, leverage and how it is applied to the chassis and housing are what make it work the way it does.
Wherever it ends up is a baseline, hopefully pretty close, but not the final answer. On track observation determines what adjustments and tweaks are needed from there. Sometimes I will go back and determine the IC of where the bars were set, but use that information just as a reference.

There was a decent discussion earlier this summer that would help. Ill see if I can find it

So do you find any trends? Do high HP powergilde cars tend to like an IC that falls on the neutral line, where that same car with a three speed, or even a clutch may want the IC in front of and below the neutral line?

If you have two different bar settings that get the same IC, how do you which bar angle to test first?

I must have missed it. I'd like to read it if you can find it.

If you read some of the chassis books they tend
to say 750hp and up you start at like 45" out
and less for lesser power.. there are things like
if you spread the axle housing bars you tend to
wheel stand.. also think where you want to lift
the weight at.. try lift just behind the CG

Depends on 3 things for me........power, tire, weight percentage.

I haven't had anything less than a 48" bar in a slick tire car in 20 years. Most books are way out of date on their info

Most of the power level cars I deal with are in the mid 60s and longer

But its not just the number. I want the bars in certain places on the housing

On some lower powered stuff (650HP) with a stick I ended up at a 100 out and 3 up IIRC but it was close to that. There are people today telling me that 48 out is LONG by today's standards. I know I didn't have the best shocks at the time, and the long IC help slow the shock down and manage some of the hit.

I started thinking about this because I went to just one race last year, and it seemed to me from what I could see, many of the faster cars were way short on IC. It's hard to tell at the track, but from watching the car and then looking at the car in the lanes made me think everyone went to short IC's. Then, I watched the last episode of S.O. and got to thinking there is no way those cars could deal with a the big hit from a short, high IC.


And that's how I ended up here. I suppose if the shocks are that much better, you could deal with the violence of the short IC, but then, you have the tendency to induce tire shake.

Very confusing.

You won't see a big power car with a short IC........at least not one that works well. Most square up the tire and shake and the only way they know to get them down is back the power off and make it a turd for 60ft.

FWIW I would agree spread and angle are things most look at these days. IC's on higher powrered cars tend to be farther out. HOWEVER on some S/G or similar pro tree cars with decent power you will find short I/c's that tend to be on the high side as well. They do it to help hit the tree, same reason they run small front tires. Makes for an ill handling car down track especially when bombing the brakes. But has been the trend for a few years.

The IC is just part of our "tool box" to make a car hook. It is just one piece of what makes the entire car work and needs to be in balance along with the shocks, power, tires, etc...

To understand it all… I view launch as two distinct pieces.

-First is the HIT.
-Second is what happens after the hit (forward/upward vehicle motion).


Hit is basically defined as the quick (less then 0.1 sec) pressure that is put on the tire BEFORE the car moves.

You can really see this in action at the 2:12 mark of this video.

https://www.youtube.com/watch?v=jr1VILyS7WE&t=2s

If you watch slow motion video, you can see that the tire will be forced into the starting line before any significant body motion (up or forward) happens. This is because the sidewall of the tire is the weakest part of the vehicle motion force path on trans brake release. It's "easier" for the car to crush the sidewall then it is to move the whole car upwards or forward.



Now that HIT is defined… There are three main things that effect hit force.

1) IC Length - This effects the geometrical mechanical advantage of the suspension during the tire hit. The shorter the length, the harder it will hit the tire.
2) Power - The more power the car has, the harder it will hit the tire.
3) Shock Extension/Rebound stiffness - The looser the shock, the harder the car will hit the tire as it tries to “throw” the housing to the ground. (This is specifically related to the HIGH SPEED portion of the shock curve.)

The Instant center length needs to be balanced with the power and shock to achieve the proper amount of pressure on the tire during the hit.

-Not enough pressure will cause the car to spin shortly after the hit because the traction is too low for the acceleration. (you can see this in run 5 of the video when the rebound of the shock is too tight. Spins almost immediately after the hit)

-Ideal pressure is balanced between traction and forward motion.

-More pressure then needed will waste ET (energy that could be used to make the car go forward, is wasted on overly aggressively pushing the tire down).

-Excessive pressure will cause the tire to deform and loose traction.


The second component to launch is the transition to forward motion. This is where the IC height comes into play because of Anti-squat %. This is a VERY hard thing for people to understand (especially without pictures) But the simple analogy I try to use with people is the chair behind the door knob example.

I’m sure everyone has seen (or tried) putting a tall chair under/behind a door knob to effectively keep someone out. You push on the door, and the chair wedges itself into the carpet preventing the door from opening. This is very similar to how anti squat works.

Think of the line drawn between the tire contact point and the IC (this could be a physical IC of a ladder bar/leaf spring, or a theoretical IC of a 4-link.) The steeper the angle of this line, the more downward force is put on the tire. This is exactly what’s happening with the chair behind/under the door knob. As the person pushes on the door, the steep angle of the chair creates huge pressure into the carpet.

If you have a steep antisquat line (high IC), the inertia of the car acts as the door, and the antisquat line acts as the chair. As the car accelerates, the intertia of the body (resistance of movement) tries to stay stationary. The axle is effectively trying to walk itself under the car. This creates additional pressure on the tire as the car starts to accelerate.

The LOWER the IC…. The less additional pressure is put on the tire after the hit. The HIGHER the IC… the more additional pressure is put on the tire after the hit.

One thing to clarify is that the HEIGHT of the IC has VERY LITTLE to do with HIT. That is primarily controlled in IC LENGTH. There is obviously a transition between hit and forward motion that the two interact, but they can primarily be considered separate because once the sidewall of the tire has been crushed, the motion of the body comes to somewhat of a balanced equilibrium.

This pressure on the tire after the hit is very complicated. You have the resistance of the low speed shock stiffness resisting this force, you have the front shock stiffness resisting this force, weight bias is changing dynamically as the body position changes, converter coupling is happening applying more power to the tire, etc…

The important part to understand is WHAT characteristics of the launch the IC position effects, and using it when needed to correct (balance) undesirable attributes of your specific launch.
Consider this... there are lots of fast (fairly high powered) leaf spring cars out there with 20~24" segment springs. Obviously, that is a VERY short instant center and hits the tire VERY hard because of the short IC. The only way to counter this is through a very stiff rebound setting on the shock. Those cars also have the front spring attachment point lowered because they don’t need the high IC. The high power alone creates a lot of pressure on the tire through general acceleration/geometry, where a low powered car doesn’t have that advantage.

Sorry for the long post, but it's a snow day here in Detroit so I finally had time to properly respond to one of these suspension topics...

The one thing I didn't want to debate was that there is multiple 4-link settings that can achieve the same IC location. What gets ignored a lot is that the suspension certainly isn't 100% rigid. The wider spread creates a more stable geometry to eliminate the flexing, and gives the shock more control. IC theory is the same still.

I admittedly don't know near as much about the 4-link stuff...but the angle and spread are still changing the IC

Good read. Good explanation.

The biggest thing that I see people failing to grasp is POWER and the fact that the more you have, the LESS bite you need. They think big power means you need big tires and a lot of bite to HOLD that power, when in fact it's the polar opposite