In helping my friend initially set up his tube chassis 4 link I took the recommendation from this forum and set it up with an IC of 40/6 (if I remember correctly). He's still looking for the IC diagram I prepared when we originally set it up. Anyway the car launches good and goes straight but it is hitting the tires very soft. I need to raise the IC for a harder hit and my question is about how far should I move it per change until we're satisfied?

Thats sorta hard to answer.. sometimes a little is
a lot.. so if you move the lower front point up
2 holes that could be a ton on change.. do either
of you have a 4 link program... if you take the
time to plug all the car measurement points and do
them right you will see all the points that will be
available to you.. granted when you find the sweet
spot/area you might have a total of 4 points that
will work for most of the tracks
EDIT
For my car(short wheel base) I have a map tapped
on the wall of my trailer to show 6 different
set ups.. 4 of those work very good.. the other
2 are fair.. anything greater that those dont work
and I show the shock settings for those settings

If at 40 out and 6 up its soft the car must be down on power. You have to have the IC at less than half the wheel base.

On clutch cars I start at about 90-92 out and 4 up and work with the shocks. On transbreak cars I usually start another 10 inches in and 2 inches up.


I never run the bottom bar parallel to the ground. It should run on a line slightly downhill and run through the bottom ball joint.

40 out and 6 up is pretty aggressive for anything with any power. Horsepower, stroke, starting line ratio, stick or auto, total weight and percentages, center of gravity height, personal preference for tuning, etc., etc., all factor in. I would look at the brackets themselves, angles, spreads, distances from axle C/L, etc. The bars can be in the car a number of different ways to arrive at 40 and 6 and they won't all work the same way. The geometry is what matters

Jerry Bickel's 4 link program is good. Once you map the car it will show you all the possibilities and you will always have that saved. And you can use it on all your other friends cars.

https://www.jerrybickel.com/4-link-wizard-professional-version.html

If you spend the time to map their car out..
it isnt just map one and it works for all...
but thats one of the 4 link set ups I use
He also explains on the HP break where is its
under a given point it should be closer to a
certain set up and if its over that point.. it
a good program.. the company that makes the
Pipe Max has a good program also

Have you thought about shock adjustments? Tire pressure?

Those adjustments can have a lot to do with how hard the car hits.