As mentioned the top/bottom hole does NOT change the instant center. On a leaf spring or ladder bar car, The Instant Center (IC) is defined by the single point in which the suspension is attached to the car (so in this case it’s the front spring eyelet).

What the top and bottom hole does in a caltrac is determine how quickly the front spring segment goes stiff. If you think of the front spring segment without a caltrac, the axle tries to rotate pinion up. There are two ways this motion can happen. Either the spring segment is completely and 100% stiff (like a ladder bar) and the pinion angle changes from body rise. Or in the case of a leaf spring, the body can stay still temporarily and the spring can flex.

The sole purpose of the caltrac is to keep the spring from flexing, and essentially create a short stiff ladder bar.

If you look at how the caltrac works, the front hanger rotates around the spring bolt. This is caused from the axle rotating, pushing the bar forward, which rotates the hanger down until it contacts the top of the spring. At this point the system is “solid” (except for some high force flexing that can occur).

When the bar is in the upper hole, it take less forward motion from the bar to make contact with the top of the spring. The lower hole requires a longer “push” of the bar before this can happen. The amount of gap/preload in the setup, and the upper/lower hole, all have to do with the “timing” of how quickly the system gets to full stiff, and how abruptly it happens.

The reason the bar in the upper hole (and more preload), hits the tire harder then a low bar hole/less preload, is because the system goes stiffer/sooner. Anything other then the upper hole and preload, is wasted motion of the suspension which will slow 60ft times down. The exception to that is if you don’t have a good enough tire/shock to hold that immediate tire hit, the lower hole/gapping can sometimes be a “Bandaid” and help with traction by launching the car softer.

If you look at any of the fast leaf spring cars, they almost ALWAYS have the bar in the upper hole. If your car isn’t spinning, it’s unlikely that the lower hole will help anything.

Also, being that it’s a foot brake car, you’re already preloading the pinion up anyway, so you’re likely getting close to full “Stiff” on the front spring segment anyway while you’re on the converter (depending how hard you’re on it).

If anything it looks like you could tighten up the front shocks a LOT if that’s an option for you. Comes up way to easy. Keeping the front down is the best way to improve the 60ft if you’re not spinning.

Hope that helps…