help me understand instant center as applies to leaf springs

I am converting my cars rear suspension to a cal-trac setup.with the mono leaf springs and QA 1 double adjustable shocks.What I am replacing a set of launcher springs that i've had on the car for several years. The car is a 72 charger weighs 3500lbs and runs mid 10's @ 126 mph. with a best 60ft of 1.47. I foot brake the car ,no trans brake.
My dilemma is the front spring hanger has two mounting holes. As a starting point i am not sure which mounting position to use. Does it even make a difference?
The current set up with the launcher springs uses the upper mounting hole . One of the reasons I'm switching to the cal-tracs is the car hits the tire so hard that I'm having trouble controlling the seperation and I'm about maxed out on my rebound adjustment with the shocks . Ultimately what I'm after is a quicker reaction time from the car and an improved 60ft time.
So what would be your recommendation or advice as to a starting point.

upper hole raises the instant center. Your real problem is the short front segment of a Mopar leaf spring, and your real solution is a shock that can control the movement of the rear housing. GET A GOOD SHOCK or get what you have re valved if possible. There are a few places that will help you with a good stiff shock for the back of a leaf spring mopar. You will be surprised at just how stiff a shock it takes to control the seperation

My mate has mono's -c-tracs in the upper hole on his RR, motor has 720ftlbs on the hit and is controlled by just caltrac 9-way shocks on 7 stiff, good DA shocks are important yes, wait till you get those mono springs onto see the difference, here's a vid of the car on a 10.35 hit, been a best 1.421 60ft.@3800lbs>

https://www.youtube.com/watch?v=9tnu9cSnf2U

so you are saying that the front spring mounting position does not matter in reducing or incresing the hit to the tire? Only a GOOD shock will control it?

Don't make the rear suspension less aggressive. Buy better shocks and control it.

Talk to AFCO or Randy at Fastshocks. You will be flabbergasted at what good shocks will do for your car.

And work on the front end. Skinny T-bars, free it all up at the control arms and strut bushings, good shocks there as well.

Stock Eliminator people have been working on this stuff forever. Copy them

Do all that, then start tuning.

Instant center location determines housing movement direction. The higher or further rearward the instant center, the more it will seperate. Housing seperation adds hit to the tire. The shock controls the speed at which the housing moves.
Doug

This is helpful. thanks.

lowering the IC or lowering the front of the leaf spring might reduce the seperation, but surely not enough that a great shock isnt needed. You will find its all about controling the movement of the rear housing.

YES you are close to maxxed out on the rebound adjustment.....this is what im talking about. YES the caltracs are great, but they wont work on their own. Whatever suspension you use, you still need a good shock.
What shocks do you have?? If they are a good, name brand, its possible they could be re valved to allow more stiffer rebound, thats control you need...

ok that makes sense, thanks. I'm using QA1 stocker star double adj. I looked into santhuff shocks....wow are they expensive!
Does a mid 10 sec car really need a shock like santhuffs? what are other good shock choices?

Viking make a good affordable shock .Thats what i have on the rear .

Tex

Get them now if you can, you won't have to buy anything else later when you run out of adjustment or struggle with the cheaper shocks unless you go to struts or a 4 link (DAMHIK)
I'm only running mid 9's but they made a world of difference on mine. We went 1.41 on the rears with a 3.73 gear @ 4000 lbs + on our last pass of the season.
We previously ran Menscer valved Afco's, the Santhuffs are a vastly superior piece.

If your shocks are in the stock location you might try moving them so they can be more straight up. My car runs 10.40s but it hits the tire real hard and my DA vikings are maxed out. I am going to stand them up before I upgrade.

This worked well for us. We built an upper bracket to move the top of the shocks closer to the frame rail and get them almost vertical. Even the Calvert shocks needed/liked this and it was easy to do.

The GTX in the picture went 1.31 60 ft on Calvert bars, mono springs and 9 way single adjustable at 3550LBS. I switched over to ladder bars and coil overs. They did improve consistency, but the car has only equaled the the same 60ft times since the change.
Always mounted my bar in the lowest hole with little to no preload.

I just watched the video, the car looks very well controlled on the launch. That is what I'm after.

Have you looked into switching to ladder bars with coil overs? If not I think you should
The reason I'm suggesting this is I use to race against John Calvert in NHRA stock when he was developing the Cal Tracks, he would have used ladder bars if they had been legal, I would have also
Look into a set of 36 inch long ladder bars and real good set of double adjustable coil over with 6.0 inches plus travel and go have fun getting your back and neck hurt from the G force on the leave
AKA, a 1.42 60 ft. times can hurt you if you're leaning forward on the leave, especially if you're a big guy that doesn't work out your upper body frequently
A 1.23 60 ft. time can and will make you grin a bunch

60 ft is also a function of power to the tire. Be it gear ratio, converter, or plain old engine power. If the chassis doesn't have excessive movement and the car hooks consistantly. The 60 ft is about what it is. Some cars need chassis seperation and pitch rotation to hook. Some don't. A lot of variables in the equation. What kind of track prep? weight distribution? A 10.50 car will generally be in the mid 1.4x 60ft range. I doubt you need Santuffs in your ET range. Are they good? Heck ya. But they're pricey. I personally run Afco's. Resonably priced. With proper valving they work well. I can't comment on Vikings as I haven't used them. Many seem to be happy with them.
Doug

I will second the info on the cal trac setups mentioned. But jut to stimulate some thinking, the main force that is planting the tire is through the front spring eye, the cal trac s keep the spring from deforming so it transfers the power. But some energy is also applied by the back of the spring pulling down on the car. So don't overlook the need for proper shackles and sufficient travel for the spring to go through the arc of motion as the body separates.

Do you mean body separation like this? Sorry about sideways pic.

Contact Ron Galbreath at Afterworks , he sells Vikings or possible he can re valve your current shocks .
Yes , top shelf parts are expensive . No way around it . I had santuffs front and rear , and just upgraded , was it expensive? Yes it was ….it really comes down to how serious you are , and what you are trying to do . Budget stuff can work , but it only gets you so far ,

The guys here are correct in all they say but the vid is clear, the RR has 20yr old 90/10's up front, barely 5 " of front travel, it runs those 10x29's at 17psi and has 2 flats preload on the bars to be precise. Dana 60 with 4.10's and a low first gear in the 727 with a 4200-ish flash stall f/brake and just 685fwhp on track. I think it could do better personally.

The GTX in the other post runs the same 9-ways and manages to 60 much faster albeit a bit lighter and probably is a mid 9 sec car with around 800fwhp at that weight.

Fit the mono caltrac system and keep your DA shocks and try it, you don't need to pay the earth for Santhuffs at that level, most buy the santhuff fronts so they can land from a big wheelie like a plane.

Here a couple of videos on Dizusters car with Cal Tracks. This 1st was early in the Savoy's development. The 2nd going mid nines with 1.37 60ft. Also my friend Mikes 64 Dodge. The Dodge has since been upgraded with cal tracks and Strange double adjustables. It now runs In the high nines with 60 fts in the 1.37 range.

https://www.youtube.com/watch?v=sjZDXBXRC9Q

https://www.youtube.com/watch?v=_Ynn9f3Bud4

https://www.youtube.com/watch?v=jr1VILyS7WE

Doug

Cal-tracs, Calvert shocks, CE 3 way adjustable fronts and otherwise stock suspension with front snubbers removed and slant 6 bars. 3.54 gears and 11" converter. Vikings on the shelf to replace these but trying to push into the 9's the way it is.

Was 1.37 60 ft. in this pic and has been 1.36.

Afterworks revalved my front Vikings.

Thanks for the videos.
My car did the same thing as the 64 dodge till I put the da shocks on the car . That helped a bunch but it is stlill too aggressive, there is a lot of wasted motion on the the launch. and it is still wadding up the tire. I'm going to mount the leaf spring in the lower hole position and start there. then look for a better set of shocks.

yes like that.

If you look real close you can see the the leaf spring parallel with the ground.

I did the straight up shock mod along with the 25" S-10 front spring on my Dart and it does work pretty well on my junk. 1.33 foot brakin so far and I have a new 8" 5800 vert to replace my 30 year old piece which should wake up my tunnel ram even more...

I don't know if this will help you any but here is a cheat sheet I got from John Calvert a long time ago.

Top Hole – Hardest Hit (start here)

Bottom Hole – Softer Hit, moves IC forward

More Pre-load, Softer Hit

Less Pre-load, air gap, harder hit

If car pulls to the right add a quarter turn of pre-load to the right bar and visa versa.

Set front shocks soft

Set rear shocks on #5, if car wants to porpoise go to 6 or 7 (harder).

If car spins, needs harder hit, go softer

Too much wheelstand…. Softer hit, restrict front travel

Draw an line from contact patch through the instant center. No caltracs the instant center is the front spring eye. Now draw a line parallel to the ground through the center of gravity of the car. Where the first line intersects the second line determines the % of rise. Farther to the rear, it hits the tire harder and tries to separate more. Farther forward it’s softer on the tire and lifts the nose more. Lowering the front spring eye, moves the intersection forward. Changing to a taller tire, moves the intersection back. Caltracs, are almost like a open ended, tune-able small ladder bar. Moves the instant center forward. I think your line for instant center would go from contact patch through the front bar eye. It could actually be farther forward of that.

https://www.youtube.com/kevinwilsonsbc

[quote=FurryStump]Draw an line from contact patch through the instant center. No caltracs the instant center is the front spring eye. Now draw a line parallel to the ground through the center of gravity of the car. Where the first line intersects the second line determines the % of rise. Farther to the rear, it hits the tire harder and tries to separate more. Farther forward it’s softer on the tire and lifts the nose more. Lowering the front spring eye, moves the intersection forward. Changing to a taller tire, moves the intersection back. Caltracs, are almost like a open ended, tune-able small ladder bar. Moves the instant center forward. I think your line for instant center would go from contact patch through the front bar eye. It could actually be farther forward of that. [/quo

so if i'm understanding you correctly , the cal-trac bar now determins the instant center not so much the front spring mounting point. That will move the ic significantly foward, which would explain the cal-trac claim of reducing /controlling seperation..
Thanks for the input , I'm attempting to gather info so as to understand what it is that I am doing with the suspension rather than just install parts and hope for the best .

Not sure if this is correct, Caltracs stop spring wrap. The IC is above Neutral line = more anti-squat (or more separation), The higher hole hits the tyre harder = more rise. At least this is the case in my experience.