Help with 4 link

[nikwho]

Hey guys,
I am in the preliminary stages of designing my rear suspension. I am leaning toward a triangulated 4 link. My intentions for the truck is to be a weekend warrior that I can take to the occasional autocross event for fun. Will be driven 99% cruising on the weekends, realistically, but would like to participate in autocross for fun. The truck is getting a moderately built 454ci/TH400/9" rear end. I intend to clearance the firewall to move the BBC back somewhere in the 6-10". I intend to have the truck set up for coil-overs, as well as bags. So, when just cruising, I can yank the coil springs and run on the air bags. I intend on running a 275-295 series rear tire and 255-275 front.

I have just started plugging some preliminary numbers into the calculator. Might be a good starting point for some critique.

Here is the information that I have:
With a 275/35-20 tire (27.6", 13.8" tire rolling radius)
Wheelbase 116"

These are just guesses for these fields:
Vehicle mass: 3,500 lbs.
Vehicle CG height: 18"
Unsprung mass F:300 lbs. R:400 lbs.

Suspension Geometry:
Upper Links x y z
Frame End 28" 16" 13.8"
Axle End 1" 8" 15.8"

Lower Links x y z
Frame End 28" 19" 8.8"
Axle End 1" 19" 7.8"

Geometry summary:
Static Anti-Squat 90%
Travel Anti-Squat: -1%
Roll Center Height 19"
Travel Roll Center: 20.15
Roll Axis Angle 2 degrees (Roll Oversteer)
Travel Roll Axis: -12.78°
Instant Center X-Axis 73"
Pinion Change: No value
Instant Center Z-Axis 10" (is this too high for good weight transfer)
Travel Amount: 5.00"
Travel Increment: 0.25

Planned on a +2.5 degree pinon angle at ride height.

With regard to instant center, I have read that the distance of the instant center from the rear axle center line (x axis) should be the percentage of the wheelbase that would co-inside with the percentage of the vehicles total weight that sat over the front wheels. I also read that instant center should be approximately 5-7" off of the ground (z axis).

So, I am estimating that my '53 that will be sitting on a '94 S10 with BBC/TH400/9" will weigh in at about 3,500 lbs. I am estimating that my weight distribution will be approximately RF-1000 lbs., LF-1000lbs., RR-750lbs., LR-750lbs. So, this puts 57% of my total mass over my front wheels. So 57% of my 116" wheelbase is roughly 66 1/4". So, by this math, my instant center z axis should be 5-7" off of the ground, with x axis to be 66.25" forward of the rear axle center line. My truck will sit low so I should probably stick closer to the 5" mark for the z axis.

-OR-
I could just build an equal length, parallel 4 bar system with a Watts link and call it a day.

I am just trying to kick this around, but I think that the first thing that needs to be accomplished is determining what type of setup to build. I have also heard about an inverted triangulated 4 link setup that offered great handling characteristics, but can't recall the name.

I may be putting my cart before my horse with regard to running numbers on a triangulated 4 link, but I am open to suggestions.

Thanks so much for the help guys!

Nik

[nikwho]

Here is a picture of the calculator:

[nikwho]

And bump geometry, assuming a completely random 4" of up travel:

[CHRIS67GMC]

Your weight is going to be closer to 4000#s. I would not run a parallel 4 bar build a more traditional 4-link so you can adjust it.

You can autocross just fine with bags ( trust me I know ) its all about the shocks.


This is my set up No-limit 4-bar on shock waves.