Suspension questions

[nicksta43]

A couple months ago I ran across the rules for the SCCA Classic American Muscle (CAM) class while poking around. The rules for the class are as follows;

Eligible Vehicles

Vehicle must be considered a “street legal” (lights, wipers, etc.), US-domestic automobile of front-engine/RWD configuration or a “pick-up” truck. Must be licensed and insured.
Vehicle must pass the mandatory safety inspection (tech) and be in compliance with Section 3, Vehicles, of the 2014 SCCA® National Solo® Rules.
Vehicles must weigh at least 3000 pounds.

Body

All body panels must be present in the original standard locations and may be modified or replaced.
EXCEPTION: High-Boys (1954 and earlier), Roadsters (1954 and earlier), and Trucks (1940 and earlier) are not required to have fenders or hood sides.
All glass must be present. Glass components may be replaced by Lexan®.
Interior must be finished and have minimum seating for two adults.
The fuel tank/cell must be separated from the driver/passenger compartment by a metal panel/bulkhead. The fuel tank/cell must not vent into the driver/passenger compartment.

Wheels and Tires

Any metallic wheels are allowed. Non-metallic wheels must be certified from an appropriate, recognized standards organization (e.g., SAE, SFI, FIA, TUV, etc.).
Only DOT-approved tires with a UTQG Treadwear Grade of 200 or higher are permitted.

Body Electrical System

Electrical components and wiring are unrestricted.

Brake System

Brake system and components are unrestricted.

Suspension and Steering

Suspension and steering components are unrestricted. Method of attachment is unrestricted.

CAM 4
Engine and Drive Train

Engine, drive train, and associated components (internal and external) are unrestricted.

Safety Inspection (Tech) Summary
Complete details in Section 3.3.3.B of the 2014 SCCA® National Solo® Rules (page 37)

Loose items removed
Hub caps (snap-on), trim rings, fender skirts removed
Seats secured
Camera(s) secured
Battery secured
Throttle return in good condition
Brakes in good condition
Tires in good condition
Wheel bearings, steering and suspension in good condition
Wheels and lug nuts/bolts secured and in good condition
Seat belts/harnesses secured and in good condition
Exhaust exit behind driver or to the side
Fluids no leaks
Roll bar (if equipped) compliant and at proper height
Helmet compliant and in good condition
Snell SA2010, SAH2010, SA2005, SA2000, M2010, M2005, M2000, K2010, K2005, K98
SFI 31.1, 41.1, 31.1A, 31.2A, 41.1A, 41.2A
BS6658-85 type A/FR
Flex fans not permitted
Alcohol or alcohol/water injection not permitted
Steering wheel “spinner knobs” not permitted
Nitrous bottles at the event (Section 1.3.2.M) not permitted

My immediate thought was a 73-87 GM truck. Mainly because I love them but also because it shouldn't be too hard to get it down to the 3000# minimum weight and those big wheel wells should hold big ol' meaty tires. Plus the proven simplicity of making decent power relatively cheaply with the SBC.

However I have really been wanting to build an SCCA Rallycross car for several years now. Then I got to thinking, I'll be doing this for fun anyway and given the broad rules for the CAM class It's unlikely that I'll ever be in the hunt for a national championship so why not build the truck to compete in both disciplines? Those same big wheel wells will still allow enough room for some aggressive off road tires even with some moderate lowering. Given that a pure Autox setup wouldn't be good in the dirt and a pure Rallyx setup would suck on the asphalt, I know I will have to make some compromises for each category.

So I've built this thing million times in my head and think I have a basic plan on how to change the setup from dirt to asphalt and vice verse. I have a basic understanding of suspension geometry and I think I can get most of the suspension figured out if not right of the bat then at least through trial and error.

Keeping it buttoned down tight like what is needed for an autox course will not work when on full attack bouncing through a field. Right now my plan is to go the old school Herb Adams school of thought with a reasonable spring rate and massive, possibly custom, sway bars with quick disconnects and adjustable shocks. That will give me a lot of roll resistance when I need at the auto cross course and by simply swapping out the wheels and tires, disconnecting the sway bars and adjusting the shocks still allow enough suspension compliance to run in the dirt.

The biggest question I have in my mind right now is how far to lower it. What I would like to know is how much suspension travel is left in the front with a 5" drop? Or to put it another way how low can I go while still retaining a decent amount of suspension travel?

I believe most of the venues I will be running are pretty flat fields so ground clearance isn't the biggest concern. Keeping the front suspension from constantly bottoming out is more important.