Home | Mailing List | Specifications | Care and Feeding | Modifications | Vendors | Literature

Active Suspension Theories

Thanks to Jim Heaton

One of the neat upgrades Ford made to the SHO when redesigning for the Generation III V-8 is the Semi-Active Ride Controller, (SARC). This system allows each of the 4 wheels to have 2 different suspension settings: soft, like a luxury car, and hard – firmer than most sports cars. Each corner of the car can be in either mode and acts independently. 1996 and 1997 cars have the full system. 1998 SHOs have 4 semi-active struts, but wheel sensors only on the front tires, presumably with an algorithm tied into the ABS speed sensors to extrapolate rear wheel inputs as a time delayed function of the same-side front wheel.

1999 SHOs have a more traditional suspension much like the original SHOs, but still benefiting from the much more rigid chassis, ZF rack & pinion and other enhancements common to all Gen III SHOs. In particular, the 1999s will be by far the easiest to modify using traditional spring/strut upgrades.

But the SARC SHOs have the most sophisticated suspensions even made for a Taurus. Very subtly, the car virtually eliminates dive on heavy breaking, drastically reduces squat on acceleration and in my experience totally eliminates pogo-sticking in every situation. In one way, it makes the car actually SEEM slower, less exciting. In reality it makes it more capable as a performance machine. To see what I mean, disable the SARC and do some sudden breaking and acceleration from a stop. While the car feels more boatish, it also feels more powerful. Changing the handling characteristics of the car is strictly at your own risk.

Remember driving a boat sedan with a big block V-8 down a country road? Remember finding those gentle rises and dips and getting a light wheel on the rebound (maybe even a little air). Not gonna happen with SARC on the job. It's happy to leave you on soft mode as you compress the suspension quickly yet softly down, but approach the end of that stroke as the car would start to pogo up, the controller snaps to firm and you lift very slowly - with better tire-to-road contact than either the soft or firm suspension alone would give you - and a better ride to boot. This car was DESIGNED to go fast and keep control at 100+ all day long (IMHO).

Here are my best guesses at how the SARC system works based on observations of front solenoid voltage on my car, a test drive in Tim Wright’s ’97 SHO with 17" rims and 45 series tires, and empirical evidence from dad’s 96 SHO with stock rims and larger, softer tires than stock.

The SARC output that energizes the shock solenoid has the following behavior: when energized it is full soft, when no power, full hard.

Here's the basic SARC algorithm:

Default is soft mode.  If we flip to firm mode, we do it full tilt for either x or 3-4x time units. No in between - either soft or firm.  x=.5 seconds (very subjectively)  In firm mode, an event can reset the timer for an additional x or 3-4x time units of firm mode.  The time to go into firm mode is very short. To a human it is 0 delay (miliseconds).  The threshold to determine whether an event switches the system to soft mode seems to vary linearly or more rapidly (geometrically?) with speed. Faster is easier to trigger firm mode. At very low speed it takes a quite large event to leave soft mode - very sharp turn or blipping the throttle up & down in manually forced 1st gear. Blipping in Drive/OD will not do it except from a standing start (special case?). This speed-sensitive behavior could also be purely mechanical and not software.

Events that will trigger firm mode.

Hard accel or decel (quite hard, typically).
Hard turn.
Road discontinuity.
Certain gentle cyclical smooth up & down road surfaces, (Though I can't *tell* it's in firm mode except by
the volt meter).

Firm mode is x long if the event is a road "feature"  Firm mode is 3-4X long if it is initiated by a driver action (turn, brake accel/decel).  If both road & driver events, longer mode is used.  If suspension travel is approaching the mechanical limit, switch to firm mode.  If suspension acceleration is very rapid such that an extrapolation of the suspension motion would approach the mechanical limit, switch to firm mode.

Theories: I believe my initial hypothesis about being sort of conditionally stable is correct. On rough roads with flaws less than .5 second a part the system will go into firm mode with the stock tires & rims for many seconds at a time. With stiffer and/or thinner tire sidewalls, I believe this would happen more often and on less flawed roads. Note the system will still go into smooth mode, and probably be there most of the time. But over harsh surfaces, it will go firm and really amplify bumps just like a very firm track setup would.

Even on a fairly poor road, if the flaws are not too large the system spends most of its time in soft mode. With less than Z rated tires and/or thicker sidewalls, it would take a pretty big event to make it go firm. It could easily get to the point that it feels a little floaty. Not bad, but probably only a touch firmer than a SLO with 15" rims and 60 or 65 series tires.

I can't yet predict which mid-range severity events will trigger firm mode and which won't. Small and large events are easy. Mid sized events will seem to trigger the system semi-randomly from a ride-quality point of view. I bet with a storage o-scope hooked up it would be easy to pick out which type trigger by their shape on the height sensor.

I'll get to try it out on the highway tomorrow. It's a pain to try to read the meter and drive. I had the kids reading "high high high low low high high" out loud to me as a drove. Might be a good AWA function!!

P.S. I take back what I said. The system is quite good, but I think given the existing black box to copy I could code it. The outputs are a few orders of magnitude simpler than I expected. You could write some pseudo random inputs for the wheel sensors and track the output for the solenoids and store which types of events trigger the x or 4x output and reverse engineer it pretty effectively, then start tweaking from there.

As has been mentioned, you'd never recover the engineering costs over the small market, but it would be easier than, say, cracking the encryption on the Gen III/Cobra engine computer. Probably you could just grab a junkyard SARC from a crashed SHO and send it to of those CA reverse engineering SHOps and they'd send you the exact program the thing uses and tell you what cross compiler you'd need to just rewrite the darn thing the right way. Where's my issue of Nuts & Volts....

I also think it would be interesting to either pre-process the inputs to change the behavior of the system – perhaps something as simple as small inductors on the position sensors to soften the ride for those with large rims. One could also post-process the outputs to the solenoids, damping out certain changes to firm mode – say when only 1 wheel is being switched (road seam), but pass through outputs when 2 or more fire (hard cornering, breaking, accelerating).


Contact Information