Six Steps for a King's Ride

The Inside Story of C4's Shock Absorber Revolution, Selective Ride Control

by Hib Halverson

The most valuable ride-and-handling option available on 1989-1995 Corvettes is RPO FX3 "Electronic Selective Ride and Handling". Known by many Vettesters as “adjustable suspension" or by its RPO number "FX3", GM engineers called it “Selective Ride Control”, "Selective Ride" or just “SRC”.

 Selective Ride's core is four, Bilstein, electrically-operated, ride-adaptive shock absorbers–ok–real engineers call them "dampers". "Ride-adaptive" means the shocks adapt their damping to changing vehicle dynamics while the car moves. Along with the dampers, SRC has an on-board computer to control them, a center console selector switch and the wiring harness that links all this hardware.

Set the Way-Back Machine for 1985

In the early-'80s, Bilstein designed this system for the $230,000 (today, about 460 grand), limited-production, Porsche 959. Clearly, Bilstein wouldn't profit much nor even recoup its development costs on such a niche-market vehicle.

Bilstein also supplied the fixed-valve, high-pressure, gas-filled dampers introduced on C4 for 1985. About the time the first 959s hit the road in Europe, Chevy was working on a 1989 project called "King of the Hill Corvette". The Bow-Tie guys wanted a technologically-advanced suspension  to go with a four-cam V8 and a six-speed they were putting in this up-level C4 which was eventually branded "ZR-1".

According the Dave McLellan, Corvette Chief Engineer from 1975 to 1992, the reason he envisioned SRC for the ZR-1 was simple: it would enlarge the car's performance envelope with improved handling in combination with better ride. The only way that could happen was if the dampers were ride-adaptive and controlled by some vehicle dynamics parameter such as speed.

Scott Allman, Lead Engineer for Corvette's ride-and-handling development from late 1987 to 1994, told us in an interview, "At low speeds with the Gatorbacks (Goodyear's first generation, ultra performance radial tire on '84-'91 Vettes), C4's rode like trucks. With Z51 and Z52, we were getting a lot of complaints about ride harshness. We were doing everything we could to take control authority out of the shock absorbers to improve the low-speed, rough road, ride and handling. When you do that, but then go autocrossing or road racing; the car is all over the road. It's difficult (with fixed-valve shocks) to get enough control authority to have proper damping at high speed without negatively impacting the low-speed ride and handling.

"Bilstein had talked to us about this variable damping with six steps," Allman continued. "The idea was at low-speed, we could have a lot less damping than we needed at high-speed."

The rest, as they say, is history. The Chevrolet/Bilstein partnership repackaged the damper hardware used on the 95, recalibrated the controller software and developed the system for C4. When ZR-1 slipped a year, SRC was introduced as a Corvette option for model year 1989. The King-of-the-Hill finally arrived a year later with Selective Ride as standard equipment. Between MY89 and '95, 35,614 C4s, including all 6939 ZR-1s, were built with SRC.

 Flimflam it's Not.

Not marketing-driven gimmickry, Selective Ride was a technical milestone for a high-volume car in the late-'80/early-’90s. Its chief advantage, a computer-controlled, wide damping bandwidth, made any C4 so-equipped ride nicer in normal driving situations and handle better, when driven hard, than Corvettes with standard dampers and even other cars with the non-computer controlled, bi-state systems of the period.

SRC shocks evolved from existing Bilstein high-pressure, nitrogen-filled, damper architecture. Adjustably was accomplished by adding a variable bypass which, as the shock piston moves up (rebound) or down (compression), allows shock oil to bypass the piston's valve stack. The more oil bypassed, the less suspension movement is damped. The less oil bypassed, the more aggressive damping becomes.

This bypass is a sleeve valve located at the bottom of the shock's piston rod. The tubular piston rod has a pair of holes in its wall, 180° apart. Concentric, inside that rod, is the valve shaft with a passage which runs vertically from its open end, up an inch or so, then T's to openings in the shaft wall which align with the holes in the piston rod.

As the valve shaft rotates, the valve opening

 varies in size. The shaft is splined at the top and engages an “actuator,” a bidirectional, DC motor with an integral, Hall-effect position sensor, which attaches to the top of the shock. The actuators are controlled by the SRC Module, or "controller", located in the well behind the driver seat. They rotate the bypass valves to one of a possible six discrete positions giving SRC up to six “steps” of damping. The position sensor feeds valve setting data back to controller.

ZR-1s had Selective Ride in two versions : '90-'91, "early", and '92-'95, "late". The late system has: 1) significantly less bypass (82° vs. 162°) travel. This quickened response because, while actuator speed was unchanged, the valves move half the distance, so they switch steps twice as fast, 2) two modes have less damping steps. Sport was cut to five steps and Performance was cut to four, 3) a revised controller, compatible with the changes in travel and steps, 4) a secondary valve stack in the bypass circuit which improves damping in the lower steps and 5) damping which is more aggressive overall because of lower spring rates for 1992.

There's even a bit of a mystery surrounding the

 early system. In '89, Selective Ride (FX3) came only in combination with the Performance Handling Package (Z51), but when you ordered those two, in a strange twist, the car was downgraded to softer, base  springs, stabilizer bars and front lower control arm bushings. In 1990, FX3 no longer required Z51 but, if you ordered both, again, you got base springs, bars and bushings. Unfortunately, this downgrade was not widely known. Many unsuspecting '89/'90 customers, wanting the stiffer springs and bars, ordered Z51 along with FX3 but never knew their cars had base parts. While there were 83 Corvette Challenge or World Challenge race cars built during those two model years with FX3 and "real" Z51 (a combination known to some as "R9G") they were not available to the general public. It was not until 1991 when a combination of SRC and the stiffer springs and bars was released. That was RPO Z07, available through 1995.

We had to dig to find why this quirk existed. Scott Allman remembered FX3 and Z51 forcing base springs and bars but couldn't remember why. We contacted famed Corvette road racer and retired GM engineer, John Heinricy, who, starting in 1989, spent a few years as Corvette Development Manager. John told us that SRC was developed in lockstep with the ZR-1 which used spring and stabilizer bar rates of the base C4. Developing and validating a system as complex as FX3 took time. GM's resources were limited and prioritized, so work focused on the needs of the ZR-1, first.  Heinricy told us that, since validating SRC for the base suspension came first, it took a while to do the same with the stiffer springs and bars so, in the interim, if you ordered both, you got base springs and bars. Once SRC was on the road in '89 and the ZR-1 in 1990, Corvette Development validated Selective Ride in combination with the more aggressive parts.

It Works Like This

 Whenever the ignition is turned on (key-up), the SRC controller tests itself, commands the actuators to 0°, then sets them at a starting point determined by system series, controller calibration and switch position. The bypass valves remain at this starting position until the SRC controller receives a vehicle speed sensor (VSS) signal.

Bypass Starting Points

Once Selective Ride sees VSS data, it begins setting the shocks according to vehicle speed, driver input via the selector switch and the controller calibration. The faster the car goes, the more aggressive the shock valving becomes. The switch skews valve positioning up or down the speed range. For example, with the late system, in the "Tour" mode, at 75 mph; the controller commands Step 2 at 15° valve rotation–pretty soft damping–but in "Sport", at 75, you get Step 4 at 54°–more aggressive damping.

Tour is pretty mild damping in all six steps. In fact, Tour 1, enabled below 50-mph, is the only time the bypass is full open. The Sport range is good for (well...duh) sporting street drivers. The "Performance" or "Perf" range (originally called "Competition" until some political correctness weenie ordered a name change) is for the most aggressive driving mainly on race tracks. The lowest step in Perf sets the shocks at 54,° applying more damping authority than any Tour step and the same as Sport 4. Performance 4 enables at 75-mph and is the only time the bypass closes.

"We spent a lot of time," Scott Allman recalls, "tuning the Tour position to be the most comfortable then, basically, made the Performance position rock hard for the race track or autocross. The Sport position–the middle position-offered improvements in body control (over tour) but unless you were on a road with body input–1-hz or 2-hz–you weren't going to distinguish much. Sport snugged the car down enough, particularly out west, at highway speeds, so you didn't have to go to Performance and its impact harshness."

If you want to feel differences in SRC damping, run 60-70 mph on a concrete highway that is “tilt-slabbed” or has closely-spaced tar strips and causes a cyclical wheel movement GM development engineers call “freeway hop.” Move the switch amongst the three settings and you'll feel damping change as the controller moves the bypass valves.

According to Scott Allman, there was a "3rd generation" Selective Ride under development. "SRC 3" would have used "high-powered actuators" to speed bypass valve movement such that response time improved to about 30-milliseconds allowing the system to act in near real-time. Additionally, accelerometers allowed that system to adjust damping according to body motion rather than just vehicle speed. When GM decided to go to a different system being developed by Delphi for 1996, (RPO F45 "Electronic Selective Real Time Damping"  or "RTD"), SRC work ceased. For Corvette buyers, that was a great loss as the '96 RTD had less bandwidth, was bi-state (only two levels of damping) and was an inferior system to SRC in-spite of it being near-10-year-old technology.

When Things Goes Wrong

Selective Ride Control has on-board diagnostics. If you key-up and the self-test fails, one or more shocks doesn't initialize, a bypass valve doesn't move quickly enough or, once you're moving, there is no VSS input; a two-digit, diagnostic trouble code (DTC) sets in the controller memory, the "Service Ride Control" light comes on and the shocks are commanded to 60° (MY90-91) or 30° (MY92-95). If the problem is a specific shock, its actuator is turned off. At any time after key-up and self-test, if a bypass valves doesn't move quickly enough, a problem with the controller occurs or the selector switch fails; a code will set. At any time, if system voltage goes below 10.0 or above 16.0 volts, no codes set and the service light says off, but no actuator movement occurs until voltage normalizes.

To display DTCs: turn the key on but do not start the engine, ground diagnostic link connector (DLC) terminal C of the '89-'93, 12-pin connector, or  terminal 3 of the '94/'95, 16-pin connector, then observe the flashing Service Ride light.

Each digit is signified by half-second-apart flashes. A second separates the two digits. Each code is flashed three times. Each code sequence is bracketed by code 12, signifying its start and finish. Say DTC13, a left-rear, time-out error, is set. Turn the key on, ground the diagnostic pin and the service light flashes once, then twice (12), three times in succession, each separated by a second. Then, it  flashes once, then three times (13) and does that three times. Finally, it flashes once, then twice, three times in succession. Numerically, it's like this: 12-12-12-13-13-13-12-12-12. To clear codes, ground the SRC diagnostic pin for two seconds, three times in succession.

To make flash-code and diagnostics work, you've got to jumper across DLC pins. In ZR-1s, it's tough to see the diagnostic connector well enough to count pins. Corvette Central has the solution: grounding plugs which snap onto the DLC. We've used a set of these plugs for '94-'95 (PN 304800) and they are a great idea. You simply push one on the DLC, then key-up–way easier than fussing with jumper wires. A set of them (PN 604259) is, also, available for '90-'93, 12-pin DLCs.

A Bosch TECH 1, TECH 1A , TECH 2 or Mastertech scan tester equipped with brake and chassis software for the model year in question (along with, in some cases, a DLC adapter) can, also, command the controller to display DTCs and to clear codes.

There are three simple electronics problems SRC may experience. DTC23 (loss of VSS signal) sets if you key-up three or more times without moving the car. If you have a 23 for that reason, as soon as move, the light will go out. If the selector switch is in-between positions, DTC42 will set. Properly adjusting the switch will turn the service light off, if 42 set because of switch position. In either of these situations, you still need to clear codes to erase any history code retained in memory which might confuse future diagnostic sessions.

It's common with recently-purchased, used C4s equipped with SRC to find the system not working and the service light not on. Because the cost of repairs can be high, a common, but idiotic solution is to remove the service light bulb which gives the driver the impression no problem exists. The service light should flash once after key-up as proof the system and its diagnostics are working. If the service light does not momentarily flash, check the bulb.

Some damper mechanical problems can be: 1) leakage–the durability of the original, concentric shaft sealing system has been inconsistent. If it fails, gas pressure forces shock oil through the gap between the piston rod and the valve shaft at the top of the shock. In the front, oil appears at the bottom of the actuator and leaks down, over the upper shock insulator and onto the shock mounting bracket. In the rear, it leaks from between the frame rail and the shock mounting plate. Leakage at those places means the shock needs to be either rebuilt or replaced. This type of leakage should not be confused with “dampness” which is acceptable on the piston rod shafts below the upper shock mounts. 2) The front, upper shock insulators can be softened by some oils or solvents and will "squish" out from beneath the retainer. If you find that problem, replace the insulators. 3) If you drive on a surface which causes "freeway hop" or has large tar strips and you move the SRC selector but feel no change in damping, check for either an inoperative service light bulb or for worn or damaged shocks.

An urban legend about Bilstein dampers is that manually compressing them is a good test of their condition. Not. Short of putting them on a shock dyno, the only way  compression is a useful test involves not how they feel, but how far the piston rod will compress before the piston rod bottoms in the shock. Bilstein uses a German word "austrait" for this and if the shock's austrait is about a quarter inch, the shock is probably ok but if it's half an inch or more, the shock has probably experienced an internal failure. An even better test is to compare the austrait of two of the same shocks. If they are markedly different, one or both of the shocks are bad.

Additional SRC service information is too lengthy to reproduce here. If you're going to do Selective Ride repairs yourself, buy a Corvette factory Service Manual from one of the Corvette parts vendors, such as Corvette Central, and use its diagnostic data to affect repairs. The Manual, also, has specifics on installing the actuators on the shocks and the shocks to the vehicle. Failure to follow proper procedure may result in damage to the actuators and/or the shocks. Aftermarket manuals, such as those from Chilton or Haynes, do not cover SRC, other than having a few sentences saying that the system is too complex and only dealers can service it which, of course, is a distortion of fact.

Bottom line: get the factory book and, in many cases, you can fix SRC yourself.

Service, Parts and Upgrades

 Replacement shocks are available from Corvette parts specialists such as Corvette Central, Tom Henry Racing (THR) or Vette Brakes and Products. For several years, actuators were discontinued, but in 2002, Bilstein had 2000 of them made. THR sells them as does Bilstein. At this writing, in December of 2008, all the shock mounts or "insulators" have been discontinued, but polyurethane replacements for SRC shock insulators are available from Tom Henry Racing, Corvette Central and Doug Rippie Motorsports.

Both the stock controllers ('90-'91, PN 22115156 and '92-'95 PN 22151010) are discontinued GM Parts, however, at this writing, there are a few dealers which still stock the early unit. Those needing a '92-'95 controller should inquire at aftermarket Corvette parts specialists, such as THR or Corvette Central, used parts vendors, Corvette salvage yards or they'll have to by a "Motorsports Controller" and a stock '92-'95 calibration (or "cal") from THR. This controller has a plug-in EPROM rather than the OE controller's soldered-in EPROM.

The early-to-late conversion allows use of the improved, '92-'95 shocks on an early SRC car. You either convert a set of early shocks to the late configuration or you replace them with late units. This conversion, also, requires the Motorsports Controller and a stock '92-'95 cal.

In addition to O.E. cals, Bilstein offers several, all-out racing calibrations for the motorsports controller intended for drag racing, autocrossing and road racing. The use of these will not alter damping beyond what’s available from the shocks. These cals only change how the controller reacts to vehicle speed and the selector switch when it sets the bypass. Motorsports cals degrade ride quality as all disable the system's lower damping ranges and some do not vary the bypass at all so, generally, these calibrations are race-only.

Bilstein offers shock rebuilding which can be less expensive than replacement. Options during the rebuilding are early-to-late conversion and revalving. One or all of these services are available for $145 per shock.

Bilstein covers shocks which are rebuilt, converted and/or revalved with only a 90-day warranty, however, aftermarket replacement shocks are warranted for life. As new, aftermarket, SRC Bilsteins are only about 225 bucks, some people who don't want revalving but need new shocks or want the late shocks pay the extra 55 bucks to get a lifetime warranty.

If you are an aggressive street driver, autocrosser or road racer, have Bilstein revalve your shocks. Several valving configurations are available, including one called "Moraca Valving" which is outstanding for dual-purpose, street/track cars. One disclaimer: if someone buys an aftermarket shock from Bilstein and immediately orders it revalved, the warranty shortens to 90 days so, in that situation, you're better to rebuild and revalve an existing set of shocks rather than buying new.

With street/track valving, damping is more aggressive overall with high bypass valving a little more aggressive and low bypass valving significantly more aggressive. Additionally, the rear shocks get an increase in rebound damping authority which reduces the ZR-1's tendency towards drop-throttle oversteer, a problem caused by a combination of: high lateral acceleration, sudden forward weight transfer, the ZR-1's 200 extra lbs. on the front end, soft spring rates and inadequate rebound damping. The approximate 50% increase in rebound more effectively damps weight transfer and makes the car less susceptible to drop-throttle, oversteer.

Road Testing Rebuilt/Revalved SRC Shocks

Several years ago, we found both front shocks

 leaking on our 1995 ZR-1. As the car had less then 50,000 miles on it, no doubt this was caused by the valve seal durability issue discussed earlier. We contacted Bilstein and they told us that rebuilding would include new piston rod assemblies having a revised seal package. Since we drive this ZR-1 hard on the street, they also suggested revalving so we ordered Bilstein's Moraca valving.

Occasionally a rebuilt shock, when first installed, will set a time-out code. This is caused by stiffness of the new bypass valve shaft seals. It takes is a little time to loosen up the seals such that actuator movement quickens to within the range the SRC controller required.

The effect revalving the shocks had on the car's handling was dramatic. In the lower valving ranges, the wheels more aggressively damped and, as a result, the "floating" one often feels from the OE valving's weak rebound control was gone. Once in a while, this car gets autocrossed and it often gets driven hard over windy mountain roads. That the upper SRC ranges are noticeably stiffer, offering better control during aggressive maneuvering, was a welcome improvement. This stiffness was especially noticeable (and desirable!) at high speed in road racing "track day" events.

After testing the car in both autocross and road race environments, our opinion is: the revalving definitely improves the car’s feel and response in transient maneuvers. On a ZR-1, the change in rear rebound, alone, is worth the cost of revalving. 

We’ve run the car about 30,000 street miles since the Moraca revalving and our opinion of its ride is positive–but with qualification. The more aggressive, street/track valving degrades ride quality. Even with the SRC selector switch in “Tour”, ride is more firm than was the stock valving. While we, here at the CAC, like this car's ride on the street, other people who think they need motorsports-derived shock valving but know they don't want a firm ride over tilt-slabbed highways, potholed roads, harshness crossing tar strips and so forth, should stick with the stock valving.

Now that you've read the disclaimer, know that those who race or who regularly drive hard on the street will benefit greatly from revalving. In fact, we've been wondering what took us so long to do this to the car. Where will our ZR-1 project go from here? Well...now, we want to upgrade the car's springs, stabilizer bars and front lower control arms to Z07 level.

And Finally

"It was always my favorite system," Scott Allman said about Selective Ride. "We didn't feel we need to go to these complex real-time damping systems and could keep it to basically a 'state device' and meet all the performance requirements.

"Roads don't change fast enough in their mean excitation that you need real time damping. I guess you could justify it on, say a Cadillac, if your trying to get that last iota of ride comfort, but if you were doin' the Corvette, you're looking for a level of control authority which is a step above your standard sedan and you don't need to have a real-time variation to get the handling effect you're looking for."

 Admittedly, Allman is somewhat biased because SRC was, for the most part, his baby. After driving C5s and C6es with ride-adaptive shocks, we're not sure we fully agree with Scott's view of Corvette not needing more advanced systems. There is a large block of Corvette buyers wanting the car for its image, not for its performance and those folks demand a soft ride.

Conversely, we share Scott Allman's view that, from an at-limit handling perspective, some of the systems which came after SRC, namely "Selective Real Time Damping" or "RTD" (both the bi-state, 1996 version, and the continuously-variable, 1997-'02 version) offered less damping bandwidth than Selective Ride. This made them a poor value to Corvette customers interested in the best handling performance for his or her dollar.

It was not until 2003, when GM introduced RPO F55, "Magnetic Selective Ride Control" ("Magnaride" or "MR") did Corvette get an uplevel suspension damping system that exceeded Selective Ride Control's bandwidth in both ride and handling.

The ZR-1 Net would like to thank Dave McLellan, retired Corvette Chief Engineer; John Heinricy, retired GM Director of High-Performance Vehicle Operations and Scott Allman, Principal Engineer, Harley-Davidson Vehicle Dynamics Group and Bilstein of America for their support during the research for this article. This story was adapted from an earlier piece published during 2006 in Corvette Enthusiast magazine. The ZR-1 Net is grateful to CE Editor, Andy Bolig, for making it possible for us to post our version of this article.

Sources