FOrt Collins, Colorado

C4 Corvette suspension tech

There are basically two "vintages" of C4 Corvette suspensions.  The "early" suspensions are '84-87 and the "late" ones '88-96.  They can be identified in a couple of different ways.  First, the early suspensions have aluminum splash guards covering the inboard side of the front and rear brake rotors, while the later ones have no splash guards.  However, that's not necessarily a reliable method to tell them apart, because the splash guards could have been removed.  Other ways to identify them are described below.  1988 was a "transition" years so some of the components remained the same as '87, and some were updated to the new suspension design. 

Front C4 Suspensions

A quick look at the k-member or engine cradle will tell you if it is an early or late front suspension....the early suspension k-members have a flat top, while the late ones have two "humps" on top.  This is because the suspension geometry was re-engineered in 1988 by GM and they raised the upper a-arm attach points on the cradle, used a taller spindle, and lengthened the lower a-arms.  This change makes the late front suspension about 1" wider than the early front suspension, and none of the suspension components except the hubs are interchangeable.  In addition, the upper a-arm shafts have "flats" on them in both vintage suspensions, however the "flats" are vertical in the late suspension and horizontal in the early suspensions.  This offers another way to identify the vintage of the suspension. 

All C4 Corvettes use front-steer (forward of the spindle) rack and pinion setups.  Steering racks are available in 1.96 turns or 2.36 turns lock-to-lock, depending on the suspension option.   Front swaybars range from 24mm to 30mm for all years.  Front brakes were 11.5" on early suspensions (retained through '88), and 12" on late suspensions, with a 13" OEM option (RPO J55).  Upper and lower a-arms and front knuckles are made of forged 6061 Aluminum alloy, reducing un-sprung weight.  The actual year of the k-member can be ascertained by looking at the date-code stamped on the front of it, under the steering rack.

Rear C4 Suspensions

The C4 Corvette IRS is considered a "5-link" rear suspension.  It consists of a differential "pumpkin" with a differential cover that has a wide truss brace cast into it, often called a "batwing".  The "batwing" ends are where the differential is mounted to the frame.  There are two halfshafts, two lower strut rods below the halfshafts, and two forward links, commonly called "dogbones" on each side. 

Many of the components interchange between the early and late rear suspensions.  The halfshafts, strut rods, and dogbones are all virtually the same and interchangeable. Rear springs are interchangeable across all years.  Strut rod inner bushing hole size varies from early to late, but they are the same length.  The inboard strut rod brackets were modified in 1988 to change the suspension geometry, but they will interchange.  The major difference between the early and late suspensions is in the knuckles and brakes.  The knuckles are of a different design between early and late C4 suspensions, and will not interchange unless you replace the entire assembly outboard of the halfshafts.  This is also where the 1" width difference is, with the late suspension being the wider of the two.  The dogbones are in the same location, and the batwing frame attach holes are the same dimensions on early and late suspensions.  Early suspensions used a drum-type e-brake while the late suspensions used a mechanical e-brake integrated into the brake caliper.  The late calipers are much larger than the early calipers as a result of the e-brake design.  The standard rear brakes were 11.5" through '88 and went to 12" for 89-96.  Swaybars ranged in size from 19mm to 23mm for the early suspensions, and 22 to 26mm for late suspensions. Virtually all of the rear suspension components are made of forged 6061 with the batwing and differential case made of cast aluminum alloy.

Rear C4 differentials

There were two differentials offered in the C4 Corvettes, all of which were limited-slip and all of them installed in aluminum housings with aluminum covers called "batwings".  The first is the Dana 36 which was offered in all years of cars with automatic transmissions.  This is an 8.2" differential but is surprisingly strong.  It would make a good choice for a smaller displacement engine with an automatic transmission, or for a car that's not driven hard.  Gear ratios range from 2.59 to 3.07.  Aftermarket gears up to 3.73 are available for the Dana 36. 

The Dana 44 HD is a heavy-duty differential that came in cars with manual transmissions, whether it was the Nash 4+3 or the ZF-6 6-speed.  The Dana 44 HD was not offered until 1985, so all 1984 Corvettes came with Dana 36 differentials.  The Dana 44 HD is the same one used in the Dodge Vipers, and is said to be equivalent in strength or stronger than the GM 12-bolt differential.  The Dana 44 HD came with 3.07 gearing up through 1988 (a carryover from '87), then the housing was changed and it was offered with 3.45, 3.33, or 3.54 gears in the later cars, depending on year.  The 3.45 gears were the most common, and it was the only choice in the last couple of years.  The Dana 36 and Dana 44 HD will interchange in the suspensions, as the batwing attach bolts are at the same width, and the yokes are the same width.  However, the Dana 44 has more pinion offset, and has a longer case than the Dana 36.  You can put any year/type C4 differential in any year suspension. We offer pinion support brackets that allows the replacement of the Dana 36 with a Dana 44HD.

The easiest way to tell a Dana 36 from a Dana 44 is that the Dana 36 has a bolt in the CENTER TOP of the rear cover.  The Dana 44 does not have that center bolt.  In the  picture below left, the Dana 44 is on top.  In the right picture, the left differential is a Dana 36 while the right one is a late Dana 44...the housing differences are readily apparent.

Why use C4 instead of C5/6 suspensions?

This is a question that many people ask.  The thought is that since the C5/6 suspensions are newer, certainly they're better, right?  Not necessarily. 

In 1997 the C5 Corvette was released with a transaxle (Tremec T-56 based) and an all-new unequal length upper and lower a-arm type rear suspension.  The driveshaft was enclosed in an aluminum tube, or torque tube, which forms the backbone of the C5/6 Corvette chassis.  An aluminum cradle supports the rear suspension components but the upper a-arm pickup points are not included in the cradle.  The front suspension was also changed considerably, also utilizing an aluminum cradle.  Again, the front upper a-rm pick-up points are not included in the cradle.  

The first obvious problem in using the C5/6 rear is that the transaxle won't fit under a Tri-5 Chevy, or any other car for that matter.  The floor would need to be cut up considerably, and the torque tube would have to be lengthened.  In addition, the C5/6 suspension is several inches wider than the C4 suspension, making it impossible to keep the tires under the car without narrowing it.  Narrowing would involve moving the a-arms inboard, resulting in a narrowing of the frame rails, and using custom, shorter halfshafts, and a custom swaybar.  The cradle would have to be narrowed, or discarded completely in favor of building the pickup points into the frame, and generally over-complicating the construction.  This adds a lot of cost and complexity to the project.  Therefore, the C5/6 rear suspension is rarely used in any but the most exotic custom car builds.

The front suspension is a somewhat different case.   The C5/6 a-arms are similar to the C4 a-arms, as are the steering knuckles.  The components are all aluminum.   Again, the C5/6 suspension is several inches wider than the C4 front suspension, so the Corvette cradle is basically useless unless narrowed, and is usually discarded when C5/6 components are used.  With the suspension narrowed so the wheels fit under the car, the stock C5/6 steering rack and swaybar can no longer be used.  Also, all of the suspension pickup points must be built into the new frame.  Again, this adds cost via a custom cradle, a new rack, and a different swaybar. The only components that can be used from the C5/6 are the upper and lower a-arms, knuckles, and brakes.  The C5/6 brakes are better than the C4, but upgrades can be bought for the C4 suspensions, or C5 brakes adapted fairly inexpensively.  

All of the above makes the C4 suspension much more adaptable to use for conversions.  The front suspension is a modular design, and the C4 cradle carries all of the pickup points for the suspension.  The width is reasonable to use under many other cars.    The C4 rear is a good fit in many frames, and it's available with a strong POSITRACTION differentials.  The factory brakes are very good, with aftermarket upgrades available. In short, the C5/6 suspensions are not worth the added benefit of trying to use them.  While they may be somewhat better than the C4 suspensions, the C4 achieves most of the performance benefits, with a lot less complexity and cost.