I've got a 1979 C3 that I'm building up and I want to convert it to the C4 rear suspension. I keep reading that the "later" (90-96?) C4 IRS was better than the "early" C4 IRS, but could use some more in depth explanation. The "early" seems to be narrower and thus more easily packaged in the narrow C3, but would like to understand what made it worse. Please talk about the "early" C4 IRS having jacking issues, but is that due to the IRS itself or a result of the pickup points on the frame?

 

The only real difference as I understand it is the tip of hub to tip of hub is larger. The center section is identical. IIRC because of that the wider one is better planted slightly because the geometry was improved? (Do not quote or hold me to that.) The overall width tire to tire remains the same however. The difference comes from back spacing on the wheel. More or less either or is about the same. I don't think the difference is substantial, an inch or 2 if that?

Jacking on a c4 is the frame pinch points only. I never saw an easy way to do it with the suspension out back.

 

The latter IRS geometry have less roll steer because the inner camber arm mount is lower. They also have less Anti-Squat because the trailing arm brackets are lower in the front.

The earlier geometry will turn in quicker and lanch harder in a straight line but be slightly skittish cornering at the limit and under braking on bumpy tracks compared with the later.

Will

 

Are the knuckles and half-shaft lengths the same?

Early C4 IRS is easy to find and relatively cheap, but the later IRS seems scarce and expensive. If I can use the earlier parts but then adjust my frame pickup points, that would save me quite a bit of money.

 

I think they are different.

 

The halfshafts are the same.

The knuckles are different enough.... the pickup points for the camber arm and trailing arm are the same, the actual difference is the bearing register/spacer/caliper bracket thickness. However the step ring that locates it on the actual knuckle is a different diameter and its not a simple swap. The outer stub axels are longer on later suspension as well.... this is where the additional width comes from on later suspensions.

People have narrowed the late suspensions inorder to use ZR1 rear wheels. Theres an old pdf writeup somewhere on here about machining the spacer blocks and using early outer stub axels. FYI C4 ZR1s all used the early narrow rear suspension widths as they had 36mm offset wheels... I've never bothered to look at the inner camber arm mount on a ZR1 to see if they have the early or late type.
Will

 

Were the ZR1a different? I assumed the reason the 36mm worked was because of the widened rear body panels?

 

Is there any benefit to the later (thicker) knuckles?

 

Absolutely. The ZR1 uses the early shorter outer stub axels. But like I said GM widened the rear suspension by making the caliper bracket/spacer wider and lengthening the outer stub axels.... thats it. Everything else is the same save the geometry changes to the inner camber arm brackets and frame mounted trailing arm brackets.
Will

 

No and the later outer stub axels are weaker than the shorter early ones. I learned this drag racing both my 92 and 84 switching parts around figuring out which geometry and springs worked best.

The only thing is if you are worried about antilock brakes...... 84-85 C4s didn't have it (only 86-88 did) and while drilling a few holes in the caliper bracket spacer for the sensor is easy, 84-85 outer stubs don't have provisions for mounting the reluctor rings. So if you're worried about that buy parts accordingly. Shouldn't be a problem with a C3 suspension swap.

I want to point out that theres nothing wrong with the early suspension geometry..... it's actually better for all out performance. GM changed the geometry to accommodate the older age group of people buying C4s and driving them at relaxed rates rather than full on track driving them. Same reason they softened up the spring rates, the 2 changes actually went hand in hand.... softer springs meant faster suspension travel and therefore faster camber change on the rear..... they changed the rear roll rate to accommodate the softer springs nedded to keep senior citizens buying the cars.

If the narrow track width works better for you then use early suspension..... if the wider works then use the later..... it's not a big deal either way for 99.9% of people.

Will

 

Lots of great info here. Just for clarity thought I would add a few things.

the 'early C4 suspension is 1984-1987. The late C4 suspension is 1988-1996.

also be aware that depending on the car the C4 suspension can have either a Dana 36 or Dana 44. I believe all 1984 cars had the D36, after that auto cars got the D36 and manual cars got the D44 (I think). There was a number of gear ratios offered thought the years so you may want to check which ratio the rear end has.

One other thing to note, that might have been mentioned already, the early C4 suspension used an Internal drum rear park brake, whereas the late C4 suspension had the park brake integrated into the rear caliper. Z51 cars from 1988 onwards also got the J55 13in (2 piston caliper) front brakes.

finally, there were a whole lot of spring rates offered in the C4, so be aware that the spring rates are not all the same, so you will want to do some research to determine what spring rate will work for your C3. The same goes for C4 anti-roll bars.

 

I would select the early knuckles. They have a parking brake that actually works. ( drum brake instead of the later caliper parking brake )

 

I always found the parking brake on my 88' worked very well, never had an issue in over a decade of ownership. Having said that the late style park brake does make it a bit more challenging to upgrade the rear brakes if so desired. Although Baer offers a nice rear brake kit for late C4 suspension, so there is that option and Street shop inc makes a C6 Z06 brake conversion hub for late C4's as well.

 

Let's clarify a few things. "Jacking effect" as the OP referred to is a function of the height of the roll centers and how that vectors lateral cornering force vertically into the chassis, thereby raising overall ride height and CG during cornering. The higher the RC, the more jacking takes place for a given G force, and that's bad for cornering and can also make for a more harsh breakaway characteristic. The earlier rear C4 suspension has more of that jacking effect. It's not dramatically or dangerously more by any means, and it can still handle very well. Also, this is affected by tire diameter: the lower the tire height, the less jacking effect.

There is no roll steer with an IRS. That's only a function with a solid axle as it literally steers the axle when the trailing arms go through their arcs. With a C4's IRS, the rear steer is completely controlled by the toe links. They don't always perfectly match the travel of the trailing and lateral links, and when that happens we get "bump steer." To my knowledge, there is not a difference in bump steer between the early and late suspensions. If there is, that can be adjusted out with a bump steer kit (I don't even know if such a thing exists, because it may not be needed).

As far as I can tell, there is no difference in rear suspension parts for the ZR1 vs the regular 92-96 C4s, and the 88-91 is the same for all C4s in those years. The only difference to the 92-96 spindles being the reluctor ring. This is confirmed in the parts manual I have. IOW, the ZR1 uses 36mm offset wheels purely because of its wider body work.

As far as I know, the early C4 front suspension is generally considered faster in cornering, but not the rear. The earlier rear has a little more (negative) camber gain with roll, but the higher roll center with correspondingly higher jacking effect.

Since the OP wants to adapt a C4 rear suspension and diff to a C3, I'd say use whatever is easiest to fit. The performance differences are not enough to worry about in this application, and either will be light years better than the stock C3 rear suspension. It's an awful thing. Also, since you'll be fabbing the frame mounts anyway, you can set the trailing arms frame pivot heights anywhere you want. And if you want to lower the diff-mounted pivot point for the lower camber rods, you can easily enough make a bracket to do that (or check out Doug Rippie lowering brackets).

 

Are you planning on running the C3, or C4, suspension width?

 

Not sure, but leaning towards the C4 width as I like the newer (high inset) style wheels like are on my C5.

 

What're your thoughts on the reasons for that? Is the prime reason the camber issue during cornering (due to the KPIA and the limits of the stock crossmember)? Or something else?

Just interested in your input. Thanks.

 

For autocrossing, I think the camber loss due to the insane KPIA (or SAI) is at least half the issue. For most road courses, it's probably less of an issue because the steering angles (and therefore the camber loss) are usually lower. The other oddity on the later front ends is the gobs of anti-dive geometry they have. You see this visibly as the upper control arm being angled severely upward toward the front where it mounts to the frame (the front mounting bolt is much higher than the rear). This makes for lots of push on braking: where most cars turn in better with some trail braking, the later C4 just plows until you get off the brakes. This issue could be corrected in other ways besides a wholesale swap to the older front subframe, if one were so inclined. Basically, you'd need to find a way to drop the front mounting bolt hole for the upper A-arm about an inch. Then you'd want to increase your front spring somewhat to compensate for the higher dive and increase turn-in on the brakes. I contemplated doing this, but for my purposes it would have probably been better to do the whole swap.

The previous owner of my car had amassed all the parts to do the swap. He never got around to it, and sold it to another C4 autocrosser in his region. That guy did do the swap, and some national champion caliber drivers have pronounced that car as the best handling car they've ever driven (and they've driven hundreds of cars competitively). It seems to me that an early front end a later rear end are the perfect combo. If I hadn't sold the car, I'd probably be working on that swap right now...

 

I appreciate your time and input here. Got a couple additional questions if you're willing.

The main issue I have ever heard regarding "lots of" anti-dive is that it tends to bind up the suspension under hard braking (effectively locking/increasing the spring/wheel rates). Are there any additional reasons that you're aware of?

I have a '96 suspension on the front. When I first installed the C4 suspension I duplicated the stock A-arm positions on my custom brackets on the frame, but I can reduce the anti-dive by moving the front A-arm bolt down one hole in the bracket. Your comments above have piqued my interest in trying out a reduction in the anti-dive. I haven't done the math yet to see what the change in static caster would be with just lowering the front of the A-arm. I suspect minimal, but I also am under the impression that "flattening" the A-arm angle would also decrease the outside tire caster gain under lateral weight transfer during cornering. I'm just trying to get comfortable with this change, so that hopefully there's a net increase in front grip. I'm running coilovers, and have heavier springs if the need appears.

Thanks again.

 

I'll tell you what I know, but keep in mind I'm not an expert.

The binding up really depends on how the anti-dive is accomplished. If the car has a lower A-arm angled downward toward the front (which is equally effective at creating anti-dive), then it will be less compliant over bumps. That's because in order for the lower ball joint to move up in compression (as it would going over a bump) it also would have to move forward, but the bump you're hitting to cause that is trying to drive the ball joint backward (unless you're driving in reverse!). So that's a bad way of doing it. I think a C4's lower arm is pretty level from front to rear, though. The upper arm being angled upward toward the front actually should make it more compliant over bumps, because as the ball joint moves upward in compression it also moves back a bit. Luckily, that's how a C4 does anti-dive.

It does act like the front spring rate is increased though, because it understeers on the brakes. I've heard anti-drive referred to as a "geometric spring," which is what it feels like to me. But I've not been able to completely work out how this would happen. The geometry should add a constant vertical force into the frame that only varies with braking force, and so one would expect the spring and sway bar rates to still determine roll resistance (which is what determines handling balance). However, as the car dives or rolls, the CG would get lower which increases the anti-dive even if the force and vector angle stay the same. I don't know if that accounts for all this effect or not, but in some way it seems to add to the effective spring rate. Note that if you took out the anti-dive and compensated with a stiffer front spring, you'd have the same effective spring rate as with anti-dive. However, you would have presumably compensated by also increasing the rear spring or sway bar rate or by decreasing your front swaybar rate in order to keep the balance. What you end up with, then, is a more consistent handling balance vs the car with lots of anti-dive, which may be close to neutral off the brakes but understeers a lot with the brakes on.

That's actually a point I hadn't considered. As it sits, the upper ball joint does move backward in compression, which the direction it moves when the car leans to that side. It's probably not insignificant: if the arm were canted upward at 45-deg the ball joint would move backward as much as it moved upward. It's not close to 45-deg, but it may be close to 20-deg (the early C4 appears to have around 12-deg). If so, it could gain roughly a degree of caster for every inch of compression travel. That's generally considered beneficial, at least with as much KPIA as is built into the later cars. That said, I'd rather build in a higher amount of static caster (even using custom or SPC upper arms if necessary) and get rid of at least some anti-dive in order to get more consistent handling, than to rely on dynamic caster gain from anti-dive geometry.