Have questions about C4 Corvette suspensions for your Tri5? Ask them here!

[NickP]

Nick, you might have but I don't recall. What kind of file was it? I have seen several of Newman's rear steer kits and I have a few tapered bushings that Gene from CT made me years ago but they're for me.

[55 Rescue Dog]

As touchy as IRS rear suspensions can be, I'm afraid to move anything from stock. Even GM has struggled since 1960 with it, but finally seems to have it sorted out, with no "jacking effect", that even the C4's can have when you mess with it. Surely the toe rod makes no difference on the geometry from what I've read. Still trying to get an idea as to what the changes in wheelbase, and mass between the C4, and the tri-five have? 2 totally different cars it seems in some ways. No free lunch.
Would love to hear how narrowing the C4 suspension would be a benefit for handling, to get a bigger tire under it? My un-informed thought is widening the rear track width would help. On my C5, and BMW M Roadster the rear is wider than the front for some reason.

[chevynut]

As touchy as IRS rear suspensions can be, I'm afraid to move anything from stock.

What's "touchy" about an IRS? The same suspension geometry principles apply to an IRS as they do to an IFS. Think about the rear toe adjuster the same as the front steering rods. When going forward, they just keep the tires centered. If the pivot points are wrong on the steering rods, you'll get bump steer. Same in the rear. If the rear toe rods are parallel to the halfshafts and the same length, they won't affect toe as the suspension moves. In reality, the toe rods are slightly longer than the halfshafts but they pivot at points further inboard on both ends. I think any toe change over the 2" or so of up/down suspension travel is minimal because the components are so long....longer than the front a-arms and rods.

As usual, you over-complicate things and think that GM got everything "perfect". If you don't understand what's changing, you probably shouldn't move anything unless it's proven. I think Newman and others have proven that the dropped toe adjuster works just fine. There are probably a couple thousand examples driving around.

I have no issue with moving the toe rods down so they clear the frame because I think any effect is minimal, and if I thought it was a problem I'd do more analysis. Of course, if you want to leave the rear of the car at around stock height you can leave them in the stock location.

Even GM has struggled since 1960 with it, but finally seems to have it sorted out, with no "jacking effect", that even the C4's can have when you mess with it.

The Corvette didn't have an IRS until 1963. GM didn't "struggle", they just kept improving it over the years. That's how designs evolve. The C3s used a solidtrailing arm setup with no toe adjuster in the rear of the differential. That caused suspension geometry issues and wasn't as "independent" as it could have been. Camber changes twisted the trailing arms. The C4 improved upon that by using dual trailing arms, improving anti-squat performance and camber gain and overall flexibility of the suspension. The C5/6/7 rear design is an improvement on the C4 with the unequal length a-arms like an IFS. That allows the rear to have a constant anti-squat since the a-arm angles don't change. It took a while for some reason, but GM finally got to where Porsche, Ferrari, and other exotics have been for a long time.

What do you mean "mess with it"? If you understand what's changing you can "mess with it". Lots of chassis manufacturers "mess with" the pivot points on an IFS and rack location. Maybe you shouldn't . C4 suspension components are commonly used with narrowed k-members and custom front crossmembers that are narrower than stock. What are you calling a "jacking effect"?

Surely the toe rod makes no difference on the geometry from what I've read.

It makes no difference on anything but toe in or out during suspension travel. A solid rear has no toe movement with suspension travel. Is that ideal, or should you have some toe in/out going into turns? Which way is "best"?

Still trying to get an idea as to what the changes in wheelbase, and mass between the C4, and the tri-five have? 2 totally different cars it seems in some ways. No free lunch.

The wheelbase goes from 96.2" for the C4 Corvette to 115" for the tri5. That's pretty significant change but it affects nothing that I know of but the Ackerman and steering radius. What's the "ideal" Ackerman? Again, there are a few thousand tri5 cars running around with C4 suspensions so the Ackerman is probably not a big issue. Nobody has complained about any negative Ackerman effects that I'm aware of. The weight of the cars is comparable...both around 3200-3400 pounds and a couple hundred more isn't going to change much imo. Both are just cars and behave similarly with the same suspension geometry. Does this keep you up at night?

Would love to hear how narrowing the C4 suspension would be a benefit for handling, to get a bigger tire under it? My un-informed thought is widening the rear track width would help. On my C5, and BMW M Roadster the rear is wider than the front for some reason.

Most cars have a wider rear track width than the front, even a stock tri5 is a little wider in the rear than the front. And "track" doesn't refer to the wheel mounting surface width, it's the center of the tires when installed on the car. An 88-91 C4 Corvette has an actual rear track width of 60.4" and a front track width of 59.6", almost identical to a stock tri5 with zero offset wheels. A 92 and later is 59.1" rear and 57.7 front according to my C4 specs book.

Narrowing a C4 rear allows the use of a wider tire, because it moves the dogbones inboard. The outboard limiter is the quarter panel and the inboard limiter is the dogbones. You can't move the quarter panel, but you can move the dogbones if you narrow the frame. I can put 345 rear tires under my Nomad and keep them comfortably inside the quarter panels. It's also an aesthetic thing, since some guys like a deeper dish wheel, and it makes the wheels easier to remove. You don't need to narrow the rear to get a 295 or 305 tire under a tri5 with a C4 suspension, but any more than that and you have to. We've built 3 frames with narrowed rails for bigger rear tires but it takes a lot of work. My Nomad rear is the same width as a stock tri5 so I can use the the same wheel frontspacing if I want. I don't believe handling is negatively affected by narrowing the rear 1.5" per side since my roll center is lower than an early C4 and I could move it even lower with a simple inboard strut rod bracket re-design.

If you think something is detrimentally affected by narrowing the C4 IRS, what is it? Of course as usual you want to confuse and complicate everything more than it needs to be and question everything I have done. Besides, you have a stock width rear (at close to stock height) and it can't be narrowed without serious frame surgery, so why do you care?

[55 Rescue Dog]

Okay, so you can do whatever you want to an IRS I guess. I would love to see a narrowed one autocross, and prove it won't jack like a swing axel Corvair, unless it has a roll center underground. I would think a 345 tire's centerline would move it inboard a lot from a stock tri-five regardless of the outboard width. A straight axel might have zero toe change, but the roll steer can be a big factor in making the car turn.

[chevynut]

Okay, so you can do whatever you want to an IRS I guess.

Lots of guys narrow the C4 IRS as I said, and they also narrow the C4 IFS. According to you that's a no-no. Lots of guys use dropped toe adjusters but you seem to have some sort of concerns about it that you can't articulate or prove. Some guys use adjustable dogbones and I'm guessing you have some kind of an issue with that too. You have an issue with poly suspension bushings, with coilovers, and with just about everything GM didn't do but many others have.

Guys use different offset wheels on a C4 suspension, depending on the vehicle the suspension is going into. There's nothing "magic" about an IRS. Yes, you can do just about anything you want with it if you understand how it works. You probably shouldn't mess with yours.

I would love to see a narrowed one autocross, and prove it won't jack like a swing axel Corvair, unless it has a roll center underground.

Yours might "jack" since it's at almost stock tri5 height . If mine "jacks" so does yours. I don't think you know what causes what you call "jacking" and the C4 rear is nothing like a Corvair or VW rear suspension. Why would a narrowed rear cause "jacking" anyhow? Can you explain it? You apparently don't even understand what changes when you narrow the IRS and keep the track width the same. All you have to do is change the wheel offset to make things fit. Mine is narrowed 1.5" per side...hardly enough to make much difference in geometry. That's less than 10% of the length of the halfshafts. Why don't you calculate what that did to the roll center? And as I said, the roll center now falls somewhere between an early and late C4, and closer to the late one. Do you understand what moving those inboard strut rod brackets does?

I would love to see you have a debate and actually quote facts and figures, instead of your unfounded opinions and beliefs.

I would think a 345 tire's centerline would move it inboard a lot from a stock tri-five regardless of the outboard width.

Well you only have so much room outboard until you hit the quarter panel, so if you want a bigger tire you have to go inboard or put fender flares on the car. This stuff seems to confuse you a lot for some reason . If I put a 345 tire on my car with 1/2" clearance at the quarter panel lip, the track width would be about 57.25". The center of the stock wheelwell with relocated springs is 59.125". That's a difference in track width of 1 7/8". Each 345 tire centerline is 15/16" inboard of the center of the stock wheelwell. The center of a minitubbed wheelwell with springs relocated is about 57.125" which would make it slightly narrower than mine if a tire was centered in it. In fact, the stock tri5 rear track width is 58.9". The differences aren't that big, and if you tried doing a little math once in a while to try to prove this to yourself you wouldn't have to bring stuff like this up.

I can put a 345 tire on my car and the CG is lower than yours is. You max out at a 295 tire with a stock C4 rear IF you have minitubs, and about a 275 without them. Big tires and a low CG help on the skidpad and slalom..that's why exotics and super cars run such big tires. Sure, you could drop yours to lower the CG but then the suspension geometry will be all screwed up. The ride height needs to be built into the frame design.

You're better off leaving everything as GM made it, so you don't fret about it so much. But don't attack what others do to theirs unless you first try to understand what you're talking about.

[NickP]

As an aside to this, relative to width at the hubs, an email I have from Bob Shetrone, AKA Progressive Automotive states, "1988-96 can be narrowed to 58.5” hub width & 1984-87 can be narrowed to 57.5” hub width"


They have a slew of them out there and seem to be fully functional.

[chevynut]

Nick, I think that refers to the front suspension, but I know Progressive also narrows rear suspensions. Front suspensions have to be narrowed a specific amount to fit the narrower racks they use. You can narrow a late rear suspension to about 60" before you run into clearance issues between the batwing and the knuckle. I actually clearanced the knuckles on my suspension to give better clearance for the batwing, since I didn't narrow the batwing. If you narrow the IRS much more than 60" you have to narrow the batwing. Maybe that's where they get the 58.5/57.5" in the rear because that's pushing it.

[NickP]

I had inquired as to the cost of narrowing the rear batwing and the half shafts.

Nick,
Narrow Rear Cover with new urethane rear cover bushing installed
Narrow toe-in (tie bar) adjuster
Narrow Half shafts with new half shaft u-joints installed
Custom built camber bars
$1,650.00

1988-96 can be narrowed to 58.5” hub width
1984-87 can be narrowed to 57.5” hub width

Best Regards,
Bob Shetrone, Pres.
Progressive Automotive, Inc.
125 West Rome St.
Baltimore, OH. 43105
Phone: 740 862-4696
8:00 AM -4:00 PM, Monday-Friday ET
ProgressiveAutomotive.com
[IMG]file:///C:/Users/Prime/AppData/Local/Temp/msohtmlclip1/01/clip_image001.jpg[/IMG]

[chevynut]

Interesting Nick. I wonder why it's limited to 58.5"/57.5" if they narrow the batwing too. Maybe it's the width of their frame rails or something like that. At 58.5" at the mounting surfaces (with discs) a late C4 suspension is narrowed almost 5" which is pretty significant. Tray Walden at The Street Shop told me he's narrowed them as much as 6" for early Corvettes. Do you think they're just "jacking" around?

[NickP]

Interesting Nick. Do you think they're just "jacking" around?

LOL. I understand the term and it's quite common with "SWING AXLE" vehicles - BTW, they might be an IRS version but yards different in reaction - that's the whole reason VW went to a true IRS.