View Full Version : Shortend IRS

10-06-2005, 07:20 AM
Can someone who has shortended a C4 IRS corvette rear please let me know what general steps they followed?

This is going into a 68 Camaro
Plan on using the Wayne Due IRS kit.
Rear IRS is from 89 vette Dana 44 rear.
Plan on putting 18x12 in the rear so shortening will I think allow for the stock 4 link to stay?

Anyways I think there are parts that need to be modified but not sure what all the steps involved here maybe so all comments are welcome!!

Also have front C4 suspension complete from 89vette does anyone know what the effect are of using the wider suspension with a wayne setup are? would it even fit?


P.S. I kinda started this thread before the forum got kill a few days ago so sorry for the repeat..

10-10-2005, 03:22 PM
what do you mean by shortened? the nature of the design is that the pumkin is seperate from the hubs. simply put the hubs were you want them, get the rear centered, and whatever length is needed for the half shafts is what you use. maybe im missing something, but i am not seeing how its any more difficult than that. well for the half shaft length anyway.

10-10-2005, 04:51 PM
Look at a site for Tray Walden at Streetshopinc.com. Tray builds replacement Corvette frames and he narrow's C4 rearends daily. He might be able to help you. The biggest issue you will have will be the rear spring IF you decide to continue to use the rear leaf spring. Tray uses coilovers. I know that Wayne Due recommends the 84-87 C4 rearend as it is 1 1/2 inch narrower than the 89-96 rearend. Tray perfers the 89-96 reared as they have better brakes and are more available. Dana 44 Corvette C4 rearends bring $2,000-$2,500 where a Dana 36 runs about $750. Tray's unique concept is to install a new frame with late model Corvette suspension under early cars with no cutting of the body ulike all the other builders that require body modification. The reason all the others require body modification is that they do not narrow the rearend.

10-10-2005, 05:16 PM
so the crossmember/rearcover is too wide and that has to be narrowed? i honestly have never measured but i didnt even think that part would be too wide. interesting, ill have to see what he does to narrow them too.

10-10-2005, 07:14 PM
Tray does narrow the rear cover that mounts the rearend to the car. When he is finished you cannot tell that it has ever been modified.

10-11-2005, 03:41 AM
OK, these are the issues that I can see as being a problem, before I have even started mockup.

Just before I start I will be using coil overs to eliminate the need for teh rear leaf spring.

1. There is a rear sway bar attached to the IRS, is there a place you can get custom sway bars? Custom to your length, Is there a need for the rear sway bar in these types of conversions? If I move the hubs in a few inches both sides, 3" per side estimate, like I said haven't mocked up yet.

2. The lower control arm is cast, do I replace with an adjustable bar? What size diameter is recomended etc.

3. Tortion bars from the rear diff to the hubs, again adjustable lenght ones, any suggestions from where?

4. A good place to order half shafts custom lenghts, any idea on costs?

5. I believe I can still use the rear 4 link bars if the wheels I get have enough backspace in order to allow them to clear the rim. Any ideas if someone has done this before on measurements? That is for a 12" rim, how much backspacing total lenght of the rear, What is was shortened too?

Answers to any of the above is great, And I already appreaciate the feed back.

Too bad Streetshopinc.com didn't have too many pics of there setups. I am finding it hard to get a detailed build of this type of setup. Even though I know there are folks on this site that have done this before... Help out please! I am not an engineer but can follow instructions pretty good, unless from my wife so I don't want to be the guy that waste money trying to do this for the first time. I don't think my project can afford to be the ginny pig. Or at least I can't afford that. I will get some pics up and circle the areas I am speaking about..

Thanks in advance!

10-11-2005, 04:31 AM
When Streetshopinc narrows a Corvette rearend for "their" application, the only piece that is not modified in some form or fashion is the rear sway bar. The stock rear sway bar is used.

I suggest you give a Tray a call as he should be able to answer all your questions. The biggest issue will be "if" Tray can help you at this point as his business for C1,C2 and C3 replacement frames is huge at this point. Try has already built a 69 Camaro with C4 and C5 Corvette suspension for customers. He manufactured his own mounting system. He is currently finishing a 67 vette with C5 front and rear suspension with an new LS-6. The build definitely required cutting of the 67 Corvette body.

10-11-2005, 08:10 AM
I don't see how he uses the original rear swaybar if the half shafts are shortened which would bring the hubs in making it impossible for the sway bar to line up. But I could be wrong!

I will try to call him, but since I am not buying a product not sure how much time he will want to spend with me :nopity:

10-11-2005, 03:34 PM
If you're going to use Wayne's kit, you should talk to him. I don't know how his kit sets up a rear sway bar, but I'm using a third gen vette sway bar facing forward and below the rear conected to the hubs by a custom bracket I made that also mounts the shocks. I'm using a 1985 rear, w/ a dana 44. 84-87 is the same mounting wise as 88-96 and does not need to be narrowed to fit in a 1st gen camaro even w/ a mini tub. (unless you start to cut into the frame rail, at which point waynes kit will not work anyway) the differences in width are only due to brake changes (88-96 no longer use a drum style emergency brake) and only requires slightly more offset in the wheel. I'm using a 9.5" wheel w/ no tub using the stock 84-87 corvette offset. I would assume when using an 88-96 rear, that year vette wheel would be a good place to start.
Take a look at wayne's site http://www.waynedue.com/index2.html it might help understanding.

10-12-2005, 10:43 AM
Hey Estamato

Can you minitub and get a 12inch rim under your car with a IRS. Looking at the waynedue site it looks like the trailing arms would go on the outside of the frame rails and would limite the tire width?

10-13-2005, 03:25 AM
Exactly why I want to shorten the rearend posibly enough to bring them to the inside or directly onto the frame. I think Waynes kit is still nice because he has the components to connect the coilover shocks to the bottom of the spindle and such. I am not a master fabricator, and will not pretend to be! I know there are some real fab guys here, I have been a member of this site for years and have continualy gathered info to get a good foundation for my project. Cause I cant afford not to!

As for the IRS I believe the lower control arm will need to be fabricated some kind of adjustable bar like they use in the 4 link suspension stuff, also for the trailing arms not sure if the location of the bracket will allow me to use the stock bones as I call them. I believe I can also use the same adjustable bars to make the difference in lenght if there will be any. There are also 2 bars that are attached to the rear diff cover and attach to the spindle, not sure what these do? The coilover shocks I believe will eliminate the need for the rear leaf spring that connect the 2 hubs to the rear diff. Just not sure about the sway bar and If I need to use the piece that attached the rear diff to the tranny?

I want the car to handle as if it had an IRS in it. not worse not better...

This car is going to be a weekend car, not a trailler queen or show car, I just want to own and drive and enjoy a kick ass car that I said I built and have people drewel all over the hood :) ! I want grown men to go home and tell there wives that they are going to buy a first gen camaro cause they just have to! kinda like me when I first saw the Mule. and then bought my camaro, cause I just had to!

02-06-2006, 04:48 PM
I too am taking this route into consideration for my second gen, i would think the main concern would be that cast LCA. how have those been shortened? just a completely new piece?

also can you just weld the half shafts (or have them welded at your local driveline shop) or would an aftermarket piece be the only way to go?

02-06-2006, 08:06 PM
Here are some pics of a 7" narrowed C4 rear in Prodigy. We made new 4 link bars and lower control arm. We shorthened the original tie rods.


02-25-2006, 09:08 AM
Here are some pics of a 7" narrowed C4 rear in Prodigy. We made new 4 link bars and lower control arm. We shorthened the original tie rods.

My Dana is narrowed , I also narrowed Rear Batwing,, PST did my half shafts,, Wayne Due also helped with angles and set up info,,, He is a Hell of a nice guy !!!!! If I can help ---yell,, Don't go to this site much anymore ,I was given a PITA status about a coulpe of cars I was defending,,,, If you buck their trend you are a PITA,,, Roger J

02-25-2006, 06:00 PM
One thing to keep in mind with IRS is that most modern IRS are designed to use wheels with a lot of offset (very deep backspacing). I believe the '88 and up C4s use about 2" positive offset, which would equate to about 8.5" backspace with a 12" wheel (obviously, those numbers are off the top of my head, you'll need to measure and verify for yourself). The reason they use such large backspacing is to minimize scrub radius. When you apply power to the tire, it will want to "twist" the rear spindle and steer the tire toward the center of the car, if the car has significant scrub radius. This will actually lead to "torque steer" from your rear wheels, similar to the "torque steer" effects noticed in poorly designed front wheel drive cars. In addition, this torque will put additional loads on the bearings and hubs that they are not intended to handle.

The reason I mention this is that folks often want to put deep dish rear wheels on their ride, and will narrow the IRS for this reason alone. Not a good idea, if you're trying to preserve the original behavior of the IRS.

02-25-2006, 07:37 PM
PTA, I am intrigued with this discussion, could you continue to elaborate? I can understand where narrowong the rear will bump / compression steer, that is simple math. What I am not quite sure I understand is how applying power to the wheels can steer the rear more?

02-26-2006, 08:02 AM
I didn't explain this very well, here's another try. Think of an IRS as a front suspension whose ball joints are replaced by some sort of "rigid" linkage. Of course, no linkage is perfectly rigid, especially when it uses bushings which are also subject to deflection. So if, for instance, you attach a three foot long bar to the hub, sticking out from the side of the car, and then push forward on that bar with a hundred pounds of force, the hub/carrier/linkage/bushing assembly will deflect slightly, toeing in toward the center of the car in plan view. Just how much it deflects depends on the design of the linkage and bushings, but even a fairly small degree of deflection toe-in is enough to make the car "hunt" back and forth under hard acceleration as first one wheel, then the other gains and loses traction over bumps and surface irregularities.

Of course, there is no three foot long bar applying toe-in torque on a real car. What there is is a moment arm whose length is determined by the distance, from rear view, between the center of the rear contact patch and the effective axis of rotation of the rear hub carrier (the effective axis of rotation is not easy to figure, and will depend on the exact design, but a first approximation for a double lateral arm setup is to draw the axis through the upper and lower attachment points like they were ball joints on a front suspension). With a high offset wheel, this moment arm can be kept to near zero length (this moment arm is called scrub radius on a front suspension, and maybe on rear suspensions too - I'm not sure). With deeper dish wheels, the moment arm rapidly increases. For example, we have a deep dish wheel that makes the effective moment arm 4 inches long. Under hard acceleration with sticky tires on a 3500 pound car, we can easily get a thousand pounds or more of accelerative force at the contact patch. Working through a 4 inch moment arm, this creates approximately the same toe-in twisting force as the three foot long bar with 100 lbs force I used as an example above.

Just how much of a problem this will be in a given real world design is hard to say - all you can say is that the suspension was designed and modeled by the OEM using high offset wheels, and by changing those assumptions, you may be creating a bit of a problem. Computer modeling of such a situation is fairly complex, but it should be pretty easy to rough out a real world test with a long bar and an alignment gauge.

Hope this makes sense.

02-26-2006, 09:22 AM
Some other obvious points I guess I should make re deep dish wheels on an IRS:

The bending moment on the hubs and bearings, due to car weight, will potentially be much greater than in the stock application for which those parts are designed.

Braking will also cause some deflection steer, but less so because weight, and therefore traction, is transferred away from the rear wheels.

Solid axles, because of their basic design, will not suffer from such deflection effects to anything like the same degree.

IMO, if you're going to use an OEM IRS setup, particularly on a high performance application (lots of street rods are not actually driven in a high performance manner), you should try to keep all of the design parameters as close to stock as possible, to avoid the dreaded "unintended consequences".

02-26-2006, 09:41 AM
That was a very good explantion example. I will let you know if it turns out to be a issue in the real world when we get on the track. I can tell you i talked at lentgh with Mark after he drove the Vette with a 10" narrowed rear to ask about things just like this. I was very interested to get his thoughts as that car is basically Prodigy in a Corvette wrapper. While my focus was on bump steer concerns, my concerns, he did not mention any torque steer issues at all. It appears my bump steer concerns went unfounded as proper alignment keeps this in check.
I'll make contact with Mark again and specifically ask about torque steer as that Corvette had a ton of lip, I think no offset at all. I guess we will just have to wait until April and see for ourselves, but you at least have me thinking.