C4 Corvette IRS in 55-59

[Houston54]

I picked up an 89 C4 assembly at the Decatur swap meet last weekend to use on the 58 big window project next in line. It will be mounted using the Flat out Engineering bracket kit but I wanted to find someone who is using this suspension system to discuss rim issues such as back spacing, diameter and width.

I am not fond of tubbing the bed or making the fenders wider to fit fat meats under the truck so my tire width will be limited to 235 to 265. I also like to keep the same size rim/tire on all four corners as I use my trucks as drivers and rotate the rubber from time to time.

If you have or are using the C4 vette IRS on your truck please share your wheel/tire information so I can keep my lessons learned list a bit shorter.

[Atomsplitter]

Sweet IRS!
How close is it to the original rear end width and how much did it cost?

[bensimmons_05]

Make sure you claim in on your taxes so the IRS knows about your IRS.

This looks like it is going to be awesome, can't wait to see more!

[bensimmons_05]

dang I thought of a better one........

Your approach is good, its always better to avoid IRS trouble by gathering all the information you can.

Alright no more.

[GRIMSS]

For some reason I thought that Nathan at Porterbuilt has done something with the c4 and the TF trucks. I might be wrong but you might check them.

[Steve@OldSub.com]

What front suspension are you going to use? Since you want to use the same wheels and tires on both ends you need to choose something that will work well on both ends.

I don't remember when the rotor size changed, but one of the constraints with Corvette stuff is that the bigger brakes require bigger wheels. In '89 the standard wheel was 17x9.5 which is larger than '88 and earlier. I don't know if the brakes were larger but I know early C4's and late C4's have different brakes.

Those are my thoughts regarding the C4 IRS. Well that and my C4 is one of the best handling cars on the road easily out cornering most new cars. Engine technology advances mean half the cars on the road have more power than my early C4 'vette but it will still beat them around the corners.

[Houston54]

The brake size issue is known and I plan to run 16 or 17 inchers. That is the main purpose of the thread though. I have seen the threads on Porterhouse's outstanding work but in my ap I am not tubbing/backhalfing or making mods to the fenders. I did not see anything in those threads that address my concerns regarding back spacing, width and diameter.

I read that some 16s work fine and others do not with this. I talked with Don at Flat Out Engineering and he was of the opinion that was only with the C4 "front" suspension.

The 58 will have a Fatman IFS kit on the front with standard 10.5 rotors at least to start. I am not that thrilled with showing off the brakes and like the look of the steel gennie rims. I am just not a fan of Wheel Vintiques as their caps seem to crappy and I have read of run-out problems with their rims. That issue can be resolved later though.

I mainly want to find out what other wheel/tire combos have been used by others with a similar project plan. There may not be any out there in which case I will find the answers through trial and error.

Thanks for comments though and I am glad to hear from a C4 owner that they love the handling performance.

The kit I am planning to use does away with the stock spring in place of coil overs. I might investigate retaining the spring or replacing it with a light weight unit if one exists.

[Gene Sharp]

If you could find one put C=4 and rack and pinion under front also
I just put new rack with the tighter ratio in my C-4 and it drives like a dream
If one could get some of that performance in a PU it would be outstanding
only problem I forsee is possibley the higher center of gravity of the truck
Please keep up the pictures and updates am very interested in this swap


Gene Sharp

[Steve@OldSub.com]

Originally Posted by Houston54

I did not see anything in those threads that address my concerns regarding back spacing, width and diameter.

I read that some 16s work fine and others do not with this.

'84 to '88 the C4 Corvettes had 16 inch wheels. Unless the rear brakes got a bunch larger I'm sure a 16 is going to fit. The Corvette wheels are offset like a FWD wheel and pretty much enclose the brakes.

Starting as an option at least as early as '84 Corvette wheels were not always the same front and rear. I have two sets of factory wheels, one that is the same on both ends and one that is wider on the rear. More recently the diameter also is different front and rear.

Both sets of C4 wheels I have are directional. One set the wheels are literally marked front right, front left, rear right and rear left. Choosing this kind of suspension may make it difficult to run an identical wheel and tire on all four corners.

I would imagine you can run most any width and offset that fits under the fender on the rear, but a similar change on the front is going to upset the steering geometry some. I really think if you are going to try to run the same thing on all four corners you have to start with what will fit and work right on the front. Front fitment issues are more likely to be a problem because of the changes in wheel position because of the steering.

Running a C4 IFS would make it easier to choose wheels that are compatible with the suspension on both ends. The relationship of king pin and the ideal location of the middle of the tire is much different in the C4 design than the Mustang II.

You might have the ride benefits of four wheel independent suspension but I have to wonder if you will achieve the handling benefits with the IFS you've chosen and the wheel compromises that may force.

My wife's car is a 2009 German thing with four wheel independent with 17 inch wheels on all four corners sold as able to handle better than most cars. And actually it does handle pretty nice, however it scares the crap out of me when I forget I'm in it and not my 'vette. The Corvette with the right wheels and tires is that much better. If I head into a corner like I'm in the 'vette but not the car tells me I pushed it too hard.

In addition to changes in brake size there was also a change in width. I don't recall the width of an '89 or the width of my own car either. You should be able to measure that width or look it up, and then working from the measurements of bed and fender get a real good idea what back spacing you need to make it fit.

The up and down motion of the rear tires on a C4 does not have the same camber change a straight axle does. For that reason the clearances you leave on the sides though still important do not need to be as generous.

I would choose the coil overs or maybe air over the fiberglass spring because tuning the fiberglass spring for your truck would not be easy if its even possible, and while a variety of different spring specs are available I would not count on one being even close enough on a completely different vehicle. Stock your truck's weight isn't a lot different than the donor car, but the weight distribution is a lot different.

I don't have a TF and am not running a C4 IRS under anything except my C4, so what I've offered here is based more on my C4 experience than anything else. I hope it is of some help.

[Coronas56]

I know a guy that used corvette stock springs on his 55 TF truck, looks good, I am using coil overs on mine.

[Houston54]

I have made a little progress on this install. Found some 16" camaro rims to use as mules to set the ride height and get the chassis to roller status. I found the front of the frame was bent so I installed a 2nd gen F-body subframe I had laying around and will save the Fatman kit for another project.

I have the IRS in place with upper brackets and pinion support fabricated and tacked in place. The only issues remaining on the rear are how to lower the ride height about 2 inches and the fabrication of the control arm mounts. Right now the rear has a lot of negative camber. Once we set the bed on this reduced it some but it still needs some help.

We placed the front clip and bed on this weekend to confirm the wheel positioning before we send the frame out for blasting. Once it returns we will complete the welding and move on to cab body work.

['58Patch]

This is pretty cool. I am picking up a front and rear end from a '85 corvette donor on Thursday. I will be using the Flatout Engineering kit on the front and rear as well.

I was also thinking about the rim size and difference between the front and rear. I am pretty new to this so please pardon the ignorance but: If you ran the same sized rims all around, wouldn't wheel spacers on the rear help balance the geometry? Or will the tire width still be a factor/problem?

[addicted2dunes]

I got one too!

[smittster]

I have a 1950 GMC and I have 85 corvette frot and rear suspension using Flat Out Engineering equipment. I have no problems so far, it looks and drives good. I just have roller wheels on the truck for now. I am using donor wheels and tires from I think a 90's
s-10 truck on the rear and it has no clearance issues. Not the same story for the front, I had to remove the s-10 wheels because the tires rubbed the fenders so I am using some 90's firebird wheels. The firebird wheels were the same size as the rear but a different offset.

[OKGMC4]

I'm using C4 stuff front and rear with C5 wheels. They are so positive offset I have to use bolt on spacers to get them out far enough.

I've had two 2nd gen trans ams and the handling charachteristics are WAY different than the vette. Oranges and apples. Maybe this will work in your case, I just fear the truck will act strange in a turn. The rear will be much more precise and ride charachteristics will be different plus the camaro clip only makes the front/ rear ratio even farther from 50/50. The Vette stuff was engineered to work together and is tons lighter...anyhow - if you want a couple inches lower put some third gen springs up front. I put 82 firebird springs in a 79 TA and it sat perfect plus the third gens usually have higher spring rates so it will ride rougher but will take some slop out of the front. You can get a C4 rear down a bit by running a tap farther down the spring bolts and redrilling the cotter pin hole farther down. Don't recommend more than 2 inches this method.

[Ipaintgrt]

Arent the 82 trans am, 3rd gen, front suspension different than the 2nd? I had an 85 and it had a strut type front end. I'm curious how you did this.

[Houston54]

Not sure who your question is directed to but the 3rd gen F-body springs have the same inside diameters as the 2nd gen. G-body cars also use the same spring design. I am using 3rd gen springs (Moog 5662) on my 54 (80 model G-body) front suspension.

The 3rd gen front suspension used Macpherson struts in place of the shock but still used coil springs between the lower control arm and the unibody.


OKGM4- good information. Although the C4 front would be the overall best option I am using what was on hand and expect the handling to be more than acceptable. Sent you a PM on something you can probably clarify for me.

[Ipaintgrt]

Thanks, thats very interesting. S

[OKGMC4]

I had that backwards - longer bolts to lower and shorter to raise. My head was somewhere else earlier. Sorry about that.

[Az. Hank]

Are you using the coil overs?

[875DVL]

47 1/2 ton corvette IRS 15x10 rear positive offset .brakes fit fine

[Houston54]

What year model did the rear come out of? Is the parking brake mechanism in the rotor hat or the caliper?

Are you planning to lower the rear further?

[Houston54]

Originally Posted by Az. Hank

Are you using the coil overs?

The plan at this time is to retain the stock spring. If it is determined that coilovers are needed to get the stance/ride quality I want change out then.

[875DVL]

MIne was out of my old 85 corvette that I owned , I used the engine trans and rear end . caliper park brake . Iam not going to lower at this time because I like the more HOt rod look I guess . But would be curious to see a bag set up for irs on these old trucks as I have not seen many bagged IRS coversions.

[Houston54]

The units with the parking brake in the caliper, like the one I am using, will not accept 15 inchers which can complicate your wheel options. I think that change happened in the 88 model year.

Will you have to make any mods to the fenders to fit them over the tires?