Affordable Independent Rear Suspension

37ford · Oct 19, 2011

I'm in the early planning stages for a 67-68 build, and I was aiming to build a car with moden technology. I'd like to put an IRS under it, but the aftermarket ones I have seen are quite pricey. I did a search here and found some information. Someone mentioned a Jaguar IRS and that led me to CTM Engineering. Looks like they make a nice product, but, again, it may be out of my budget. There was also a mention of a 2003ish Cobra IRS unit.

Does anyone have any suggestions on what the best donor cars might be to find a unit? Which Jags? Which Mustangs? Any others? Are there any affordable aftermarket units?

rmousir · Oct 19, 2011

Independent Rear Suspension Forum

you should find the info you need here.

buening · Oct 19, 2011

The cheapest route possible would be the 2003-2004 Cobra IRS. Dave at DVSrestorations has brackets for this install. A VMF user Latamud has a thread on here somewhere where he did his own brackets and IRS install. Do a search and you might look that thread over.

cmefly · Oct 19, 2011

Also look at the 2001 cobra It is the same as the 2003 minus the stronger axles and the center diff brace that can be added. it has 3.27 gears instead of the 3.55 from the 03/04. The 99 Cobra has 28 spline axles and 01-04 has 31 spline.

mustang125 · Oct 19, 2011

Their is someone local that has one for sale. check Huntsville, al craigslist to see if it still is available. I can workout the shipping if necessary.

Klutch · Oct 19, 2011

What's your objective? I remember reading team Shelby built a GT350 prototype with IRS and it didn't handle any better better than the live axle cars. So, I'm just curious if you want IRS because it's really cool, or if you're expecting a significant performance improvement.

robert_t_wallace · Oct 19, 2011

I bought a 1999 Cobra IRS on Craig's list (got a good deal) to "attempt" to put in a '66 Coupe I have. I was doing it for the "cool" factor, but realized real quick that it's beyond my skill level to put in. I also bought the rear section of the Cobra with the frame rails and brackets. The coupe needed new frame rails anyway and I thought I might be easier to graft in the frame rails than buy the brackets from DVS. I have looked at it hard and done a lot of measurements and it is possible, but think I am going to back out of the work...

Paul1958 · Oct 20, 2011

Everyones opinion of costs is different. Clearly what you might call 'a little pricey' may be a sum that I am living on right now. You need to state in $ what you can spend. Personally Im at the end of the spectrum where the terms 'affordable' and 'independent rear suspension' are incompatible and shouldnt be in the same sentence.

Unless you are RACING an IRS is for BRAGGING.

supershifter2 · Oct 20, 2011

why would irs be modern technology and a solid axle not be modern technology ? a straight axle rear set up properly will OUT CORNER an irs set up properly. a straight axle rear is predictable and an irs rear isnt. a properly set up solid axle mustang will out corner a properly set up irs vette, cobra mustang, etc. the ONLY advantage the irs has is a little smoother ride. just cause something is new doesnt mean its better.

pdupler · Oct 20, 2011

Not advocating this, but always fascinated by some people's ingenuity....

Sean's Suprang Page

37ford · Oct 22, 2011

Thanks for the link to the IRS forum. There's a treasure trove of information there.

Klutch, my objective is to build a car with modern performance, handling, and technology, but still respect the look of the classic version. As for the testing of the IRS in the original Mustangs, every account a have read said that they weren't any faster on a smooth track, not that they didn't handle better. They were selling so many, anyway, that they just scrapped the idea.

I know things have come a long way since leaf springs on a straight axle like coil-overs, 4-bars, and the like. That's why I'm asking these questions. I just want to find see what's out there and make an informed decision.

jimeast · Oct 22, 2011

IRS vs Solid Axle

The IRS did not handle any better than a solid axle on a "flat" race track with no bumps, etc... The IRS should excel when used in a street ride.

Klutch said:
What's your objective? I remember reading team Shelby built a GT350 prototype with IRS and it didn't handle any better better than the live axle cars. So, I'm just curious if you want IRS because it's really cool, or if you're expecting a significant performance improvement.

DazeCars · Oct 22, 2011

buening said:
The cheapest route possible would be the 2003-2004 Cobra IRS. Dave at DVSrestorations has brackets for this install. A VMF user Latamud has a thread on here somewhere where he did his own brackets and IRS install. Do a search and you might look that thread over.

That is hands down the simpelest install with the least fab work but it is by no means the cheepest option. I have less in to my entire Jaguar install (that includes all the parts machine shop work, the rear end it self not to mention a set of Qa1 adjustable coil overs and a limited slip differential upgrade) than the price of the DVS brackets alone.

For my project rear end is in the car. Has been for about a year. I got it in and got 90% of the bugs worked out and then went on to do some work on the Galaxie. As promised it is a complete bolt in and uses the leaf spring mounts as attachment points.

I used the 9/16" front sway bar that came on the car as a rear sway bar

I was doing a really good job of documenting my project on my web pages on the subject but I have not updated it lately because of the time involved in making a technical web page and because much of what I have been doing has been available on my IRS forum www.IRSuspension.com Let me know if you have any questions about the project I would be happy to answer them.

jimeast · Oct 22, 2011

IRS VS. Solid Axle

I don't think this is a correct statement. An IRS takes a lot more effort to sort out and in practice not much difference over a solid axle on a flat, smooth race track, but simple geometry diagrams and applied friction forces support results contrary to your statement. The Vette's IRS also had built in design flaws to keep manufacturing costs down.

I live in the northeast where all the roads are bumpy and crappy in general and I'd like to have an IRS keeping my tires parallel to the road, and the ride more "eloquent." If I was on the track, I'd opt for a solid axle, because it's much easier to sort out and roughly equal to an IRS as you state.

Don't forget about the benefits of un-sprung weight using an IRS either. As cars get lighter and wheelbases get shorter, the IRS becomes more beneficial on high performance street cars. If I had a Cobra kit car, I would always opt for an IRS, unless it was a strip only car.

supershifter2 said:
a straight axle rear set up properly will OUT CORNER an irs set up properly. a straight axle rear is predictable and an irs rear isnt. a properly set up solid axle mustang will out corner a properly set up irs vette, cobra mustang, etc. the ONLY advantage the irs has is a little smoother ride. just cause something is new doesnt mean its better.

slim · Oct 22, 2011

37ford said:
I'm in the early planning stages for a 67-68 build, and I was aiming to build a car with moden technology. I'd like to put an IRS under it, but the aftermarket ones I have seen are quite pricey. I did a search here and found some information. Someone mentioned a Jaguar IRS and that led me to CTM Engineering. Looks like they make a nice product, but, again, it may be out of my budget. There was also a mention of a 2003ish Cobra IRS unit.

Does anyone have any suggestions on what the best donor cars might be to find a unit? Which Jags? Which Mustangs? Any others? Are there any affordable aftermarket units?

Very interesting! By chance do you have a 37 Ford? I bought a near mint one owner 37 Ford Delux Business coupe in 1967. Sold it in 1989. My first car was a 46 Ford Sedan Coupe. Interesting especially because Fords still had transverse buggy springs front and rear through the 48 model. Kinda the opposite of IRS!

Nothing to add on IRS. I'm behind the times with my stock 68 vert.

Slim

buening · Oct 23, 2011

Good work on the Jag IRS install Daze! I should clarify that I think the Cobra IRS is the cheapest IF you buy a used IRS AND you make your own brackets for install. The cost of the DVS brackets are more than the IRS will likely be. How common are the Jag IRS??

My biggest complaint with most IRS is the weight of the entire thing. Pretty heavy and bulky IMO, compared to a solid axle with any choice of suspension package.

DazeCars · Oct 23, 2011

Thanks!! most Jaguars from 1960 to 1987 used the same basic IRS design so really they are fairly easy to find and fairly cheep. I got mine for about $400.00 including the cost of having it truck freighted all the way from the east coast to MT. When I was first looking in to this I found most Cobra IRS units were about $1000 and most T-bird Mark VIII units were $600 to $800 They all cost when it comes down to it and with all those moving parts the cost does add up as the project goes. I agree they are heavy, no argument there.

68_GT · Oct 23, 2011

pdupler said:
Not advocating this, but always fascinated by some people's ingenuity....

Sean's Suprang Page

what ?!?! wow but why did they do that ????

look on SVTPerformance in the market place for complete IRS rear ends. Those guys occasionally swap out their IRS for SRA's or solid rear axels. I did when I had an 03 IRS. Should cost you about $1k to get an IRS assembly. Personally I'd find a better upgrade to the car than an IRS. That's a lot of trouble for minimal benefit if any, plenty of drawbacks (wheel hop / still weak designs even the 03/04 has weaknesses), and they weigh more than a solid axel set up also.

37ford · Oct 24, 2011

Slim, yes I do have a 37 Ford Slantback. I used a stock rebuilt MII front suspension and a 9" with leaf springs in the back. It's a very light car, so the ride is okay. I didn't want to get into anything too complicated or expensive for my first project. I built the car with good quality parts, but a fancy suspension would have really pushed back the completion date (which still ended up being 7 years). Plus, so many other guys were running that setup.

So, for this project, I want something much better. The setup I had wasn't adjustable. Both the front and rear springs settled on the 37 after driving, and adjusting the ride height on that setup wasn't easy.

DazeCars · Oct 27, 2011

Paul1958 said:
Im at the end of the spectrum where the terms 'affordable' and 'independent rear suspension' are incompatible and shouldn't be in the same sentence.

anything is affordable if you do it a little at a time. A Jag IRS is like a new engine. You can pony up all at once and get a stroker create engine and drop $5000 or you can find a good block have it machined, buy parts as you have the funds and put it together your self. the rear end is the same you can drop $5000.00 for a hidits or Mustang IRS unit, or you can buy a used Jag unit $200.00 and then rebuild it your self a little bit at a time.

buening said:
My biggest complaint with most IRS is the weight of the entire thing. Pretty heavy and bulky IMO, compared to a solid axle with any choice of suspension package.

You are absolutely correct. On average the weight of an IRS is 150% that of its live axle counterpart, however it is the right kind of weight. It cuts your unsprung weight buy at least half over a live axle if not more AND a little extra weight in the rear end is not necessarily a bad thing. It may be counter intuitive but adding weight to the rear end of most cars will improve handling performance, by improving the weight bias. To prove my point take your car and put it through its paces on a spirited drive. Then add 100# to the trunk and do it again. Assuming the rear suspension is not tired and is up to the task of the extra weight the car will perform better. it will corner better and hold the road better. Yes that 100# might reduce your 1/4 mile time by .01 seconds but the better weight bias will give you better handling on a road course and improve those times.

Affordable Independent Rear Suspension

37ford

rmousir

buening

cmefly

mustang125

Klutch

robert_t_wallace

Paul1958

supershifter2

pdupler

37ford

jimeast

DazeCars

jimeast

slim

buening

DazeCars

68_GT

37ford

DazeCars

Top Contributors this Month

Recommended Communities