Suspension Considerations

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I’m planning to buy an M4 and looking for suspension advise. I used to own a 66 Corvette with IRS and I liked it. Would an IRS suspension really be that much better in a Cobra than a straight axle if I’m just doing street driving? Also, I much prefer the look of taller sidewall tires with 15” wheels. Is it true that an IRS suspension won’t work with 15” Halibrands? Thanks for any input. Tom

[CraigS]

There is no question that IRS is better in every way but...a 3 link from FFR isn't all that bad. I happen to prefer softer rear springs than the 350 that FFR provides so I ran 200 or 250 when I had a 3 link. I 'think' you are correct that irs precludes 15" wheels. I know my irs w/ the OE rear brakes needed 17" wheels to clear. But a call to Gordon Levy would see if he has any irs brake calipers that would allow 15" wheels. OTOH, I'd get used to 17 or 18" wheels just for the much greater tire choices available.

[TrackDay17]

John In KC in the build threads is building a car right now with IRS and 15 inch tires, I'd check out his build thread.
I think he got some parts from Gordon Levy to make it work.

[Jeff Kleiner]

FFR offers a brake package (#16765) which allows the use of 15” wheels on the IRS.

Jeff

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FFR offers a brake package (#16765) which allows the use of 15” wheels on the IRS.

Jeff

Thank you Jeff I appreciate it.

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Thanks Craig

[CraigS]

If you can get the 15" look you like w/ irs and the $ are in the budget, I'd go for it. It will be better especially in a turn over rougher pavement so you get to enjoy that. But, although hard to think of now, it will help w/ resale. If I were looking for a completed FFR, I wouldn't even go look at a solid axle car.

[Blitzboy54]

My first MK4 had a 3 link, the second I am building now is IRS. The straight axel was awesome but had it's limitations. I cannot wait to rip it with the IRS.

[Jim Wehr]

I have been very happy with my 3 link. Never a problem.

[edwardb]

I've had 4-link, 5-link (the Levy setup), 3-link, and two with IRS. On a budget, I'd do the 3-link. But the best ride and handling for me, by far, have been the IRS builds. Resale is a good point too. You will get good reports from 3-link owners and they're deserved. But IMO the most accurate testimonials will be from builder/owners who had have had both.

[cob427sc]

I agree with edward. Having built with all the various options, the IRS is the best riding/handling and obviously the most expensive setup.

[BEAR-AvHistory]

The 3 link has been fine for the last 10 years. That said had the current IRS been available I most likely would have bought it. Had no interest in the old one.

[JMD]

FFR offers a brake package (#16765) which allows the use of 15” wheels on the IRS.

Jeff

They do and that's what I originally ordered, but wasn't crazy about them. I never put them on...they're probably fine, but I intend on doing some performance driving and didn't think they would be the best setup. If the car was a cruiser they are probably fine. Just my opinion.

I did get a set of Wilwoods from Gordon Levy that fit the 15" Halibrands. I believe they are 4 piston dynalite calipers with ~12" rotors. They do take a bit of custom work, but if you can drill and tap you won't have any problem. I have some posts up on my build thread regarding the installation process. The only drawback is they won't work with the FFR e-brake setup. I got a valve to put in the rear brake line to use as my parking brake instead. It works just fine...my only concern is whether they will flag it on the highway patrol inspection. They're pretty lenient with these cars as the 1965 safety standards were minimal, but I'm sure that varies by state and/or the person doing the inspection. But if it's a problem I'll figure something else out.

Edit: the entire package from Levy was MUCH cheaper than any of the FFR Wilwood options. I think I got all 4 corners for the price of the big front Wilwood package from FFR. Just FYI.

[JMD]

John In KC in the build threads is building a car right now with IRS and 15 inch tires, I'd check out his build thread.
I think he got some parts from Gordon Levy to make it work.

That's me!!!

[Avalanche325]

I have driven both, and auto-crossed both. To be fair, that was the old version of IRS. I haven't driven the new IRS.

I would bet that 95% of the people couldn't tell which one they were driving if you put them in someone else's car. If you are the type of person that can tell if your tire pressure is off by 1 or 2 PSI, you might notice the tiniest bit of a smoother ride.

The 15" tires are going to make a MUCH bigger difference in ride quality than the rear suspension.

[Jim Doak]

I'm mid-build with IRS and FFR's 15" wheels and brakes.

[CraigS]

I have driven both, and auto-crossed both. To be fair, that was the old version of IRS. I haven't driven the new IRS.

I would bet that 95% of the people couldn't tell which one they were driving if you put them in someone else's car. If you are the type of person that can tell if your tire pressure is off by 1 or 2 PSI, you might notice the tiniest bit of a smoother ride.

The 15" tires are going to make a MUCH bigger difference in ride quality than the rear suspension.

You make an excellent point about 15" and their effect on ride.

[Jeff Kleiner]

They do and that's what I originally ordered, but wasn't crazy about them. I never put them on...they're probably fine, but I intend on doing some performance driving and didn't think they would be the best setup. If the car was a cruiser they are probably fine. Just my opinion.

I did get a set of Wilwoods from Gordon Levy that fit the 15" Halibrands. I believe they are 4 piston dynalite calipers with ~12" rotors. They do take a bit of custom work, but if you can drill and tap you won't have any problem. I have some posts up on my build thread regarding the installation process. The only drawback is they won't work with the FFR e-brake setup. I got a valve to put in the rear brake line to use as my parking brake instead. It works just fine...my only concern is whether they will flag it on the highway patrol inspection. They're pretty lenient with these cars as the 1965 safety standards were minimal, but I'm sure that varies by state and/or the person doing the inspection. But if it's a problem I'll figure something else out.

Edit: the entire package from Levy was MUCH cheaper than any of the FFR Wilwood options. I think I got all 4 corners for the price of the big front Wilwood package from FFR. Just FYI.

'Performance driving" and 15' tires are mutually exclusive.

Jeff

[JMD]

'Performance driving" and 15' tires are mutually exclusive.

Jeff

Yes, that's why I also have a set of 17" wheels that I'm going to swap on when I intend to do that kind of thing. The 15's are for the street as I love the classic look, but I also wanted the best brakes possible given that limitation. Swapping on another set of wheels for a fun event is easy, changing brakes not so much! If it was more of a track focused car I would have gone with bigger brakes and never run 15" wheels...but that's not the way it's going to be used 99% of the time.