Rear Springs

[nicknitro71]

I've notice the stock rear springs are quite undersprung @ 350 lbs. Is anyone running 650-700 lbs rear springs?

[CraigS]

Since you mention 350# I assume you are running a solid axle as the IRS uses 750#. I think you will find that the 350s are plenty stiff. FFR started using 350s when they started supplying the specially developed Konis 3-4 years ago. Prior to that the stock spring was 250.

[nicknitro71]

Thanks, I will go 750 and IRS as I'm planning to race this thing!

[68GT500MAN]

You may not like using the IRS in a racing application. I know that many have used it, but we had so many problems with it in Karen's Coupe that when she built the new one she went back to solid axel. There are some threads in the history about the teams struggles using the IRS, look under Thunder Valley Racing.
Doug

[loeffler1]

If I were doing my coupe over I'ld go to 25# springs in the rear. My 3 link is always banging over bumps.

[loeffler1]

I meant 250# !!!

[Gbeck]

I guess this all changes with the 2015 Mustang-based IRS.

[nicknitro71]

So I should go with the 3-link for racing applications?

[wallace18]

So I should go with the 3-link for racing applications?

Since the challenge cars use the 3 link, IMO it would be what I would use to Road race. JMPO.

[Daytona Dan]

Yes, based on Karen's true racing R&D experience (cracked control arms, bushing and/or spherical bearing wear, etc.) I also suggest you go 3 link if you're really going to seriously track day or race it. I bought a fully built car years ago that had pin drive width IRS and I track prepared it over several years but it never felt as good as my friend's solid axle coupe. I learned on this forum that the suspension geometry of pin drive width was inferior to standard width IRS but it also seemed the standard width took a bigger beating due to the arms being 2.5" longer so that was going to be high maintenance in the long run even though I am not racing it. I finally decided last winter to have Whitby cut off the IRS and install 3 link with a 9" Moser rear end. I set a personal best yesterday at WGI by over 2 seconds so I am convinced...watch this to motivate you in your build! https://www.youtube.com/watch?v=le2fgAnzB5A

[narkosys]

As another option there is also Gordon's 5 link suspension (http://levyracing.com/store/levy-racing-5-link-rear-suspension).

P

[CraigS]

My take on this is that trying to utilize a suspension that was designed for a 4000# car, w/ 6 inch wide wheels and 50,000mile tires, and very low power didn't work out so well in racing applications. It works fine, w/ a few upgrades in the bushings, for a street FFR. I think it can also work pretty well for autocross since braking is minimal and, of course, the duration of the loading is so short. Unfortunately the T-Bird IRS isn't up to long term racing stresses.

[nicknitro71]

I'm still very debated about what rear end to get.

The engine is going to be a 351W block, the new 460 cui dry-sump. Ford claims 575/575 for HP and torque with mid compression and pump gas.

Originally I was going with a LSX, 398 twin turbo but the space under the hood is limited for the large inter-cooler I needed. I could have pushed 1,000 on pump gas this way.

The car is going to be used for street and road course racing, no intention of doing any drag.

Handling, braking, and acceleration are the paramount here. There's got to be no under-steer in any condition!

[cjcnj01]

Running a 3-link. I've had mine set-up with 500's and with 350's in the rear and found the 500's to be way too stiff... the car handles better on the track with the softer set-up.