Please, Need advise on Coil-over Springs

[bittenbycobras]

Maybe some members familiar with the independent rear suspension on the MKIV roadster can offer some advice as to what strength springs to use. This is a 2015 version of the MKIV which I recently purchased from the original owner who never started any assembly work. The kit has 500 lb. springs in front with IRS. The wheels are 9” and 10-1/2”.
There seems to be two recommendations on which rear coil-over springs should be used for the IRS, the printed manual shows 350 lb. but the on-line manual shows 750 lb. coil-overs. The springs delivered with the kit are 750 lb. As this is being built as a street car, probably not driven on the track, it seems 750 lb. springs would be pretty stiff.
Afraid I’m in over my head on this one and would like some help. Current progress is: the front suspension and brakes are in and completed, the rear pumpkin, CV axles, and lower control arms are in place.
Thanks, Charles Mowry

[Jeff Kleiner]

Which IRS is it? A 2015 Mk4 could have either the T-Bird or Mustang suspension depending on when it was produced. T-Bird suspension uses 750# and I believe the Mustang derived IRS does also. Straight axle 3 or 4 link is 350#

Jeff

[CraigS]

750 for the T-Bird IRS and when I did the 2015 Mustang based IRS retrofit on my MkII FFR sent me Konis w/ 400# springs.

[rich grsc]

Yep, Jeff is correct, again. :eek: IRS has the heavy springs, straight axle has lighter springs and longer shocks.

[bittenbycobras]

Thanks guys, so the 750 lb. Springs are correct as this has a T-Bird differential, pre-2015. This will be a street car for fun driving, not for the track. The IRS will make the ride better but with the stiff springs I don’t want to find out it rides like my pick-up with an empty bed. I was thinking of putting 400 lb. Springs on the front and 600 lb. On the rear but there’s a lot to consider when designing a suspension. Does anyone have experience with softer springs on the MKIV, any recommendations or is it smarter to stick with the FFR design? THANKS.

[rich grsc]

USE what they sent you. You have no experience with these cars, yet you want to change something that has been used on every car FFR ships. ????

[danmas]

I would use what they sent you. Its pretty easy to change out the springs/shocks later on if it is too stiff....

[proeagles]

For what it's worth I think you have a valid point as 750lb springs are about twice as heavy as a race spring setup on a Fox Body Mustang with coil overs and that car out weighs yours by 1000 lbs or so. Maximum motorsports recommends subtracting 150 from the front spring rate for street driving and 125 for high performance driving. So if you are running 500 then rear would be 350 and even those seem too stiff for a 2300lb car. Maybe someone with more experience with your setup will chime in.

[rich grsc]

For what it's worth I think you have a valid point as 750lb springs are about twice as heavy as a race spring setup on a Fox Body Mustang with coil overs and that car out weighs yours by 1000 lbs or so. Maximum motorsports recommends subtracting 150 from the front spring rate for street driving and 125 for high performance driving. So if you are running 500 then rear would be 350 and even those seem too stiff for a 2300lb car. Maybe someone with more experience with your setup will chime in.
Bad advice. He has an IRS rear suspension, they use heavier rate springs because of the geometry of the control arms and the position of the shock. AGAIN these are rear shocks, not front

[Jeff Kleiner]

For what it's worth I think you have a valid point as 750lb springs are about twice as heavy as a race spring setup on a Fox Body Mustang with coil overs and that car out weighs yours by 1000 lbs or so. Maximum motorsports recommends subtracting 150 from the front spring rate for street driving and 125 for high performance driving. So if you are running 500 then rear would be 350 and even those seem too stiff for a 2300lb car. Maybe someone with more experience with your setup will chime in.

I'll be happy to ;) There's a whole lot more to it than just the pound ratings of the springs. What really matters is actual "wheel rate" which is calculated by factoring the length of the control arm the spring is on, the distance from the pivot point of that arm to the spring and the angle of the spring (i.e. vertical or X degrees from vertical). When this is all taken into consideration and actually calculated the 750 spring is not "too stiff for a 2,300lb car"...at least not this 2,300 pound car. Trying to compare spring rates for a FFR/T-bird derived IRS to a Fox body Mustang truly is an apples to oranges comparison.

Jeff

[proeagles]

Thanks for chiming in on that. My only concern was that I've used 400lb springs on a E36 BMW which has IRS and they were damn stiff and that car weighed 3400lbs. Even my full race 600lb spring was so stiff the car was undriveable on anything but a race track.

[rich grsc]

Lets compare bmw to FFR

[proeagles]

Since you are missing the point, I'm just going to respectfully back out of this topic and let the OP decide what he wants to do. I know I would like to do a little more research on that if it were my car and since I plan on building a FFR car with IRS someday for cruising. 750 may be in the ball park as a GT350 with IRS is 850 or something like that and it's a pig at 3800 lbs.

[Jeff Kleiner]

Thanks for chiming in on that. My only concern was that I've used 400lb springs on a E36 BMW which has IRS and they were damn stiff and that car weighed 3400lbs. Even my full race 600lb spring was so stiff the car was undriveable on anything but a race track.

Again, unless the car has exactly the same geometry it's apples and oranges to compare spring rates.

https://www.focusrs.org/attachments/screen-shot-11-18-17-at-10-27-pm-png.240034/

Cheers,
Jeff

[proeagles]

Thanks for the chart. I would still like to ride in one that is already built and see for myself. I'm not into rock hard rides anymore. The chart gets you into the ball park but ride comfort varies quite a bit as there is no single rate that works for every application.

[CraigS]

My experience w/ a couple of friends FFRs w/ the T-bird IRS and 750s is they have a very nice ride. Also be sure that you check that the Koni shocks are set to full soft before you install them.

[edwardb]

Two 2015+ IRS builds with the kit supplied 750# springs. Roadster and Coupe. The ride is surprisingly comfortable and not harsh at all. The best advice you received in this thread, other than the technical description by Jeff (always helpful) is start with what the kit provides. How many have they built again?? Over 10,000? Maybe they know a little what they're doing. Change things afterwards if you want, I guess, but re-engineering before building, driving, or riding isn't the best idea IMO.

[rich grsc]

Paul, you just don't understand. :rolleyes: The wheel needs to be reinvented. ;):p , no matter how round it is.

[bittenbycobras]

Just want to write ‘thanks’ to everyone who took the time to share their thoughts and experience on the coilover springs. This forum is very helpful, learned from your comments and decided to go with the original springs as set up by FFR. Jeff Kleiner the chart is good, shows just how deep in over my head I am when I think about making a change, thanks. Rich grsc, you definitely know a lot more about this than I know so thanks for your valuable input. Craig S, yes I checked the clicks to be sure the shocks are on the softest setting, thanks. Edwardb, you have actual experience with a similar setup so your comments are greatly appreciated. As you wrote, danmas and edwardb, the shocks/springs should be relatively easy to change out if ever they need to be changed.
Charles Mowry

[danmas]

Good to ask questions and seek information and I learned a lot listening to the grand masters here. Thanks for asking the question.

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Dan