Talking with MM...

[TheJeanyus]

...this is what the guy said about rear coilovers:

I do not know where you heard that "using rear coil overs with IRS can result in less traction", this is absolutely false, in fact it is quite the opposite. I am assuming that the person who said this was running too stiff of a spring for his combination, resulting in reduced compliance of the rear tires. When correct spring rates are used with coil-overs there is nothing on the market that can match their performance or excellent ride quality.

It is ideal to run coil-overs front and rear, this will produce the best results and most balanced ride. Coil-overs can be used in the front only, however you must use an extremely stiff stock location spring out back or a very soft coil-over spring up front to keep the car feeling neutral.

And some other advice he had:

If wheel hop is an issue for you the Delrin Control Arm Bushings are what you need. Wheel hop is caused by the original soft rubber bushings, urethane will reduce this deflection but not eliminate it. Delrin Control Arm Bushings will offer the largest reduction of wheel hop for your money. Some people are intimidated by them because they think that they will bring NVH (noise, vibration, harshness) into the vehicle, this is false. In this particular location delrin is very acceptable, any NVH will still be isolated at the four subframe bushings. This means the delrin is no more intrusive than stock.

If you are experiencing a driveline vibration, like most IRS equipped Cobra owners, at higher freeway speeds the 3-Piece Aluminum Differential Mounts are for you! These bushings will offer pinion angle adjustment to eliminate that annoying driveline vibration. Due to the fact that they also solidly mount the differential means that you will never experience any differential housing/cover failures (common with other 2-piece kits on the market). These will transmit a bit of gear whine into the interior, mainly in reverse and when cruzing on the highway with the windows up. It is not nearly as intrusive as the supercharger whine.

He never did recommend a spring rate, but it sounds like front/rear coil over conversions with soft springs are the way to go?

[Sendero]

I had always heard to skip the rear coil-overs on the IRS cars, but this was of course before MM started to get involved. But it makes sense considering some people were running a 1100lb conventional spring in the rear!

When you say soft springs, please don't be confused with them actually being soft.

Coil-Over Math

Wheel Rate = Spring Rate * (Motion Ratio)^2

Conventional spring motion ratio is 1/4 (.25)
Coil-Over spring motion ratio is 9/10 (.9)

Your car has 600 in-lb Front / 600 in-lb Rear springs. So lets get the class started.

600 * .25 = 150lb wheel rate with a Conventional Spring

They are probably going to recommend a 375lb front spring, so lets try that one first:

375 * .9 = 337.50lb wheel rate with a Coil-Over Spring. See the difference?

Originally Posted by TheJeanyus

No.

Ok so lets back this out. To achieve the same wheel rate with a conventional spring:

337.5lb / .25 = 1150lb conventional spring to do the same amount of work.

I'm willing to bet that they will recommendation of 375/375 or even 375/400 for your car considering the IRS has a longer moment arm than a stick axle.

Originally Posted by TheJeanyus

Ok, I see that. But won't all that wheel rate make my car ride like crap?

Ahhh, my young Padawan, you have many things to learn. Warning, long answer ahead.

If you look where your conventional springs mount, they are very close to the control arm attachment points. From "Statics and Dynamics", you should remember this causes a great deal of load forces on the bushings in these locations. (You know, resultant force increases as you move towards the point in question) These forces in turn cause static friction on the bushings and create bind. Whenever you migrate to coil-overs you are reducing the static friction and causing less bind on the bushings. In doing this, you allow the bushings to do their job which is allow the Control Arm to move freely. The spring is also allowed to do its job more efficiently because its not fighting the bushings (static friction) as the arm moves up and down. With the system working in harmony, you get a better ride with awesome handling potential.

One last thing; I would suggest skipping the Bilstein "Sport" Valved shocks that they sell with the kit because at 375lb you are at the limit of the rebound valving. I am experincing this problem right now with my car. I'd look into either: the Race Valved Bilsteins or Koni DA's.

Hope this helps!

[TheJeanyus]

Thanks for the reply, Nathan. The wheel rate and control arm bushing stuff all makes perfect sense.

I figured the springs wouldn't be soft, as in Crown Vic soft or even stock Mustang soft, but softer than most people think they need to be. It seems there are some guys out there who think the springs need to be so stiff they might as well have welded a big steel cylinder in place of the springs.

I was going to simply have my stock Bilsteins modified to use with the coil overs. This may be a temporary setup for a while until I go to the Race Valved Bilsteins.

[Sendero]

Originally Posted by TheJeanyus

I was going to simply have my stock Bilsteins modified to use with the coil overs. This may be a temporary setup for a while until I go to the Race Valved Bilsteins.

The stock Bilsteins on the front will be ok. (My rear shocks are the culprits) I would look at upgrading the rear shocks though, but don't get out of balance!