...Looking for some input on rear springs for my SE.

I've recently finished the front suspension overhaul/rebuild and am getting ready to start the shot rear. I'm running 300# Eibachs in the front on GC coilovers with MR2 Koni yellows. In the rear I want to stick with the stock spring perch and run these Koni Mustang shocks.

My question is specific to the rear springs... I was planning on using these Landrum race springs. I'd prefer to stick with Eibach in the rear but the shortest Eibach 5" spring is 11" long. I'm not keen on cutting longer ones down...

I used Hooke's equation to get a feel for how my chosen spring rate of 175# compares to stock at 3 different free lengths, here's a breakdown:


Stock rear springs = 12.74" free length, 1.8kg/nm=~101# spring rate (FSM tech data section)
Stock rear deformation length assuming 600lb force (approx. rear corner weight) = 6.74" compressed height.

Aftermkt 8"@175# and 600lb force = 4.57" compressed height.

Aftermkt 10"@175# and 600lb force = 6.57" compressed height.

Aftermkt 11"@175# and 600lb force = 7.57" compressed height

I think we can all see the relationship here when keeping all variables static and only adjusting the spring free length.

Based on these numbers I'd think that I'd be best with an 8" long spring and adjustable perches.

I don't want the car slammed, just dropped 1.5-2".


You guys have any suggestions or see any problems with this?
As it stands now I'll likely go with the Landrum 8" 175# race springs, Mustang Koni yellows and likely some adjustable perches like this.

VVV Down here VVV you can see the rear height and gap at stock spring height with Enkei ApacheIIs [15x8 25mm offset] and Falken Zeix 225/50 E-912s. Overall wheel/tire dia is close to stock.

 

you are correct, I was running 150# 8" springs with adjustable perches like that. if you use the 10" you wont have room for those perches and still drop in a little. im now dropping the car more so in going stiffer but still with the 8" x5"id springs

 

Ground Control sells pro7 springs (I'm pretty sure they are made by Eibach). They are 175# ,5" diameter. The springs have several dead coils that you can cut to get the ride height you are looking for. Give them a call and they can probably find them for you. They will work perfectly with your 300# fronts.

 

Great guys thanks for the input, glad to know I'm on the right track.. With the front done I have a taste of the greatness this car is capable of and can't wait to get crackin' on the back.

 

To be OCD it is more like 500# on the rear springs. Don't for get the weight of the axle/tires is un-sprung weight.

 

I calculated my car's rear unsprung weight to be 960lb: Shallow Pockets Rallycross Team's RX-7 - Page 24

I know from driving on/off scales that my car has 1350lb front weight, 1200lb rear weight. That is with the battery mounted in the bin area.

I have never had a 1st-gen that was near 50/50 weigh distribution, they have always been at least 100lb heavier on the front.

 

Pooft... revived

Okay so maybe 150# would be more appropriate accounting for the unsprung weight? I know the 300:150 is popular... likely for a reason.

I have a little rub issue occasionally on the driver's rear since the damper is shot. I figured a little stiffer spring rate would insure against rubbing especially when lowered. Time to roll the fenders I suppose

I hope these rear Mustang Koni dampers work well with this rate... the 1 way adjustability will help like it did w/the MR2 Konis in the front I'm sure.

I don't really want it any stiffer so I'm perfectly fine w 150 .

 

Edited my sig pic... those are the stock SE rims which I still love... bent one

The ride height is the same but now w the Enkei Apache IIs. So the rear gap is similar..but wider with the 8s under there.