Spring rate question for suspension guru's

[olympic]

Ok, one way or another the SHOpaz is getting a coil-over suspension conversion. It's just too damn far off the ground! I found a kit that I think will work and it comes with 450lb/inch front springs and 335lb/inch rear springs. The specs on my car are 3100lbs with driver and a 62/38 weight split.

My question is, are these spring rates appropriate for a daily driver/weekend road racer type of car? Any opinions are welcome.

<small>[ September 25, 2002, 04:50 AM: Message edited by: olympic ]</small>

 

[Toolman]

Sounds a little stiff for a daily driver to me. My 95 has the IPT coils, and 600/F, 350/R. But my car wieghs over 3700 lbs with driver (approximately), and it is too stiff for daily driven use IMO. Especially do the rear 335 lb springs sound stiff for a car with only around 1000 lbs on that end. I guess it all comes down to what you are willing to tolerate from day to day. Maybe the Canadian roads are in better shape than Oklahoma's (can't be worse). BTW, which coilovers are you using? (didn't know anybody made coils for Topaz's).

 

[olympic]

The kit I'm looking at is just a generic kit that comes with springs, collars and perches. So I'd have to make some custom strut hats. I too thought that the 335lb/inch springs for the rear sounded a bit harsh but I normally have about 200lbs of stereo equipment in the trunk which is not included in the 3100lb weight I stated above. So maybe things would equal out when I'm on the street with the stereo and on the track without the stereo.

I think I'm going to buy the kit and see how I like it. The springs are just ordinary 2.5" I.D coil overs so I can swap them out for softer springs if I want too.

 

[ThrillSHO]

Like most other decisions, spring selection is a compromise. You have to begin by asking, “What am I trying to accomplish?” A smooth ride? Minimum ride height? Maximum wheel travel? These are all competing criteria, but not necessarily mutually exclusive. You mentioned lowering the car, so we will start there. You need to know the desired ride height, the available suspension travel, the car weight (ideally the corner weights) and the road characteristics you are likely to encounter.

Measure your currently available suspension travel. Let’s assume it is 5”. If you want to lower the car 2”, this will leave you 3” of suspension travel. In reality it might be slightly more or less depending on suspension geometry, but for simplicity let’s figure 3” left. If the car weighs 3100lbs and is biased 62/38 front to back, the static weight on the front wheels is 961lbs each. Let’s assume you are going to be careful where you drive (avoiding potholes and such) so we design for a maximum 1.5g bump. Based on these assumptions, you would be absorbing a 1441lb force over 3”, therefore requiring a minimum spring rate of 480lb/in.

As you can see, suspension travel has a dramatic impact on spring rate. In this example, if you lower the car another inch you would need 720lb/in springs. You can go with softer springs and stiffer shocks, but then you are relaying on shock dampening to keep from hitting the bumpstops. Konis work well for this because they are adjustable for bump, but fixed for rebound.

FWIW, I have about 2” of travel and run 700lb/in springs in the front and 450lb/in in the rear. How stiff is it? If a 200lb man sits on the front fender, the spring compresses about a ¼”.

<small>[ September 27, 2002, 10:17 AM: Message edited by: ThrillSHO ]</small>

 

[RI-SHO]

I have about an inch of gap between the fender and tire upfront, the rear sits basically at tire tread level, all with SS springs. I have never rubbed before but my springs defitnaly feel stiff as ****.

Does anyone know what spring rate my SHO should be at with "regular" type springs(compared to coilover springs), and what specs the SHO Shop springs had?

<small>[ September 27, 2002, 03:10 PM: Message edited by: RI-SHO ]</small>

 

[shojuan]

ThrillSHO,

That was a great post. I think with a calculator and a little thought, just about anybody could visualize what setup they need with that succinct little "netucation"!

Rick

 

[olympic]

Yes, thank you ThrillSHO! That definitely answers some questions for me.

 

[ThrillSHO]

Thank you guys! Technically, the procedure I described yeilds the "wheel rate." This is then translated thru the suspension geometry to determine spring-rate. Fortunately, with Mcpherson strut suspension, wheel-rate is a close approximation to spring-rate with spring-rate being slightly higher

RI-SHO here are some different spring rates:

Stock SHO Springs - 200lb/in front, 160lb/in rear
SHOShop Linears - 240lb/in front, 190lb/in rear
Eibach Progressives - 150-250lb/in front, 150-190lb/in rear

I don't have info on Intrax springs.

<small>[ September 30, 2002, 03:51 PM: Message edited by: ThrillSHO ]</small>