custom offroad springs, need some advice

[UP2MTNS]

Hey all,

So I'm getting some custom springs for my Bilsteins. The out-of-the-box springs are nice, but a little harsh and there's not enough travel for rally x/off roading. I bottom out way too much.

So I'm talking to a Bilstein dealer and he says Eibach are the best and will fit on the PSS9's.

I need to know spring height and spring rates. its only going to be $300 for all 4 corners, so not too bad.

DMS50's rates are 224/180lb/in. But I don't know the spring height....any idea?

Also, if a typical WRX is what....3,200lbs, what's the distribution of front to rear? 60% on the front, 40% rear? or closer to 70/30?

EDIT, another question:
lets say you hit a bump....enough to fully extend the suspension and get your tire about 1" off the ground....anyone care to try and figure out the amout of force your car comes down and hits the ground? 2x the weight? 3x?

[I Like It Sideways]

I have 10" eibach springs (with ground control coil sleeves), 225lb front and 200lb rear. 10" should raise your car if nothing else.
Are you talking about the DMS 50mm coilovers?
Those are beefy... I wish I could afford a set


If your car is ~3200 lbs, and you are 1 foot off the ground, your car will land with a force of about (this is very generalized, as the average force is dependent on the distance the car travels vertically after impact) is about 3881lbf So effectively, each tire sees ~970 lbf(varies front to back from weight distribution) Then you get into the effective pressure each tire sees (Force per unit area), which is dependent on the area of contact, and that varies per tire, and the contact patch changes with time as the tire is loaded and unloaded

I think I did that right... hopefully someone else will be able to chime in.

[409industries]

Hes referring to the Bilstein PSS9's (which i am planning on getting, and YES the DSM 50mm's are siiiick) So I am interested to see the result of this spring swap.

[UP2MTNS]

So I did all this in excel and it seems to cut and paste nicely in here. Assuming 3,200lbs and a 58/42% front to rear weight distribution (from someone off of the Cobb forums) here are my calculations. Assuming I use 250lbs/in in the front, and 200 in the rear, that's just a little stiffer than the DMS set up.

________________________________________
All units are LBS and/or Inches, assumes linear springs
Car Weight 3200
Front/Rear Dist. 0.58 0.42

Weight on 1 front tire 928
Weight on 1 rear tire 672

Maximum force = 3x
Front 2784
Rear 2016

Spring Rates (lbs/in)
Front 250
Rear 200

Static Compression (in)
Front 3.712
Rear 3.36

Max Compression (in)
Front 11.136
Rear 10.08

Normal driving spring travel (in)
Front 7.424
Rear 6.72

______________________________________


SO, what this means is if I purchase 12" springs for the front and 10" springs at the rear, at the given rates, the springs will compress 3.7" and 3.6" in the front/rear respectively when you're just sitting there or driving normally.

Giving me 7.4"/6.7" of travel before I bottom out (so I could hit a speed bump pretty hard and not bottom out.

However, if I hit a bump and get a little off the ground (so the springs are at full extension)....at impact (assuming 3x the weight of the car)....the springs will compress to 11.1"/10.08". Which means the front will have another .9inches to go before bottoming out, and the rears will just bottom out slightly.

I dunno....sounds pretty freakin' cool to me. Maybe that's too easy? The math is right, but is the logic sound?

[I Like It Sideways]

Well.. sort of. Unfortunately you don't get the full lengths worth of travel in a spring. Even from full droop to bottomed out. it would take fairly decent jump for the initial impact to be 3x the weight of the car (FWIW weight is different than force, so its the force that I'm actually talking about). I don't quite know how to take into account the damping forces however... but in theory all they are supposed to do is slow down the travel of the springs. You get into friction and heat and viscosity of the fluid etc etc and it becomes a much more dynamic problem. Dang me for not paying more attention in class

[UP2MTNS]

Well.. sort of. Unfortunately you don't get the full lengths worth of travel in a spring. Even from full droop to bottomed out. it would take fairly decent jump for the initial impact to be 3x the weight of the car (FWIW weight is different than force, so its the force that I'm actually talking about). I don't quite know how to take into account the damping forces however... but in theory all they are supposed to do is slow down the travel of the springs. You get into friction and heat and viscosity of the fluid etc etc and it becomes a much more dynamic problem. Dang me for not paying more attention in class

yeah, I'm making a lot of assumptions that aren't listed as well, but that's just to keep me from going insane.

I just took a bunch of measurements....it looks like there might not even be enough room for 12" springs anyways....so its going to have to be 10" springs I think. we'll see.

[I Like It Sideways]

Well.. sort of. Unfortunately you don't get the full lengths worth of travel in a spring. Even from full droop to bottomed out. it would take fairly decent jump for the initial impact to be 3x the weight of the car (FWIW weight is different than force, so its the force that I'm actually talking about). I don't quite know how to take into account the damping forces however... but in theory all they are supposed to do is slow down the travel of the springs. You get into friction and heat and viscosity of the fluid etc etc and it becomes a much more dynamic problem. Dang me for not paying more attention in class

yeah, I'm making a lot of assumptions that aren't listed as well, but that's just to keep me from going insane.

I just took a bunch of measurements....it looks like there might not even be enough room for 12" springs anyways....so its going to have to be 10" springs I think. we'll see.

Yeah we both were making assumptions, but it helps to give us a general idea of what you're looking for. Thats the problem with problems like this, there are too F'ing many assumptions to make
12" springs are pretty long.. a friend of mine cut and re-located his spring perch lower so he could fit longer springs (less heat generated by the spring by doing this)

10" springs will do the job just fine though, especially for a strut/spring combo

[Dynapar]

I know someone locally who was running full length springs on his impreza ice race car. I know that they were full lenght so that would make them the same as a stock springs which is like what 10" - 12"? The spring rates were at about 200/180. hope this helps

[UP2MTNS]

one of my dumbest assumptions above is that a 12" spring will have 12" of travel. so what do I expect, that the spring will compress into itself into a flat piece of paper? LOL dumbass

anyway

so I ordered 12" springs for the front. 250lbs/in
10" for the rear, 200lbs/in

talked to folks at bilstein, vt sports Car, and All Wheels Driven.....rates and lengths are good. what will be interesting is how the shocks do.

will hopefully have them installed by Antioch 1. BBQ folks on the 23rd may be recuited to help out!! 8)

(BTW, gotta love the rally community......Dustin at VT Sports Car and Barrett at All Wheels Driven both took about 10-15 minutes to talk to me on the phone about what they do for their stage rally suspension set ups. great help/feedback. Dustin even offered to help me out with used parts, etc if I need anything. And Barrett remembered me from Paul E's school. awesome guys. )

[409industries]

one of my dumbest assumptions above is that a 12" spring will have 12" of travel. so what do I expect, that the spring will compress into itself into a flat piece of paper? LOL dumbass


We all forget easy stuff sometimes. The paperthin spring comment actually had me laughing out loud. Funny stuff. You know "upgrading" your car the night before the rallycross could make for some hustle and bustle...ill lend a hand for sure. especially if theres beers ;)

[BlueRu]

If I make it then I would be glad to lend a hand! Although thats about all I would be good for in this matter. Handing everyone else stuff.

[UP2MTNS]

If I make it then I would be glad to lend a hand! Although thats about all I would be good for in this matter. Handing everyone else stuff.

You can hand us beers :)

If I get everything in by Friday the 22nd, I'll probably start that night and do 2 corners and then do the other two the next morning. Since I've taken my suspension on and off a few times already, I've got it down pretty good....hopefully no suprises the day before the race.

[UP2MTNS]

bump/update.

ok, measured the inner diameter wrong. Bilsteins are german...hence metric. 2.5" inner diameter springs were too small....too one corner off, and they were 2.75" (which is actually a 70mm ID spring).

SO, eibach only makes 2.5, 3.0", or 5.0" springs. make another call to Bilstein...."oh yeah, you either want Swift springs or H&R springs." great.

do a search, find these guys:
http://www.upgrademotoring.com/suspension/swift_coil_70.htm#11in

Swift doesn't make 12" springs (length), so I'm going with 10" springs on all 4 corners, but slightly higher spring rates (280f/224r). They'll be in next week, so I'll have more to report then.

A little more expensive, but the Bilstein guy says they're the best and worth it. We'll see. Oh, he does send me all the valving specs on the PSS9's at full soft and full stiff....not sure if it makes sense to me yet, but this combo should be pretty cool.

[409industries]

Sounds like it will be awesome when its done. I know that the H&R's are badass springs as well. probably just as expensive as the swifts, if not more.