308 Coil Spring Suspension Data Sheet

Mike, I have been looking into the same issues. There are many factors such as car weight, distribution of the weight that determines the spring rate and length used. I don't think you can compare GTBs to GTSs without compensating for those factors. When I did the calculations for the GTS it turns out that the laden length of the springs in front was longer then that of the rear. That maybe why our cars have that pitched up look up front. I am trying to work with a coil spring manufacturer to reverse the appearance by altering the specs of the coils without sacrificing handeling. This is the calculations that I sent for the stock springs. It is a copy of the email.


Dear Sirs,

Please check my math, although I have a degree in physics in years past I do not do this as a profession as you do. Bear with me. Also please use these numbers to see if you can produce the coils and give me an approximate cost. I will measure the actual coils on the car now to get the weighted lengths for the final numbers which should be pretty close to those calculated below. I am still undecided about dropping the front about 1/2 inch.
Here are my calculations:




The chart I sent attached specifies spring rate as .32 front .30 rear in mm/kg this needs to be converted to more commonly used units please see below**:




As far as loaded height, that should be determined by the weight of the car and the distribution. You should not need this for making the spring given the total length of the coil and the other given variables but guessing:




Weight total 3229 lbs. (84-85 308 GTS QV) assuming ~ 40/60 weight distribution front/back (best guess based upon web search)






front coil =Free length: 14.50"
Spring Rate: 175 pounds / inch

The operating height of the front coils will be: 14.50" - (645.8pounds / 175 pounds per inch) = 10.8" (remember to halve the weight)




rear coil =Free length: 15.19"
Spring Rate: 187 pounds / inch

The operating height of the rear coils will be: 15.19" - (968.7 pounds / 187 pounds per inch) = 10.01" (remember to halve the weight)






**

I believe the math is to take the reciprocal of the values in the chart I sent and multiply by 55.998 (constant for converting kg/mm to lbs/in) so:


1 / .32mm/kg = 3.125kg/mm 3.125kg/mm X 55.998 = 175 lbs/in for the front

1 / .30mm/kg = 3.333kg/mm 3.333kg/mm X 55.998 = 187 lbs/in for the rear

 

Unfortunately, you need to also take into account the mechanical layout of the suspension elements and how they interact -- i.e.: 1) the spring acts closer to the inboard a-arm pivot axis than the tire, and 2) the spring is not mounted vertically. Both effects tend to increase the load on the spring vs the load on the tire so the springs gets more compressed than you have indicated (especially the front spring that is inclinded at a rather large angle). See pages F6 and G2 in the 308GT4 WSM which give the working 308GT4 spring lengths as:

front = 205 mm (8.1")

rear = 227 mm (8.9")

 

Don't have any useful data to add, but I believe that the extra "flexibility" of the GTS chassis is the reason why they had to soften up the GTS suspension a little compared to the GTB -- i.e., the higher the wheel rates, the lower the chassis torsional rate appears so the front and back end start to feel more like two rigid bodies connected by a flabby spring (and can vibrate more violently wrt each other torsionally). By lowering the wheel rates, the whole system behaves in a more mushy way -- just a thought...

 

I've found this list recently (can't remember where exactly..). It gives an overview of all technical details of the springs, euro and US.
Image Unavailable, Please Login

 

In case this thread ends up going in this direction, the springs that came out of my Aussie glass GTB were around the 150pounds/" mark. I've installed new springs which are around 200pounds/", lowers the car by about 20mm all around too and obviously had the shocks revalved to suit. Made a big and pleasant difference. It was far too soft and rode too high for my liking as standard.

The last photo is a 'before' shot.
Image Unavailable, Please Login
Image Unavailable, Please Login
Image Unavailable, Please Login

 

Okay, great points. Back to the calculator. This should be a straight vector problem with proper angles if published. I will check the WSM and get back. Thanks Steve! Also, to make these springs the information given in the manuals should be enough to reproduce them. Why would a manufacturer need the laden spring height?

 

Because that's how the spring is used. Coil springs do not always have perfectly linear force vs length behavior (especially those with squared and ground ends) nor do all springs (even though made as identically as possible) have the exact same spring rate -- you can note that F calls for sorting springs in matched pairs and marking different pairs with different color paint stripes. So best to specify the laden spring length and the desired spring load at that laden length (which is not the wheel load) rather than the spring's free length at zero load (and rely on a theoretical calculation and all springs being exactly identical). Although free length at zero load is often included in the specification just as a way for the Designer to communicate as much information as possible to the Manufacturer -- but it's usually not used as something that gets inspected as a critical feature and/or it will have a much larger tolerance than the laden spring length.

 

not agree completely

we did extensive testing with springs from different brands.

all the "cheap" ones turned out very unreliable regarging liniarity. Per cm of pressure in the real value of tension turned out out of spec to both ways ( so less or over expected kg value) with sometimes 40%. on the same spring it was sometime far over and another cm further it was well under the expected value.

furthermore it turnede out that after use, sometimes for only half a year of tracking the line was completely different.

the only exception we found were eibach springs. Not only did they follow the expected line withing 5% but even after a full year of tracking the line kept about the same.

so for me it's not mainly about ordering it right but far more, ordering at the right place. The market is full of very cheap chineese made springs: forget them!

 

Would the temper of the spring be the responsible factor in what you found?

 

don't know what you mean with temper but we bought all sets as matched sets for "racing purpose"
most of them turned out to be of poor quality in both rating and durability