here is a quick excel sheet i whipped up to help people pick spring rates





here are some helpful tips.

the weight in the box is for a stock IROC as an averge of the different ones listed online in various places... your mileage may vary.

the front end weight is again, for an average IROC, assuming a weight distribution of 57% front 43% rear which is an average

if you have an accurate front end weight you may enter it in the box.

the total vehicle weight box has no effect on the final calculations except to figure out the front end weight assuming a 57/43 distribution



the spring rate box is where you put the known (or calculated spring rate) of the springs you intend to run
then enter the free length of the spring

you can compare up to 5 springs (or more if you want)

stock, in this case is for a moog 5664 spring




ok, so i fixed the issue hopefully.
the chart is now inserted into a word document.
see if this works

 

Not having much luck opening the zipfile (work computer thinks the file has been corrupted).

Is this a ride frequency or "flat ride" sort of calculation?


Norm

 

Latop at home doesn't like it either.


Norm

 

no, its nothing that sophisticated.
just a rate and free length calculator to figure out theoretical drop.
try opening the .doc

 

the word doc doesnt work on my comp either... just tell me the theoretic drop for the moog 5664s... since thats what im running

 

Sorry, *.doc no workee either.


Norm

 

jesus....
well i give up.
it worked in my other thread.
the 6554 will give 0 drop because i used that as a refrence.
but, if you tell me your original spring PN then i can find the info and give you the answer

i dont get it
i can download it just fine




when you click, what happens?
on mine it says, "would you like to open or download" or something
then i click open with word
then it opens
i click on the table click edit
it opens the table for editing in a new version
i edit the table

 

After a little detective work, I found the other thread. But I also had to play around with that one a bit to get it opened up as a usable spreadsheet.

I needed to save "spring_chart(1).xls.doc" as "spring_chart(1).xls" first, and then open it in Excel; otherwise it tried to open in Word as a mass of ASCII characters with undecipherable meaning.

I have one immediate question - Are you considering the portion of the free length that becomes inactive portions of coils (in contact with the seats and cannot deflect no matter how much load) in your deflection computations? If so, the values in column F are probably a little off.

Do you mean spring #5664?


PS - don't take these comments too hard - I do technical and independent review of engineering calculations as part of my day job in an industry that is kind of picky about such things.

And FWIW, I have a spreadsheet copy of the Moog paperback catalog table that I'm occasionally adding features to. Number of active coils is already done, back-figured from the dimensions and an assumed value for Young's modulus. And it can be sorted a little like a database. PM me with an e-mail address if you're interested.


Norm

 

the reason i uploaded it as .xls.doc is because TGO wont upload a xls directly
i should have mentioned that the rates i am using are advertised rates of circle track springs which are compatible with our front suspension and only cost slightly more than standard moog springs.


yes, i do.
i have a come down with something and for some reason i have been feeling half drunk for the past few days.

anyway, the front end weights and vehicle weight in that version are for a buic GN, and i completely forgot to think about that when i first uploaded the chart.
i was looking at a chart on a GN site and thought, wouldnt it be great if i could see the theoretical drop of whatever springs i want?
so i made this.
apparently TGO hates me and wont let it work though.
who knows

 

I downloaded your doc and xls just fine, so it works for me.

So to make sure I understand this correctly, a STOCK spring is 13" free and #750 right?

So on your last example in the spreadsheet, you have 12" free spring with 1000# rate and that "should" drop the car about an inch correct?

I'm thinking of getting 1100 lb spring rate front and 1000 lb rears with 11" free height, is that just stupid or will it work?

 

Here's a screenshot of the chart with some extra additions at the end:

 

this is only for the fronts.
for the rears, you want to run something in the 300# range max

 

300# max? would you explain why? If there's roughly a 55/45 distribution then if the fronts are 1000 the rears to be equal ratio should be close to 850-900# right? Why run such a soft spring in the rear

 

It's not the springs themselves that you're "balancing", it's the corner frequencies of the car. Meaning that it's the "wheel rates" that really matter. Up front, the wheel rate is much lower than the spring rate, by virtue of the "motion ratio" for the spring. At the wheel, a 1000 lb/in spring only gives a wheel rate of 250 lb/in if the spring's motion ratio is 0.5 (spring would be about halfway out on the LCA and would compress half an inch when the wheel moves 1").

A 250 wheel rate up front makes a 300 wheel rate in the back look a lot better, no?


Edit/disclaimer - the above motion ratio number is a crude approximation for purposes of illustration only.


Norm

 

sorta kinda but not really...
you got the math backwards.

motion ratios are bigger than one.
a ratio of 2:1 means that for every inch the spring compresses, the wheel moves 2"
in a live axle rear suspension, the motion ratio in direct compression (no roll) is always 1:1 unless there is a truck arm setup or something else funky like that.
up front, we have about a 1.4 ratio or something... it might even be higher than that.

then, realize that you are going to need some safety margin for oversteer.
if the wheel rates are equal front and rear, the wheels will break free at the exact same time (gross simplification, but here, it works)
that is assuming that you are not giving the car any gas or breaks at the time.
if you are giving it any gas at all, the rear will break free much sooner.
if you are on the brakes at all, the rear will want to break sooner.
if the rear bar is stiffer than the front in relation to the mass each has to control, then the rear will break sooner.
...
it goes on and on.



ideally, the car should be set up so that with no sway bars at all, the car will corner more or less neutrally with a hint of understeer.
then you add swaybars to keep the car level.
we have the luxury of enough power to break the rear tires loose with the gas pedal in just about any circumstance if necessary, so unless you are trying to squeeze the last bit out of a corner, understeer is the safest way to set your car up... not too much, but just enough.

 

Great info for a suspension beginner like me, I appreciate it.

My question then becomes what is the "optimum" rates for front and rear for our cars by your definition? Assuming an example rate, such as 800# front and 250# rear (just an example) then what is you increased each spring by 20%, so you now have 960 fronts and 300 rears.

Am i misunderstanding your logic?

 

i couldnt tell you what is optimum.
my reason for getting into this was because i wanted to lower the car.
a lot of spring manufacturers will give you shorter springs with the same rate and call it a day.
when you lower the car, the CG gets lower, but the roll center lowers faster, which is bad.
in order to improve handling, you must increase the spring rates as you lower the car.

i dont know any specific numbers, but the springs i have now are 700# springs.
my front ride height is in my opinion way too high.
i want to lower the front about 1.5 - 2inches.
the formula for cutting springs is around here somewhere but i cant remember it off the top of my head.
the point is that i figure that if i were to assume i had 750# springs (moog 5664, the stiffest you got in our cars)
and i cut them to the correct length to get the ride height i wanted, the overall effect on the car would be minimal because the springs were stock rate that i cut.
blah blah blah, somehow i figured out that at the length i wanted, i would need a rate of about 870# to keep the suspension happy.
then i rounded it up to 900# so that i will actually decrease the roll in the suspension.
i may go with 950# but that seems awfully high for a completely street driven daily driver car.

 

Now you're going to make me drag out my RCVD when I get home to make sure I'm not mixing motion ratio and installation ratio. Picky little detail that doesn't really matter as long as you get the wheel rate OK.

Actually, I was guessing at the 0.5 figure (2.0) with some reason - that number is about what the MR for my '79 Malibu is (the fact that the 'bu is a SLA design introduces only a 2%-ish difference at ±1.5" wheel travel) and I suspect that the LCA for the 3rd Gen F-body was either derived from or taken directly from the downsized G-body cars.

From a manufacturing standpoint, if you're not going to do either a SLA or a full strut but stop at a modified strut suspension arrangement instead, it makes a lot more sense if it's because you're raiding the parts bin for the LCA, LBJ, and at least the basic spring dimensions. Otherwise you're putting the spring loads on the chassis at one point and the damper loads at another for little gain (and forcing yourself to provide adequate structure in two places vs one).

FWIW, the 4th Gen F is about 0.58 (1.7).


Notation differences aside, I think we have rather similar chassis tuning/chassis development philosophies.


Norm

 

im just wondering why you cant open the daggon file lol




oh, and if the 4th gen ratio is 1.7, then i would assume ours is too.
i would assume GM in its infinite wisdom would not want to have to spec a whole new line of springs for the 4th gen line.

 

oh, and for what its worth, a 900# 11" spring with a 1" spacer will have a different ride height than a 900# 12" spring
the shorter the spring, the more each coil has to deflect.
longer springs spread the deflection out more and have to have thicker coils to make up for it.
the difference is minor, but its there. id say about .25" or so

 

So Dragon, if you're going to get 900# fronts what # rears would you get? I'm sort of in the same boat, I cut my stock z28 springs to lower my car about 2.5" or more (not sure exactly) but I now want to go buy springs with a higher rate that will keep the car about the same height but as you put it "keep the suspension happy"

I'm assuming 250# was stock rear? So going from 700# fronts to 950# fronts is a increase of 35%. 250# increased by 35% would be 338#, rounded up or down to whatever the maker offers.

 

stock rears are in the neighborhood of like 175# or so...
i have no idea.

i dont plan to get new rear springs yet...
i only want new front ones so that it will be lower.
i am happy with the rear so far.

for some reason my car has a "nose up" kinda style going on

 

If this is primarily a street car, there is a relationship between front and rear wheel rates that naturally gives a "flat ride" at a certain speed. "Flat ride" refers to NOT having a front-to-back head-snappy ride like being in an old pickup truck with stiff springs, dead shocks, nothing in the bed, being driven down a rutted/potholed dirt road. Generally, that means that the rear ride frequency is slightly higher than the front.

You'd choose spring rates to hit ride frequencies suitable for your purposes (and perhaps your own tolerance for a stiff-legged ride) Ideally, you'd choose front and rear springs to make the flat ride speed fall well within the range of speeds that you normally drive, without going too far outside your preferred frequency range.

1.0 Hz = pretty soft, traditional domestic sedans
1.5 Hz = firmish
2.0 Hz and up = race or serious autocross territory


I'm guessing that a 700/175 combination puts you around 45 mph flat ride speed (pretty good place for a street-driven car to be, IMO). To match this with 950 fronts, you'd want 255's out back, so I think you be looking at 250's. The rear increase is proportionately a little greater than the front increase, so the handling could be expected to loosen a little. Or you could choose 225's and tighten it a little (the flat ride speed would go up into the high 50's though).


Norm

 

I'm a little confused by this, after reading it a couple times I think I understand your logic. If you want a stiffer ride without sacrificing flat ride speed, you have to keep the ratio correct as you increase the spring rate. So to keep a 45, like you said, you would have to go from 700 to 950 fronts and 175 to 250 out back.

Now you also said something about loose handling if the ratio is incorrect. To make a tighter handling car you said to ease up on the rear rate. So cars that have stiffer front than back springs (ratio-wise) means tighter handling yet bumpier low-speed ride?

So what does the car feel like with stiffer springs, such as going 35% stiffer on both front and back springs? I have a very high tolerance for a stiff ride, I actually hate riding in cars that are even remotely "squishy" in their ride.

Also what would shocks do to your ride, if you had 950/250 rate springs and medium shocks vs. the same springs with stiff shocks?

I have stock springs cut for lowering the car, and very stiff shocks and I love the ride. I am therefore thinking stiffer springs and very stiff shocks would be even more stiff of a ride = better (I want a stiffer ride).

 

Short answer, yes.

It's a bit more complex than that, but for the simple comparisons we're working with here (spring swaps only) if the front springs were made 35% stiffer and the rears only 25% stiffer, the handling would tend to get "tighter" (or shift toward more understeer). My use of "tighter" was in circle-track language where it means the opposite of "loose", (IOW, oversteer, or what the tail feels like if you've got a bad enough case of this)

Vertical bumps will hit the chassis 35% harder, and the ride frequencies will go up about 16%. High-strung/nervous is one way to think of it. My current autocross car at 1.5 Hz front, 1.75 Hz rear is starting to feel like this, although I know that a lot of it is the Tokico struts - they tend to be valved a little on the harsh side. I can live with it OK, but it's a lot more harsh than my other DD (and my wife doesn't like to ride in it so much any more). I think 950/250 puts you around 1.5 / 1.85, 700/175 is good for about 1.33 / 1.6.

This one is WAY more complex than soft/medium/firm, and I'm not a shock engineer. The stiffer springs probably want firmer low speed rebound damping and less high speed bump (compression) damping. That's shock piston speed, BTW, not car speed.

There is such a thing as too much here, and at some point stiffer = slower no matter what sort of driving you're doing.



One other thing - don't go over-interpreting any of these numbers, or even numbers based on lots more measurements than I was mostly estimating at. Sooner or later you have to drive the thing, and you may find that you actually prefer something other than what corresponds to the answers to the mathematical equations. Maybe you want a little extra "slack" to cover for getting a little sloppy from time to time and don't know it. IOW, this kind of thinking from Red Dragon

Norm

 

I'm impressed by dragon and norm's knowledge on this subject, thanks for taking time to help.

 

everything i have learned so far is from reading and asking questions.

NORM, one thing that i have noticed is that if your front and rear frequency rates are exactly the same, and your damped frequency is the same front and back, the car will tend to pitch front and back more than it should when you hit a bump with both front wheels and both back wheels, like a speed bump.

typical design pracitce is to set the spring frequency a bit higher on the rear than the fronts so that the rear will "catch" the front
what i mean by this is:
say you are driving at 45 mph and hit a bump that causes 2" deflection in the front and rear suspension, but the front is .25 seconds ahead of the rear.
if the frequency is the same and both are similarly damped (in relation to the rate) the car will continue to follow a sine wave type form where the front goes up, back goes up, front goes down, rear goes down.

this can make the car hard to control and cause motion sickness if the sympathetic frequencies are right.

common practice then is to make the rear frequency 1.1 times higher (about) then you normally would.

to make the math easy, fronts are 2, rear would normally be 2, but for tuning, you make it 2.2.
this will give you some STATIC oversteer, but you can tune it out with bigger front swaybars and shock tuning.
you can also play with the roll centers to get it to feel "right"

i dont really know or understand the math behind most of what you were saying about the 45 stuff, but it could be because i was too impatient to sit and read it (my ADD must be flaring up again lol)



TPX, before you go and run out and buy new springs, keep a few things in mind.
1) i made up all the numbers i have given you for rear springs. if it were me, id pull one of the rear springs, go get about 5 50# weights and do some testing. record the length with no weight, add a weight, record, add a weight, record, add a weight, record.
you can then do some math and determine the spring rate.
i say get a bunch because it gives you more data points to work with and you can account for rate creep ect...

2) shocks.
if you get 900# springs as i plan to, you will absolutely positively need new shocks. and gabriel red ryders wont cut it.
i am going to get KYB AGX adjustable shocks front and rear when i eventually get around to tackling the suspension.

3) alignment.
you will lower the car significantly in most cases and that will make it harder to get a "correct" alignment. in the front, you will need new caster camber plates if you lower it more than, oh, id say 2 inches.
for the rear, figure some LCARBS and a new adjustable panhard bar at the min, and adjustable LCA's and torque arm are recommended.
the torque arm is not required because it is going to keep the pinion angle at -2, but you may run into a situation where the wheel needs to move foreward or back in the wheel well to look right and thats where the LCAs come into play

4) comfort.
true, a stiff car is more fun on a smooth road, but that fun quickly wears off when you have 3" deep potholes.
keep that in mind

 

Thanks for the extra pointers. As for your first point, I dug up some info on the rates but I can't decipher it. You know what it means? Here's an exceprt followed by the link it came from:

"Suspension: Stiffer bracing and bushing material than base camaro.
58/32 N/mm Front/Rear Spring Rate (SC = 58/18, F41 = 58/18)
31/21 mm Front/Rear Sway bar diameter (SC = 27/12, F41 = 29/12)"
http://www.iroczone.com/specs/camaro/camaro82.html

A worth note, I do not have control arms or anything like that in the rear, I have a solid rear axle. The springs might be stock but I have no way of telling.

For shocks, I was going to get some high performance brand that was adjustable. I have red shocks that came with the car that I have no idea what brand they are. Pretty stiff though.

Alignment, I will look into the plates for the front but I'm not sure if I need to buy a new torque arm. I have a panhard bar. (what do panhard bars even do)

As for the comfort, what is that? haha, I've been driving my car up here in arcata (aka pothole central) and it has to have the crappiest roads I've ever had the misfortune to have to drive on. I go back home and the roads are all smooth and newly paved. No cracks or potholes for miles. It's alright, I don't plan on living up here in Arcata but I do plan on keeping the car so stiffer ride is a must.

 

You understand the concept well enough, the rest is just nuts and bolts detail. The math isn't that tough, and that can be worked up from the information in Fred Puhn's book. 45 mph just happened to be where my best guess at enough numbers puts the flat ride in this particular situation; the speed that lets the rear suspension complete exactly one complete cycle exactly when the front suspension does. I've just written it into a number of spreadsheets so that all I have to do is plug in a few numbers and right away I'll have a number I can "talk" with. In this case, a ?? mph flat ride speed is probably more easily understood than separate front and rear frequencies. 10% is a ballpark estimate, pretty good in many cases but may be a bit off if your car has a particularly long or short wheelbase, or if extremely high speed pitch behavior (pitch behavior = what this whole "flat ride" thing is really about, actually) is being investigated.



Norm

 

331 lb/in front . . . . . are you positive about this - I was expecting about 550 lb/in here
183 lb/in rear


331 lb/in front
103 lb/in rear


Norm

 

Well I'm not positive, but click on the link there and see for yourself. The info could be incorrect but I see no reason why it would be so. It does seem low huh?

 

Took me a while to find it, but here's a summary of what Road & Track had to say about 3rd Gen suspension tuning back in January 1982, meaning that this was at least the plan about three months earlier than that. I'm not trying to claim that R&T is the ultimate authority on this particular item, but they do have a pretty good reputation for reliable reporting.

Model - front spring/rear spring . . . front bar/rear bar

Camaro
Base - 331/103 . . . 27/none
Berlinetta - 331/103 . . . 27/none
F41 - 365/103 . . . 29/12
Z28 - 548/183 . . . 31/21


Firebird
Level I - - 331/103 . . . 27/none . . . . (Base)
Level II - 365/103 . . . 30/12
Level III - 548/134 . . . 32/21


I do know that GM kept tinkering with the springs and bars on these cars, so all sorts of possibilities exist. FWIW, 331/183 gives a flat ride speed somewhere down around 10 mph.


Norm

 

the easy way to get your spring rates is to go to NAPA and ask them for replacement springs for your model, trim, engine, ect.
if you have an iroc, fronts should be 5662 and rears are like 5665.
either way, whatever you get, then go look up the part numbers on the MOOG spring chart.
ill try and upload it, but i seem to be having issues with attachments