First post here. I've got an '88 firebird that's all stock for now. Picked up some interior, susp. & exhaust stuff lately. Got a set of Intrax coils for the back end at a good price, those will give me the drop I want and they are 184lbs. Been looking at lowering coils for the front but they seem kind of soft, I'd like to get 650-750lbs. with a 2" drop up front. Checking specs on Moog coils, looks like there are 3 possible sets #s 5660, 5662, 5664, they're in the lbs. range I want. I know cutting springs increases the spring rate, But How Much. I want a 2" drop that wont drive like my truck and will corner flat. Spring rate on the 5660 is 598lbs. and the 5662 is just over 700lbs. I think the 5664 is already about 750lbs. I'd cut 1-1 1/4 turns first and try that first and go from there. I've cut over a dozen set of springs in the past and from experience I've learned to cut a little at a time. Also bought a set of C6 five spoke I'll throw on in the future. These springs seem to be linear and if anyone has some kind of formula to figure out spring rates ? Later.

5660 - 650lbs/in
5662 - 700lbs/in


There isn't really an accurate formula for the increase in spring rate based on how much you cut. I'd leave spring cutting for the honda-matic drivers, and just throw on the 5664's and enjoy.
If you really need to drag on the ground, get the 5660's and chop them an inch or so. They'll be plenty hard for ya.

Yeah Sonix, that kind of confirms what I planned, the 5662 or 5664 would keep the stock height so by trimming the 5660 I should get the right height and the right spring rate. Later.

Just got my Moog 5660 coils and will try to install next week. Going to put them in stock height first and then trim from there, going to be extra work but no problem, it's not my only car so no rush. I should get the spring rate and height I'm looking for 2"- 2 1/2" 700 lbs. Later.

Well I got tired of seeing my car sitting up high so last night I decided to put all my springs in, took me five hrs. about two hrs. too long. I planned on putting them in stock height and then trim from there to get the stance I wanted but without a spring compressor that didn't work so I trimmed 3/4 of a coil and it still wouldn't go after trying for a while so I trimmed another 1/4 turn (total 1 full turn) and that worked out, the hardest thing was lining up the two bushing bolts on each. To get the stock ones off I just took out the bushing bolts with jackstands under and jack under the arm, I put a length of chain with a bolt in it just in case something went flying, didn't even take wheels off. I was determined to do it last night and did, the height of these after trimming is 12 1/2" (trim bottom) stock is 15". I wanted about a 2" drop and got 1 3/4", so I'll let it settle for a couple weeks and may trim a little more. Still got 3 1/4" clearance with 3" exhaust, when u lower remember to cut down ur front rubber stops 3/4- 1" otherwise they just limit some movement. Pretty much happy with how it sits. Later.

Alright, just got some BMR LCA's and some J&M relocation brackets, that'll take care of that nasty wheel hop. Just put in some carpet, seat belts and working on my console swap now. Later.

Can you please takle some pix from the side of the car?

Im planning on doing the same, because i know the stock springs would ride too high. I like the height my car is now (with orginal factory suspension) ie: 21 yrs worn out. But i dont want it to be much higher than now.

As for a formula.

search for posts by BmonteSS, he had some mathematical stuff to say about it.

1full turn is quite a bit of a cut. His "formula" suggested that 1/2 a coil was worth about 1-1.5", and a 30%? increase in spring rate. Dont quote me on that, but it seems to be what i remember.

Yeah I took some pics, so I'll post when I figure it out. One full turn brought it down just about 2"(just right). Later.

Here's a pic. Later.

Oops, here it is. Later.

Nice car, i thought it would be a little too low, but that actually looks pretty good. Seems like you also have plenty of clearance.

Did your car not come with spats? You have the special appearance package already.

Hey thanks, there were 3 spats, one missing but I like the look without. I've got lots of clearance all around. Like I said I used the 5660 and trimmed one full coil. Later.

Nice job. That's about how low I'd like mine. Where did you find the spring rates for the Intrax?

I found the Intrax on a wrecked Z28 and they happened to be about the correct spring rate I wanted. Later.

spring rate = (11,250,000 X wire diameter^4) / (8 X active turns X mean diameter)

Since the only variable being changed by trimming your coils is the number of active turns, simply divide the original number of active turns by the number of remaining turns after trimming then multiply by the original spring rate to get the new spring rate.

Example:

Given:
Original spring rate = 700 lb.
Original number of active turns = 10
Active turns after trimming = 8.5

Solution:

10 / 8.5 X 700 lb. = 1.1764706 X 700 lb. = 823.52941 lb.

Happy computing!
----------
Oopsie! I forgot to define active turns!

Active turns are the number of coils not including anything that touches the spring seats.

Cheers

Thanks 84IMSA, by the way do you mind posting a couple pics with the wide body kit, thnx. Later.

Do you want pics with or without the car cover?

Okay, I'll post some pics in a couple of days. They'll be in the Body tech board.

Cheers!

I was shopping for springs for my 70 Esprit and it turns out it uses the same configuration springs as the 3rd gens.

I found 28 Moog coil springs that have inside diameters of 4.080 to 4.090 inches and installed heights from 10.50 to 12.00 inches.

Spring rates (unmodified):
5660 - 598#
5662 - 748#
5664 - 707#

Applying the spring rate formula from my previous post, I came up with the resultant spring rates for 1" and 2" drops after cutting these springs:

Spring Rates (1" drop):
5660 - 659#
5662 - 825#
5664 - 780#

Spring Rates (2" drop):
5660 - 735#
5662 - 919#
5664 - 869#

919# Better make sure your dentures are glued in securely!



Based on the stock installed height of these springs, here is how much lower the springs would be per full coil cut:

5660 - 1.90"
5662 - 2.04"
5664 - 1.80"

So it looks like approximately a 1" drop per half coil cut for these three part numbers.

Please note that these figures are based purely on mathematics. YMMV! Every article I've read on cutting coil springs has recommended cutting only a half a coil at a time, reinstalling, driving the car, then cutting more as required.

To paraphrase the warning in the Eaton Springs article: There is no known way to add coils to a spring.

Cheers!

This is wrong. These numbers were widely spread in a catalog, once upon a time ago, and they keep spreading like wildfire. The last two need to be swapped, 5664 is 748, 5662 is 707./

I was just about to make that correction also, but 84IMSA' s formula seems to be right on though. When I cut one full coil on the 5660 I did get just about what his formula cam up with. Later.

Hi guys and gals!

I do not claim to be any sort of authority on coil springs (yet.)

I do enjoy crunching numbers especially when trying to make technical decisions. My raw data came from this web page:

http://bbb-carb.com/moog_Coil_Springs.htm

I am a firm believer in GIGO so if anyone has a more reliable source, please let me know.

Does anyone have part numbers for rear coils?

It is entirely possible that the numbers were transposed by whoever generated the table at bbb-carb.com. These are the part numbers from bbb-carb.com listed according to spring rate from lowest to highest. You can see they are not all sequential. There are several part numbers with the same spring rate and these are listed sequentially.

5378 5384 5278 5332 5276 5330 5376 5406 5402 5596 5598 5600 5602 5604 5404 5270 5268 5612 5642 5606 5608 5610 5556 5534 5658 5660 5664 5662

Since the bottom line we are looking for is the spring rate regardless of the part number, the one with 748# spring rate has a wire (bar) diameter of 0.765". The 707# spring rate units have 0.750" wire (bar) diameter. I would definitely measure the wire (bar) diameter before leaving the store with my goodies.

Calipers are your friends.

Hi Fords,

How does the car ride now that you have increased your spring rate?

Now I'm thinking of getting a couple of pairs for my 70 and 84.

Cheers!

Well the 5660 ride just fine for me I probably could have went with the 5662 but would have felt more of the road. You can feel the're stiffer than stock but pretty comfortable around town. Later.

How does the car ride now that you have increased your spring rate?

Now I'm thinking of getting a couple of pair for my 70 and 84.

Cheers![/quote]

Fords:

Which springs do you have in the rear? Did they get any trimming?

Online170, I didn't use Moog springs for the rear, I found some Intrax 184lbs spring rate from a wrecked Z28 and they work just fine. Later.

Good info. Subscribing for future reference!

How did you find out the rate of your intrax springs? By calculation or just have the info?

Just pure luck, they are marked on the springs, LOL. Later.

Close... Just 1 correction... spring rate = (11,250,000 X wire diameter^4) / (8 X active turns X mean diameter^3) ... Mean coil diameter should be cubed... He probably forgot to write it in.. Later

Im so glad i came across this. Im going to lower my 92 bird. Now these Intrax springs you talk about. What are these, are they something that is special? Please excuse my lack of knowledge on suspension parts.

Storm, Intrax are just a brand and I just happen to find some at a wrecking yard (and they were cheap). They're purple so you know what they are when you see purple springs. Later.

Thanks man. Since i have come across this thread, i know what im going to do with my front springs, now time for the rear. Any suggestions on what i should look for, i work at advance auto so i have access to lots of stuff.