Cutting a spring is fine if done properly and within moderation, i.e. don't cut too much off and use the proper tool to cut it. It's not wrong. Cutting a spring actually increases the spring rate; so cutting too much makes it too stiff, that's where moderation comes in. Cutting a 1 or 2 coils will make the spring rate a little stiffer than stock, which is needed since the spring is shorter.

Eibach, Steeda, etc. make lowering springs to make money. Way back before there were companies that made lowering springs, all the hot rod type magazines recommended cutting springs (using the correct method and amounts) to lower a car.

Then when companies started popping up offering lowering springs and they started advertising in said magazines, Guess what what the articles in the car magazines started saying? "Cutting springs is WRONG! Buy lowering springs from Company XXX! Its the only correct way to lower a car!"

In the book "Chassis Engineering" by Herb Adams, who was the chassis engineer on the Pontiac Trans AM WS6 package, one of the better handling cars of the '70s, he recommends cutting stock springs, gives instructions on how to do it, and gives the technical reasons why it's acceptable .

It also seems nobody can give a real, concrete, technical reason why cutting stock springs is "wrong"; all they can say is "Don't do it! It's BAD, VERY BAD!" Can someone who is against cutting stock springs give me any real technical reasons why its not a good thing to do?

That said, I do have Eibachs Pro-Kits on my '98 and I wish I would have just cut my stockers. I would have saved myself some money for other things!

 

I followed the write-up above (very good, by the way! ) and I LOVE the way my stang looks/feels now. I took her out and finally got some 360 burnouts with her. I didn't have to coax them out either, she wanted to let loose! The cornering and steering response is great!

I do have a question, while the car looks great (even from both left/right and front/back perspectives), the before/after measurements I took are a bit strange.

I've attached a bitmap (excuse the crude drawing) showing the measurement from the concrete in my driveway to the highest point in each wheel well.

[attachmentid=35398]

First off, I left everything in 16ths for ease of comparison - I really do know how to reduce fractions . The top numbers show the stock measurements and the lowered measurements are in bold. I cut precisely 1 coil from the bottom of the front springs and exactly 2 coils from the top of the rear springs. I really took my time and measured the coils I was cutting.

Did I do something wrong, or is this as close to "exact" as I'm going to see?

Thanks again Dave for the write-up. Great job!

 

You know when you cut a coil in the spring you up that rating you're talking about? It will also make the spring weaker.

Cutting springs is for ricers, when you start having problems with your suspension you'll see that you get what you pay for. They'll know you cut the spring when the ride quality isn't as good as aftermarket springs BTW.

 

How do you know it "ups" the rating and makes the spring weaker? I'm not calling you out, I'm truly interested in where you are getting your facts. I'd like to read more about this.

<!--QuoteBegin-TehSLeeper@October 3, 2005, 12:13 AM
Cutting springs is for ricers, when you start having problems with your suspension you'll see that you get what you pay for. They'll know you cut the spring when the ride quality isn't as good as aftermarket springs BTW.

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Now, about the "ricers" comment. :nono: Cutting springs is not "ricer". Neon is "ricer". Cutting springs is an attempt to achieve a degree of performance while learning about how your car really works (not just reading about it). It's also a great way to achieve an awesome sense of accomplishment in the process. Any adverse affect on the suspension geometry can be mitigated with a professional alignment, which is a good idea when installing a $250 set of after-market springs, anyway.

I could take my car to a shop and shell out thousands of dollars on parts and installation. In my opinion, that's not modding, that's having your car worked on. While there are certainly things you don't want to do yourself (gears, alignment, etc.), if I had the tools I'd pull my engine, do the headers and cams, etc. But then again, my wife would also kick my ****.

 

Has anyone checked into how much the weight bias shifts when lowering the back of the car more than the front?

 

I did a little research on this to make sure I have the facts to back up my statements.

The spring rate for a helical (coil) spring -is- increased by cutting off coils.

Spring rate is defined simply as the amount of weight necessary to compress a spring 1 inch, which is why spring rates are usually exrpressed in lbs/inch.

The spring rate for a helical spring is

k = G*d^4/(8*N*D^3)


Where
G = the shear modulus of elasticity (strength) of the material
(use G= 12,000,000 for steel springs)
d = the diameter of the wire
N = the number of coils
D = the diameter of the spring

From: "Mechanical Engineering Design" by Shigley & Mischke (McGraw-Hill, Inc.)

Increasing the number of coils will make the spring constant smaller,
making a softer spring. Decreasing the number of coils will make the
spring constant larger, making a stiffer spring. It does not "weaken" the spring, it increases the amount of force needed to compress the spring.

If someone can fill in the info about the stock springs (diameter, # of coils, wire diameter) we can calculate exactly what the new, increased spring rate is when cutting 1 coil.

 

You are absolutely correct about cutting the springs increasing the spring rate. However, the rate will still likely not be as high as most aftermarket springs. Other than that, I just cannot put much validity in anything else that you said.

The late Herb Adams actually advised that owners would generally be better off to cut their stock springs rather than installing aftermarket lowering springs. Why? Because IHO cutting the stock springs works fine (so long as they aren't progressive, which he hated anyway) and most aftermarket companies raise spring rates needlessly high.

With due respect, if anybody here thinks they know more about handling than Herb Adams please fell free to say so. Of course, that would be a bit like saying that I know more about Vettes than Dave Hill does. If you disagree with this line of thinking that is your prerogative, but the silly ricer comments get thrown around too much and I would hesitate to call Mr. Adams a ricer..

 

Thanks John, that's good information! I will get the info about the stock coils as I'd be interested in the resulting spring rate of my cut springs.

So cutting the stock springs will make them stiffer, not weaker. What exactly do the Eibach and Steeda kits give you that a cut set of stockers don't (other than a warranty and a sticker to put on your car)?

As I said above, I love the way my car handles and looks now. I'm extremely happy with the results.

 

Well i see this thread progressed from uninformed to nearly informed. To fill in some of the gaps, see what follows:

Looking at the fronts (1st post above) and ignoring they are slightly progressive, there are about 4-1/2 active coils (i.e coils that can deflect or flex ). The coil resting on the seat does not flex so it is not counted). 1 coil was removed, so now we have about 3-1/2 active coils left. From above the factory front spring rate is 136.

The formula above is the basic coil spring formula. For comparing specified rate to changed rate we can throw everthing away except the rate and no. of coils and end up with this formula:

rate (new) = rate (old) x Ncoils (old) / Ncoils (new)

The new spring rate is 136 x 4.5 / 3.5 = 174 probably a little light a lowered ride if you want to do the job right. If he gets into some rough stuff, suspension bottoming will result because there is less suspension travel and the spring is not stiff enough to handle it.

The rear spring with 1-1/2 (or 2) coils cut is increased in rate as well. Not sure how many coils that one has.

And as some have clicked on, the spring is not weaker or stressed more ... just stiffer and shorter

I seriously doubt that the likes of Steeda, Saleen, Roush and others just pulled a 'outragous' spring rate number out of the the air to design to. So what you get from them is an engineered part, not a backyard modification and shot in the dark. Nothing personal ... but if a job is worth doing, it is worth doing right. And hey, you have a $26,000 and more invested in the base vehicle, why skimp on something as important as the suspension when most are willing to drop $2,000 or more on a set of tires and trick wheels.

ebrand -

Just how level is the floor where you took your measurements? Was the car exactly in the same place when you took your measurements? The four contact points of the tires on the floor must be exactly level (e.g. run a bubble level between the 4 spots before you put the car there and shim as necessary) with respect to each other for your measurements to be meaningful. You need to bounce the chassis really good to get it to settle before taking the measurements

 

at the very least, I give the guy credit for posting the pics and doing the write up (which is basically the same procedures for someone who wants to replace with aftermarket springs)...I know I'll find it useful when I begin my suspension mods.

everything else is personal preference...

 

preferential ... maybe. My preference is not to have the suspension hitting the bump stops while the car is in a high speed turn. Such preference ensures or reduces the likelyhood the car will go off the road as it instantly transitions from stable to unstable when the suspension bottoms out after engaging a small bump and radically changes the tire loading and vehicle balance.

I do agree though is was a nice capture of How To.

 

Awesome Write up! Gonna see about getting it linked on the site as a how-to maybe.

 

Thank you for filling in the blanks. So the new spring rate of the cut front spring 174 lb/in vs 136 lb/in for the original or about a 28% increase.

Quote:

Originally posted by RadBOSS@October 4, 2005, 10:12 PM
The rear spring with 1-1/2 (or 2) coils cut is increased in rate as well. Not sure how many coils that one has.

And as some have clicked on, the spring is not weaker or stressed more ... just stiffer and shorter

I seriously doubt that the likes of Steeda, Saleen, Roush and others just pulled a 'outragous' spring rate number out of the the air to design to. So what you get from them is an engineered part, not a backyard modification and shot in the dark.

What are the aftermarket lowering spring rates for the '05?

I know the Eibach Pro-Kits I have on my '98 are not much stiffer than the stockers. I don't have the exact numbers off the top of my head, but it looked like the spring rates were nearly the same as the spring rate increase I would have gotten with a cut stock spring. My Eibachs also sagged about a half inch after a few months.

Whether the spring is a cut stocker or a shorter aftermarket spring, you're still going to have the problem of bottoming out due to reduced suspension travel. Most aftermarket springs either recommend trimming the bumpstops or want you to buy shorter bumpstops to help with the bottoming out. Of course there are probably much stiffer aftermarket springs that might help compensate, but then ride quality will suffer.

There's nothing wrong with backyard modifications... as long as the theory behind them is sound. That's the one of the things wrong with "car people" these days, a lot of people don't experiment or have ingenuity, they just open their wallets and make "mods". But why do that if you can achieve the same results with a little work and save some money at the same time?

<!--QuoteBegin-RadBOSS@October 4, 2005, 10:12 PM
Nothing personal ... but if a job is worth doing, it is worth doing right. And hey, you have a $26,000 and more invested in the base vehicle, why skimp on something as important as the suspension when most are willing to drop $2,000 or more on a set of tires and trick wheels.

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Thats's good point, but a lot of aftermarket parts are really overpriced for what they are. If you can achieve the same results with backyard modifcations that cost less or are free, why not? A good example of this are the turbo boost "bleeder valves" people were making out of aquarium valves for $20 or so. Now the aftermarket has caught on and is now selling blue anodized valves as "manual boost controllers" that do the same thing for $129...

 

vermillion98 -

Based on what I could see on the cut coil spring, that is roughly the right percentage increase for that front spring.

Will have to go back thru some of my research. Steeda gave me thier spring rate numbers and I was shown a MM article on the Eibachs. The Eibach numbers are actually referenced in post 23 above, and they are stiffer than you think ... and they are progressive, where the Steeda's are not. Progressive rate gives you a nicer ride under normal conditions and as you raise the bar in speed, etc, the more they deflect the stiffer they get. Interesting to hear your comment about Eibachs sagging, because the MM article brags about how Eibachs don't have this problem and I'm thinking, wow, I should get those, because the Koni Sport springs I put on my XR4ti sagged in a big way on the rear of that car.

Modifying the bump stops is a way (system mechanical linkage permitting) to get some extra travel ... but puts your [stock] front strut at more risk of bottoming out ... generallly not good for them. other things you need to check are ball joint angles ... do they get into a bind at extreme travel?

Nothing is wrong with doing your own thing to save some money and you get the result you want and feeling of accomplishment as well. For some people, it may be just the look is enough ... or what they don't know about the compromises they may have just made, doesn't bother them. Some people are better equipped to see and understand the compromises made along the way and clever enough to mitigate them too.

Nothing is simple.

 

RadBOSS,

Thanks for the info on measuring. The car was in the exact same spot before/after measuring and I'm pretty sure the ground on which it stood is level (although no scientific instruments were used). The point was a comparison of the before / after measurements. The stock measurements seem much more similar side-to-side and front-to-back than the after measurements and I was wondering if this was to be expected or do the Steeda/Eibach springs yield these same results?

For me it really wasn't all about the money. You were right in a previous post, $250 is not a lot to invest in an aftermarket product for a $30,000 sports car; that is, if you believe what you are buying is worth the money spent. However, wouldn't you agree that to simply purchase and install an aftermarket product is not going to guarantee a problem-free experience.

You're also right about the sense of accomplishment. I would also add a sense of education and adventure to the list of benefits as I certainly learned a great deal about how my car was put together by taking it apart.

Anyway, the point was: great write-up, Dave! Thanks for the useful information.

Some things actually are simple. For me, installing a set of springs on an '06 Mustang GT now falls under that category.

 

vermillion98

got this from Gus at Steeda several weeks ago

"Sport springs are the same. 1 inch lowering. Same spring rate. Rate is 200lb front, 175lb rear.

The Ultralite sport springs are also 1 inch lowering, slightly lower spring rate at about 195lb front, 175lb rear. They are also a lighter weight wire for less unsprung weight. This reduces inertia. The slightly softer rate combined with the low inertia make the Ultra-lite sport springs the best choice for ride quality.

The street/comp springs increase the spring rate to 225lb front, 185lb rear. Also about 1 inch lowering."

Also the spring rate numbers he cited for the 'stock' springs was not as referenced much further above. So I checked my copy of Motor Vehicle Manufacturers Specifications for the 2005 Mustang and discovered why. Spring rates get talked about and sometimes interchangably two different ways. There is the rate of the coil spring by itself and there is the wheel rate. Based on the number referenced by Gus at Steeda (wheel rate), I am now confused and asking him for a clarification.

What is cited way up above from a MM article about Eibach springs makes me think they were comparing actual spring, spring rates.

When the author does not qualify, you never know for sure if its spring or wheel they are discussing ... so for comparative purposes you have to ask!!!!!

Wheel rate is dependent on spring rate but is different because of the mechanical advantage of the suspension linkages. So for the 05 GT, Ford indicates the front coil rate is 136.5 lbs/in and the front wheel rate as 170.7 lbs/in..

For coil cutting comparative purposes that ratio of the coils still works whether you are talking wheel or spring.

ebrand

if the spot was the exact same and the floor is level, you should get nearly the same variation result ... or at least within 1/16th or so. When you reinstalled the springs, was there any kind of coil 'end' indexing tab or feature in the spring seat? If there was and you missed registering in it, that might explain the reason for the major difference side to side. What is even stanger though about your measurement results 'after' is the difference is diagional ... WELL that is hard to do unless the floor reference is not level anymore or the car is now major twisted ... i take it you did not run it into a ditch? Try taking your measurements again and tell me about the spring seats (i have not lowered this one yet so i am not quite there).

 

Just thought I would post an update on mine. Cut my stock spings in my '05 GT nearly 1,000 miles ago. No problems to report.

After lowering it, I naturally took it to have it aligned, they said they "maxed" it out, but the camber is still within specs. But I went ahead and finally got around to installing Steeda's Billet Camber Adjusters so that I could have it better aligned.

My dad just bought an '06 Pony Package V-6. I installed JBA's true dual exhaust on it and cut his stock springs for hime also.

The way I look at it, if I can save some money by cutting my stock springs and not compromise anything, then why spend $200+ on replacement springs? That money saved could go towards something else. With the stock springs cut, it does ride a little stiffer, not as stiff as Eibach's though, does handle better, looks great, and I saved $200+ by cutting them myself. I do understand that Eibach, Saleen, Roush, etc. all do extensive testing and R&D in their products, but cutting springs has been around forever. I got the idea from my dad, who got the idea from his dad when he was my age, and his dad got the idea.....see what I mean?

 

This thread has really brought out some good info. Just comparing front springs, here's what we have found:

stock = 136 lbs/inch
stock w/ 1 coil removed = ~174 lbs/inch
Eibach = 171/229 lbs/inch (variable rate)
Steeda Ultralight = 195 lbs/inch
Steeda Sport = 200 lbs/inch

Bullitt1609's results from cutting his springs seem to reinforce our calculations of the spring rate increase when cutting one coil from the stock spring: stiffer than stock but not as stiff as the variable rate Eibachs with the high end spring rate of 229 lbs/inch.

I think we've proven that cutting one coil is a perfectly sound modification for those folks who want to lower their cars but keep a good ride quality and aren't hard core road racers (who want much higher spring rates than stock).

IMHO, the real life results and calculations show that cutting the stock springs 1 coil is not "ricer" or "wrong" and that it works fine.

 

Contacted both Steeda and Eibach on spring rate terminology. Confirming both when discussing thier kits refer to the actual coil rate and not the wheel rate. If you guys want to know about wheel rate, let me know and i'll try to put it in layman terms so the whole audience can benefit what its different but directly related.

As a side point one interesting thing i got from Gus at Steeda is the nominal stock front spring rate. While gus could not reveal details, he indicated that even within the GT coupe there are several spring packages. Not sure why, other than there might be some regional considerations(?). Anyway the long and short of it is the nominal front spring rate stock is about 165 rather than 136. This is a bit mysterious to me because the Motor Vehicle Manufacturers Specification document indicates the spring rate to be 136. I suppose it could be wrong, or maybe even Steeda.

 

So then we have these front spring rates:
stock = 165 lbs/inch
stock w/ 1 coil removed = ~212 lbs/inch
Eibach = 171/229 lbs/inch (variable rate)
Steeda Ultralight = 195 lbs/inch
Steeda Sport = 200 lbs/inch

The formula for calculating wheel rate from Herb Adam's book:

Wheel Rate = Spring Rate * ( (a/b)^2 ) * ( (c/d)^2 )

Where:
a = distance from control arm inner control arm pivots to the center point where the spring acts on the lower arm

b = length of the lower control arm from the ball joint to the front suspension instant center.

c = the distance from lower ball joint to the front suspension instant center

d = distance from the center of the tire contact patch to the front suspension's instant center.

So it looks like wheel rate takes into consideration the leverage the control arm and tire/wheel have on the spring's rate...

Man, I wish my '06 that I have on order was here already; I'd take a tape measure to it. This is really interesting stuff.