I was wondering if I cut my front OEM springs maybe 1/2 coil or 1/4 to get .75" lower I have swift springs and they are great, but the front is not great while driving in S.F roads. I know swift springs lower 1.2" in the front and I would only need .75 to get a little lower stance to match the rear swift springs. I already cut my bump stops. Would cutting this minor thing cause problems?

 

Cutting springs = ghetto

 

Cutting springs raises the spring rate and it's a bit unpredictable. You're taking a car that already rides stiff, you live in SF, you're proposing reducing the suspension travel and raising the spring rate all at the same time. How about just putting it back to stock?

 

Cutting springs is ghetto, but its very minor cutting of 1/4 or 1/2 coil only. It is stock now, but reducing the gap very little would be nice. I know the JDM sport suspension for GE8/GD3 only lower 20mm.

 

NO it doesn't. Cutting springs lowers the spring rates.

 

It lowers the ride height and raises the spring rate. Do your homework before you emphatically disagree. If you want to cut your springs than go ahead. Danger 69 was asking about cutting springs and I told him what I thought with a simple fact thrown in about making the car ride stiffer.

 

Cutting a linear spring increases the spring rate proportionally to what is cut. Shorten a linear spring 10%, you increase the spring rate 10%.

OEM springs are not necessarily linear and the loops at the end touch each other (like a ball point pen spring), they don't do much of anything.

John

 

NO. Cutting spring lowers the spring rates. Think about it. You're weakening the spring.

Just because the ride is harsher does not mean that the ride is stiffer.

 

YES. It does.

Google is your friend:

http://www.google.com/search?q=short...f&aqi=&aql=&oq=

 

just cut them!!! forget about this people... they got money, mines are cut 1-1/2 coil in the front and 1 coil in the rear on skunk2 and my car dont bounce, i got the normal bouncing of going low, and it got a lil stiffer...

yall going all mathematical and shit ... just cut them and have fun

 

Yes because a bunch of morons on car forums vs. a guy who studied mechanical engineering in college.

HMM

 

What about us "morons on car forums who studied mechanical engineering?"

Will no one think of us?

And on a linear spring the K-rate should stay... wait for it.. linear. Which is to say the same. For every 1" of compression you should see xxx lbs of force across the board.

But cutting the coil, like heating it weakens the structure. How much? Tough to say. Do your ghetto fab work and cut that shit! I want to hear about you banging the car onto the bumpstops by next week!

But the coils that are already bound (in contact) do not necessarily affect the spring rate, just the overall height of the spring end to end.

 

You sir were able to say it much better than I was.

 

"Yes because a bunch of morons on car forums vs. a guy who studied mechanical engineering in college"

Maybe a different university would help?

Try reading Herb Adams book "Chassis Engineering" where he goes into great detail on how shortening springs increases the spring rate. Cutting coil springs doesn't require softening them with heat. A cut-off wheel works just fine, you don't need to mis-use a torch.

Or, try this thread from the morons on a Bimmer forum where they actually go through the thought process:

cut coils springs - Bimmerforums - The Ultimate BMW Forum

John

 

That is some rather circular reasoning you got there..

or are you intentionally touting the book they talk about in the thread before hand hoping we won't notice? You are also treating all springs the same with your statements.

If you actually read that thread yourself the majority including many who had actually cut their own springs, advise against it.. like we do.

You are doing all sorts of things when you cut the springs, because among other things you have to heat the end where you cut it and bend it flat to sit in the perch properly.


And again, on a linear spring the K-rate is consistent (usually within 80%) for every inch of compression.

Only on progressive springs, and specifically when you cut the wider spaced coils, are you going to see an over-all increase in spring rate, because the spring rate on progressives is not fixed and you have removed part of the "softer" section.

 

swift springs too harsh? I run skunk coils in the city daily... *shrugs*

 

Let me try this.

Yes a linear spring is linear. For example 100 pounds of weight compresses it 1" and two hundred pounds compresses it 2". That is a linear spring rate.

Now lets say that 100 lb per inch spring has 10 coils. When you put 100 lbs on the ten coils, each coil compresses 0.1" for a total of 1".

Cut one coil off so you only have 9 coils. Now put that 100 lb weight on the 9 coil spring. Each coil compresses 0.1" as nothing has changed for each coil for a total of 0.9" compression.

To get the 9 coil spring to compress 1.0" you need about 110 lbs of weight. The spring rate is now about 110 lbs. per inch. An increase of about 10% in the spring rate. Still linear but the rate of the spring has changed. That's it. That's all.

The problem with cutting springs is the usual, "if a little is good a lot must be better". People then cut their springs in half, which is usually not good as it throws the suspension out of its operating range in terms of bump stops, camber angles, shock range, etc.

All I was trying to point out is that shortening a spring increases the spring rate not decreases.

Think of it as shortening a torsion bar. Take a 4 foot long torsion bar and twist it 10 degrees. If you shorten that same torsion bar to 2 foot long, it will take twice as much force to twist it 10 degrees. Same with a coil spring.

John

 

Yes, that is what happens when you do it on paper. The formula is as follows:

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

So:

d = diameter of the wire

G = Bulk Modulus (material property)

D = Diameter of the spring

N = Number of turns

k = spring rate

But I am referring to what actually happens when you ghetto fab your springs. Especially springs that have already seen the abuse of the elements and the forces of daily driving. Results will vary, sometimes there can even be a wide spectrum across 4 springs. Which could be a lot of fun at speed on a traffic worn road when harmonics come into play.

Cutting springs and the implications of which are more complex than just saying the rate automatically increases when you hack at the coils. You are weakening the spring structurally when you cut it, even without a torch. Not only that but you have to fit it to the spring perch among other bits after modification.

It is not as straight forward as comparing it to a torsion beam because of the other stresses involved in the coils. Even if you assume the same module of elasticity (178,000 for steel or something similar) the torsion bar has fewer vectors to deal with than a wound rigid steel wire, load is being applied differently.

All of this is also neglecting the fact that if you manage to increase the k-rate without changing the valving or the oil you are gonna have a crappy bouncy ride because if the k-rate increases you will be overpowering the damping.

There's theory and then there is practice. I am familiar with both.

Just get some proper shocks and springs you cheap bastards.

 

"Just get some proper shocks and springs you cheap bastards. "

Unadjustable shocks and springs with unpublished spring rates from two different companies? With every company having their own idea of "proper" spring rates.

So is there really a difference?

And, as you pointed out as N gets smaller, K gets bigger. Which is what I said (simply) in the first place.

Thanks,
John

 

There are a couple differences, and the a big one is consistency.

And I agree, on paper, the numbers are inversely proportional, but what I am trying to say is: in practice strange things happen.

And the numbers might not be available on a website or through a simple google search but they definitely exist.

If FF members are going to be doing suspension work they should be doing research, which often involves calling manufacturers and figuring out whos product does what if you actually care about what you are doing.

Otherwise you will end up with a wheeled trampoline and a very unsettled ride.

For the hellaflushtards who would be lucky to get up to the speed limit before a bump, pothole, driveway or a large pebble has them on the brakes.. cutting and hacking might be the way to go, for everyone else I want them to do it properly.

For safety if nothing else.

That is my main issue with this.

This is unfortunately one of those you have to pay to play things, so just do it right, do it once.

I have had everything on my cars over the years from cut springs on stock suspension to Moton External's on my old road race car.

I definitely appreciate that you know the formulas though!

So I apologize if it came off as condescending, but cutting springs is not so straight forwards, and consider the people who will be doing this.. people who probably don't understand or care about the math behind it.

So if we are going to put this out there for public consumption I want people aware of what they are getting into.