Somebody check me on this...
 

I'm getting ready to cut my front springs, hoping to get about a 2" drop in ride height along with a stiffer front spring:

Front springs: 8 coils, 4.0 diameter, 0.515 diameter wire, 13.5 inches long

Spring rate = (Gd^4)/(8ND^3) = (11.25 x 10^6 x 0.515^4) / (8 x 8 x 4.0^3)
= 193 lbs

Assume the car weighs 2500 lbs, 50:50 weight distribution; each wheel then carries 2500/4 = 625 lbs.

625 / 193 = 3.238, so the compressed height of the spring is 13.5 – 3.238 = 10.26 inches; this results in a ride height (at the front cross member under the oil cooler) of 7 inches.

Cut 1.5 coils from the spring…

Spring rate is now 238 lbs., 625/238 = 2.63”, new compressed height is 8.34 inches … which should result in a new ride height of 5.08 inches … right where I want it.

1.5 coils is also the amount (if I remember correctly) that CRL says to cut from the stock springs.

Sound right?

THANKS!

Ohhhh, I read it, now I have a splitting headache %/ %/

BOLDLY GOING NOWHERE

Petek,

Think I was on my 3rd or 4th pair of front springs into the dumpster before switching to coil-overs, but anyway...

The car has to be fully assembled when you install the springs, and the rear set where you want it. Otherwise the ride height will change when you add something else, or change the rear setting.

Remember, on the front, the load on the spring is NOT the load on the wheel. Rather it's about 1.5X the load on the wheel. You're applying a load (moment) through a lever (lower control arm). Distance from lower control arm inner mount to wheel mounting face is ~ 13". This is the up moment. Distance from lower control arm inner mount to spring centerline is ~ 8". This is the down moment. At equilibrium, the load on the spring is ~ 900#, where the load on the wheel is ~ 625#.

Anyway, if the book says cut 1 1/2 coils, then make your first cut at 3/4 of a coil. Then measure ride height. Cut another 1/4 coil and test again. Leave the ride height about an inch higher than where you would ideally like it because it will drop as you accumulate some miles. After about 6,000 miles, if it's still too high, make final cut. Chances are you'll have the coilovers in before then.

Petek,

You've got it down to a science! Now pass the aspirin and cut at least 3/4" of a "coil" and NO more than a 1 1/2!

I suggest a full coil. It will settle a little more within a few miles.

Coil overs are in your future, but in the interim, cut and be happy!
DV

Petek,
I changed mine to coilovers front and rear and have readjusted ride height several times due to settling. I think it takes me about 3 minutes to adjust. OOOOH do I love my coilovers, maybe because I was never very good at math. Good Luck!
BrianR

Pete, The manual shows 2 different coils from the donor. The 1600cc Pinto had a .531 wire diameter. The Mustang had a .565. The book says to cut a coil and a half from the .565 coil. This does work but the ride is very stiff. With your wire diameter of .515 I would start with a 1/2 coil. The Pinto spring with the .531 diameter sets the height pretty well. You can get 2 fingers in between the tire and wheel lip with a 245x60 tire on the front. I think 1 1/2 coils would drop you so far your pan would scrape. If you want to go with the tubular lower arm and the coilover maybe we can work something out on the friendly payment plan! Then all the trial and error will go away.
Don

PETE,

MY EXCALIBUR - BASICALLY THE SAME AS A CLASSIC ROADSTER - HAD THE OPPOSITE PROBLEM. I COULD RUN OVER A DIME - BUT NOT A NICKEL. I NEEDED TO RAISE THE FRONT END. THE CAR HAD MUSTANG REAR SPRINGS ON THE FRONT END. THEY MEASURED (NOT CALCULATED) 460# AND 480#. AFTER A LOT OF FOOLING AROUND WE ENDED UP WITH MUSTANG FRONT SPRINGS - CUTTING OFF TWO COILS. (IN 1/2 COIL INCREMENTS). AND INCREASING THE RIDE HIGHT BY 2.75 IN. - EXACTLY WHAT I WANTED. THE NEW SPRING RATES MEASURE 600# AND 610#. THE RIDE IS "DIFFERENT". NOT QUITE AS CRISP AS BEFORE. IT HANDLES WELL - JUST SLIGHTLY DIFFERENT. OF COURSE - BE SURE TO HAVE YOUR FRONT END RE-SET AFTER ANY CHANGES.

THE ENTIRE PROCESS TOOK JUST ABOUT EIGHT HOURS.

LONG STORY SHORT - - - LISTEN TO THE GUYS THAT TOLD YOU ABOUT THE COIL-OVERS. IF I EVER HAVE TO FOOL WITH THE FRONT END AGAIN - FOR ANYTHING - THAT'S WHAT I'M GOING TO DO. (SHOULD HAVE DONE).

Y'ALL HJAVE A REALLY GREAT DAY,

BLACKJACK

Thanks everyone!

Jack, a BIG thanks... I had completely forgotten about the lever formed by the suspension.

Coil overs up front are on the list of improvements, possibly for this winter; probably with tubular upper and lower a-arms and maybe even get rid of the strut rod.

I installed the rear coil overs from Don in about a half hour yesterday ... the rear springs are sooooo much easier to work on than the front springs!

I'll probably cut between 3/4 and 1 coil from the springs and let you know how it turns out.

THANKS AGAIN!

Someone told me when I started this project, that one does not need to be an engineer to build one of these things, but it does help if one is, or have friends that is. That's what I love about this forum.

My reluctance to go with coil-overs initially was the entire weight of the front of the car rests on two 1/2" bolts through the lower control arm, and a rubber bushing in the shock. It seems to be working, but am wondering about service life. Polyurethane, with a steel lubricatable insert might work better.

I was the one woofing about the strut rod with polyurethane bushings being too stiff. Oddly enough, it softens up with mileage. On the CR, all the strength of the front suspension is in the strut rod mounting point. The steel that the upper and lower control arm mount to is not particularly beefy. Looking at "Moments" again, the strut rod is where most of the resistance is when you hit the brakes. The upper arm wants to twist foreward, and the lower wants to twist back, and follow the wheel around. The strut rod resists the braking torque. Anything used to replace the strut rod has to be equally beefy. One vendor, Pole Position, uses a 5/8" grade 8 bolt, and some pretty beefy bracketry that gets welded in. RCC only uses 1/2" grade 8, and less bracketry.

Another thing that will drive you nuts is front end alignment. Honestly, pick up a pair of Beroths upper control arm brackets for $100, and be done with it.

And welding that close to fiberglass makes me nervious. Take a cigarette lighter to one of the little glass strands, and you'll see what I mean. Rather do this before body goes on.

Well I set up a spread sheet and varried everything from the number of coils, the moment arm ratio, corner weight, and phase of the moon.

Finally decided to cut one coil.

I'll let you know how it turns out!

PETE,

AS LONG AS YOUR GOING TO HAVE THE SPRINGS OUT ANYWAY - YOU MIGHT WANT TO CUT THEM AT A SHOP THAT HAS THE CAPAPBILITY TO MEASURE THE SPRING RATES. A LITTLE EXTRA TIME/WORK AT THIS POINT CAN MAKE A BIG DIFFERENCE IN YOUR HANDLING.

GOOD LUCK AND, Y'ALL HAVE A REALLY GREAT DAY,

BLACKJACK

Grumble, grumble ... if this is a double post, blame it on Comcast.

I need glasses! I remeasured the wire diameter of the springs and sure enough, the diameter is NOT 0.515 but 0.565. Looks like I have the Mustang springs.

So, what did I do? I cut one full coil out of the spring, right from the middle of the spring. (Ok, just kidding, I cut one coil from the end.) Used a Dremel motor tool with a fiberglass reinforced cut off disc. Worked great.

Much easier to get the spring back into place with one coil cut out. Can't wait to get the Cobra back onto it's feet (Cobra have feet?).

Anyway, thanks again everyone!

PeteK,

BTW, Your spring rate should not change because you cut the spring. The spring should just get shorter. Force=K(Delta x) where K is the spring constant (rate). You could use conservation of energy as well, but I don't know if that will help with your assumptions or not without thinking it through. Honestly, I think Jack21 and DV have the plan.

The Dreamer

PETEK, I have the same springs as you. My car looked like an SUV! The book said if you buy the springs from CR just put 'em in! NOT!! I measured the springs and came to the same conclusion - gotta cut the the coils. The book said to cut 1 1/2 coils off which I did. Now the front frame "U" in front of the rad is 3 7/8" off the ground!! TOO LOW. I have coil overs on order. If you want to stick with the CR springs cut off 3/4 to 1 full coil. Hey, do you want to try my springs?;)

Dreamer ... that's what I thought too but I had that formula in my collection and it seemed to work (for another application); now I'm going to have to dig out the physics text and see what happens when you change the number of coils in a coil spring :)

Mike ... SUV? Yup, that's what it looked like! 1 1/2 coils = 3 7/8 so 1 coil should be right where I want it (4 1/2 -> 5"). Coil overs are in the future ... perhaps my wife can get them for me for Christmas!

How are you all measuring spring rate? You need to compress the spring to about 20% it's travel, measure force. Compress to 80% travel, measure force. Difference in force over difference in travel. Removing a coil will increase the spring rate on a given spring.

I wasn't measuring the rate, I was attempting to calculate the rate. And after thinking about what Dreamer said ("Your spring rate should not change because you cut the spring.") I think I've come up with a reason the spring rate changes when you change the length of the spring; not using any math :-)

The formula I used earlier indicates that the spring rate (K) changes when you change the length of the spring. I believe this to be true as when ever someone cuts the springs on their car, the car rides rougher. The question is why would this happen and I think I've got an example that would explain it...

Unroll the spring into a long, straight rod. The rod will have a certain amount of flex to it, "springy" rod as it were. Heck, think of a fishing pole. (We're going to ignore the mass of the rod in the next part.)

Hold one end of the rod fixed and press on the opposite end with a known amount of force. The rod deflects. Now cut the rod so that the length of the rod is 1/2 of the initial length of the rod. Hold one end fixed and press on the other end. More force will be needed to deflect the rod an equal distance. Two things in play: lever action and strain.

So, changing the length of a spring changes it's K value.

Now, having said all of this it could all still be pie-in-the-sky BS as I haven't the foggiest idea of what I'm talking about (there are times when I wished I had not slept through physics)!

Hooke
 

By the law of Hooke (not related to Cptn Cook) a sring has a constant rate derfined in force per length:

lbs per inch.

well most sprigs. fancy springs may have a progressive rate.

if you load your springs (i.e.springs with a value of 500 lbs per inch) with 500 lbs it will lower one inch, 1000 lbs equals 2 iches.

if your spring is mounted preloaded, as is often in coil overs, your car will not lower before the amount of preload is exceeded.

A common automotive spring, although, may have due to manufacturing, another rate for the first 1/2 inch or so. that is why cutting is a try and error thing, plus the ride is harsher for small bumps as mentioned earlier.

call KONI or Rockwell springs for more information.also you may read the chapter on springs from Carroll Smith's "Engineer to win".

What was the outcome of cutting?

dominik

Refreshing myself on Mr. Smiths fine collection of books is on my list of things to do... paying special attention to the section on springs. I particularly like the working title of his fastener book: "Screw to win".

Haven't dropped the car back on the ground yet. Just finished installing the side pipes (HPC coated, look great). Want to add a safety pull to the electric door poppers... then back on the ground!

pete,

you hit a weak spot: I do not have "screw to win", but all the others.

it is not listed in amazon.com - shame.

dominik