I have been doing a little research to clarify some confusion I am encountering with spring rates. It is my understanding that you cannot compare spring rates of standard (stock strut size springs) with 2.5" I.D. Race Springs. Is this true?

If so is there a conversion formula to compare "ride feel" on different type springs? For Example would a Racing Beat Front spring of 145# spring compare to a 2.5" ID 7" spring at 250#? THere must be a difference, because standard springs are more than 7" or 8" tall which is a common length for an ID race spring, so the higher spring rate makes sence, because more pounds are required to compress the spring an inch.

Am I making any sence? I am confused? Someone please Help. The reason I am wondering is because I just ordered a set of Ground Control Camber Caster Plates, with the weld in Coil Over Kit. I didn't know which spring rate to get so they recommended a 7" 250# spring, as I told them I had Racing beat 110# springs in the rear.

THanks for any help,

Spinner-D

 

Spring rate is spring rate. Doesn't matter the diameter, length, number of coils, torsion spring, coil spring, leaf spring etc.

You can directly compare the spring rate of any spring to the spring rate of any other type of spring; it's the same.

 

Not according to Ground Control you can't.

Anyone else know what I mean? Obviously Spring Rates are spring rates, and can be compared, but my question was what is the ride feel on a 145# stock diameter spring compared to a 250# 2.5" ID spring? Or what would the ride feel be of a 145# stock spring to a 145# 2.5" ID spring?

 

145 pounds is 145 pounds. It doesn't matter the size or how the springs are made; if their rates are the same then the springs are equally stiff. They would feel exactly the same.

The 250 pound spring will be stiffer than the 145 pound spring. What that actually means for ride varies with every car because of mounting differences which affect wheel rate (note that this is different than spring rate!).

Then you must misunderstand something. Spring rate is the direct measurement of a spring's stiffness. It is represented by how many pounds of weight it takes to compress the spring one inch. A 145 pound spring will compress one inch when 145 pounds of weight is set upon it. To make it compress 2 inches you would need 290 pounds. To make it compress 3 inches it would take 435 pounds. Spring rate is a property of each individual spring and does not change unless you modify a given spring; like cutting coils off for instance. Whether the 145 pound spring is 6", 10" or 36" long it's spring rate and thus "feel" is exactly the same. The longer springs merely allow more displacement.

A 145 pound spring that is 6" long and 2.5" in diameter is the exact same stiffness as a 145 pound spring that is 10" long and 5" in diameter; they will feel exactly the same when installed on the car.

 

OK I guess Ground Control must have said that because they assumed I had progressive springs, but Racing Beat Springs are supposed to be linier, unlike Suspension Techniques Progressive springs.

Ok so now that that is settled, is a 250# spring in front ok for auto-x suspension? I have 110# springs in the rear. Is that a good match or too stiff up front or too soft in the rear?

If it matters the tires I am on are 225/45-15's, and I usually run 35lbs of pressure.

 

This is interesting to me.I just replaced my racing beat linear rate springs with a ground control setup. The eibachs that I asked for were 500 front and 400 rear.

The ride is considerably softer than the racing beats. It is more comfortable, turn in is not as sharp. It seems to me that these are not as stiff? Or they are non linear?

Matt

 

Well there is a good question! I hope the 250# I got for the front won't be too soft.

Anybody have an explinaton for this???

 

OK so Ground control forgot to ship springs with my kit, so instead of getting the 250# for the front, I asked them to send me 350#'s for the front, and 175#'s for the rear. That should be good aye?

 

want to know something really weird? There is no standard method of measuring progressive rate springs!

 

Same shocks? Are you sure you got 500/400s (check the part number on the springs)? Springs are springs as DamonB pointed out. The rating in pounds per inch is what tells you how stiff they are, period.

-Max

 

Yes Max, same shocks. The springs do have an eibach part number on them, at the end it says either 400 or 500. This is what we discussed on the phone when I called to order. In the box they wrote which ones to put on rear and front. 500 in front of course.

I can get the exact part number if you think it makes a difference.

I was not complaining about them, just noticed my ride felt softer than the racing beats.

-Matt

 

ebb, it sounds like you got the right ones. I don't need the part number -- just making sure the rates match what you ordered.

I don't have any explanation for why they would feel softer than the RBs.

-Max

 

You have to be careful with spring rates. Some vendors quote wheel rates (as spring rates), which will be 4 to 5-fold LOWER than the actual spring rate because of the suspension's lever action against the spring. Thus a 110-lb/in wheel rate spring could actually be a 440- or 550-lb/inch spring rate.

Suspension tuning is a complicated topic. Those thinking of buying new suspension components should read up on the subject (Carroll Smith's tuning series of books). Many vendors aren't necessarily up on the subject; their aim is to sell product, and let the customer deal with tuning.

ebb, from your description the springs might be progressive rate springs?

OC, yes you can measure the spring rate of progressive rate springs, and plot the rate as a function of load. With linear rate springs you get a straight line (y = mx + b); with a progressive rate spring you get a curve described by a nonlinear differential equation.