Coil Spring advertised rate vs installed rate

[AlleyCat]

Been using this forum as a research and educational tool for years now, but haven't posted much. Well, now I'm confused and could really use some guidance.

I'm building an autocross/open track '66 Mustang with a 1.75" UCA drop and am having difficulty wrapping my head around a few things.

Just purchased a set of 750 lb/in front coils from Mustangs Plus. The springs have a little more than 6.5 coils, including the ground-flat top coil.

I've been using the following formula to help work out the installed rate compared to the advertised rate:



K = [11,250,000 * (wire dia)⁴] / [8*(# avail coils)*(mean spring dia)³]

The wire diameter is .750" and the OD is 5.450" (so 4.700" mean spring dia). Since the flat-ground top coil rests against the upper perch and about half the bottom coil rests on the lower perch, that leaves about 4.5 or 4.75 available coils. So crunching the numbers, the formula spits out 902 lbs/in. The reading I've done indicates this is the spring rate the car actually "sees" based upon the number of coils available to absorb load (installed rate).

The formulas for wheel rates and suspension frequencies all include spring rate as a variable, but am confused about which number to use.

[j-rho]

Hi AlleyCat,
The only way to know for sure the effective rate of the spring at its installed height in the car, is to put it in a spring rate tester and preload it to that height. Pro teams and others who tune to very high degree will always test their springs for effective rate before putting them in the racecar. From folks involved in those processes, you'll hear stories about how Brand X is always off by 10%+ + or - and how they had some bad handling they couldn't figure out until they tested the springs, and now they never use Brand X.

902 is about 20% stiffer than 750. If a given car was perfect and you went up 20% in front spring you'd probably notice - with something you're just putting together, you probably won't. So long as the spring doesn't encounter a coil bind condition, and the ride frequency isn't over about 3hz, it probably won't be terrible.

[David Pozzi]

Your wire diameter measurement is critical to getting a good answer. Are you just measuring a powder coated coil? The powder coat thickness will throw off the calculation. Just .005" makes a difference. The spring modulus needs to be correct for the type of steel used. It's far better to actually measure the coil loaded to ride height.

[AlleyCat]

Thanks a bunch, guys. At least that gives me a direction. Yes, the coils are powder coated and the .005" buildup is consistent with my experience as well. The problem then becomes, as you both mentioned, ride height. These coils are way too tall, easily 2" higher than where I want the ride height. By the time I cut them I fear the rate and resulting frequency will be out of the ballpark.