C2 Front springs

[tsarbar]

Anyone have information on front spring rates for C2's with big blocks?

Thanks,
Chris

 

[WayneC]

This is an interesting question that I am curious about, myself. Unfortunately I don't have that information, but I do have some spring info and I found a formula for calculating spring rate.

http://personal.riverusers.com/~miracerros/mustang/springs_calcs.htm

Turns out it is highly sensitive to the number of free coils, as well as the wire diameter.

I had always assumed that the F41 heavy-duty spring rate was around 450 lbs/inch based on extrapolating information in the Vette Brakes catalog, but my calculations produced wildly different results.

I calculated the standard 63 Corvette spring at 243 lbs/in, standard 67 Corvette spring at 217 lbs/in, the original F41 (heavy duty) spring at 670 lbs/in, and the replacement F41 spring #340519 at 664 lbs/in

I don't know whether to trust those numbers or not, since I would not have expected such a wide variation.

 

[JohnZ]

The A.M.A. Specs for the '67 show the front spring rate as 195 pounds per inch for small-blocks, and "To Be Provided" for big-blocks. The small-block spring (3851100) is .600" diameter AISI A-5160 steel wire, 3.8" inside diameter, 16.82" free length, and its working height is 9.98" @ 1340# load. The big-block spring (3888250) is the same steel, wire diameter and inside diameter, free length is 14.26", and working height is 9.98" (same as the small-block) @ 1490# load (same working height at 150# more load than the small-block spring). That's all I can dig up.

 

[WayneC]

Thanks for the spring info, JohnZ... now I wish I knew how many COILS the big block spring has.

If one uses the free and working height differences of the small block spring: 6.82 inches (16.8 - 9.98), with each front corner holding perhaps 28% of vehicle weight (that's a guess on my part, so if anyone knows the weight distribution for certain, please post it), or about 950 lbs, then one can calculate a spring rate of about 139 lbs/in (950/6.82), which doesn't match the published spring rate of 195 lbs/in.

...So, assuming the numbers to be accurate and the spring rate quite constant, there are likely some geometry issues with the way the spring is scissored in between the frame and the A-arm... in other words, the spring actually holds a greater load because of adverse leverage on the lower A-arm. This adverse leverage can be calculated to be somewhere in the neighborhood of 195/139, or 1.4 times the vehicle weight, so the spring actually holds 1330 lbs (950 x 1.4).

For a big block, that means the spring is depressed 4.28 inches (14.26 - 9.98) by about 1435 lbs (1025 lbs x 1.4, after adding 75 lbs for half the extra engine weight) for a spring rate of about 335 lbs/in.

In my prior calculations using the formula, I had been guessing there were 2 non-free coils (ie, 2 working coils less than the total number of coils), but that guess could have been incorrect, so I tried adjusting that to 1 non-free coil (half a coil at each end) to force the formula to work out close to the 195 lbs/in number.

If my assumptions are true, then the big block spring, using the info provided by JohnZ, needs to have 7.3 coils (in the formula) to achieve a rate in the area of 335 lbs/in. 7.09 coils (like the F41 spring, except this spring has smaller diameter wire), yields a rate of 346 lbs/in. Either of those spring rates are probably "close enough"!

... so who knows how many coils that 3888250 big block spring actually has, so I can verify my numbers?

 

[JohnZ]

More '67 small-block specs: The wire length the SB spring is coiled from is 168.5" long, and the spring rate (at the wheel, which factors in the leverage of the control arm) is 80#/inch. Just to confuse the issue further, this is a progressive-rate spring, with the closely-spaced coils at the top. The BB and F-41 springs are not progressive-rate; they're constant-rate, with even coil spacing.

Weight distribution: SB Coupe - 1555# front, 1600# rear
SB Convt. - 1535# front, 1640# rear

Adds to TOTAL weight (no splits shown):

A/C - 93#
Hardtop - 50#
427/390 - 215#
427/400-435 - 150#
L89 - 75#

 

[WayneC]

JohnZ, where did you find the weight distribution? With the mass of the engine up front, and a lightweight body, it's hard to believe the rear weighs more than the front on a midyear... not saying it isn't so, just hard to understand why. Unfortunately, mine isn't in any shape to make the trip to a weigh station to check it out.

 

[tsarbar]

Thanks for the info guys.

My goal is to raise the front end of my big block 61 to an acceptable level. Hopefully without creating a harsh ride.
My logic... flawed as it may be... is to find out what spring the C2 BB carries and have a corresponding set made for the 61 from eibach or some other company.

Again thanks for all the help

Chris

 

[JohnZ]

JohnZ, where did you find the weight distribution? With the mass of the engine up front, and a lightweight body, it's hard to believe the rear weighs more than the front on a midyear... not saying it isn't so, just hard to understand why. Unfortunately, mine isn't in any shape to make the trip to a weigh station to check it out.

It's in the Engineering Specifications and in the A.M.A. Specifications package, both of which are included as part of the "Restoration Package" you can get (free) from Chevrolet by calling 1-800-222-1020 and giving them your VIN.

 

['63split]

Hey Guys!

With all that said, where is a good source to acquire a set of front springs for a '63 sb coupe? And how much should one expect to pay for a set?

Thanks,
Mike