'69 BB front coil spring dimensions/spec's?

[G A.]

For my application, JG says the part number was 3931825. BB, no A/C, w/F-40. Also references that the spring was installed with the closely spaced coils up. Well, I'm looking for a repro set and would like to have an idea of what spec's the original spring met for comparison (free height, installed height, rate etc.). Only 1 vendor I have seen so far with a spring that has the coils closer on one end. All the rest (so far) have evenly spaced coils.

Would anybody here have that data on an engineering drawing or technical sheet for accurate reference??

Dan

[Terry M.]

Re: '69 BB front coil spring dimensions/spec's?

Dan,
The coil springs with the coils more closely spaced at one end are called progressive rate springs. The springs with the coils evenly spaced are called constant rate. Progressive rate springs will have a compression rate (given in inches per pound) that changes as the spring is compressed. Thus x pounds will compress the spring A inches, but 2X pounds will not compress the spring 2A inches.

I believe the spring rates (perhaps at design ride height) are given in the AMA (Automobile Manufacturers Association) specifications which are available on the GM Heritage Center web site:

http://www.gmheritagecenter.com/gm-heritage-archive/docs/Corvette/1969-Chevrolet-Corvette.pdf

Have fun searching. Don't give up the good stuff starts some pages into this pdf.

[G A.]

Re: '69 BB front coil spring dimensions/spec's?

Terry, your right, LOTS of good information. Here is what I found: Part Number 3931825/RH Helix coil/Steel alloy/Cut-off length 138.75/Wire Dia. .618/Coil Dia. 3.80/Working Height 9.99"@1540#/Deflection rate 284#/inch.

So lets see, now I THINK know the wire diameter, the total length of the bar (wire), inside diameter, working height (installed height?) and rate (284 seems light). Still do not know the free height.


Dan

[Bill M.]

Re: '69 BB front coil spring dimensions/spec's?

Still do not know the free height.

9.99 + 1540 / 284 = 15.41

[G A.]

Re: '69 BB front coil spring dimensions/spec's?

Wow Bill! Obviously you understand something I don't, would you mind explaining to me how that works??
I see where the numbers come from, but why does that give you the free height?
Does it work all the time??

EDIT: After some thought, I see that the load of 1540 divided by the rate of 284 gives a compressed height loss of 5.42 inches. Add that back to the loaded height of 9.99 and you get 15.32. Close??

[Bill M.]

Re: '69 BB front coil spring dimensions/spec's?

Wow Bill! Obviously you understand something I don't, would you mind explaining to me how that works??
I see where the numbers come from, but why does that give you the free height?
Does it work all the time??

EDIT: After some thought, I see that the load of 1540 divided by the rate of 284 gives a compressed height loss of 5.42 inches. Add that back to the loaded height of 9.99 and you get 15.32. Close??

That's it. (This works on a constant-rate spring, but not on the variable-rate springs used from '64 - '67.)

[Terry M.]

Re: '69 BB front coil spring dimensions/spec's?

I thought the variable rate (progressive) springs were used through 1972, but maybe I am wrong. The note about which end is up is in the AIM through 1972, but maybe some draftsman was too lazy to remove it.

[John M.]

Re: '69 BB front coil spring dimensions/spec's?

wouldn't the 1540 be 770, 4 wheels

[G A.]

Re: '69 BB front coil spring dimensions/spec's?

I thought the variable rate (progressive) springs were used through 1972, but maybe I am wrong. The note about which end is up is in the AIM through 1972, but maybe some draftsman was too lazy to remove it.

In my case did a '69 have a variable rate spring or not, is one of the issues I have been looking into. My question initially came from the constant rate style of spring offered as a "reproduction of original" offered by many vendors. I saw the note in AIM as well as the '69 TIM and what I have found in the archives leads me to TEND to believe that a 3931825 was a variable rate spring.
There are 2 other BB springs for 68-69 (w/A-C, w/F41) so the ONE variable rate spring I have found offered as an 'exact reproduction' only has a 33% chance of being what I need.

Dan

[Bill M.]

Re: '69 BB front coil spring dimensions/spec's?

I thought the variable rate (progressive) springs were used through 1972, but maybe I am wrong. The note about which end is up is in the AIM through 1972, but maybe some draftsman was too lazy to remove it.

The formula should work on a variable-rate spring too as long as you use the initial spring rate and the coils haven't started to bottom at design load. I would expect a variable-rate spring to have a range of rates. I have 195-295 for the variable rate springs used in '64-'67.

[Bill M.]

Re: '69 BB front coil spring dimensions/spec's?

wouldn't the 1540 be 770, 4 wheels

1540 is the load on the spring at design load (design load is with passengers and luggage). The load at each front tire would be about 1000. That may seem high, but I have a road test on a '70 454 where the test weight was "over 2 tons."

[Timothy B.]

Re: '69 BB front coil spring dimensions/spec's?

Is the 3931825 spring the same one that's used for a 68 427/400hp w/o ac. I think I have the two that I took off my 68 back in 1984 to install those F-41 springs. I think the part# on the HD springs ended in 518 and I remember you could remove them by jacking the car up because they were loose.