Early C3 Coil Spring - Advice Needed PLEASE

[Patrick H.]

Re: Early C3 Coil Spring - Advice Needed PLEASE

All will be.

I'm waiting.....

[Patrick H.]

Re: Early C3 Coil Spring - Advice Needed PLEASE

Ride height dimensions are given in the AIM's. The 1969 specs (3931823 springs) are essentially the same as the 1973 specs (331316 springs).

Silly thought I had this morning -

As the 73-and-later Corvettes had a different body-to-frame cushion which was taller, would that then affect the design of the front spring height at all, so that if installed in an earlier Corvette it would ride a bit higher?

Again, just a thought.

[Joe L.]

Re: Early C3 Coil Spring - Advice Needed PLEASE

Don,

The "P" dimension for my 72 LT-1 (non A/C) is 27 1/4", sitting on its original springs. I hope you get this figured out.

Dan.

Dan------


Keep in mind that your springs, if original, are now 45 years old. There's no doubt that they suffer from at least some "sag". So, whatever the "P" dimension is now won't be the same as when the car was new.

[Ralph S.]

Re: Early C3 Coil Spring - Advice Needed PLEASE

2 original springs

[Joe L.]

Re: Early C3 Coil Spring - Advice Needed PLEASE

2 original springs

Ralph------

Thanks. I thought that the 3931823 springs were about the same length as the 331316 and you have proved that.

Also, I measured my 331316 length at the very end of the top coil. From your photo, it looks to me like if you had measured your 3931823 at that point you would have gotten a measurement even closer to the 15-7/8" I got.

[Joe L.]

Re: Early C3 Coil Spring - Advice Needed PLEASE

All-----


Here are photos of an NOS GM #331316. This spring is 15-7/8" overall length. It has 10 coils and the wire diameter is 0.600". It is progressive rate. So, except for a slight difference in length, it seems identical to the 3931823 that Ralph pictured.

Most, if not all, of the "reproduction" and replacement springs I have seen for the 68-72 small block w/o A/C application have been single rate, shorter, and with fewer coils. Some have a slightly greater wire diameter.

[Jimmy P.]

Re: Early C3 Coil Spring - Advice Needed PLEASE

Thanks a bunch Joe. Is your car a BB or SB? AC?

Does anyone else have a "P" dimension (ground to fender lip at wheel center line) for a '68-'72 that they can share?

I've been wondering about this all weekend. It occurs to me that with the 1973 MY, federal impact regulations required the rubber front bumper. I was young then, but kinda recall how there was also a bumper height mandate around that same time. I remember this because it purportedly threatened the continuance of several vehicle designs. Might the General have raised the bumper height in 1973 to comply with a bumper height mandate, and might they have made a spring change to accomplish a bumper height increase? It would be helpful to know that a 1973 AIM shows for the "P" dimension. Anyone?

Hi Don,
Here are the 'P' measurements listed in the '73 AIM.
Coupe Standard Sus.- 27.31 Curb - 27.75
Conv. Stand. Sus. - 27.39. Curb - 27.84
Coupe &Conv. with F-41 Sus. - 26.73. Curb - 26.73
All - F-70 tires 28.32 Curb - 28.37
All GR-70 tires 28.30 Curb - 28.35

Front bumper to ground: 12.2 inches.

I'll be sending you an email too!
Hope this helps!

[Don L.]

Re: Early C3 Coil Spring - Advice Needed PLEASE

Ralph------

Thanks. I thought that the 3931823 springs were about the same length as the 331316 and you have proved that.

Also, I measured my 331316 length at the very end of the top coil. From your photo, it looks to me like if you had measured your 3931823 at that point you would have gotten a measurement even closer to the 15-7/8" I got.

My head's beginning to hurt with all of this. I'm actually glad to see the original 3931823 spring at 15.5" (or more). It better explains how GM chose 331316 as a replacement. I'm puzzled as to the 12.08" free length dimension as posted prior. As I suggested, it came from GM specs. I've attached a scan of the suspension page from the 1972 CORVETTE SPECIFICATIONS document, downloaded from GM Heritage web site. Obviously, at least the free length listed in their specs is inconsistent with what's being observed.

I still expect to have properties analysis for the 331316 springs that were removed from my car over the weekend. BTW, the light yellow part number tag on those springs is identical to what's shown in Joe's photo here.

[Bill M.]

Re: Early C3 Coil Spring - Advice Needed PLEASE

My head's beginning to hurt with all of this. I'm actually glad to see the original 3931823 spring at 15.5" (or more). It better explains how GM chose 331316 as a replacement. I'm puzzled as to the 12.08" free length dimension as posted prior. As I suggested, it came from GM specs. I've attached a scan of the suspension page from the 1972 CORVETTE SPECIFICATIONS document, downloaded from GM Heritage web site. Obviously, at least the free length listed in their specs is inconsistent with what's being observed.

This does compute from the specs:

Design load 1395 / spring rate 250 + design height 9.99 = free height 15.57

[Duke W.]

Re: Early C3 Coil Spring - Advice Needed PLEASE

Thanks a bunch Joe. Is your car a BB or SB? AC? I've been wondering about this all weekend. It occurs to me that with the 1973 MY, federal impact regulations required the rubber front bumper. I was young then, but kinda recall how there was also a bumper height mandate around that same time. I remember this because it purportedly threatened the continuance of several vehicle designs. Might the General have raised the bumper height in 1973 to comply with a bumper height mandate, and might they have made a spring change to accomplish a bumper height increase? It would be helpful to know that a 1973 AIM shows for the "P" dimension. Anyone?

1973 was the first year for federal bumper standards - "2.5 MPH" (increased to 5 MPH in '74). Height from the ground had to be within a specified range, but implementation method was up to the manufacturers. Chevrolet choose for the Corvette a design with a bolt and die with an endura cover over the bumper bar. On impact the bumper was pushed back and energy was absorbed as the bolt was pushed through the die, so it was a one shot deal. The required bumper height range is probably why the spring was redesigned, but I have to question that the replacement spring rate nearly doubled to nearly equal the HD 550 lb/in spring. I suspect an error. There is a formula that you can plug your data into to compute the rate. I know it's in the Chevrolet Power Manuals chassis section and you can also probably easily find it with a Web search. The formula is for constant rate springs, but you can probably get a ballpark number if the springs are variable rate. There's a common misconception that the 2.5/5 MPH bumper standard specified no damage. That's not true. It only specified no damage to "safety equipment". So if you hit a wall you could do unlimited body damage as long as a two-dollar sealed beam headlight didn't break... your government at work.

[Jimmy P.]

Re: Early C3 Coil Spring - Advice Needed PLEASE

1973 was the first year for federal bumper standards - "2.5 MPH" (increased to 5 MPH in '74). Height from the ground had to be within a specified range, but implementation method was up to the manufacturers. Chevrolet choose for the Corvette a design with a bolt and die with an endura cover over the bumper bar. On impact the bumper was pushed back and energy was absorbed as the bolt was pushed through the die, so it was a one shot deal. The required bumper height range is probably why the spring was redesigned, but I have to question that the replacement spring rate nearly doubled to nearly equal the HD 550 lb/in spring. I suspect an error. There is a formula that you can plug your data into to compute the rate. I know it's in the Chevrolet Power Manuals chassis section and you can also probably easily find it with a Web search. The formula is for constant rate springs, but you can probably get a ballpark number if the springs are variable rate. There's a common misconception that the 2.5/5 MPH bumper standard specified no damage. That's not true. It only specified no damage to "safety equipment". So if you hit a wall you could do unlimited body damage as long as a two-dollar sealed beam headlight didn't break... your government at work.

Hi Duke and Don,
Here is a photo of the 73 Suspension data page from the "1973 Corvette Specifications"info from the GM Heritage Center. It lists the spring rate for the base car. Hope its legible enough for everyone to read. Also, you mention the bolt and die design above used by GM for the 73 corvette. That bolt was called the "Omark Bolt". It was a very interesting design. As a matter of fact I just recently wrote a little tech article on it for submission in a future Restorer Magazine. You are correct, once it did its job, it was used up and had to be replaced. I've included a photo of it below, along with one of it on my 73.
Regards,

[Patrick B.]

Re: Early C3 Coil Spring - Advice Needed PLEASE

1973 was the first year for federal bumper standards - "2.5 MPH" (increased to 5 MPH in '74). Height from the ground had to be within a specified range, but implementation method was up to the manufacturers. Chevrolet choose for the Corvette a design with a bolt and die with an endura cover over the bumper bar. On impact the bumper was pushed back and energy was absorbed as the bolt was pushed through the die, so it was a one shot deal. The required bumper height range is probably why the spring was redesigned, but I have to question that the replacement spring rate nearly doubled to nearly equal the HD 550 lb/in spring. I suspect an error. There is a formula that you can plug your data into to compute the rate. I know it's in the Chevrolet Power Manuals chassis section and you can also probably easily find it with a Web search. The formula is for constant rate springs, but you can probably get a ballpark number if the springs are variable rate. There's a common misconception that the 2.5/5 MPH bumper standard specified no damage. That's not true. It only specified no damage to "safety equipment". So if you hit a wall you could do unlimited body damage as long as a two-dollar sealed beam headlight didn't break... your government at work.

Having had some experience with NHTSA rulemaking and the power and influence of auto industry lobbyists, I would expect that the industry brought congressional or legal pressure to put aesthetic damage outside the scope of NHTSA's authority. That left damage to safety equipment as the only crash damage criteria subject to regulation. At least the very expensive headlamp rotation system of Corvettes could be included in the protection of the safety function of the headlamp system. It's a case of the good versus the perfect, and the perfect can't survive politics. There were plenty of Corvette enthusiasts at NHTSA, and they understood that $2 headlamps were not what caused public outrage at the cost of damage due to very minor impacts.

[Don L.]

Re: Early C3 Coil Spring - Advice Needed PLEASE

After reading all the good inputs to this thread (thank you all), I think I'm beginning to figure out some of the puzzle.

Based on some analysis I had done in our spring lab in North Carolina, it looks like both the 250# and the 494# rates are actually correct. The difference is in the way the rates are specified. In so much as the springs are dual rate, the initial rate is specified as 250#. It's the second rate that is spec'd at 494#.

I want my car to sit at specified height. I having my guys do some shot peening and setting work and then I'll re-install them. We already have the working height down to 10" at 1395#. We've cold pressed approx 1/4" out of free length. Once the work is done, I'll measure and report back. At the same time, we're doing some design work that I think will yield the ability to manufacure a dead-nuts replica of the 3931823 spring, giving it the right rates, the proper free length, OD, wire size, ride height, etc.

Stay tuned...

[Don L.]

Re: Early C3 Coil Spring - Advice Needed PLEASE

UPDATE: I completed the rework of the 331316 springs so that the free length shortened from approx. 15 7/8" to 15 3/8". Did this by shot peening them and cold pressing (to solid) a half dozen times or so. After install and a shakedown, I am happy to report that a the front ride height has dropped from the "out of spec" 28 1/8" to right at 27 1/2". With the "P" dimension spec at 27.379 +/- 0.5", I think it turned out nicely.

During the rework project, we did a spring design and I think we can now make a spring from scratch that will yield GM specified ride height. After I get over all the hard work involved in removing/re-installing coil springs, I may have our guys make a set and see if I can get 27 3/8". I will install on my car or another at some time in the future.

Here's a before and after photos of the same springs.



Thanks again for everyone's help on this. I learned a lot and reached a great outcome with your help.