Early C3 Coil Spring - Advice Needed PLEASE

[Don L.]

Hi all.

Recently, I decided to tear into the front suspension on my August 1971 built '72 L48 (M20, no AC) convertible to fix a jacking scar in the cross member from a previous owner and to see why the car sits high in the front. Where the AIM calls for a ride hgt of 27.39" +/- 0.50”, my car rides at approx 28.125". It's been that high since I bought the car in 2005.

I pulled the springs. What I hoped to find was the factory GM3931823 spring. What I found is the replacement GM331316 spring. According to my archive research 331316 replaced 3931823 in Nov 1972. Also, research shows that many people are dissatisfied with ride hgt from 331316.

An A/B comparison is as follows:

A) GM 3931823:
Free Length: 13.08"
Spring rate: 250 lbs
Coil Count: 10.0
Working Hgt: 9.99” @ 1325lbs
Wire diameter: 0.600"

B) GM 331316:
Free Length: 15.50"
Spring rate: 494 lbs
Coil Count: 10.0
Working Hgt: ???
Wire diameter: 0.600"

With a 2.5" increase in free length and a spring rate that's double that of the original, I’m not surprised to see the car sitting high.

I'm in the spring business, so I have access to product manufacturing and can have a spring made to almost any design. My question is, should I have a couple new springs made to the GM3931823 specs and should there be a high likelihood that this would resolve my ride hgt issue? might there be other factors that lead to ride height?

I can’t imagine how I could go wrong here, however, as anyone that’s done a coil spring R & R would agree, it’s not a job you want to do twice unless absolutely necessary.

Any advice is greatly appreciated.

Thanks much.

[Joe L.]

Re: Early C3 Coil Spring - Advice Needed PLEASE

Hi all.

Recently, I decided to tear into the front suspension on my August 1971 built '72 L48 (M20, no AC) convertible to fix a jacking scar in the cross member from a previous owner and to see why the car sits high in the front. Where the AIM calls for a ride hgt of 27.39" +/- 0.50”, my car rides at approx 28.125". It's been that high since I bought the car in 2005.

I pulled with springs. What I hoped to find was the factory GM3931823 spring. What I found is the replacement GM331316 spring. According to my archive research 331316 replaced 3931823 in Nov 1972. Also, research shows that many people are dissatisfied with ride hgt from 331316.

An A/B comparison is as follows:

A) GM 3931823:
Free Length: 13.08"
Spring rate: 250 lbs
Coil Count: 10.0
Working Hgt: 9.99” @ 1325lbs
Wire diameter: 0.600"

B) GM 331316:
Free Length: 15.50"
Spring rate: 494 lbs
Coil Count: 10.0
Working Hgt: ???
Wire diameter: 0.600"

With a 2.5" increase in free length and a spring rate that's double that of the original, I’m not surprised to see the car sitting high.

I'm in the spring business, so I have access to product manufacturing and can have a spring made to almost any design. My question is, should I have a couple new springs made to the GM3931823 specs and should there be a high likelihood that this would resolve my ride hgt issue? might there be other factors that lead to ride height?

I can’t imagine how I could go wrong here, however, as anyone that’s done a coil spring R & R would agree, it’s not a job you want to do twice unless absolutely necessary.

Any advice is greatly appreciated.

Thanks much.

Don------


The GM #331316 springs were used in PRODUCTION for 1973+ Corvettes with small block, 4 speed, without A/C, and without F-41. They became SERVICE for 1968-72. I used them on my 1969 and I'm 100% satisfied with the ride height. I think they're an improvement over the original springs.

[Bob W.]

Re: Early C3 Coil Spring - Advice Needed PLEASE

Joe Did you see any increase in ride hight or stiffer ride ? Maybe Dons problem is weak rear spring. Can you still order the 331316 spring from GM?
Bob

[Joe L.]

Re: Early C3 Coil Spring - Advice Needed PLEASE

Joe Did you see any increase in ride hight or stiffer ride ? Maybe Dons problem is weak rear spring. Can you still order the 331316 spring from GM?
Bob

Bob------


Very slight increase in ride height. However, I attributed that to slight "collapse" of the original springs. I had no way to compare the ride height of the car when new to the ride height of the car with the 331316 springs.

The GM #331316 springs are long-since GM discontinued.

[Bob W.]

Re: Early C3 Coil Spring - Advice Needed PLEASE

Joe Thanks for the quick reply Also do you go to Corvette Charlisle /If so i would like to meet you.
Bob

[Joe L.]

Re: Early C3 Coil Spring - Advice Needed PLEASE

Joe Thanks for the quick reply Also do you go to Corvette Charlisle /If so i would like to meet you.
Bob

Bob------


Once-in-awhile I go to Carlisle but it's been quite some time since I was last there.

[Bob W.]

Re: Early C3 Coil Spring - Advice Needed PLEASE

Well I think it"s time to plan a trip for 2017!!
Bob

[Don L.]

Re: Early C3 Coil Spring - Advice Needed PLEASE

Bob------


Very slight increase in ride height. However, I attributed that to slight "collapse" of the original springs. I had no way to compare the ride height of the car when new to the ride height of the car with the 331316 springs.

The GM #331316 springs are long-since GM discontinued.

Thanks so much for the feedback so far, guys.

Bob, I suppose the height at the front could be somewhat due to the rear spring, however, know that I replaced the rear spring several years ago, using the Eaton spring. From the old rear spring to the new one has not changed front height. I'm far less than enamored with the Eaton spring's ride height, however, I know maybe too much about performance and durability of rusty and pitted springs, which is what we typically see in used parts from the era. This is why I chose the Eaton spring. I may re-arc the Eaton leaf spring if I can get the front end down. Right now, I am level at all four corners, but to my eye, too high.

Joe, what is your '69 (coupe/convertible, small block/big block, M20/M40, C60/ non-C60, etc)? As well, you note how you attributed the change in height when moving from the 1823 to the 1316 springs to be due to a weak 1823 spring. What is your "P" measurement with the 1316 spring?

There's a substantial difference in spring design between the 1823 and the 1316 parts.

Thanks again!

[Joe L.]

Re: Early C3 Coil Spring - Advice Needed PLEASE

Thanks so much for the feedback so far, guys.

Bob, I suppose the height at the front could be somewhat due to the rear spring, however, know that I replaced the rear spring several years ago, using the Eaton spring. From the old rear spring to the new one has not changed front height. I'm far less than enamored with the Eaton spring's ride height, however, I know maybe too much about performance and durability of rusty and pitted springs, which is what we typically see in used parts from the era. This is why I chose the Eaton spring. I may re-arc the Eaton leaf spring if I can get the front end down. Right now, I am level at all four corners, but to my eye, too high.

Joe, what is your '69 (coupe/convertible, small block/big block, M20/M40, C60/ non-C60, etc)? As well, you note how you attributed the change in height when moving from the 1823 to the 1316 springs to be due to a weak 1823 spring. What is your "P" measurement with the 1316 spring?

There's a substantial difference in spring design between the 1823 and the 1316 parts.

Thanks again!

Don------


The "P" dimension on my car is 28.0". However, the actual dimension may be a bit lower as the suspension is not completely normalized with the car sitting on ramps at all 4 wheels.

Also, the car is currently equipped with 245-60-15 tires. However, these tires are within .06" OD of the original F-70-15.

[Bill M.]

Re: Early C3 Coil Spring - Advice Needed PLEASE

Don:

There is something wrong with your spring specs. Spring rate is dependent on wire dia. and no. of active coils (assuming same OD) so they both should have the same spring rate if the dimensional numbers are right.....?

Bill

[Don L.]

Re: Early C3 Coil Spring - Advice Needed PLEASE

Don:

There is something wrong with your spring specs. Spring rate is dependent on wire dia. and no. of active coils (assuming same OD) so they both should have the same spring rate if the dimensional numbers are right.....?

Bill

Hi Bill.

Since at least the 331316 spring is a dual rate spring, rate comparison would depend on where/how the rate was measured and whether or not the 3931823 was also a dual rate spring. I am having a rate measurement taken today on the 331316 parts that I took off my car and will post once I get the report. I am also getting a pitch profile analysis for this part as well. I will share that too.

So far, I can confirm that the 331316 uses 0.600" (actual mic dimension is 0.607") wire. I can confirm it has a free length of 15.5". I can confirm it has 10.0 coils. I can confirm it has an OD of 5". The specs I got for the original spring came from original GM charts. I don't have an original spring to confirm all that. Does anyone have the ability to confirm what I posted for the 3931823 spring? Can anyone tell us whether or not the original spring was dual or single rate? A phot would be great.

Thanks.

[Don L.]

Re: Early C3 Coil Spring - Advice Needed PLEASE

Don------


The "P" dimension on my car is 28.0". However, the actual dimension may be a bit lower as the suspension is not completely normalized with the car sitting on ramps at all 4 wheels.

Also, the car is currently equipped with 245-60-15 tires. However, these tires are within .06" OD of the original F-70-15.

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?

[Dan A.]

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.

[Anthony F.]

Re: Early C3 Coil Spring - Advice Needed PLEASE

The 'P' dimension for my 72 LT-1 with A/C, original springs, is 27 3/4". I am running BFG 225-70 15's (diameter is about .5" more than F70-15), so with original size tires I'd probably be closer to above poster Dan's 27 1/4".

[Joe L.]

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?

Don-------


It's an original small block. Now, it's an all aluminum big block. However, the BB assembly is about 30 pounds lighter than the small block. No A/C. Hood, carburetor, and AIR are not currently installed. All will be. As I mentioned, I expect the "P" dimension I measured to be slightly less when the suspension is fully normalized.

The 3931823 was a dual rate spring. Most 68+ coil springs were dual rate. The only exception I can think of is F-41.

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