Rear Spring Arch

[Gerald C.]

Re: Rear Spring Arch

Gary,

Should I then use the 10 3/8" measured spring to be closer to the GM spec?


Jerry

[Gary B.]

[Gerald C.]

Re: Rear Spring Arch

Great. Thanks

[Mark M.]

Re: Rear Spring Arch

Good timing on 9 leaf info. I've been working on 69 427 car with 47k miles. It has a tired rear spring. For years I've always looked at C2,C3 half shaft angle as good indicator of proper rear suspension geometry. The outer ends slightly lower than the inner. A smiling rear spring usually suggests its fatigued. This of course if there's no other issues with the car. The AIM's show suspension or body ride heights depending on year and options. The 69 AIM pages sheet A12-13 has body heights from sill molding to the ground. Well I tried to have this original 69 spring ( 9.5" free arch height) re-arched to factory 10.5 at local spring shop. The car sits a little to low and I know it will only go south soon. I decided to try an Eaton Detroit Spring 9 leaf. They have an outstanding podcast on their site with Franz Estereicher on early corvette suspensions. He is an engineer who worked with one of the main suppliers for c1 c2 c3 springs in that period. It's 51 minutes long and worth listening to. The podcast should change the way we see these rear leafs painted. I asked the Eaton sales person how much to reheat treat my spring and was told their replacement was far cheaper and looks like the original. Well since starting all this a few weeks ago I found two original 9 leafs I've been saving in good shape with 10 5/8 free arch. Today I've been measuring springs apart and came up with the same info as Jeff Salz. I'm going to try the Eaton spring in the 69. Eaton suggests 50-100 miles for spring settling. This sagga may take weeks before alls told.

[Gary B.]

Re: Rear Spring Arch

Mark,

I’m curious where the 10.5” free arch spec comes from that you cite. I believe the GM spec is 10” +/- 1/8” for the GM 9-leaf spring.

Also, I think you’ll find that the shape of the ends of leaves 2 thru 9 on original GM springs is not reproduced the same on the Eaton-Detroit repro spring. The GM spring ends are referred to as taper rolled, and they have a gradual upsweep to them. The Eaton spring ends have a distinctive, somewhat sharp angular bend to them. Having said that, I believe the Eaton spring is the best 9-leaf repro spring available.

The 9-leaf spring on my 66 was created using a new, Eaton main leaf and good, used GM leaves for leaves 2-9.

Gary

[Mark M.]

Re: Rear Spring Arch

Gary, The shop that re-arched the spring mentioned that spec. It was vague and this shop has been around for a long time with a good reputation. The original spring out of the 69 showed 9.5" so the shop said they would make it a little more than 10.5". The good originals I have show 10 5/8". Where could I find the GM spec.? As you mentioned the Eatons ends don't look quite like the originals. It's seems a little more effort by them could make them the same. With a little work the Eaton ends could be shaped as originals. I like your idea to use their bottom spring or as many as needed to get the right stance. This is time consuming as I've had this conversation with a local restorer a few times. John Hinckley wrote an excellent article called Spring Thing were he mentioned Eaton-Detroit Spring and how the original thickness spring steel is no longer available so Eaton uses the closest and compensates with less arch. The Eaton 9 leaf spring assembly is around 5/8" thicker than originals. So the correct 9 leaf still isn't available. My gut feeling is this 69 will end up with Eaton bottom under original 2-9 springs as you have done.

[Gary B.]

Re: Rear Spring Arch

Mark,

My notes from 15+ years ago say I got the free arch spec from the Spring Research Institute (SRI). Among other things, the SRI is a repository for spring specs provided by the various manufacturers, which allows other manufacturers to make the specific springs.

Back to the taper rolled ends, the attached is part of a GM engineering drawing that shows the specified geometry for the end treatment. I can think of no reason why Eaton can’t reproduce that end treatment accurately. Note that there is no sharp angle to the end treatment like exists on the Eaton repro.

Gary
16D00730-4F34-4137-B85F-5520AA271CDC.jpg

[Ron G.]

Re: Rear Spring Arch

Just an FYI - Many years ago I too purchased an NOS rear spring for what I believe was for a 1963 & 1964. I put it in one of my cars and it also sat too high. I am still in possession of this spring.

[Mark M.]

Re: Rear Spring Arch

Spoke with Mike Eaton today and he was very helpful. As you have mentioned Gary, the free arch should be measured from center-line of the holes to center of the tension side with spring together. The spring shop I took the 69 spring to placed straight edge on spring bottom edge which yields 1/2" or so more to the 10" spec. given. That method could be less accurate if the leaf was cut too long. The two good originals here both measure 10 1/8" from hole centers. Mike said tooling expense was reason the end taper and curl are different. It would be interesting to see GM specs. with changes for say the 69 rear 9 leaf. The 63 and 64-74 9 leaf can be challenging to get just right.

[Gary B.]

Re: Rear Spring Arch

... It would be interesting to see GM specs. with changes for say the 69 rear 9 leaf...

[Gary B.]

Re: Rear Spring Arch

... It would be interesting to see GM specs. with changes for say the 69 rear 9 leaf...

[Mark M.]

Re: Rear Spring Arch

In the podcast with Franz Estereicher, he mentioned 63 was first design and 64-74 second. Through those years GM made numerous small change orders. Pages of them by the end of 74 design. Small details we may not notice. (SRI) may have these! I'm just curious to see some.

[Patrick H.]

Re: Rear Spring Arch

I suspect that if you install new liners in the original spring and use new cushions, the ride height will be good.

The Eaton springs are notorious for riding high just like the other replacements, no matter what they say.

[Gary B.]

Re: Rear Spring Arch

In the podcast with Franz Estereicher, he mentioned 63 was first design and 64-74 second. Through those years GM made numerous small change orders. Pages of them by the end of 74 design. Small details we may not notice. (SRI) may have these! I'm just curious to see some.

[Mark M.]

Re: Rear Spring Arch

Patrick, I did replace those parts and checked all rear suspension. The original spring right out of the car measured at least 1" fatigued. The re-arch didn't get up enough. The Eaton bottom spring replacing the original as Gary mentioned may be best chance. They do sell that part separate but not the others. I'm going to try the Eaton assembly to learn what it does. I ordered a set of longer spring mount bolts from Paragon and will make sure the length isn't too long. The 2 originals here are for other projects. Gary one must wonder why they bothered, considering they went with composite spring from than on in 81.