Transverse rear spring rate adjustment ponderings.

69427

69427

The Artist formerly known as Turbo84
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Mar 30, 2008
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Clinging to my guns and religion in KCMO.
I've been running a variety of C4 rear springs under my '69 as they're reasonably inexpensive, look stock, and fit the lightweight D36 batwing I adapted to my stock differential. I'd like to be able to "adjust" the effective spring rate, as the selection of stock springs is limited, and due to needed modifications to the spring to fit this narrowed C4 suspension, once I buy a spring I own it forever, as I'm not aware of anyone else using/needing narrowed C4 springs. With this background info, here's my ponderings and questions below.

My current spring is nice on the street, but a bit soft on the track. My next stiffer spring on hand is the base '84 spring, which is a bit stiff with the current rear weight of the car. I'd like to soften the effective spring/wheel rate of it somehow. An idea popped into my head, but there's significant issues that I don't have answers for yet. As we learned in freshman engineering, if you put two springs in series you effectively reduce the rate. With this, my first thought was an experiment to essentially put an engine valve spring between the leaf spring and the hanger bolt bottom washer/mount. This would reduce the effective rate, but a valve spring has very limited travel before coil bind, and would almost certainly bind just from the static weight of that corner of the car. I haven't searched around yet to see what valve spring rates and travel are available (and I'm not optimistic that a suitable part exists), and I haven't yet looked at McMaster-Carr to see what industrial springs are out there.

While I'm searching around to see if any spring candidates exist, I welcome correction, input and thoughts on this topic. (I thought about a bellcrank/rod system, but it just doesn't seem like it would package very easily.)
 
The C4 spring has a rigid section in the center of the spring (where it mounts to the differential) correct? Maybe you could use the softer spring but make the rigid center section wider to increase the stiffness of the spring:

1723550318362.png
 
Maybe something like the old VB&P dual mount spring kit they had for the C3? BBShark's description rung a bell in my head on what I remembered seeing in their catalogs. I remember a spring & mount kit for the C4 that had multiple positions to set pins to adjust the spring rate like the W2 dimension in his sketch. I'll see if I have an old catalog on the shelf at home to see if there is any info about it.
 
And, when Red77 finds the catalog details I have a VB&P dual mount spring sitting in the corner of the garage.
Cant recall the "rate," but let me know. Shipping only then it's yours.

Cheers, Jim
 
I count 12 leaves on this car. He stacked them for more stiffness. IMG_2721.jpeg
 
phantomjock said:
And, when Red77 finds the catalog details I have a VB&P dual mount spring sitting in the corner of the garage.
Cant recall the "rate," but let me know. Shipping only then it's yours.

Cheers, Jim
Click to expand...
I appreciate your generosity. (y) If I can trouble you first, could you give me the measurement of the distance between the spring outer holes for the trailing arm hanger bolts? My suspension setup is pretty bastardized, so I can't measure any stock C3 dimensions to compare to the present setup dimensions. Thanks again!
 
rtj said:
I count 12 leaves on this car. He stacked them for more stiffness. View attachment 57045
Click to expand...


My spring is a composite unit, so stacking layers is a risky option. The only safe option I see is adding the single steel bottom leaf under the composite spring, and then having both the center and ends of each spring vertically spaced slightly apart so there is no inter-leaf contact and wear. I also wonder what the bottom leaf rate availability would be, and what the weight of that steel leaf would be. Seems doable, but I don't have answers on some of the details.
 
69427 said:
My spring is a composite unit, so stacking layers is a risky option. The only safe option I see is adding the single steel bottom leaf under the composite spring, and then having both the center and ends of each spring vertically spaced slightly apart so there is no inter-leaf contact and wear. I also wonder what the bottom leaf rate availability would be, and what the weight of that steel leaf would be. Seems doable, but I don't have answers on some of the details.
Click to expand...


That car is for sale with Tom’s 12 bolt, cage, new 572, etc. I think it was set up for wheels up launches.

Maybe look at some of the overload springs attach. I searched on progressive springs, but not long enough to find anything applicable.

The two support springs others mentioned might be the best.
 
Mike, Here is my Dual Mount spring setup. The center to center for the bolts is ~ 46" installed. The good thing about this spring is that it is a constant cross section, but that is the only thing. I had to ditch the mounts because of a number of problems that I became aware of after VBP went under.

20240816_151742.jpg
 
To elaborate on the difficulty with the VBP design, these are the cushions for the spring mounts after being on the car with zero miles:

I was getting ready to put the car on the ground and looked under the front suspension. There were small chunks of plastic laying on the ground. The source was cushions that are sandwiched between the spring and the spring mount blocks. These have been installed for a week!

Here are the remains of the "cushions".

461d1d1f369129.jpg

So that is probably why they use polypropylene cushions that allow the spring to slip. That is probably why my cushions crumbled when I had the car on the ground. The spring slipped through putting a shearing force on the cushions.

461d3333ce0c7b.jpg

I'm thinking your design might be made more complex due to the shape of the spring (non-constant cross section) but you shouldn't have this problem
 
Got the numbers from my 2 springs.
The rear VB&P spring is 44 1/2 inches center to center - with a 5 inch "drop" to the end points from center.

The front VB&P spring is 42 1/4 inches, center to center with a 2 inch drop. It mounted at a wider location than the rear spring, but can be adjusted (I believe). You can see the mounting fitting in the edge of the pic.
Here is the spec on the front spring:
front spring.jpg

Cheers - Jim
 
Quickie over coffee this morning. Thought might better explain the dimensions... Rear spring. Cheers.

quickie - over coffee.jpg
 
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Gentlemen, thank you for your input and your patience with me. I started (what I thought was) a minor chore last week on the rear suspension to see what I could do to increase the rear suspension travel before it hit the rubber bumpers. In the process I found that the rear shocks may have been doing some of the bump stop work when driving on the rougher parts of the track. I cut and modified the shock mounts to get rid of that problem and increase the travel, but then several other items (that I won't bore you with) then had interference issues with the frame, batwing mounts, trailing arm vertical travel plane, and even the inner sidewalls of the wider tires I'm now running. After multiple days I'm almost done with the feasible changes I can do on the RR suspension, and then I have to duplicate those changes on the other side.

Back to the main topic here. I measured my current spring width, and I get 42 inches between the two hanger bolts. It appears a 44-44.5 width spring would fit between the tires, and the hanger bolt angle wouldn't be too far off from vertical. The biggest issue fitting the spring is the mounting. AFAIK, it's meant to be used with brackets that connect to the differential crossmember rather than the cover. That crossover is obviously not there with my C4 suspension. I suspect that a spring bracket at the batwing could be fabricated to be compatible with that variable rate spring.

Phantomjock, I'd be interested in looking closer at that spring to see if it could work with my C4 suspension. I have no desire to make any modifications to the spring, and if I can't make it work as is I would certainly contact you about returning it. Just let me know. Thanks again. Mike.
 
No luck finding the old VB&P catalog on my end. I must have tossed it when we last moved since they were defunct by then anyway. Good to hear you've made some progress despite uncovering new issues. That's what good R&D is all about.
 
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