Transverse rear spring rate adjustment ponderings.

Update: Received the spring from Jim a while back (Thanks! (y)). Digging under the bottom cushion I saw some writing that indicates it is in the vicinity of the rate I was looking for. Been busy getting the car back together for a track day this weekend, but I'm also mocking up some ideas on a spare rear end to adapt this C3 spring to fit the C4 D36 batwing I'm using under my '69. Hopefully I'll be able to get some serious progress made on it next week.
 
Well, got some aluminum pieces cut for the spring mount to adapt this spring to the batwing. The top portion of the mount will locate/center the spring and transfer the force/weight to the batwing. I'm still kicking around ideas on the bottom/clamp portion.
 
I was never quite clear on VBP's measurement of spring rate on dual mount springs. The cen to cen distance of the spring mounts is 18" and with 4 holes, spaced 1/2 apart. VBP said that moving the blocks, that are clamped onto the spring, out 1/2 inch would increase the spring rate 25lbs. I think the issue with these springs is that, as the spring is loaded (static weight of the car) at bends into an arc (between the clamp centers) causing the spring chord length to increase and cause the spring to slide back and forth through the clamps.

I think I solved this problem but I don't have any miles on my car so maybe not. If that fails, I might try to use a C5 style design. A C5, which also uses a dual mount spring with compliant rubber cushions molded onto the spring. This was a good discussion on this (starts at post 161):

www.vettemod.com/threads/long-overdue-progress-rebuild-doors-latch-mods-10-02-24.11639/page-9


1728650645425.png
 
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Just a quick progress update. I have the spring brackets welded up to adapt the C3 spring to my C4 batwing. I'm currently striking out looking for some half inch thick rubber sheet material to provide a cushion between the spring and the lower bracket. Will probably make a temporary cushion out of whatever I can find in the mean time. A test fit of the spring shows it will package well, although it looks like the spring will need each end shortened by about two inches to line up with the suspension hanger bolts. I'm trying to finish and install this setup before the nice weather disappears and I can't take the car out for a test drive.
 
Harbor Freight? Floor cushion? Pretty low durometer but cheap and easy.
Keep us all posted!

EDIT -- Just found this at Mcmaster:
1731707334987.png
www.mcmaster.com

McMaster-Carr

McMaster-Carr is the complete source for your plant with over 595,000 products. 98% of products ordered ship from stock and deliver same or next day.
www.mcmaster.com www.mcmaster.com
the package is listed as "UV-Resistant Vibration-Damping Pad Sets' uNDER 20 BUCK SET OF 4.

Cheers - Jim
BTW - Just gotta go sailing this weekend, so I may get back to work next week.
 
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Just adding a thought on cutting/drilling/modifying the spring.
If you have access to a tile wet-saw, I've used those to cut thick carbon fiber plates - works well. Maybe not as good as a glass cutting saw, but good.
Likewise drilling wet would be my approach. You probably knew all that - Also MASK and Goggles!
Safety first - I hate those trips to ER!

Cheers - Jim
 
phantomjock said:
Harbor Freight? Floor cushion? Pretty low durometer but cheap and easy.
Keep us all posted!

EDIT -- Just found this at Mcmaster:
View attachment 57277
www.mcmaster.com

McMaster-Carr

McMaster-Carr is the complete source for your plant with over 595,000 products. 98% of products ordered ship from stock and deliver same or next day.
www.mcmaster.com www.mcmaster.com
the package is listed as "UV-Resistant Vibration-Damping Pad Sets' uNDER 20 BUCK SET OF 4.

Cheers - Jim
BTW - Just gotta go sailing this weekend, so I may get back to work next week.
Click to expand...

Hey, thanks! I suspect that M-C will have the better product, but I'll stop by HF while I'm running errands today to see if I can find something cheap to tie me over until I can get something shipped from M-C.
 
Update: I finished narrowing the spring this afternoon and wanted to do a test fit today. I don't have the "official" lower rubber pad mount yet (McMaster Carr had so damn many durometer options that I would have just been guessing which one to pick), so for now I just cut up an old lawn tractor tire tread for a temporary pad. Got the spring installed (hot damn, everything fits!), and have the car back on the ground. Hopefully the weather clears up and I can take the car out on a test drive soon.
I'm anxious to see how this spring rides. I've run three different C4 rear springs as I've pulled weight off the car. The two '84 springs (about 410 and 500 #/in) are too damn stiff now (rear axle weight 1300#), but the other C4 spring is a base suspension spring (about 230 #/in), which gives a nice ride, but the car will bottom out too easily. The spring I got from Jim looks to be about 335 #/in if I'm correctly reading the markings on the bottom of the spring. This should be an acceptable ride, and also a bit more roll stiffness compared to the C4 base spring.
As all the threaded bolt holes are metric on the D36 batwing I'm using, I collected up a bag full of different size metric bolts at my last trip to my local Pick And Pull to mix and match when installing the spring brackets. These bolts look like they have the correct plating/coating for use in aluminum, and they also give a more stock look to the suspension.
 
Mcmaster has a durometer and applications guide that might be helpful:

www.mcmaster.com

McMaster-Carr

McMaster-Carr is the complete source for your plant with over 595,000 products. 98% of products ordered ship from stock and deliver same or next day.
www.mcmaster.com www.mcmaster.com

1732109815474.png

1732109849941.png
 
phantomjock said:
Pics are ALWAYS appreciated!
Just curious - using the spring to replace an ARB too? More weight reduction.

Cheers - Jim
Click to expand...
I'll try to get a picture posted up. I'm rather rusty posting pictures, so I might need to brush off some cobwebs in the process.

I don't have a rear bar on the car. I've tried to figure out a way to package one back there, but the combination of a C3 frame and a narrowed C4 suspension just doesn't play well together.
 
The new format of the forum greatly facilitaes loading pics I've found.
The transverse spring will give some ARB-like efforts to the rear end - so maybe you can forgo sorting one out.
Looking forward to your reports.

Cheers - Jim
 
phantomjock said:
The new format of the forum greatly facilitaes loading pics I've found.
The transverse spring will give some ARB-like efforts to the rear end - so maybe you can forgo sorting one out.
Looking forward to your reports.

Cheers - Jim
Click to expand...

Nothing fancy with the bracket. I just had to make it attach to the same four threaded holes used by the stock C4 spring. I did have to mount the spring an inch lower than I preferred, but I had no choice. Unlike C3 differential covers, the C4 batwings don't have lube fill plugs in them (it's in the differential housing), so I had to put one in the batwing when I modified it to bolt to the C3 differential housing years ago. It's in the rear, just below the two lower bolts attaching the toe-rod bracket. Below the two bolts is a "mouse hole" access to the plug. After the test drive yet to come, I'll do a bit of cosmetic work on the bracket, and probably shave a few ounces off it, and also find one additional right-size bolt for the bottom plate.
While I was under the car I took an additional picture of an experiment I did a few months back. I had some low RPM drone from the exhaust that I wanted to quiet down, and I thought I'd try a surge volume/resonator/"fluid capacitance" box. It's an empty volume connected by a short section of drilled holes to allow the exhaust pressure waves to enter and exit it, to hopefully reduce the low RPM drone and noise level at all other RPMs. I was pleasantly surprised at how well it worked. The weight penalty was minimal, as all of the exhaust rearward of the transmission is aluminum tubing, and there's no added restriction in the system.
 

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BBShark said:
Interesting exhaust drone solution.
Click to expand...

Well, a few years back I finally got to use a particular item (analogs) from perhaps sophomore engineering. If you recall we were shown that several items in the electrical/mechanical/physics world all share the same basic formulas, with only the units changed [like Energy = (AxB^2)/2]. I've found, as a double E, that if in my head I look at the engine as a variable frequency/pulsewidth/magnitude current source, and the exhaust system as a collection of resistances, capacitance, and inductance, it looks like any of a number of circuits I've dealt with before, and much of the mystery is gone. Of course, in this circumstance, the only instruments I have available are a pressure gauge and my ears.

Admittedly odd, but it works for me. 😊
 
I like the observation that all engineering disciplines are really inter-related. Years ago I read a book by Buckminster Fuller. He wrote about education and proposed there should be a "Unified Design Curriculum". Basically it was a re-definition of roles (kind of a Leonardo Da Vinci model). He didn't believe that there should be "disciplines" where education is specialized. After all, specialization is what killed the dinosaur.

Later in life, a coworker told me if he had his choice, he would only hire math majors, they can understand anything.
 
Last decent weather for a while, so I took the car out for a drive. Nothing broke, stalled, or fell off, so it's a successful drive in my book. The main things I wanted to check out were the ride quality and the bump/wheel travel (I had moved the upper shock mounts an inch higher to keep the shock from bottoming out). The ride was very acceptable (Thanks, Jim!) and despite a few bumps in the road I didn't detect any serious issues with bottoming the shocks.

Other observations: I love how the car handles on track days, but due to the gearing (3.08 rear with a close ratio Muncie) taking off from a stop light is miserable. I had contemplated over the years putting my original M21 on the shelf so I don't break it on track days, and buying a used M21 to put in the car. But, after today's drive, I'm contemplating looking around for a wide ratio M20. I've got all winter to think about it and do the math for the RPM@speed difference in third gear, as it seems most of the time third and fourth are the only gears I use when on the track.
 
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