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.
 
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