Transverse rear spring rate adjustment ponderings.

[phantomjock]

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

Cheers - Jim

 

[BBShark]

Mcmaster has a durometer and applications guide that might be helpful:

https://www.mcmaster.com/products/durometers/

 

[69427]

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

Cheers - Jim

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.

 

[69427]

Larry, that's a damn helpful chart. I wasn't aware of it. That ought to help me make a decent first guess. I appreciate it.

 

[phantomjock]

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

 

[69427]

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

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.

 

[BBShark]

Interesting exhaust drone solution.

 

[69427]

Interesting exhaust drone solution.

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.

 

[BBShark]

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.

 

[69427]

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.

 

[metalhead]

Sounds like a good outcome. Catching up on the thread here, and you've already got it in the car so probably not all that relevant - but I was just going to say to keep in mind that shortening the spring stiffens it.

 

[69427]

Sounds like a good outcome. Catching up on the thread here, and you've already got it in the car so probably not all that relevant - but I was just going to say to keep in mind that shortening the spring stiffens it.

Oh, certainly. This is the fourth spring I've modified to fit my narrowed suspension. I estimate it changes the rate by 5-8 percent, and as I don't have a convenient fixture to actually measure it, I just compare the non-modified rates to each other when trying to decide which way to go and how much when considering a spring change.

 

[69427]

Spring update: Had a track day yesterday, and things went exceedingly well. Nothing broke, and the car was truly enjoyable to drive fast. The new rear spring (Thanks again Jim ) worked very well, and survived the abuse. The car was very well balanced, with just a hint of controlled oversteer which helped rotate the car around the tight corners. I've owned this car for decades, and was never able until just recently to flick the car back and forth through a slalom environment like it will do now. I did experience a bit of tail wagging under heavy braking, but I think I can clean that up with some geometry tweaks to the rear toe rods. I'm suspecting I'm getting a touch of rear toe changes under braking, so I'll do some measuring of things to see what I need to do to rectify this.

Had an enjoyable time mixing it up on the track with a guy in a new 911. A very pleasant guy, after that first session he stopped over where I was parked and he complimented me on the car's performance, and was curious how my antique (my label, not his) handled that well. We had an enjoyable discussion.

One other item. One of the spectators stopped by and complimented me on how my car sounded. I thanked him for going to the effort to walk over to say that. I didn't bother him with details, but I was happy that my efforts fabricating the mufflers and resonator paid off. I know the system flows well with minimal (measured) backpressure, but I've never actually heard my exhaust at WOT without it being filtered through a helmet and a lot of wind noise.

The car is far from perfect, but I'm finding I'm still having to look at smaller and smaller items to remove weight or make handling improvements. (A first world problem, and I am not complaining. )

 

[phantomjock]

From your track day analysis - sounds like you won't be needing the rear bar. Less weight, fewer items to check off the "smaller" list.

Cheers - Jim

 

[69427]

From your track day analysis - sounds like you won't be needing the rear bar. Less weight, fewer items to check off the "smaller" list.

Cheers - Jim

Due to packaging constraints of a C4 suspension under a C3 frame I've never been able to package a rear sway bar under my car. I've been fortunate that I've been able to get the car to handle decently by just tweaking spring rates and the front bar.