Using progressive coil-over springs for ride/ performance compromise

[midlife]

Does anyone have experience using progressive coil-over springs for a compromise of soft ride and handling in an early 911?

I would like to preface this with the fact that while I am an engineer, I have no suspension design experience.


I am starting on a suspension refresh/ upgrade on my ‘69 Targa. The car currently has stock ‘84 suspension.

The car is a daily driver and most likely will remain 100% street.

I am installing ERP rubber bushings, and bump steer. I got a set of RSR front struts from a fellow pelican to replace the Boge ones. These are set up for coil-overs.

I would like to increase my rear torsion springs from 23mm to 27mm, and install progressive coil-over springs on the front so that the ride is decent for the street.

The first dilemma is getting the right spring rate to correspond to the rear torsion springs.
Wil Ferch created a nice chart of the spring rate at the wheel for the different torsion bar sizes. This does not correspond directly to the coil-over rate because the coil-over is not at the wheel. What I am using as a point of reference is the stock 964 coil-over which is 150-200 lb/in. While Porsche gradually increased the rear torsion springs over the years they did not increase the front much, so I am assuming this is still the case for the 964.

The next dilemma is achieving the variable rate, and getting the proper crossover point. The 964 and I believe most cars now have a single variable pitch spring or variable diameter spring that increases in rate as some of the coils begin to bottom. I have not seen this as an option for aftermarket coil-over springs. They use 2 linear springs. Usually for race applications the smaller spring remains bottomed most of the time, and only is used to prevent the main spring from coming loose under full suspension extension. For street applications the smaller spring can be a tender spring to create the 2 rates.
If the tender spring is allowed to bottom out to create the increase in rate, the chance of that happening at the right ride height with what springs are available seems pretty slim. For instance the 150 lbs/in Eibach tender spring options bottom out at 263 or 338 lbs. They would be bottomed the majority of the time.
It appears the off-road guys solve the crossover point adjustment by not relying on the tender spring bottoming, but rather putting in stop blocks to cancel out the tender spring.

Another issue, is the way the spring rates add up. With two linear spring you cannot get the 150 – 200 lb/in rate like the 964. If you use a 200 lb/in main spring, and a 200 lb/in tender, the rate will be 100-200 lb/in. Maybe a compromise would be to use both 250 lb/in springs to get 125-250 lb/in.

I would be interested in hearing if anyone else has done a progressive coil-over, and how they accomplished it.

Thanks,

Mike

[hoss4659]

I live in an area that suffers from poor road maintenance with lots of rough surfaces. I have wondering about the same thing to improve the car's ride quality. Finding a shop that can do an engine drop and weld in the rear shock mount reinforcements is my biggest stumbling block. Either progressively wound coils or a two stage set of coils where the lower rate stage would dampen out some of the rough road harshness is high on my list. Thanks for posting the question. I will be watching postings to your questions with interest.

[Tremelune]

Dangit. I'm interested, too.

[Driven97]

Instant-g has a coil spring to TB conversion chart, hopefully it helps out a bit:



Notice you plan to go from 23-27 out back is nearly doubling the spring rate, so if comfort is your goal, be aware of that.