ideola

Using the way-back machine, rescued this fabled article from Paragon's now-defunct tech session site. Would post it natively here, but the tables don't lend themselves to posting in the forum.

Rear Spring Rates vs. Effective Rates

75ohm

Sweet!

Mark944na86

I wonder why it became defunct? Strange...

333pg333

Dan, this is from Karl at Racer's Edge when I was asking him to calculate my rates. Another guy called Bob suggested that my setup wouldn't work so I ran it past Karl.

"The info on Paragon's site is stuff they got from me a few years ago. In any case, the 47% is a number that I have calculated by taking measurements. I am now pulling out one of Porsche's own Motorsport sheets that shows all the rates of thier Turbo and "Cup" cars back when they ran competitively. Porsche lists the 25.5 mm bar as 31 N/mm which is 177 lbs/in. They also then give the variable rate coilover helper spring rates at 34-65 N/mm which is 194 lb/in - 371 lb/in. They then give the total Rate at the wheel(T-bar plus coilover) as 45.4 - 58.5. Back out the rate at the wheel due to torsion bar which they list as 31 and you have 14.4 - 27.5 at the wheel due to the coil over. So take your pick, 14.4 / 34 is approx 42% or 27.5/65 is 42%.

Bob is right about them being inboard but his numbers are off. Actually they are correct I think in that the motion ratio is about 65%. But when calculating wheel rates from spring rates it is the motion ratio squared that is used. So 0.65 ^2 is , guess what,... 42.25% which is the number that Porsche's own sheet claims as I outlined above. So your torsion bar is 177 at the wheel, and your helper spring rate is a 285 which is 119.7 lb/in at the wheel. So working backwards 177 plus 119.7 is 296.7 pounds per inch at the wheel. Divide this by .42 and that is you equivalent coilover, or 706 lb/in coilover (initially I had 661 lb/in which is attributable to my measurement error - I had 47% and Porsche lists it at 42%).

So it may be a bit stiffer in the rear than I might run but with the ability to tune sway bars etc... you should be Fine. Your setup is actually much stiffer in the front than the Porsche cup setup which ran progressive front springs (200 - 371 lb/in) with the rear setup I desribed above which is not that far from what you have( yours is 296 at the wheel and theirs was 259 - 334 lbs/in at the wheel).

Hope this make sense, but believe me, what is above is 100% correct. I can fax you the Porsche motorsport sheet if you think it will help."

Van

Dan, do some searching on this subject... that formula in the paragon article for "motion ration" isn't correct.

Karl's is - and I know this because I went through all of this a few years ago. So, based on Karl's suggestion, I took very detailed measurements in a controlled environment of all the suspension movements. To make a long story short, the motion ratio of the rear is (by my measurement) 0.64. This means, for 1" of wheel travel, the shock compresses 0.64".

The calculation of effective coil spring rate is (spring rate)*(motion ratio)^2. Which is (spring rate)*0.41.

A 500 lb/in spring will then have an effective rate of 205 lbs/in.

ideola

Didn't mean to open a can o' worms! A few folks were looking for this and I thought it would be useful to post. In any event, when I get time, maybe I'll take Karl's info and update the content...main point being, we get questions like this all the time here, on Pelican, and on 924board, so at least we now have a handy reference!

Van

That's all good! I just think you should update the formula and the table of spring rate equivalents.

Otto Mechanic

So it would seem that this article was only rescued briefly since hitting it gets me a 404 not found error.

I'm trying to figure out how to set up a combined T-bar + coilover rear suspension on a stock '89 S2. I think I might start a new thread on this subject but I'll explain what and why here just in case since I was mostly looking for advice from people who've participated in this thread on the subject.

I'm building a "street friendly" track car and right now I'm thinking about the suspension while I wait for a transmission re-build. I'm seriously considering front and rear Konis to replace the stock Sachs struts and dampers on the car (yep, looks like they're the originals). I'd like to build a suspension along the lines of the M030, but I understand the S2 CS cars shipped with a 25.5 T-bar that somehow allowed height adjustment using rear Koni dampers; how this was done without rear coilovers is a mystery to me.

My guess is they ran a combination of the 25.5 bar and rear struts with "helper" springs. I'd like to hear from anyone that could:

- confirm that suspicion

-- and --

- provide some insight into how it would have been set up in terms of the spring rates for the helpers and amount of height adjustment available.

I assume the T-bar would need to be adjusted to lower the car to a point that allowed the spring perches to be used to control the ride height through it's complete useable range? My reason for wanting to keep the T-bars is I've heard the design of the frame depends on them and that deleting them will weaken the frame and ultimately cause problems over time. Confirmation of that understanding would also be welcome.

Dimi 944

This is a somewhat common set up. There is a modification to the spring plates where you can make the re-indexing of the T-bars on the fly. This website will give you guidance on choosing the combination of t-bars and coils to achieve targeted wheel rate. http://www.paragon-products.com/kb_results.asp?ID=58

I have the 25.5mm and 27mm t-bars in stock as well as a variety of 2.5" coils for your rear Koni shocks.

Van

For a car that's running torsion bars, height adjustment is done by indexing the torsion bars.

The only factory cars that I know of which had "coil over helpers" were the Turbo Cup and Firehawk cars - and these were not threaded body shocks - meaning the ride height still had to be adjusted by indexing the torsion bars.

If you get a set of modern rear coil overs, they will be threaded so you can adjust ride height.

For what you want, you might either want to just get larger torsion bars (like 31mm) or just do the coil over conversion (like Ground Control).

I've never done it, but I imagine it's hard to set up a "dual' system properly without lots of setup and test time. Here's the issue: you want the right amount of preload on both the spring and the torsion bar so it's a pretty linear rate throughout the travel. If the torsion bars are fully preloaded at ride height and the helper springs are at their rest position (or vice versa), then the rates are different throughout the full travel (below ride height is only one spring, but above ride height is both).

I think all the major vendors can give you solid recommendations for a setup - don't try to reinvent the wheel.

mikey_audiogeek

You're on the right track. Listen to Van.

You should be aware that the factory most likely used this approach to smooth homologation for racing. Nogt necessarily the best soluution to the "problem".
As Van says, keeping the torsions and running coilovers is more work than either upgrading the torsions or going full coilover. The adjustment available depends on coilover spring rate compared to torsion bar rate. Most likely only about +/- 1 inch.

Plenty of info around on the CS setup, just as easy for you to find it as me.
Personally, if I was heading down this path, I would use these: http://shop.forza.se/bilstein-porsch...2-c-18155.aspx

https://rennlist.com/forums/944-turb...ml#post7195914

Cheers,
Mike

mikey_audiogeek

https://rennlist.com/forums/944-turb...ml#post7313749

Mike

Otto Mechanic

Hi Dimi -

Thanks for the updated link to the Paragon paper, I guess you've revived this thread again!

I think that would solve my problem. I assumed the combination set up used the rear coilover threaded body perches to adjust ride height, I come from a mostly 928 background and that's how front end ride height is controlled on that car.

So I've now heard from you, Van and Mike that even with rear coilovers, in a combined T-bar + coilover setup, ride height is controlled by indexing the T-bars.

The preload issue Van brings up is the one that bothered me, it seemed clear this setup was going to be much more complex from a design perspective than either system by itself. To be honest my main interest in rear coilovers was to gain easy ride height adjustment. I assumed incorrectly that it was the coilover that made it possible so thanks to everyone for correcting that. I'm still interested in modern coilovers though since they offer easy changes in effective spring rate by not requiring T-bar changes.

I'd be very interested in knowing more about the spring plate modifications you mention. I haven't opened up the torsion tube yet but I think it's safe to say I have the stock 24mm bars installed since there aren't any signs this was an M030 optioned car. I have the rear suspension down now for the torque tube/transmission rebuild so this is a great time for me to look at this. I'm also thinking of replacing the torsion tube bushings with Lindsey racing solid bushings since the rubber bushings are looking a bit frayed. Any anecdotes based on experience with those would be welcome.

What's the advantage of changing the 24mm bars for 25.5 or 27 (or 30/31) if adding coilovers anyway? It seems to me that if I were to add coilovers, I could just keep the 24mm bars and adjust the coil springs to get my target rate? The biggest advantage I see with moving to the 25.5mm bars would be authentically reproducing that part of the factory CS car?

It sounds like the most flexible and low maintenance solution would be a combination T-bar + coilover setup with the modified spring plates? That would allow fast and easy spring rate changes, easy ride height adjustment, and no threat of stressing the frame by doing a T-bar delete. It also gets me closest to what I understand Porsche did with the Firehawks.

At least that's what I'm getting as the take home message.

Otto Mechanic

For this build I'm using the Firehawk as a template so in some sense I'm looking to discover how they were built. If they used a combination system as you suggest I'm interested in it just for historical reasons. That said, I'm not stupid enough to do it for hysterical reasons. If it was a bad idea I'd rather not repeat it.

OK, so now I've misunderstood this twice. The coilovers are used to adjust ride height in a coilover equipped car; this make sense to me, but it still leaves the question of how to manage preload. Put all the load on the T-bars at rest, adjust height with the coils, and your spring rate becomes unpredictable as you've mentioned. It's now rocket science.

I sort of like rocket science though so it doesn't scare me

That seems like the simplest solution but after hearing about Dimi's modified spring plates I'm not ready to pull the trigger. If I went that route I think I'd get rid of the T-bars for simplicity and go with the coilovers, but that leaves me worried about wear on the frame.

At this point I'm not trying to re-invent the wheel, it's more of an archeological dig; I'm trying to discover instructions for building it.

Van

On the coil over cars, the torsion bars are removed completely.

To quickly index the torsion bars, you can cut holes in the sides of your rocker panels.