996CAB

Paul,
can you address my question ref your calculations - am looking to replicate your efforts and thus far the theory on the calculations does not make sense to me...

Would appreciate any feedback.

Thx

William

996CAB

To add to my point above,
when I do the calculations using your 228/385/400 example, I get a 143/196/400 against your 105/196/400.

when I do the calculations using your 228/385/600 example, I get a 143/234/600 against your 115/234/600.


In my calculations am following the natural progressive push of each spring on the other thus my calulatiosn are as follows;

for 228/385/400
(228*385)/(228+385) = 143 - this is expected as the compression is from the top.
(385*400)/(385+400) = 196 - this assumes that once above is copressed it will then be a 385 force on the 400.
400 = 400 - this assumes that there cannot be any further compression beyound this point.


for 228/385/600
(228*385)/(228+385) = 143 - this is expected as the compression is from the top.
(385*600)/(385+600) = 234 - this assumes that once above is copressed it will then be a 385 force on the 600.
600 = 600 - this assumes that there cannot be any further compression beyound this point.

996CAB

OK Paul,
I have worked out why you have the different numbers and here goes an explanation...

You are assuming a moving/variable 'main' spring thus the ‘385’ number, IMO this is wrong.

So your calculations assume that the 196 or 234 (which are derived from the calculations using the ‘385-progressive tender spring’ & 400-main spring’ or ‘385-progressive tender spring’ & 600–main spring’ numbers) will be the new 'main' spring as the suspension transitions from one rate to the next. This assumption has given you the numbers as shown below. The only issue I see with that assumption is that it is not being applied to the middle tier number (196) calculation....that is the wrong bit...you either apply to the 1st tier and 2nd tier or apply to none. The movement of the suspension does not fit in with your calculations - either the tender 'main' number of 385 being used is variable as per your calculations or it is static as per my calculations.

105/196/400.

115/234/600.

Hope am making sense. Either way, when I do the calculation using a progressive tender springs, the middle tier is too soft for my liking and the main spring (400 or 600 as in your calculations) is too firm and may cause compression & rebound issues…going from very soft to 2.5x as hard does not look right.

IMO, the better transitional spread is a constant X 0.5 so as an example a 200 Ib/in > 300 Ib/in > 450 Ib/in looks right…

Just my 2 cents worth….

William

997gt3north

William,

Not not answering - wanted to find my calcs - will find them today or tomorrow and go through them.


Paul

996CAB

OK and thx Paul...

As noted on PM...I, like others, appreciate all the work you did with PASM which mirrors my own work with the Audi MagneRide. In essence, I found out that the OEM dampers are valved to suit the car however the springs were too soft (front & rear) unless one drives below 50MPH at all times...which is not much fun for a 2-seater 'sports car'.

...your springs theory is much welcomed by me however looking at the numbers closely it does not make sense to me...thus the theory is not giving me enough confidence to venture out and make a change...well, am not sure what change to make...

I can go high rate progressive tender springs rate however that will give me a 'very hard / very soft / hard' spring rates (70Nmm / 30N/mm / 60N/mm) when I use your calculations and the desired springs / height etcs.

When I change the calculations to what I 'think' may happen using a progressive tender spring rate I get a 'soft / hard / very hard’ spring rates (30Nmm / 40N/mm / 60N/mm) - desired however my calculations may also be wrong.

Anyway,
I look forward to your calcs because this is a very interesting area am venturing in to…in my case I want to keep MagneRide for the convenience offered with being able to soften the ride when on streets and harden it when on track all from the comfort of my drivers seat with a simple press of a button…that is called progress…!!!

997gt3north

Will,

I believe my #s are correct - it may be that I am presenting them in a way that is not standard notation - but I am 98% certain they are correct.

To figure out the combined rate using multiple springs:
The formula for two springs is 1/K1+1/K2 =1/K3
For three springs it's 1/K1+1/K2+1/K3=1/K4
(K=spring rate)

Thus in my example of a Progressive Tender 228 and a 385 touching a 400
k1 = 228
k2 = 385
k3 = 400

Therefore, (1/228 + 1/385 + 1/400) = 1/K4, so K4 = 105

So, in my notation, a 228/385/400 will first compress like a 105 - and this will be the rate until the 228 section is fully blocked.

Once the 228 section is blocked, there are now 2 springs to worry about in series a 385 and a 400)

Using the formula
1/k1 + 1/k2 = 1/k3
1/385 + 1/400 = 1/ k3
Therefore K3 = 196

And thus, after the 228 Progressive Tender section has reached Block, the next 385 section will act like a 196# spring until it reaches block height - and only then will it jump to 400#

Thus, in my notation a 228/385/400 can be thought of as a 105/196/400.


If you think about it a little (it took me a while to get it), I think you can see that your 143# is too high - as when you add yet another spring in series (that is not a solid but a spring) there is yet another spring to push against and thus lowers yet again the lowest of the 3 springs 'actual' rate until it is fully compressed.

Also remember, these are still not the final calcs you need as you have to get into motion rates and wheel rates - which brings in more complexity but it is all very doable with a Google search as the 911 platform has all the required data on some internet page somewhere.

The basic stuff is the calcs above together with your Sprung Corner weights (not unsprung) with you in the car and the load required to compress each tender - this will give you a basic idea when your tenders and mains come into play.

My original IDEA of this thread was to come up with a 2 or 3 spring setup that would allow the car to ride around on the Tenders and then when a certain amount of additional load was put on the front or rear of the car, the Tenders would then fully compress and the Highest Rate Spring would then fully kick in to help when (braking, turning and accelerating) but that they car could otherwise remain more softly spring with less damping to soak up the bumps.

Paul

996CAB

Well done Paul...you are spot on...I see your calcs now and it is perfectly valid from a theory stance point...you are adding the 3rd number to make the calculations...

Thx for clearing things up...much appreciated.
These are exciting findings IMO...really exciting findings with endless possibilities...!

996CAB

Where did you get the formulae for the 3 stacked springs from ?
Any source to verify such calculation can be used or are you applying logic here?

Mvez

The problem I see for this type of complex setup is just that, the complexity leads to inconsistency in the handling of the car. Typically, you want the car to react the same, hence the linear rate. When you get the car loaded up in the middle of a corner, and the rate quickly spikes up, or down, the handling becomes inconsistent, and less confidence inspiring IMO. Very similar to how the car handles using the stock rear spring.

Hell, I don't even like using bump-stops, because they even change the spring rate mid-corner, and in high load situations. The only reason I use them on the RS is because these OE shocks are pricey, but on my M3, I don't use bumpstops at all with my Koni coilovers. Handling is precise, and the same everytime.

This is why the .2RS guys enjoy the stock setup at Sebring and other bumpy tracks, because the rates are low enough to handle them. All they need to do is swap to a linear rear spring (the Swift 672#) for even more consistent handling on track (and add a bit more front rate). Then add some GMG bars to allow for additional stiffening/adjustment of the setup when needed.

It's honestly all this car needs for anything but all out racing where minute shock adjustments are needed.

996CAB

OK,
Paul...your calcs above works well using my target spring rate...only issue now is that my target rates are as folows;

Initial Rate is a touch low at 25N/mm;
Middle Rate is ideal at 43N/mm
End Rate is of 61 N/mm is over the Damper valve rate of 50N/mm

...only issue here is that Eibach only offer the following progressive tender rates which are suitable;

44/96 N/mm
50/120 N/mm
60/150 N/mm
105/228 N/mm

...got to play around a little with above...

996CAB

I entirely agree with you however what I can tell you is this...

My oem springs came out and were upgraded with a lowered linear springs kit all round from Eibach in an effort to get the handling balance to my liking - and it worked to a point.

I was on the verge of ordering a KW Clubsport kit to give me greater adjustability until I read Pauls 're-valving thread'...that lead me to re-visit my suspension...I ended up making a change to the rear suspension ONLY using a part that cost £30 UK Sterling. The rationale for the change was to simply increase the rear spring rate. The item was a spring assister from Grayston p/n GE13. The reason for the changes was because the fronts leaned too much on hard cornering. I have Audi MagneRide - works in same principle as PASM.

What became obvious after the above change was that the car was much balanced and planted. I can now 'throw' it around corners with greater confidence.

I then lent the car to my tuner and simply told him I have made a change. His feedback was that the car felt like it had a KW V3 kit fitted and behaved like his car with KW V3...planted however still a little nervous in the harder suspension settings. His view is...IF ONLY I could get a progressive spring rate on the rears ONLY, then I will have a solid setup...

Here we are and that is what am researching....

Sure, i can still go KW Clubsport however we will probably spend ages getting that setup right so why not spend the time on MagneRide...we got nowt to lose...

997gt3north

If you Google Search (Tender Spring Calculation) - a bunch of links show up. Most of the links show the 2 spring calc as:

(Tender x Main) / ( Tender + Main)

But this form of the 2 spring equation is in the 'prettier' form - when you solve things from Mathematical First Principles the equation for N springs in series would derive out in the form

1/k1 + 1/k2 + ... + 1/km = 1/kn - when m=2 you can write it nicely as written above.


Google is your friend, there are a few links that show up that will give you the required formulas you need to enable you to build a quick excel file (if you are really into it).


I actually think that Porsche Inc, as the look to expand their Brands and create better 911s, they should absolutely be adding a Rally Suspension setup on the 911 platform - to me, the Targa brand should be entirely changed to focus on this.

If I was in charge, I would have the 991 Targa set up as a Targa GT3
- 4wdr
- 8500rpm
- non turbo'd
- trick rally suspension
- gear box geared for rallies
- LSDs and vectoring diffs setup for rallies

With enough ride height and suspension valving adjustments, this car would be a riot.


Paul

996CAB

Thx Paul - already got an XLS calcs running that allows me to play...theories shows following rear spring kit system setup;

30.51 N/mm Initial Rate
43.00 N/mm Middle Rate
53.00 N/mm End Rate

My fronts will stay at linear 37 N/mm for now...just following the 'one change per time' principle...

The only concern I have now is that the Initial Rate may introduce low speed understeer as the springs will be lower rate compared to the fronts...so think of 1st & 2nd gear corners where tha theory is that the softer rear may cause some mild understeer...just a theory...!

997gt3north

What is the block weight for each spring? And what do you think your sprung rear corner weights are - and with you in the car?

996CAB

You asked me Lord...?;

FRONTS
Eibach Progressive Tender - p/n 0175.250.1300 /// 1513 Ibs load at Block Height per corner
Eibach Linear spring - p/n 120-60-0040 /// 773 Ibs load at Block Height per corner

REARS - OPTION 1
Eibach Progressive Tender - p/n 0175.250.1300 /// 1513 Ibs load at Block Height per corner
Eibach Linear spring - p/n 140-60-0050 /// 1068 Ibs load at Block Height per corner

REARS - OPTION 2
Eibach Progressive Tender - p/n 0175.250.1300 /// 1513 Ibs load at Block Height per corner
Eibach Linear spring - p/n 0600.225.0300 /// 1135 Ibs load at Block Height per corner

CAR LOAD
Front axle /// 1040 Ibs
Rear axle /// 870 Ibs

Car weight with me in it and 1/3 fuel weighs 1650kg. I have not corner weighed the car as yet.