I'm seeking some of the expert opinion available out there for spring rates suggestions, and to confirm that I am making these calculations correctly. I've been watching docwhite's thread on the same topic over the past few days, but I have decided to stick with Koni's and try to keep my T-bars in place for now.

Tires are stock Turbo S sizes with 225's up front and 245 to 255's in the rear. I may try 275's on the rear when I buy dedicated track tires, but I don't plan on going that wide in the rear for the street. Rear T-bars are believed to be Turbo S 25.5, but that has not been verified. Sway bars are 26.8mm front and 18mm rear. I would like to upgrade to 30mm front and 19-22 rear at some point in the near future.

My goal is a relatively tight street car that will do well for 2-3 DE's each year and a few TT's. The car is mostly driven on highways, with some backroads that have decent pavement, but I would like to have some compliance for those occasional potholes and expansion joints.

I will be starting with Koni double-adjustable struts & shocks, and I have 400# and 550# springs immediately available. However, when I do the math, the setup looks like I would have quite a bit of understeer.

Am I calculating this correctly?If my calculations are correct, the above setup would result in an oversteer ratio, so I should really look at something like this?
First off, am I approaching that math correctly? I understand there are different values that I can plug in for front and rear motion ratios, and I've put this all into Excel, so that's easy to tweak.

I'm not sure how much to factor street tires into these numbers, nor do I know how to add sway bar effective rates, but I'm hoping that as long as I stay close to stock-sized tires and "matched" sway bars, I won't be far off.

Opinions? Suggestions? What are those of you with rear coil-overs and T-bars installed running for street & track setups?

 

https://rennlist.com/forums/9965570-post25.html

Patrick's info is the best you'll get. Remeber that wheel rate = spring rate x motion ratio SQUARED. Rear motion ratio is 0.65 so wheel rate = spring rate * 0.4225.

Hope this helps!

Mike

 

So this is better?

400# front coil rate * 0.91 motion ratio = 364 effective front rate
400# rear coil rate * 0.4225 motion ratio = 169 effective rear coil rate
177# rear t-bar rate + 169# eff coil rate = 346 total effective rear rate
Front / Rear Eff Ratio = 364 / 346 = 1.052

 

yep that's it!

 

Now you guys have got me thinking...I've got 450 lb fronts and 350 lb rears ready for fitting. I've got some 27mm t-bars & I've ordered some 30mm bars. Going with these calcs I get a 1.12 ratio with the 27s and 0.85 for the 30mm bar. The standard bars & springs are currently on the car. I've got some Tarrett sway bars & a Guard 50/80 to throw into the mix. I'm aiming for occasion street so that I can drive to tracks from southern UK.

 

Should I also apply the squared motion ratio to the front to get the correct wheel rate? So change my 0.91 to 0.91^2 = 0.8281 and re-calculate the fronts?

I've put all of the different variables and everyone's suggested motion/wheel ratios on multiple tabs on a spreadsheet, so it's easy to compare the results with different combinations of springs and/or motion/wheel ratios. For instance, using the best set of numbers for 0.4225 rear and 0.8281 front wheel motion ratios gives decent results for 400# fronts and 350# rears with 25.5mm t-bars:
Compared to using 0.9 and 0.56 motion ratios as suggested by IanG in docwhite's thread, the desired rear spring rate should be closer to 300# when using a 400# front rate:It's interesting to note that one set of numbers may work for someone running 275's at all four corners, but another set may work for someone running 245's up front and 295's in the rear. Not to mention how sway bars will affect everything.

 

Time to debunk another old ‘myth’….
Choosing the ‘best’ spring rate is not a math calculation to get the same rate for the front and rear. People get caught up in the ‘perfect 50/50’ balance concept. In reality it just doesn’t matter that much. In fact for the best grip you want a rear bias for braking and acceleration – people that like to poke fun at 911’s for their heavy rear bias don’t understand that under barking with a lot of weight transfer the 911’s end up with a nice balance and those of use with static 50/50 balance end up very nose heavy. Look at classic racing 911s –close to the same rotor and caliper size front and rear. Then when its time to accelerate the 911 transfers lots of weight back to the driving wheels….and the rest of us have to deal with tire spin!
In a track situation a well driven 944 should only have an actual 50/50 weight distribution when it is parked. If you are driving well you should be either accelerating or braking – not coasting.

SO, in reality you need to select the best spring rate for suspension control for each end of the car and then use sway bars and other technics to balance the car. There is some grey area here, you can ‘t go way overboard with different rates front to rear. Most well set up club racers end up with full coil overs running 6-800 front and 600 rear. 600 in the rear seems to give the best balance of stiffness to reduce roll and suppleness to absorb road surface variations. The front spring rate is more determined by driver style and preference. I have seen fronts set up at 600 – 1200 that give good performance.

If you do the math you can see that this is far from the ‘balanced’ approach that the typical equal effective rate would dictate.

As for street set ups – the nice balance I have found is a closer to equal than the track set up. 350/550 spring rate feels good to me – but that really depends on the condition of the roads in your area. 350/550 over expansion joints is obnoxious. On smooth paved roads its nice.

A good piece on news is that springs are relatively cheap – try a couple out and see what works for you.

 

Thanks Chris, and I totally get that point. All I'm trying to do with the math part is to help chose my initial spring rates before I start to experiment. I suspect there are many people out there who are in the same boat as I am, in that they enjoy spirited driving in the street while attending the occasional track day. And most of us are starting with stock suspension of one form or another, and we aren't sure where to start when it comes to the largest variable - springs.

In the end, I want to be sure that I'm not creating an unsafe situation on the street while I try things out. My car will always be primarily a street car, so I won't have the luxury of testing various spring rates during frequent track outings.

I'm faced with the need to buy springs soon, and I need to know where to start. I have a set of 400/550's that came with a set of Escort Cup shocks that I've decided not to use, so they be sold soon. I'm trying to figure out if I should 1) start with 400's on the front (which I suspect will be too stiff for my local roads), and/or try the 550's on the rear. At this point, it seems as if I won't be able to use either set and I'll likely and up with something like 300 fronts and 400-450 rear. This is where the math comes in for me... I'm trying to make a wise purchase of springs ONCE and hopefully end up with something safe and fun while I continue to "dial it in" once I am able to get some track time in.

So maybe I should simply ask this:

What REAR spring rates should I purchase to use with 300# fronts for a moderately well-balanced car that will see mostly street use? I plan on keeping the stock 24 or 25.5mm t-bars in the car. Would something like 400, 450, or 500# rear springs be a good place to start?

 

Hey, so long as they have their heads out the window a little barking is fine....ears flapping...tongues lollygaggling...

 

Chris's 350/550 combo sounds like a good place to start. However, how about trying the 400/550 that you already have? You can iterate from there.

Good luck and good fun!

Cheers,
Mike

 

Couple things:

If the 400's came on bilsteins, they will be 2.25" ID; konis require 2.5" ID springs on the front struts.

If keeping the 25.5mm t-bars, you will not want to go that high w/ the rear spring rate to balance a 300 front spring. Probably will want to start somewhere in the 100-200 lb/in range.

 

Now I'm leaning toward removing the t-bars. I didn't realize that I'd have to re-index them to bring the rear height down after installing rear coil-overs, so I may as well just pull them out. If I do that, I'll probably try the 400/550 combo, but as you point out, I'll need larger diameter up front. Dangit!

 

Hi guys - I intended to post in this thread but mistakenly posted in the thread about Spring Rates for Bilsteins with T-bar delete.
Van, provided some helpful advice about the major parameters for suspension set-up - for which I am most grateful.
I thought someone on this thread might be able to sanity check that my figures are a reasonable starting point & hold water.
I have some Leda coilovers with 450 lb fronts & 350 lb rears. The rates were recommended by Leda (who supplied the whole kit) as suitable for a club-racer or road/track car - I'm assuming with standard t-bars.
Running the calcs the 450 lb fronts will have an effective wheel rate (EWR) of 410 lbs and the rears using standard t-bars (177 EWR) will have an effective rate of 324. This yields a F/R ratio of 1.26. The rear effective rate equates to the equivalent of 773 lb springs w/o t-bars.
I also have some 27mm t-bars (220 lb EWR) - if I run the calcs again with those I get an effective rate of 367 on the rear, which yields a F/R ratio of 1.11. The rears will equate to a 876lb coilover w/o t-bars.
Using standard t-bars gives a softer rear but a much more front-biased F/R ratio. Using 27mm t-bars gives more even F/R ratio but a very stiff rear which might be too oversteery.
I also have Tarrett F & R sway bars & a Guard 50/80 ready to go on this car.
The Tarretts are equivalent to a 28mm solid front & 22 mm solid rear bar.
I'll be running R-compound 225/40/18 front & 265/35/18 rear (Yoky A048).
Any advice as to which set-up to go for?
Tim

 

The first piece of advice is to ditch the T bars.
The torsion bars are a nice theoretical solution since they do not add any unsprung weight to the suspension; unfortunately the way Porsche chose to use them in the 944 makes them a problem. There is no easy adjustment for height and its very time consuming to change the bars for different spring rates.

Corner balancing a 944 with T bars is a very time consuming effort…and not much fun! The problem with the ‘helper’ spring concept is that it can only raise the rear of the car (unless you reindex the torsion bars). You also will end up with an interesting situation where you have a dynamic difference in spring rates from one side to the other. When cornering you are compressing the Torsion bar and helper spring on the outside wheel but on the inside you are relaxing only the torsion bar (the helper spring will be rattling around on the shock with no effect on the suspension). Its not a huge problem, but it does make the sway bar deal with multiple effective spring rates.

If you ditch the torsion bar you can change the rear springs in under 30 minutes, and that is pretty handy compared to half a day to swap or reindex torsion bars!

Another thing to keep in mind when looking at spring rates that have been used by other people, especially track people; the main reason you see production cars use high spring rates for track use is to limit the movement of the suspension. The more you allow your suspension to deflect the great the camber and toe may change. On cars with not well developed stock suspensions the change in camber can be quite significant and the best way to control it is by using very stiff springs so that the suspension will not move from the static position. If you look at a picture of a 944 cornering hard on a track you can see that the inside front wheel has horrible negative camber and is usually scrubbing the inside shoulder of the tire. Look at the pictures to see this in action (the Garnet Red car is a picture of my street car from 10 years ago running the 350/550 combo with 2.5 degrees of negative camber and weltmeister sways – it still shows inside front bad habits!
My advice would be to keep the 400 fronts, ditch the Tbar and go with 600 rear coil overs.

 

^ THIS.

Technically, you can absolutely calculate the spring rate required. In order to determine the best rate for all dynamic loads to maximize traction on all four wheels at all times, the spring rate becomes a linear algebra problem (believe it's a 4x4? Can't recall and my brain is too foggy to figure it out). Really tough calculus requiring Matlab to solve.

I've got a simplified Excel-based model which uses finite data points. Been working on adapting it for the 951; it's currently constructed for multilink and dual A-arm suspensions. Does the job though.