[stretch]

I had been making this for my own use but figured I'd clean it up and share. There are five (and counting) worksheets in the spreadsheet, one each for calculating:

• Spring rates, rates at the wheels, corresponding spring frequencies, and optimal damping
• Sway bar rates and rates at the wheels
• Motion Ratios
• Tire deflection
• Weight Transfer in a steady-state turn, body roll, and required linear suspension travel

By far the most interesting and complex is the last one. When you first load the spreadsheet, all the values are what I believe to be stock. You can see weight transfer occurs more quickly in the rear, but that it'll keep all wheels planted through at least a 1.2G turn. At 1.2G's, should a stock-spring STI manage that, you'll have over four degrees of body roll.

To use this spreadsheet, enter your spring rates on the "Spring Rate Calculator". Then enter your sway bar size on the "Sway Bar Rate Calculator". Then, click over to the "Weight Transfer Calculator". Almost everything will be filled out for you.

Lower your car's sprung center of gravity if you've lowered your car, and lower the roll centers 2.5 times as much. The only thing left to play with is the "tire deflection" parameters- just play with them until they match the suggested values, unless you understand what they do.

Anyway, to download in Microsoft Excel format:
http://www.fromsteve.net/carstuff/su...ensionCalc.zip

Let me know what you think- be it an error you've found or how I can make it easier to use. I hope to, some day, convert this all into an easy to use web page. Microsoft Excel's "save as web page" functionality isn't very good, but here's a non-interactive preview of the spreadsheet.

Javid is going to tell me my body roll calculation is too conservative, and I know he can produce pictures illustrating that. Here's why: once you lift a wheel, your inside tires unload much more slowly. This means you effectively have a progressive spring rate and body roll skyrockets. My spreadsheet, regrettably, only handles steady-state cornering where wheel lift is not occurring, as the dynamics get too complicated after wheel lift occurs. I think my calculation is pretty close for cars in sweepers on a smooth course.

[import111]

I am gonna have to try my setup on that thing when I get home tonight or some time this weekend. My setup is stiff as hell and I get more body roll from tire sidewall flex than anything else.

[SWortham]

Man that is cool. So with the stock sway bars the car should total about 3.67 degrees body roll at 1.1 lateral G's cornering force. But it should total only 2.3 degrees body roll with my 26mm front and 24mm rear sway bars.

[fullerton]

cool! i'm going to have to give this a try.

[SWortham]

Your calculator also shows what I've been suspecting about my setup. I'm not running any rear sway bar right now since I've been waiting for a part. And as a result I think I'm actually getting some inside front tire lift when I try a low-speed powerslide.

And it also shows that at the factory height you'd have issues trying to pull more than 1.5 lateral G's without lifting both the front and rear inside tires. That's OK, I don't think my car will be capable of 1.5 lateral G's on street tires, ha.

[Turninconcepts.com]

Cool! I'll have to give this a try this weekend.

[kungura]

Wow really cool stuff!!

way to go

[stretch]

A few things surprised me:

• Going to wider tires or a lower tire pressure in all four corners will tend to cause understeer. This is because the tire will deflect less, which lessens load transfer. The effect of this grows the stiffer your suspension is.
• It doesn't matter if the CG height is different in the front of the car or rear, the CG is just one spot.
• A car with drastically higher roll resistance up front unloads its tires fairly evenly.
• The stock dampers are properly damped for the stock springs.

And it also shows that at the factory height you'd have issues trying to pull more than 1.5 lateral G's without lifting both the front and rear inside tires. That's OK, I don't think my car will be capable of 1.5 lateral G's on street tires, ha.

Well, I don't know how accurate my assumed center of gravity height is. I entered 22 inches after reading that a Corvette's CG is 20 inches. Most of the weight in the STI (just about the entire drivetrain) is below 22 inches.

If you could actually maintain 1.5 G's after lifting both inside wheels, you'd roll the car. Not good! It's amazing how those F1 cars corner at 4G's with what looks like minimal weight transfer.

I updated the spreadsheet again to make a seperate tire deflection calculator.

[flatthump]

Oh dude....sweet, thanks

[SWortham]

Can we get a sticky here? Or could this be added to the "Suspension, Handling, & Stiffening Threads of Interest?"

[import111]

2.12 degrees of total body roll for me. I probably could have filled it out more exact, but I got close I think. Loaded tires front and rear are pretty well balanced, 1299.84 loaded rear and 1546.26 loaded front. I might have to toy with spring rates to see if I can even that out some more. This is an awesome little toy. Thanks for posting strech.

[import111]

I thought the swaybar #'s were for aftermarket, oops. I upped the swaybar stiffness a little to what it should be for my setup. 1.94 degrees of roll during a 1.1g corner. My weight on the front and rear loaded tires is a lot better with my evenly setup swaybars too. 1364.20 rear, and 1479.97 front for the loaded side.

[import111]

haha, I changed the front/rear weight distribution a little bit simulating a gutted trunk with my setup. Inside rear tire in the air at -6.83. No wonder my rear end was all over the place when I gutted my trunk. Much faster lap times for me with everything in my trunk, and easier to drive at the limit.

[4banger]

Originally Posted by stretch

You can see weight transfer occurs more quickly in the rear, but that it'll keep all wheels planted through at least a 1.2G turn. At 1.2G's, should a stock-spring STI manage that, you'll have over four degrees of body roll.

According to the gyro in my Nav system, I have done around 1.3-1.4 G's during brief transients on lowering springs. Now, I don't know how accurate the gyro is, but I think this may help explain why some people can produce pictures of STi's with 4 or more degrees of body roll. (I'll double check tomorrow the scale of the gauge again, I'm not 100% sure of the numbers)

[waktasz]

Interesting...I'm think I might have to change my spring rates on my fancy new suspension I just ordered now...

Does anyone know what the max lateral G's a WRX could be expected to pull assuming big arse R-comps?