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Old 06-25-2007, 03:18 PM   #1
stretchsje
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Exclamation Weight transfer, spring frequency, damper, body roll calculator... and more!

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. All the values entered when you open the spreadsheet are what I believe to be correct for a stock 2005 STI, my car.

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. There's still lots more I'd like to add, and I continue to work on this thing. Again, feedback is appreciated.

Javid is going to tell me my body roll calculation is too conservative, and I know he can produce pictures illustrating that. Here's my theory on that: 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.
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Last edited by stretchsje; 06-25-2007 at 04:37 PM.
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Old 06-25-2007, 04:10 PM   #2
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Pretty neat steve!
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Old 06-25-2007, 05:10 PM   #3
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pretty cool, now if we can get a faq explaning what all that means to the non-tech guys
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Old 06-25-2007, 05:14 PM   #4
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Quote:
Originally Posted by deebee8 View Post
pretty cool, now if we can get a faq explaning what all that means to the non-tech guys
If you're just looking for definitions of everything, I think that searching here or Wikipedia will probably be helpful. If that doesn't work for you, let me know what you need help understanding. I'd be glad to help, but simply explaining everything would be a bit much!
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Old 06-25-2007, 05:48 PM   #5
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Wow. Now that there's a cool home project.
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Old 06-25-2007, 07:14 PM   #6
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That is an awesome spreadsheet bravo! Very interesting stuff.
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Old 06-25-2007, 09:28 PM   #7
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Excellent. Thank you for that. I'll be playing with this all night.
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Old 06-25-2007, 10:17 PM   #8
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Nice,,,, that was a great job
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Old 06-25-2007, 11:15 PM   #9
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Nice...if you don't mind I'd like to add this to my upcoming Ninja Spreadsheet. It contains all the carcentric dorky spreadsheets I have found.
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Old 06-25-2007, 11:40 PM   #10
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Thanks guys!

Unabomber: Go for it, no problem.

Last edited by stretchsje; 06-25-2007 at 11:46 PM.
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Old 06-26-2007, 12:18 AM   #11
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steve- thanks for that!

couple of questions- tire deflection- where is the 1800lb figure coming from? Four tires- you would think there would be less weight/tire

damper calculator- can you elborate a little- my Koni dyno (front) shows @ ~ 50 (actually mine are in m/sec, but that should be an easy conversion) full soft ~ 250 full firm 650, @ 100 full soft 600 full firm 1200 @ 150 full soft 900 full firm 1700, 200 full soft 1150 full firm 1900

your #'s seem a little low- where did you get the #'s from? The #'s I have are alos in Newtons so I don't think that is an issue. Also when entering in your #'s- there is nothing that is changing (on the spreadsheet) correct? Also at 65% damped your showing higher rebound rates in the rear- most struts are tuned w/ more rebound in the front

anything you can help shed light on in the damping model would be greatly appreciated

don't even have time to mess w/ the others yet

Mike
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Old 06-26-2007, 02:53 AM   #12
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Quote:
If you're just looking for definitions of everything, I think that searching here or Wikipedia will probably be helpful. If that doesn't work for you, let me know what you need help understanding. I'd be glad to help, but simply explaining everything would be a bit much!

I dont need help in the least, i was just pointing out that it would be helpful to others!!!
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Old 06-26-2007, 09:37 AM   #13
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Quote:
Originally Posted by BIGSKYWRX View Post
steve- thanks for that!

couple of questions- tire deflection- where is the 1800lb figure coming from? Four tires- you would think there would be less weight/tire
1800lbs on one tire happens mid-corner on your outside front tire. That's the purpose of the tire deflection calculator: to see how much body roll is added from tire deflection. If more roll due to tire deflection happens in the front than the rear (usually the case), then there's also some load transfer that's going to shift to the rear. In the weight transfer worksheet, that's exactly what I estimate for you: the difference in tire deflection between your inside and outside tires.

Quote:
damper calculator- can you elborate a little- my Koni dyno (front) shows @ ~ 50 (actually mine are in m/sec, but that should be an easy conversion) full soft ~ 250 full firm 650, @ 100 full soft 600 full firm 1200 @ 150 full soft 900 full firm 1700, 200 full soft 1150 full firm 1900
Well, the values I plotted are for the stock STI shocks which Whiteline provided years ago.

The other value plotted is what your optimal damping would be IF you went with linear valving. Unfortunately, I'm not a shock expert and so I don't know how to predict what a digressively valved shock should look like. My guess is that it would still somehow average out to 65-70% critically damped. However, it's quite rare to see strong digression in the rebound side of a damper- generally, it's the bump valving that is extremely digressive.

65-70% critically damped is considered by most engineers to be ideal. Anything more and you're preventing the tire from reaching the ground quickly enough, anything less and you don't have enough control. Sure, there's some wiggle room in their for individual setups, but that's the general rule of thumb. You may have already read this, but Dennis Grant has a good bit on shock tuning and reading shock dynos:
http://farnorthracing.com/autocross_secrets6.html

Koni's at full stiff are immensely firm and very few people should ever actually use them at that setting. Even at full soft, they're still quite firm. The guy I worked with on this spreadsheet showed me the WRX Koni's have an ideal range of ~350 to ~850lb springs up front and ~150 to ~6000(!!!) lb/in in the rear (based on 3500lb race weight, 58% front weight distribution). I haven't verified his plotted values, but his math was accurate.

If you find your plotted values way above the "optimal" line (which is based on your spring rate), you should probably back off on the rebound damping a bit. Do you find your car getting a little light over crests? If so, you're likely overdamped. Being overdamped is very bad for performance and ride quality.

It's kind of funny how some people talk about riding around on their new coilovers at full stiff, when that's often the absolute worst place to be.

Quote:
your #'s seem a little low- where did you get the #'s from? The #'s I have are alos in Newtons so I don't think that is an issue. Also when entering in your #'s- there is nothing that is changing (on the spreadsheet) correct? Also at 65% damped your showing higher rebound rates in the rear- most struts are tuned w/ more rebound in the front

anything you can help shed light on in the damping model would be greatly appreciated

don't even have time to mess w/ the others yet

Mike
The "optimal" line I graph for comparison has to do with your selected optimal damping, which should usually be between 65% and 70% critically damped. That value is set to the left of the graph. It plots the damper resistance it would take to create that damping coefficient based on your chosen spring rates and vehicle weight. So, if you use firmer springs, the recommended damping will increase.

The second line plotted is just your own data for comparison's sake. The two lines should be fairly close for the range plotted.

There's a neat little application here that will help you visualize what 65% critically damped means:
http://www.engin.brown.edu/courses/e...mpedvibes.html

Scroll half way down the page. Set the natural frequency as high as it'll go, then move the damping coefficient to 0.65. Click "start". That's roughly the behavior you want.

Hope that helps.

Last edited by stretchsje; 06-26-2007 at 10:39 AM.
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Old 06-26-2007, 10:29 AM   #14
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now the 1800 lbs makes sense

The STi (WRX too) are indeed much more linear in rebound than the Konis. Also the numbers for the STi fronts I have are higher- 50 ~ 400, 100 ~ 750, 150 ~ 1100, 200 ~ 1500

Agreed- there are plently of folks runnning too much damping

that link didn't work for me

Another question- you have a calculation for recommended rear spring rate, but not for front- is there a calculation for front?

I do certainly appreciate the effort that went into this spreadsheet- I for one am having fun w/ it
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Old 06-26-2007, 10:41 AM   #15
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The recommended rear rate is based on the front. It the spring rate you'd want for a "flat" ride, meaning the front and rear springs finish oscillating at the same time after a bump. The general rule of thumb there is to pick your front rates, then use rear springs with a natural frequency of 0.2hz higher. My calculation is much more precise than that, but it needs to know your front spring rate as a starting point.

Sorry, fixed the link: http://www.engin.brown.edu/courses/e...mpedvibes.html
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Old 06-26-2007, 10:57 AM   #16
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OK that makes sense- yup link is working now- thanks
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Old 07-24-2007, 12:16 PM   #17
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There was a major error with regards to sway bar rates in my spreadsheet. With regards to body roll, the sway bar rates should have been double what I originally reported. The sway bar flexes twice as much, but whereas I was dividing that force between two wheels, that force is actually applied equally to both wheels. Thus, sway bars are actually way more effective than originally predicted.

I apologize for the error. The engineer that helped me discover this mentioned, "Don't feel alone, I've only seen one hard core circle track based web series where they had it right."

Well, make that two now!

My body roll estimations now seem quite low, but it just goes to show how much compliance comes from bushing and chassis flex. Quite a bit!
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Old 08-09-2007, 11:40 PM   #18
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Thumbs up Shock Tuning and Shock Comparo

This is a very worthwhile read. Thank you for sharing!

Quote:
Originally Posted by stretchsje View Post
...You may have already read this, but Dennis Grant has a good bit on shock tuning and reading shock dynos:
link not shown (sorry spamers ruin it for all of us)

...Hope that helps.
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Old 11-24-2008, 04:15 PM   #19
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Just found this spreadsheet, I need to play around with it
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Old 03-31-2009, 10:07 AM   #20
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Came across this yesterday and ti's really helpful. Thanks a lot for making it!
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Old 10-06-2010, 04:06 PM   #21
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Cool - can someone help me a bit on this? I'm trying to find the 'ideal' starting point for the dampers on dspecs with RCE wagon springs for my 03 wagon. Whiteline 22's f/r (rear on full hard). Running p7's which are a bit heavy, and 225/45/17 gsd3's.

I got the weight at about 3300 (driver plus about 35 pounds 'stuff' in the back)

balance at 60/40

springs at 275/259

But I'm not clear on how to then find the best starting point for the d specs damping (after break in)?

Perhaps I'm just being dense, but I would appreciate some help from the gurus! (from what I've read, it should end up around 4-5 TFFS)

thanks for the help
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Old 04-13-2011, 06:19 PM   #22
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Excellent resource, thank you stretchsje.
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Old 05-26-2011, 09:12 PM   #23
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The link isn't working, anyone have a mirror?
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Old 05-28-2011, 05:44 AM   #24
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Anyone else have a copy of this?
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Old 03-03-2012, 07:22 AM   #25
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I somehow managed to stumble on a copy of this while searching through a bunch of hard drives for some other files.

http://dl.dropbox.com/u/16608/Subaru/SuspensionCalc.xls

I haven't looked at it in ages but I may have played with various numbers at some point so I can't guarantee these are the default values. Do make sure to change all pertinent info to reflect that of your own vehicle and double check all inputs.
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