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Brakes & Suspension Forum sponsored by The Tire Rack |
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06-25-2007, 02:18 PM | #1 |
Scooby Newbie
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Vehicle:2014 Honda Odyssey |
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:
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 03:37 PM. |
06-25-2007, 03:10 PM | #2 |
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Pretty neat steve!
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06-25-2007, 04:10 PM | #3 |
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BAIC
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Vehicle:2002 Wrx Land of the dead |
pretty cool, now if we can get a faq explaning what all that means to the non-tech guys
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06-25-2007, 04:14 PM | #4 |
Scooby Newbie
Member#: 107389
Join Date: Feb 2006
Chapter/Region:
TXIC
Vehicle:2014 Honda Odyssey |
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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06-25-2007, 04:48 PM | #5 |
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Wow. Now that there's a cool home project.
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06-25-2007, 06:14 PM | #6 |
Scooby Specialist
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That is an awesome spreadsheet bravo! Very interesting stuff.
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06-25-2007, 08:28 PM | #7 |
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Excellent. Thank you for that. I'll be playing with this all night.
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06-25-2007, 09:17 PM | #8 |
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Nice,,,, that was a great job
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06-25-2007, 10:15 PM | #9 |
Big Ron
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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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06-25-2007, 10:40 PM | #10 |
Scooby Newbie
Member#: 107389
Join Date: Feb 2006
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Vehicle:2014 Honda Odyssey |
Thanks guys!
Unabomber: Go for it, no problem. Last edited by stretchsje; 06-25-2007 at 10:46 PM. |
06-25-2007, 11:18 PM | #11 |
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Member#: 7958
Join Date: Jul 2001
Location: Plains of Eastern Montana
Vehicle:09 GH B+ |
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 |
06-26-2007, 01:53 AM | #12 | |
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Join Date: Apr 2005
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Vehicle:2002 Wrx Land of the dead |
Quote:
I dont need help in the least, i was just pointing out that it would be helpful to others!!! |
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06-26-2007, 08:37 AM | #13 | |||
Scooby Newbie
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Join Date: Feb 2006
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Vehicle:2014 Honda Odyssey |
Quote:
Quote:
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***37; 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:
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 09:39 AM. |
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06-26-2007, 09:29 AM | #14 |
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Join Date: Jul 2001
Location: Plains of Eastern Montana
Vehicle:09 GH B+ |
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 |
06-26-2007, 09:41 AM | #15 |
Scooby Newbie
Member#: 107389
Join Date: Feb 2006
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Vehicle:2014 Honda Odyssey |
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 |
06-26-2007, 09:57 AM | #16 |
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Join Date: Jul 2001
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OK that makes sense- yup link is working now- thanks
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07-24-2007, 11:16 AM | #17 |
Scooby Newbie
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Join Date: Feb 2006
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Vehicle:2014 Honda Odyssey |
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! |
08-09-2007, 10:40 PM | #18 |
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Shock Tuning and Shock Comparo
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11-24-2008, 03:15 PM | #19 |
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Just found this spreadsheet, I need to play around with it
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03-31-2009, 09:07 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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10-06-2010, 03:06 PM | #21 |
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Vehicle:2003 Wrx Wagon |
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 |
04-13-2011, 05:19 PM | #22 |
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Excellent resource, thank you stretchsje.
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05-26-2011, 08:12 PM | #23 |
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The link isn't working, anyone have a mirror?
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05-28-2011, 04:44 AM | #24 |
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Anyone else have a copy of this?
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03-03-2012, 06:22 AM | #25 |
Scooby Newbie
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Join Date: Aug 2002
Location: Los Angeles, CA
Vehicle:2004 STi White |
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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