Suspension Spreadsheet
 

Hi Everyone,

I have been working on a suspension spreadsheet over the last few weeks. I am mostly doing it as an exercise to help me understand the complex beast that is car suspension. I have got it a point where I think I sharing it to get suggestions is a good idea. Its similar to FCM's sheet, but my intention is go a bit further with the calculations:

-include helper springs
-adjustable sways
-maybe tires
-roll couple as a calculation of CoG, roll axis/center height, and wieght.

Just to iterate, I am trying to learn more, so any suggestions related to suspension are greatly appreciated too. If you guys find any math errors, please let me know and I will fix them.

You can play with it online (Google Docs).

You can download a .ods copy here.

I have included links in the bottom left corner. Most of the links are related to formulas or general suspension discussions. Have a look!
Attachment 185587

Nice! I support this venture.

I have included my measurements for motion ratios. I came out with:

Front Spring MR: ~.67
Rear Spring MR: ~.75

Front ARB MR: ~.50
Rear ARB MR: ~.56

I measured it a couple of times, trying different locations. I think the most accurate is measuring at rotor face where the studs are (that's what I am going with).

I was thinking about figuring out the coilover angle, but ours seems so insignificant, I am not sure its worth it. The big thing seems to be a difference of angle between the front and rear coilovers (as it creates a bigger gap between the two sets of springs).

Has anyone tried/calculated our roll center or CoG? I imagine its not easy with all the shit in the way.

NA Rear Suspension Geometry in WinGeo - MX-5 Miata Forum

That link has some pictures of a WinGeo file that someone made. It shows that the rear has a roll center of just over 3". I've heard that Miatas have a roll center of 3" in the rear and 2" in the front, and a CoG of around 17". That's just from what I remember reading on forums from a while ago.

Sometime when I have some extra time, I'm going to try to measure all the suspension pick-up points and make a WinGeo model of the front and rear suspensions. I'm really curious to see how it looks.

Spreadsheets are awesome. Mine is about 8 tabs large so far. But, it's for design of a Formula SAE suspension, so it incorporates tire data. I can take my WinGeo model, export data to the spreadsheet, and it will do iterations based on the progressive motion ratios I have designed (all calculations assume fixed motion ratio, which is somewhat worthless with a changing ratio).

Very nice, I figured the locost forums would have some good info on this. Mind sharing your suspension spreadsheet?

What has me really baffled so far is that balance seems "irrelevant" of weight distribution up to this point. I am really wondering when that starts to affect things and the calculations done at that point. I was thinking it would show up with weight transfer, but from my reading it seems weight transfer is almost always calculated at the CoG using total weight and distributing it based on roll stiffness.

I plugged the miata suspension geometry in this nifty little webapp.

The rear suspension is pretty close I think. I have scoured the internet for data on the suspension mount points and subframe. Majority of data came from the locost forums and a post on enderw88. I grabbed the roll center from a government safety test.

Stock Roll Centers:
Front: 2.17in
Rear: 3.5in
CoG: 17.7 in

VSusp is 2d only, so there is definitely a limitation on accuracy here. Plus the front UCA is really hard to figure out since its angled and not parallel. I am not sure how accurate the front is over all, but I think its pretty close.

You can adjust the ride height for your car (measured at the subframe, its .25" lower than the pinch weld).

I will add my spreadsheet later, it's super messy right now (I make VERY messy spreadsheets at first), and in the next few days I'm reorganizing it so someone other than myself may have a clue of what's going on.

Weight distribution becomes very important when you start talking about yaw moments. The vehicle rotates about it's center of gravity. Think about how the Deltawing corners. It's front tires are only 4 inches wide, and yet it doesn't understeer at all. The CG is very rearward, making the distance from the front tires to the CG much longer than the rear. The lateral force generated at the tires is putting a moment about the CG, and since the front wheels are so much farther away from the CG than the rear, they affect the yaw more.

Neato! That's what I want to do too! Unfortunately that link just took me to vsusp.com's default suspension settings. Where can I find data on Miata front and rear suspension chassis bracket positions, ball joint locations (front) and bushing locations (rear) on the spindles (relative to wheel location), and control arm lengths? I'm newer than new here and I don't even have a turbo on my Miata yet (though I have one on my Locost) but this info will be much appreciated.

I suspect your browser is acting up in that case...

I would use firefox or chrome to do it. If all else fails, copy the link address and paste it directly into your link bar.

You can find most of my sources in the spreadsheet on my google drive. (first post).

I found all the data on the internet and verified some of it on my car. I didn't try too much, nor did I change it if my measurement deviated, so you can probably retrace my steps.

If I found multiple values of the same measurement, I tried both and used what seemed reasonable (based one expected RC).

I have the rest of the site saved, but I am not near my laptop at this time.

Track, I want to thank you for your remarkably prompt response. Man, I'm such a luddite, but I'll work with the info you linked before I bother you again for more details.

You'd think that accurate measurements of Miata suspension geometry would be easy to come by...maybe if I was a spec racer or sumpin' I'd know just who to ask. Sigh.

Here are the rest of my sources. Some of these overlap with what's in the spreadsheet. Some of these were informational and/or the data was not used...

center of gravity (NA) - MX-5 Miata Forum
http://www.fatcatmotorsports.com/Mia...er_huffman.txt
NA Rear Suspension Geometry in WinGeo - MX-5 Miata Forum
Automotive suspension engineering - What does a "race" suspension really do?
Lowering car and the changes in instant centers/roll center - MX-5 Miata Forum
LocostUSA.com • View topic - First gen Miata suspension measurements.
NA Rear Suspension Geometry in WinGeo - MX-5 Miata Forum
MX-5 Miata Forum - View Single Post - NA Rear Suspension Geometry in WinGeo




Other stuff:
How to Make Your Car Handle - Fred Puhn - Google Books
Anyone measured the Miata's unsprung weight? - MX-5 Miata Forum