Thread: MCarp22's suspension math / engineering thread

Ok, so in my travels since my last B13, I've come across a bunch of cool general info on suspension tuning. I can attribute much of my knowledge to Dennis Grant's excellent Autocross to Win ebook: Autocross to Win (DGs Autocross Secrets) - ATW Home Page I HIGHLY reccomend checking this out if you have even a casual interest in how to set up your car's suspension.

Keep in mind that everything that I'm discussing here is academic, and there's a certain point where you actually have to put the stuff on the car and test it.

Suspension Frequencies:

The first step to tuning the suspension is to figure out what springs you have to work with. Different cars have different spring rates due to things like motion ratios and weight distribution, but you can do some math to come up with suspension frequencies that can be compared directly. Generally speaking, 1hz = comfort -1.5hz = sport - 2hz = competition. For an autocrosser or track car 2.2hz up front is generally regarded as a good starting point for maximizing tire grip on a sedan type car without downforce. The rear suspension has to react to bumps after the front, so we generally want to run the rear frequency slightly higher to keep the chassis "flat" over bumps.

Here's some examples applied to a B13:

Stock SE-R springs:


Hyperco:


Road Magnet:


Now, I've guessed at the unsprung weights and rear motion ratio (i need to know the angle of the strut) so this isin't 100% accurate. What we can observe from this is that the SE-R springs are actually pretty sporty, the hypercos (at the stiff end of their progressive range) are at the top of the range for a sporty spring, and the road magnets are just about a racecar spring. The hypercos also have a bit more forward bias to them, so they probably like slightly more rear swaybar than the stock or road magnet setups. Being somewhat of an outsider, this stuff more or less confirms the praise that everyone has for both of those setups.

I'll talk about shock forces next, once I have some time to play with the spreadsheet some more. If anyone has any B1# shock dynos, we can take a look at them.

Fantastic, I love you. When I leave work I'll have some math and questions for you. Have we met?

did you make that spread sheet?

keep this stuff coming!

I don't think we have, unless you were at one of the nashville meets.



Nah, stretch over on iwsti.com made it. I've just learned to use it well.

Shock Forces:

It seems like shock forces are a black art to most people, but setting them up for racecar springrates is actually pretty simple. OptimumG has a pretty neat white paper on how and why it's done:

http://www.optimumg.com/OptimumGWebSite/Documents/TechTips/Springs&Dampers_Tech_Tip_4.pdf

Paying attention to the "Transmissibility" section we can see that having too much shock force will result in the body of the car moving too much. On my miata i threw on stock mazdaspeed bilsteins with 550 front 350 rear spring rates (remember suspension frequency? that actually works out to pretty close to the same as road magnets since the miata double wishbones have "more" motion ratio), and came up with this:



Basically, the mazdaspeed bilsteins were too stiff at low speeds. On the road, the setup wasn't too bad over fast bumps, like an expansion joint on the highway or something similar that you'd hit at a high speed. But because the damping force was WAY over 100% at low speeds, things that the suspension reacted slowly to, like dips / transitions in the road would cause the chassis to move a whole lot. This was uncomfortable to say the least. On the track, the car felt pretty good, but was also very unforgiving at the limit. That made it hard to get everything out of the car, as it was very tough to find the limit without going over.

The cure would have a revalve to reduce the low speed damping force. One thing that you'll discover, is that performance shocks intended for stock springs are almost certainly overdamped for a stiff spring. Overdamping is a compromise that you have to make to get performance out of a low suspension frequency (soft spring). Once you can run stiff springs with a better frequency, you don't need to be overdamped anymore, and the suspension can move a bit, giving your tires more grip.

Since the road magnet springs are pretty close to what i'd consider "right" for a racecar, let's go to the bottom half of steve's spreadsheet and look at how we'd set up the shocks for a B13. You'll want to read that OptimumG paper to really see what's going on here. Basically i'm punching in numbers until i get my baseline at 65%, and then adjusting it for less compression / more rebound to deal with the stored energy of the compressed spring. starting at about 3"/sec, the slope is decreased to reduce transmissibility, you'd need a digressive valved shock (most are! ) to make that happen.



Plotted on a shock dyno, it'd look something like this:



Now, I don't have access to any dyno plots for the various B13 struts, so it's hard to apply this to the real world just yet. I wouldn't be shocked to discover that the progress coilovers are valved pretty close to this though!

This is good in theory.

Have you seen shock dynos or the theory's that AST, Penske, Koni, Moton, JRZ, etc etc use? They aren't all the same and none of them match the theory you are using (yours is the textbook theory to line up with % critical). Not saying anybody is right or wrong.

I personally subscribe to the Moton/JRZ theories of dampning. I have my shocks made accordingly.

I'd be interested in hearing more about that!

Reading along and attempting to get a handle on the details. LOL.

Killer work, appreciate your efforts.

I won't share dyno plots of my shocks because well they are my secret sauce, but I will say some things. It may be worth looking at a bunch of dynos from different cars to see how they differ (between shock manufacturers). I spend alot of time looking at S2k, Miata, etc dynos and driving different cars before making up my mind.

The big thing that I look for now is TONS of low speed compression with a very sharp knee and flat blow off. There are many advantage to this for quick transition, taking a "set", and just keeping the chassis flat. Just to give you an idea... I have 170 lbs by 1in/sec but it only goes up to 195 by 5in/sec.

This is popular with Moton/JRZ, lots of low speed with flat blow offs.

AST has been also developing good digressive pistons lately and are starting to also experiment with flat blow offs. They are finding that they can go with lighter spring, lowering the natural frequency, then add low speed to get the same feel but more grip.

I've driven miatas on bilsteins, tein (monotube), koni, ast, jrz, and custom bilsteins... all on the same spring rates. I can tell you that they do not handle or feel the same. I have driven tein, custom bilstein, and jrz all back to back... The tein was out of its league.