Ran across the CAD drawing for the AST 4150 (front right) strut. For the sake of building our database of dimensions, some reverse engineering was done. This data is provided for non-commercial use. If you try to rip off AST (one of the premier suspension manufacturers), expect their lawyers to sue you, the 86 community to shun your products, and numerous internet photographs of your product being teabagged in public places. Save us (and our giblets) the trouble, and just don't do it.

Distance between strut bolts is 60.5 mm --> 2.4"
Thickness of lower flange is 25.4 mm --> 1"
Distance from strut center to lower bolt is 60.7 mm --> 2.4"
Length from lower bolt to upper mount top is 392 mm --> 15.4"
Upper thread is a M12x1.25-25, with 5 mm of thread relief
Diameter of damper rod is 22 mm
Diameter of spring perch is 60 mm
Length of spring available is 181 mm --> 7.1"
Distance from lower bolt to sway bar mount is 115 mm --> 4.5"
Distance from strut center to sway bar mount is 50 mm --> 2"
Diameter of sway bar mount is 10.2 mm --> 0.4"

http://vorshlag.smugmug.com/Cars/Sub...8-Sheet1-M.jpg

Edit: Main post updated with only the relevant parameters!

Shankenstein - this is great. Thank you for starting it.

What products are you aiming to model first to work on your own personal suspension goals? Just curious.

First order of business is to analyze the stock setup. Find its strengths and weaknesses from a geometry point of view.

Second order of business is to analyze the effects of lowering springs, coilovers, sway bars, spacers, wheel offsets, camber plates, etc. Each one changes the geometry and system dynamics. I want to know the effect.

Third order of business is to connect the model results with the track. Start with the model's optimal setup, see how it performs on track/autocross. We'll watch tire temperatures, wear rates, and lap times. If you tweak the camber, and tire temperatures are more evenly distributed, you should probably stick with it (unless lap time dropped significantly).

The tough part here is keeping things objective. Getting feedback about 0 vs 1/8" toe, with the same driver on the same track, on the same day... which one feels more stable and yields better lap times. That's subjective, but still has value.

Fourth order of business is to identify where the model was wrong. Identify where a computer can be helpful and where it leads you astray. I guarantee that some stuff will look great on a computer but will perform poorly.

Examples:
Currently, some people will say that front springs should be stiffer than rear springs. Some people will say 10k springs are a great idea. Some recommend wide tires + offset + camber (because grip!). Hopefully we can see what effect 255 vs 215 tires make.

The FT86 forums are hoppin'. Theory nerds, track Stigs, and some people with too much $$ to spend. Let's put all 3 types to work. I think we can develop a set of recommendations for new guys and old guys... for each budget and purpose.

*steps off the soapbox*
I'm still waiting on my FR-S to arrive. I did get one of those extendable magnets and some huge paper, for doing the coordinates in XYZ this time.

A geometry thought:

There was talk about use of asymmetrically deforming bushings in the car. Do the geometry programs take bushing deformation into account, or would you have to manually offset the pivot points? Which would also vary by the amount of force, right?

Anyone out there want to code up an open source force-based suspension-geometry/FEA hybrid program? There are OS CAD and FEA programs that could be used as a starting point.

I wouldn't worry about getting sued. The magic happens on the inside. Anyone with a ruler can figure out what's in that diagram.

There are advanced programs out there that do suspension models with K&C data so they take bushing deflection, installation stiffness, component stiffness, etc. into account.

With the STI strut brace being introduced, I thought it would be good to bring them up in the discussion... since it's theory and all. Much of it is copy-pasta from that thread, so if you've read it there, disregard.

I'll use this discussion as a source: http://www.e30m3project.com/e30m3per...bar_theory.htm

Strut bars have 2 purposes:
1) Stiffen the tower to tower lateral movement (tower to tower bar)
2) Couple strut flex to a neutral chassis position (tower to firewall bars)

Many people discuss the chassis surrounding your suspension as being a parallelogram. While it's not completely true, it provides a useful illustration.

http://www.e30m3project.com/e30m3per...ompression.gif

Point 1 makes the top line in the parallelogram more rigid. In most cars, the other 3 lines are already very stiff. This turns your trapezoid into a parallelogram (a good thing).

Point 2 makes the angles at each corner of the parallelogram more rigid.

During compression, the strut towers want to bend inward. A strut bar is put into compression and resists this quite well.

During cornering, one strut tower wants to move inward, while the other moves outward. A strut bar ties these deflections together, stiffening the one that deflects more (like a sway bar does for vertical motion). Since the inside tire's camber is less relevant to maximum grip, you sacrifice some deflection to improve the situation on the outside tire.

All this is dependent on the struts flexing. Modern chassis design is rather rigid, so it's not as important as in an 80s Civic or my RAV4 (an SUV version of the Corolla).

When I installed my strut brace (1.5" square tubing w 1/4" plate on each tower), the handling limits did not really improve... but the consistency of when the tire broke loose was much better. Strut flex begets more camber and grip, and in many cases is not a bad thing. I value the stability and consistency more than ultimate grip. Go kart vs. muscle car, if you will.

Point 2 is irrelevant if you consider the front and rear suspension as independent parallelograms. They aren't... so preventing either from differing too much from the unstressed geometry is important. Also, triangles are stronger than parallelograms, so tying into the firewall (however flimsy it may be) is still good for structural rigidity of all components in the party.

Modern cars indeed make strut braces pretty irrelevant, since the structures are inherently more rigid. That said, if the M3 guys estimate 0.5 degrees camber change due to tower flex, I'm willing to add some reinforcement to prevent it. Some brave soul can install a strain gage on their cusco bar and see that there are indeed forces through it... but the magnitude of the deflection just isn't what it was 30 years ago. Bushings are another story for another day though.

Wow, awesome work everyone! I don't know how I missed this thread before. I was hoping to create a crude version of this to simulate various spring rates.

I think I've still got my Matlab model I made when I had my miata. I'll see if I can find it on an external hard drive somewhere and plug some of these numbers in.

I bookmarked this thread and will take some time to read it over the next few days. If there's anything I can do to help contribute, let me know!

I have been wanting to do the same, but school has kept me too busy. I want to create a simulation of the 1/4 car model posted a few posts back to see how different spring constants and damping curves change vehicle response to bumps. I also wanted to use it to try to quantify the effects of reducing unsprung weight.

Shankenstein, not too important but I noticed a small mistake in the first post. I think the tire radius should be ~12.3" or .31 meters, not 9".

That's basically what I did in college. Made a model in matlab with the spring rate, damping rate, motion ratio, unsprung weight, etc. Then simulated hitting expansion joints and potholes at different speeds to see how it would react. I did this all this my NC miata back then.

I made it all in Matlab at the time, but it's been years since I've played with it. If you have any questions or just want someone to bounce ideas off of, feel free to shoot me a PM!

I definitely applaud this tread because it looks like a great ressource for the gt86 engineering types to get information about their vehicle's geometry and available methods for modelling vehicle kinematics.

Whether or not it's actually necesary is a whole other story. But who cares, nerds rule! Let's keep it going. +1 for Shankenstein and his/her efforts to start this thread.

Well.... I was just on my way to the shop with my plumb bobs to measure all the pickup points and model the suspension when I found this. This is going to save me hours of measuring! Thanks guys

Now.. next is to find strain gauges to see some forces around the bars and chassis and go from there :)

sorry for the double post. But would you care to explain why you think this is not good race car dynamics?

Unless the camber gain in roll is so high to gain some benefit, controlling roll to keep the desired camber and manage weight tansfer is a good thing . Softer springs (with the correct frequency / rate) can help tires stay on the road longer instead of jumping about like the local tracks we have here in Taiwan and some of our public roads in US. I've always try to design my cars with springs on the soft side and use the bars as the major tuning tool.

Another point to add to this fantastic post is that it is also a good idea to maintain the relative distance between roll center and the center of mass for both front and rear when you start messing with the ride heights and such. It can help maintain the car's characteristic that many of us like so far in stock form :)

By the way, if you have had experience with Matlab, look into Octave. Octave is an open source program that does numerical computations. The language is very similar to Matlab. The only issue is that it runs on linux (not an issue if you are like me and already run linux :D).

So basically all this suspension modeling could be done in octave by many people who share the coding. The code could be combined into one awesome suspension model. Just my 2 cents though ;)

I believe the typical explanation for this is that with a sway bar your spring rate is different if the car is rolling than if the car is hitting a bump so you have to compromise between these two situations when choosing damping curves.

I may be way off here, but I would think that even on a race car you would run digressive or regressive damping curves to soak up high speed bumps while allowing stiffer damping for low speed suspension movement, so it doesn't seem like you would really be compromising.

Of course, with soft springs and stiff sway bars, you might end up with more dive and possibly more squat than you want for ideal handling on a track, especially with sticky tires.

If I am thinking correctly here, I think the R variable is supposed to be the axial distance from the center of the swaybar to the center of whichever mounting hole you are using (in case of an adjustable bar). Think of it like attaching a lever to the end of a pipe, and the measurement would be as if you are looking straight down that pipe and taking the measurement of the length of the lever that way. So that means that using the Miata figures will not be accurate.

The motion ratio should be calculated by measuring the distance from the inner control arm pivot point to the swaybar endlink mounting point, and comparing that to the length of the entire control arm. It probably also won't be 0.6, as it is on a Miata.

Also keep in mind that the motion ratio is not a constant value. It will vary with wheel travel. This is one of the benefits of kinematic solvers because they will plot the motion ratio vs. wheel travel/shock travel curve.

To clarify what I'm talking about think of a piston vs. a crank shaft. the piston position is not a direct ratio of the crank postiion. Similarly, the rotation of the anti-roll bar won't be a direct linear relationship of the wheel travel (or wheel travel deltas).

u/Josh and @mad_sb, Thanks for all the contributions guys! I'll update the original post.

@AlexTheGreek, I appreciate the kind words. I am indeed a "he".

@plucas, As much as I love open source... MATLAB is just a superior program in terms of functionality (toolboxes) and coding efficiency (multi-core utility). If it's a commercial project, and buying a $2k license is overkill... Octave is the way to go.

@MrH, would you be willing to share the m-file with the group? I'll dig through it and maybe someone can get it running in Octave :D

Lonely Sushi , I've always been taught that sway bars are a band-aid... albeit a very useful bandaid. If you can get the proper wheel rate without sway bars, the car can articulate across bumps more effectively.

Using soft springs and thicker sway bars (as modern vehicles do) gives you a sporty roll rate without the NVH of stiff springs. The other benefit is that (with certain geometries) the vehicle feels like it's "biting in" during turns which modern manufacturers seem to value highly. For me, I'd prefer the wheel dynamics to be as uncoupled as possible (so minimal sway bars).

Certain racing leagues (ex: SCCA) may not let you change springs for certain classes, but sway bars can be upsized. This does boost the wheel rate, but these cars have trouble staying at their performance limit in cambered turns or rough pavement.

Thanks for the explanation. When I say softer springs I do mean having the proper wheel rate before all else, just not too crazy with it (a lot of locals here go by "it feels stiff" = "it handles" and go way too crazy with their spring rates). But having the roll bar (adjustable one at that) to tune the characteristic of the vehicle is quite major to me rather than view it as a band aid.

Anyways, back to the topic of building a model :) I just realize you said the dimensions you posted are estimates from pictures? maybe I should find another time to try and use pumb bobs to measure the pick up points to compare the result with yours

A little note on the "band aids". Because a car with a strut suspension has so much trouble finding enough camber gain in roll, and a high spring rate on the inside tire not resisted by a swaybar tends to jack the inside tire down in droop (making this even worse). I use sways to combat the overall lean angle. This is especially so with cars that use strut suspension front and rear...just experiance with the real world talking here.. lol

Updates anyone?

I'm looking forward to this. A miata vet gave me some numbers for how to set my coilovers when I install them, and some alignment numbers to hit. I haven't had the time to get them in yet, but I own a 94, which I replicated in Forza (don't laugh, it's a *decent* simulator) and drove with what I THOUGHT were good settings, then his, and shaved nearly a second or more off of laps, and at the limit, the exits were SO much better. For the NA miata, he recommended running NO or stock rear sway bar, which sounded weird, but he stands by it for lower powered miatas and claims that if springs and dampers are adjusted correctly, all will be well, and Forza said he was right. Anyhow, I'm trying to read this, but to even understand this thread, will take me months of research and learning. Regardless-
I am curious what the "ideal" setup will be. I'm not going to be buying suspension till I get the advice here that I've had in the Miata community. People smarter than me helping me out haha. Thanks a ton everyone. I'll learn what I can and try and contribute eventually....

Forza is a good simulation for getting setup ideas.

This may not be the answer you were expecting, but I believe it largely depends on the track. The tuning of your parts will also have to be based on driver preference. While a specific setup may not be the fastest one possible, if you are more comfortable with it, you will be faster around the track.

Models are one thing and provide a 'departure point' but this can be way the heck off too...

adapting to your tires, track, climate etc are another can or worms

Ok. Figured. More looking for a good baseline to start at. Looking at going with Ceika Coilovers, which are made to order. Check em out. Might bite a bullet and try one of their BBKs too. The IS300 and supra guys seem familiar and haven't had any issues. Biggest downside is the cost of pads and rotors compared to a Wilwood kit. But at least for the coilovers, I could choose what damping rates and such I want, which might be better than some or worse. As it is always said, am I really going to be able to make a better choice than a company that has thousands in R&D and hours on a track? Probably not. The idea is still tempting...

I've got a setup I'm proving each time I track, and pretty happy with where its at. Soon i will try and get out to play with some other properly built cars I see floating around in SoCal to see what fine tuning is needed. I am of the opinion there are two slightly different schools of setup thought, drift and grip, and everything in between so be careful who you listen to as it can be good or bad advice depending on what you want.

Then you can go more exotic with a third spring on the bar, to resist dive. I wouldn't suggest this for the at home tuner.

or go with a front lower control arm solution that resists dive (and is adjustable so you get the anti dive that you want..).

but that is just one more thing to tune i guess...lol

Suspension tuning is much like a can of worms :)

Valid points, but a good model will take all those in to consideration. With Forza, you cannot change those aspects, but it's still useful to get ideas. It's not meant to give you the exact setup sheet.

Good drift setups aren't that different from grip setups at the suspension level. It's not all stiff rear oversteer like people believe it to be. They will actually tend to understeer at corner entry. High powered drift cars will actually be quite soft in the rear to get traction under throttle application. Where they largely diverge is in the drivetrain. They use a much more aggressive differential setting to get the rear wheels to lock.

You may want to ask in the general suspension section about specific brands; this thread is for the nerds who want to theorize, analyze, and bounce ideas off each other.

That would be extremely challenging to package on the front, you'd probably have to ditch the MacP system altogether lol. The rear wouldn't be as bad.

Antis are very appealing from an engineering point if you can get it right. I've heard a few driver complaints though, saying they lose feel if the anti is too aggressive, due to forces being transmitted through the arms rather than the dampers. However, I cannot verify since I've not aggressively changed the anti on my personal car. Did you notice a change in feel?

lol, can of worms is easy. Finding the right setup is more like finding a needle in a stack of needles.

Yes the front tire planted better without the front end nosing down so hard.

Sorry for leaving the thread unfinished. Been busy at work and weekends have been crazy. AutoX 2 weekends ago, my old lady got her PhD last Friday (woot! for smart womenfolk). I also started a thread of race videos, which are a wonderful way to get faster without spending any money.

Suspension points will come as soon as I get a free Saturday. I'm looking forward to some pretty camber curves and roll centers, so that we can start picking them apart.

Eventually, it would be nice to connect an optimized model result with coilover conditions (spring rate, damping rates, ride height, free camber angle, etc).

@robispec , thanks for all of the info and feedback. You're doing a great service to the 86 community with all of the test-n-tune time... and your willingness to communicate with us.

In the mean time, here's a puppy!
http://www.cars101.com/subaru/dog-lulu612b.jpg

Well I never mind being beat by customers....until my fat old slow ways catch up with me I share enough to attract customers...lol...
My car is a moving target though...lol I have MAJOR ADD when it comes to freezing a project!

Robi

new products

STREET no sheetmetal triming
Adds 2" rear travel

http://i240.photobucket.com/albums/f...psb9beb778.jpg

RACE requires rear sheetmetal trimming
Adds 3" rear travel

http://i240.photobucket.com/albums/f...pse6631b0e.jpg
Last edited by robispec; Today at 10:50 PM.

Since tire damping is so low and so so very hard to quantify, you can use this simplified model for the transfer function:

http://i1304.photobucket.com/albums/...ps2fae98f8.jpg

NOTE: Should say MASS not WEIGHT

This is basically what I did for my capstone. However it was damping a chassis (no suspension, only chassis and tires for the spring). I used Matlab for the transfer functions, and I used the root locus method to design the desired damping ratio and frequency of the system.

NERD! :lol: So that's what you and Jeff did, eh?

Everything checks out, to my eyes. That's the traditional generalization about suspensions... all masses are point masses, all forces are 1D, all spring rates are constant.

If you want to trick it up, you can do:
- progressive spring rate = f(x-x_0)
- linear damping = k * (x-x_0)'
- digressive damping = f((x-x_0)')
- damper hysteresis = nasty equations)
- pressure-sensitive tire rate = k_sidewall + k_air * pressure (absolute)
- temperature-sensitive tire rate = k_sidewall + k_air * n*R/V * temperature (absolute)
- tire temperature estimation = T_atm + k * log (T_tread - T_air_in_tire)
- tire tread temperature = T_atm + k * (heat generated - heat dissipated)
... lots of ways to complicate the problem.

Too many deltas! :barf: I was thinking state-space might be a better way to model the suspension so you can do transient analysis. This, including modeling the driver-vehicle feedback loop as the base block diagram is what I want to do my masters (or possibly doctorate) research and thesis on. But that's just too much math and I'm going to say EF IT for now lol.

This IS state-space. People like to use that term to indicate magic.

Variables take specific states, which interact based on a set of differential equations (some complex some simple).

What are your states? You can consider either the ground or the car body to be fixed... it's just perspective (although it matters that you pick one and stick with it). We care about tire deflection (x) and shock displacement (y). Thus the states are:
Q =
[ x ]
[ y ]
[ x' ]
[ y' ]
[ x'' ]
[ y'' ]

Your state space matrix will have diagonal elements to define the derivatives:
Q' =
[ 0 0 1 0 0 0 ] * Q
[ 0 0 0 1 0 0 ]
[ 0 0 0 0 1 0 ]
[ 0 0 0 0 0 1 ]
[ 0 0 0 0 0 0 ]
[ 0 0 0 0 0 0 ]

You can fill in the rest of the elements using the force balance equations in any level of complication you desire... but stiffness and damping components will appear, scaled by mass.

Solving the state space version is the same as solving the diff-eq version is the same as solving the numerical version. I'm sure you know all this, but it's good to lay it out there for the forum to glance over.

Good luck with the Master's btw. Where are you planning to go to school (or are already there)?

this is awesome

however a question/concern

with these, the wheel can now compress 2-3" more than it could in stock, is there enough clearance for the tire? Furthermore, the control arms too can now move significantly more upwards into the chassis.

is none of this a concern? how does this affect geometry under extreme cornering?

or am i overthinking this?