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I've updated the numbers but I noticed as I decrease the dampening the system seems to become more damped. Check out the difference in the step response when I change damping constant 'b' from 6000 to 25000. That can't be right?

http://i.imgur.com/Gcy40xR.png
http://i.imgur.com/EmQAMX4.png
http://i.imgur.com/f8kATc1.png

Are you using the standard definition of MR or the "racing" definition. Racing definition is WMD or Wheel = MR*Damper

Damn, I was hoping to not have to review controls, but I will do so now... :[

Quote:

Originally Posted by EarlQHan (Post 922271)

Are you using the standard definition of MR or the "racing" definition. Racing definition is WMD or Wheel = MR*Damper

Damn, I was hoping to not have to review controls, but I will do so now... :[

I've seen it represented both ways.

The way I've always heard is from the suspension design perspective:
motion ratio = (distance from pivot to shock mount) / (distance from pivot to knuckle)

OptimumG and others explain it from a vehicle dynamics perspective as:
motion ratio = tire travel / shock travel

Both versions make sense, but just a matter of convention. Which version do you plan to use?

I prefer tire/shock since that's just the way I've learned it. Plus WMD is easy to remember, weapons of mass displacement ;]

Quote:

Originally Posted by EarlQHan (Post 922271)

Are you using the standard definition of MR or the "racing" definition. Racing definition is WMD or Wheel = MR*Damper

Damn, I was hoping to not have to review controls, but I will do so now... :[

That only has a minor effect on the spring rate, and is done before the rate is even put into the program (for now). The problem with the damping is larger than that, although this does remind me that I need to consider the motion ratio of the damper as well.

Quick question, and I apologize if I missed it in the thread...also looked on the google doc spread sheet...

Does anyone know what the OEM scrub radius is? I'm looking at wheels, and want to discuss the possible offsets vs. scrub changes with my boss.

Quote:

Originally Posted by Shankenstein (Post 916974)

Mistake was exactly what you said. Used an online calculator, and I shouldn't have trusted it.

The original post is updated.

spring rate = 4 * pi^2 * ride frequency^2 * sprung mass * motion ratio^2
22970 = 4 * pi^2 * f_r^2 * 280.4 * (1/0.92)^2
f_r = 1.325 Hz (front)

Notice that the motion ratio is inverted. If you leave it as 0.92, you will get 1.5 Hz as you got.

The rear comes out to be 3.8154 Hz. It is definitely stiffer in the rear, but it doesn't quite feel THAT stiff. Once we can confirm the rear motion ratios, we can state it with confidence.

The rear ride frequency is wrong, that's why it seems so high. Using that formula you get 1.51 Hz for the rear, not 3.8.
I used K=36998 N/m, sprung weight=244.5 kg, motion ratio=1.299.

Quote:

Originally Posted by Shankenstein (Post 673321)

Updates made to suspension parameters, but not specific coordinates. Can I get a second set of eyes on this?

Requires peer review:
Bump Travel = 4 in or 0.1 m
Rebound Travel = 6 in or 0.15 m
Max Roll Angle = 6 deg
Braking on Front = 60%
Tire Rolling Radius = 9 in or 0.225 m

Still looking for suspension geometry coordinates (X,Y,Z), if anyone has logged them

since you asked, the Tire Radius should be 12.3" for 215/45r17, but I guess it squishes down a little with the weight of the car if that matters.
sorry, I'm a little late in the game here, but this is a really interesting thread:thumbsup:

Springs, Swaybars, Wheel Rates and Frequency

I put together some calculations to give some context to spring rates, swaybar rates, wheel rates, and undamped frequencies.

Here are some assumptions I used for to calculate wheel rates due to springs and bars:

Front Spring Motion Ratio
1.0
Front Spring Angle (based on nominal SAI)
15.5 degrees
Front Bar Motion Ratio (same as Front Spring combined MR due to attachment to the strut):
0.964
Rear Spring Motion Ratio (measured)
0.78
Rear Bar Motion Ratio (measured)
0.59
Rear Spring Angle (still needs to be verified – I know it’s not perfectly vertical but it doesn’t look that large either; as long as it remains smallish it shouldn’t affect calcs much)
4 degrees

BRZ Spring Rates (measured by Vorshlag http://www.vorshlag.com/forums/showp...16&postcount=5)
160 Front / 200 Rear lb/in
FRS Spring Rates (measured by Vorshlag http://www.vorshlag.com/forums/showp...16&postcount=5)
125 Front / 220 Rear lb/in

Front Swaybar Diameter
18 mm
Rear Swaybar Diameter
14 mm

Front Swaybar Spring rate (measured by Eibach http://www.phastekperformance.com/20...-sway-bars.htm)
141 lb/in
Rear Swaybar Spring Rate (measured by Eibach http://www.phastekperformance.com/20...-sway-bars.htm)
113 lb/in

To get wheel rates, I multiplied the rated spring rate by the square of the product of the Motion Ratio and Cosine of the Spring Angle. The calcs:

Code:

             Spring  Spring  Wheel   Wheel

             Rate    Rate    Rate    Rate    Front    Rear

             Front   Rear    Front   Rear    Dist     Dist

Springs      (lb/in) (lb/in) (lb/in) (lb/in) (%)      (%)

-----------------------------------------------------------

OEM BRZ      160     200     149     121     0.55     0.45

OEM FRS      125     220     111     133     0.46     0.54

Code:

          Bar   Bar    Bar  Bar    Wheel   Wheel

          Dia   Rate   Dia  Rate   Rate    Rate    Front Rear

          Front Front  Rear Rear   Front   Rear    Dist  Dist

Bars      (in)  (lb/in)(in) (lb/in)(lb/in) (lb/in) (%)   (%)

-------------------------------------------------------------

OEM FRZ      18   141    14   113    262    79     0.77  0.23

The wheel rate for the bars is the wheel rate in roll. For one-wheel bump, those bar wheel rates would be halved. The reason they're doubled in the roll calculation is because in roll, as the loaded side (outside) travels in bump, the unloaded side (inside) droops the same amount, so it's actually double the "twist".

The "Front Dist" and "Rear Dist" columns indicate how biased the total wheel rates (front + rear) are front to rear. This isn't that relevant without weights (and hence frequencies which I'll touch on below), but it's interesting to compare it to the 55/45 weight distribution of the car. I've seen with other cars how wheel rate front/rear bias closely matches the weight distribution of the car, which makes sense.

Notice how much work the front bar is doing – it provides a lot more wheel rate than the springs do. So looking at the springs and bars combined in roll:

Code:

                                            Front  Rear

                                            Spring Spring

                                 Front Rear Roll   Roll

System    WRf    WRr     WRtot   Dist  Dist Share  Share

Config    (lb/in)(lb/in) (lb/in) (%)   (%)  (%)    (%)

---------------------------------------------------------

OEM BRZ     410   200    610   0.67   0.33  0.36  0.61

OEM FRS     373   212    585   0.64   0.36  0.30  0.63

WRf = Wheel Rate Front
WRr = Wheel Rate Rear
WRtot = WRf + WRr

The BRZ is overall about 4% stiffer than the FRS, but it is also more front biased. What the “Roll Share” columns are calculating is how much of the wheel rates in roll are due to the springs and bars (i.e. for the BRZ, in the front, the springs are providing 36% of the roll resistance, while the bar is providing the remaining 64% of the roll resistance – so ~2/3 of the roll resistance is coming from the bar). For the rears the springs are doing more work than the bar. If you run the calculations for front and rear combined, the bars are providing 56% of the total roll resistance for the BRZ and 58% for the FRS.

Adding a little more context, it's useful to calculate the undamped natural frequency of the suspension as a normalization to determine "how stiff" a car actually is. Calculating the NF takes into account spring rates and weights and results in a metric that can be compared across different cars. And in general, there are ranges of frequencies that are desirable based on what you want to do with the car (smooth ride, sporty street, low speed track, high speed track, etc).

Assumptions: I started with the published curb weight of the BRZ Limited (2776 lbs), assumed a 55/45 weight distribution, 90 lbs front unsprung mass, 83 lbs rear unsprung mass, 60 lbs less over the rear axle due to lower fuel, and added 145 lbs to both axles for driver weight. Also assumed symmetrical left/right weights. This comes to corner weights of 707 lbs per front and 548 lbs per rear. Obviously more accurate numbers could be attained from corner weighting. The equation to calculate NF is 3.13*sqrt(kw/m), where kw is the corner wheel rate in lb/in, and m is the corner weight in lbs. Using these corner weights and the wheel rates above:

Code:

System      Front NF  Rear NF  Front NF  Rear NF

Config      Ride      Ride     Roll      Roll

------------------------------------------------

OEM BRZ     1.43     1.47     2.38     1.89

OEM FRS     1.24     1.54     2.27     1.95

The “Ride” NFs (Hz) are based on the spring only, the “Roll” NFs are for the springs and bars in roll. I actually have never seen that “Roll Frequency” metric anywhere before, but thought it would be useful to see the overall roll stiffness normalized.

The information isn’t that surprising – Ride NFs are right in line with sporty cars (similar to a stock 2008 STI), Roll NFs are also similar, although for the twins there is a lot less rear roll stiffness (mainly due to the disproportionately small bar).

So knowing all the equations and motion ratios, it should be straightforward to calculate individual setups. E.g., adding only a Strano front bar to the mix (advertised as 85% stiffer than the stock front and common for stock class autocrossers) increases overall roll stiffness by 36% and biases the overall roll rate to 76% front (up from 67%). Another setup – the RCE T2s that come in 400/400 result in NFs of 2.27 Hz Front and 2.16 Hz rear.

Check my numbers?

Much appreciated guys!

I have updated the original post to reflect the corrected numbers and included Wepeel in my "Thanks" line. Excellent work, and it's obvious that you know ALOT about suspension design.

Much appreciated Grodenglaive! The tire diameter in my wheel section does line up with your number. The natural frequency calculation was definitely off. Thanks!

So the Strano sway bar may only add 36% stiffness and RCE definitely makes for a more harsh ride... but the suspension is no longer dominated by sway bar dynamics.

Quote:

Originally Posted by Wepeel

Just wanted to say, this was a pretty good post!

- Andy

Great looking thread, guys. Kudos to @Shankenstein for starting this and adding a lot of useful information.

I stumbled upon this while looking for a computational model of the BRZ suspension. I am a mathematician with a slight theoretical physics background and strong computer science skills. So not much engineering experience.

Does anyone have suggestions on a resource for rapidly learning the ME side of a good suspension setup? Specifically what the ultimate, theoretical goals of an ideal setup are, and the fundamental theory that drives suspension design. Books, technical papers, online articles, forum posts, etc would all be appreciated.

I would be happy to help out with this project once I get up to speed, looks like it's off to a great start!

Quick update. Added the stock shock manufacturer and shock dyno data from RaceComp Engineering. It's old news, but I didn't read the "official" suspension/brake thread from here: LINK

Quote:

Originally Posted by Shankenstein (Post 1613966)

Quick update. Added the stock shock manufacturer and shock dyno data from RaceComp Engineering. It's old news, but I didn't read the "official" suspension/brake thread from here: LINK

typo in the ride frequencies (FR-S rear is listed twice).

i'm annoying...sorry! :lol: