suspension lateral compliance 1

[sierra4000]

Hello experts,

I am very interested about suspension lateral compliance data,

exist some database or real examples toe and camber lateral stiffness for modern passenger cars or racing cars?

or can we determine some ussual target values we want to achieve?

Thanks for the answers

 

[sierra4000]

Supplementing and specifying

if in fact some rear compliance will be replaced with wheel alignment to achieve necessary understeer
what is the real impact on performance and resultant effect on driving feel?

Thank you for explanation

 

[cibachrome]

There is not really a lot that wheel alignment can do for you to increase understeer. This would mean reducing the rear cornering compliance (oversteer) somehow. Instead, the sources of rear cornering compliance are tire slip, roll steer, lateral force steer, camber, and aligning moment (toe) compliance and rear weight (again tire load sensitivity). Unfortunately, almost all these things are oversteering, so you need to stiffen them. Changing roll steer to increase roll induced understeer is not good because as you near max lateral capability, tires don't 'listen' to slip angle changes very well. Reducing roll induced camber oversteer could be useful, but you need tires with camber stiffness to generate effective force levels. And, some race tires Slicks)have little or no or NEGATIVE camber stiffness. Changing camber alignment might get you something (again if the tires have such stiffness). You could alter toe alignment or if really clever alter roll steer side to side symmetry to mimic Ackermann or anti-Ackerman) to increase the net rear sideslip stiffness but there are additional side effects and this tactic introduces problems with compatibility with track curvature, track speed and straightaway drive ability. A wider wheel often helps and finding the optimum hot tire pressure does, too.

 

[sierra4000]

I was thinking (just from Bundorf table) can be get some understeer from the rear toe-in ,
but seems (for dynamic events) no exist appropriate equivalent to structural stiffness.

i understand correct?

(wider wheel can helps because increase cornering stiffness?)

 

[cibachrome]

If you had a twist axle with Panhard Bar for lateral support, placement of the Bar behind the wheel centerline can get you some deflection understeer if the control arms are flexible laterally (blades).
Keep in the back of your mind that NOTHING beats a great set of tires (or sets of tires if the front axle and the rear axle should just happen to have different tire constructions).

 

[sierra4000]

well,
i was doing short test in my Dynatune-XL , here is Step steer result
if my car have improved front axle
car 1 use extra rear toe (dark blue curve)
car 2 have stiffer rear axle (light blue curve)

i think difference is evident

(car with softer rear axle but with more rear tire cornering stiffness can get same result like car 2 )

i hope i understood

 

[cibachrome]

Your DynaTune is actually a Dyna-Tuna because something is fishy.

First of all, your 'Yaw Gain' is actually 'Yaw Velocity Response Due to 20 Deg SWA Input at (I'm figuring) 120 kph'. Same for Lateral Acceleration. The 'Gain' terminology would/should be put on the ratio of Output to Input. For this metric, the Gain is about 4.6 g/100 deg (to use an industry standard). This is a rather high number, BTW.

Second, Lateral acceleration for the constant speed conditions you have created consists of a yaw velocity component and a body side-slip component (probably the one labeled 'Slip Angle' and rightly so). Notice that the 'Slip Angle' figure contains a response trait at Time == 0+ due to the thrust of the steered axle. This is the normal signature of the side-slip characteristic. So... Where is this signature in the Lateral Acceleration graph ?? It's missing. That tells me that the software is not presenting true transient response, but is simply the yaw velocity channel multiplied by the set speed . Your 0.91 g (steady State) translates to about 120 kph in this case. The steady state is correct, but everything up to the steady state value is false (as in your response time and overshoot numbers are SNAFU). The 'glitch' (as we call it) in the initial Ay transient is a player in road feel.

Third, I don't know which coordinate system Dyna-Tuna is using, but it indicates a +Steer Angle produced a +Yaw Velocity' (not 'Yaw' as in Yaw Angle) a +Lateral Acceleration (both correct) but a +Side-slip Angle (say it ain't so). My Side-slip Angle transducer would produce a negative steady state value, not a positive one. Besides, my sideslip angle would be positive at low speed, negative at high speed (certainly at 120 kph) with it being ZERO at the so called "Tangent Speed". The goal of producing a high Tangent Speed is usually on everybody's To-Do List. A constant radius ISO4138 Test Procedure can hand this to you.

The middle Figure says that the light blue line Sideslip Angle is lower for the same steer and same speed and that the lateral acceleration achieved is also about the same, so the Gain is the same so the understeer is about the same, meaning the Front and Rear Cornering Compliance DIFFERENCE is still about the same. It looks to me like are merely seeing the effects of stiffer tires ALL AROUND.

Post some car details: Weights, wheelbase, steer ratio, estimated tire properties (at least some cornering stiffness info) maybe some tire aligning moment values and I can demo what you should be expecting. Stick to the steady state analysis with this program. Response times are clearly of high value to a driver, but not ones from this program.

 

[sierra4000]

I do not know,
maybe my input data are bad?
maybe is software limitation?
also it's very likely that I'm doing this test wrong?!!
then my result can be incorrect?

difference is small because changes are small also ? (rear toe change from 0,3 Dg to 0,4 Dg total)(also rear stiffness change is small)

car weight 1055kg (56% front)
wheelbase 2610 mm
steering ratio 12
tire cornering stiffness 2000 N/Dg
tire aligning torque stiffness 200 Nm/Dg
(these tire data are only estimated............i do not know if are good..........no have real tire data)

 

[cibachrome]

Well, now we can all have a little bit of fun. With the info you provided, let's throw a Hoosier 225/45-R17 tire on your ride (250 kPa on a wide wheel. At your loads, these tires spit out cornering stiffnesses of about 1390 N/deg front and 1256 rear. Aligning moment stiffnesses are a meek 34 Nm/deg front and 23 Nm/deg rear.

Rubbing your emerald slippers together should give you a front cornering compliance of 2.11 deg/g and 1.78 deg/g at the rear. This recipe was lightly sweetened with a bit of steering compliance. Turning the crank gets you an Ay response time of 0.29 sec at 120 kph and with a constant radius test sim I get a tangent speed of about 77 kph. Your 12:1 gear gets you a steering gain of 3.74 g/100 deg-swa at 120 kph. A bit quick if I do say it. In a nutshell, here is what Dyna_Tuna should get you: (more to follow).

 

[cibachrome]

And with a little more software grunt, you can get serious enough to go racing. BTW: These programs are open source Matlab. I provide them for entertainment to FSAE and its Tire Test Consortium members in order to embarrass the Design Judges who often know less about the subject than my 3 dogs.

 

[sierra4000]

Thank you Cibachrome,
although i do not understand all ,i think your responses was useful
so we can say ..........car total compliance is crucial for race car manipulation
as in steady states ALSO during transitient maneuvers, that seems really NOT the same in terms of understeer level

 

[sierra4000]

BTW: i am stay with Dynatune
is Great tool!!