ARB Mayhem...!!

[AllenGeorge2008]

Well, i'm finding myself in deep soup as far as Anti roll bar calculations are concerned. After going through quite some books and papers, the online tech tips on the OptimumG website seemed to make quite some sense. At least looked simple...
I've gone through multiple number crunching sessions now and have not been able to really interpret the results it provides.
The following is the full set of calculations I have made:

We are using springs of stiffness 250lbs/in = (250/2.206)*9.81*1000/25.4 = 43795 N/m

And designed our suspension for a roll of 2deg at a lateral acceleration of 1.5g

Therefore the roll gradient [?/A] = 2/1.5 = 1.33deg/g

My total desired roll rate therefore from the relation

K?(des)= (WxH)/ [?/A]
= 350*9.81*.274/1.33
= 707.35 Nm/deg

where W is weight if the car in Newton
H is distance btw roll centre and cg in m

The next step indicated is the calculation of the total ARB stiffness which is to be then divided between the front and the rear using the Majic no. The tutorial mentions to take the majic no. as 5% (a baseline figure) higher than the static weight distribution which in our case is 40:60 (front to rear)
So my majic no comes out to be 45%.

Now, calculating total ARB stiffness from the formula

K?(total ARB)= [[K?(des)xK(tyre)x(t^2)/2]/{K(tyre)xt^2x?/180 - K?(des)} ] - W

Where W= ?/180* K(wheel) t^2 /2

yeah, i know it looks awfully incomprehensible…

plz refer

http://www.optimumg.com/Optimu...mpers_Tech_Tip_2.pdf (http://www.optimumg.com/OptimumGWebSite/Documents/TechTips/Springs&Dampers_Tech_Tip_2.pdf)

taking t(track)= 1.3m and tyre stifness K(tyre)=1030lbs/in = 180435.4 N/m

K(wheel) = spring rate / MR^2

substituting all these values, the results seem rather absurd..

total ARB roll rate= -1.84 Nm/deg http://fsae.com/groupee_common/emoticons/icon_confused.gif

* Pardon my sensibilities, or maybe the lack of it, but doesn’t -1.84 seem too small a value.. moreover what’s with that negative sign there..??

* In the Torsion equation T/J=?/R=G*?/L; where does the above value (-1.84) fit in..?? i thought G .. but then the dimentions just dont match up..!! http://fsae.com/groupee_common/emoticons/icon_mad.gif



One more favour.. could anyone provide some ball park figures on ARB motion Ratios..??!!

[Asymmetric]

did,t look through all of the numbers but your spring rate seems a bit stiff, what are your ride frequency targets? With that stiff of a spring I dont see you using much of a bar, let alone 45% of your roll stiffness.

[AllenGeorge2008]

i'm looking at somewhere close to 2.3Hz..
We won't be using wings.. but then Ours is not too light a car... so track reaction force on the tyres will be substantial and hence i believe 2.3 is a rather decent figure.
springs: If you say they are relatively stiff, can this be the reason why I’m ending up at negative ARB values… I mean, can this be interpretted as over compensation …? making the ARB twist while the springs are good enough to do the job..??

[bahous]

a relatively low positive or negative value for ARB stiffness could be interpreted as an overcompensation.
so, for a 250lb/in spring and a desired ride frequency of 2.3Hz, no ARB is needed.

the positive side of that is that you will have less components on the car to manufacture, to package and to fix (if it breaks).

the downside is that you don't have much tunability. in the case where you want to change your roll stiffness, you will have to change your springs which is time consuming and if you don't have many springs, you will not be able to fine tune your car. the arb will allow you to have a "normal" stiffness spring and the ability to easily play with your roll parameters.

as a quick advise, there are a lot of set of equations for calculating suspension parameters (there are 2 in RCVD). so, choose one set of calculations, in your case OptimumG's, and stick to it. don't use 2 set of equations simultaneously to compare and decide on your suspension. this will only increase your confusion as to which numbers make sense and which don't.

goodluck

[AllenGeorge2008]

Great help..! That’s quite a few problems solved... Thanks a lot.

A few queries still left though.

We are designing or car for FS-UK and considering the time crunch we are all aware of, it seems a bit on the difficult side that we bring down or spring rates to incorporate an ARB. But is this a viable compromise...? That’s the question fiddling my grey matter.

Is it really a big downside to have a spring rate as high as 250 lbs/in..?? Apart from tuning issues… Yes, the suspension does get a bit stiff but it does offer me lower ride heights (lower cg, better aero etc) Also, manipulating the MR, we can get a split in the front and rear frequencies to offer better transient response (keeping front freq a touch on the higher side). “Dump the ARB, toy the MR…” Am I coming well with such a bargain..??

An answer to this should put a stopper to my dilemma ..!

[The Stigg]

Hi Allen,

in my opinion spring rates should rather be related to ride frequencies and more to damping!
Thats why most of the FSAE cars I've seen running MTB dampers are on the overdamped side. Too stiff damping is worse than high frequencies.
And it's not a secret that you can still have low ride frequencies while using stiff ride springs http://fsae.com/groupee_common/emoticons/icon_wink.gif

You can solve this problem by using a proper MR. Well I know that this brings hysteresis into the game because of lower damper piston speeds but the value depends on the damper you are using.
And to be honest the damper hysteresis on most FSAE cars is on the lower side regarding the complete hysteresis in the suspension system.

Regards Andy

[exFSAE]

Springrates don't matter. Wheel rates mater. You can have a springrate of 350 lb/in and still have a reasonable wheelrate.

You're designing the car for FS-UK? This is at Silverstone? Pretty smooth surface? I wouldnt be too worried about high wheel rates. You'll need them relatively high to prevent the chassis from bottoming under braking with a super short wheelbase.

Anyway. If it's a time crunch... don't bother with the ARB. Don't worry with front and rear ride frequencies.

Buy a set of 250 lb/in springs, if that's what you want. Buy some 200 lb/in springs, buy some 300 lb/in springs. Might be able to get away with 8 springs total.

Between swapping springs and bumping your tire inflation up and down.. as well as brake bias.. you should be able to get the car reasonably balanced and driveable.

Get it built and driving, test it, tune it, fix it, and away you go.

[AllenGeorge2008]

It’s a big relief that has come to me after a long time … I guess I won’t go on begging for “round about figures”… m pretty decided in my head now…

Thanks a lot