ARB calc confusion

[Garre88e]

Hey guys,

I'm trying to figure out the ARB calculations for our car and am getting confused. Ive been using RCVD and my OptimumG binder as reference.

290Kg Car, 45-55% F-R Weight Distribution
50-50% F-R Roll Distribution
Front Natural Frequency: 3hz
Rear Natural Frequency: 3.3hz
Front Wheel Rate: 19.27 N/mm
Rear Wheel Rate: 28.69 N/mm
Front Track: 1.37 m
Rear Track 1.27 m
CG to Roll Axis: .254m
Desired Roll Gradient: 0.9 Deg/G
Tire Stiffness: 131,345 N/m

Using these numbers, I come up with:

Front Roll Rate Springs Only: 293 N*m/deg
Rear Roll Rate Springs Only 403 N*m/deg

Desired Total Roll Rate: 800 N*m/deg
Front Desired Roll Rate: 400 N*m/deg
Rear Desired Roll Rate: 400 N*m/deg

From these numbers I would assume that a rear bar is unnecessary, and that a front bar with 107 N*m/deg resistance would be required. However, using the equation on pg. 592 of RCVD, I get 1088 N*m/deg of required front ARB.

Is my assumption of not needing a rear ARB correct in this case? Also, why is it telling me I need such a stiff roll bar to make up the small difference between desired roll and roll with springs only? Am I missing something here?

Thanks

[R. Alexander]

You're riding very stiff when compared to my numbers. Maybe it's not so bad though...

But yeah, 400 is close enough to 403.

Have you looked at the tech tips on the OptimumG website?

[Garre88e]

Yes, I have seen the tech tips, but have been working out of the binder from the seminar. There pretty much identical.

Our car weight more than the typical fsae car (290 kg) do to the hybrid components. This may explain why it seems very stiff.

For the calculation regarding required ARB rate:

Pi/180*(Kdes*Kt*t^2/2)/(Kt*t^2/2*Pi/180-Kdes)-Pi/180*Kw*t^2/2

They refer to the 'Desired total roll rate' do they mean the desired for the front and rear respectively , or desired for the entire car? I was previously using the total for the entire car and have since changed it to the total for that end of the car, giving much more acceptable results.

The required front ARB is now 174 N*m/deg, while the required rear ARB is 85 N*m/deg.

The front seems acceptable (i still don't understand why it isnt just the desired rate minus the roll due to springs 400-293=107).

The rear seems very large considering the desired is 400 and the springs resist 403..

[The_Man]

Originally posted by Garre88e:

The front seems acceptable (i still don't understand why it isnt just the desired rate minus the roll due to springs 400-293=107).

The rear seems very large considering the desired is 400 and the springs resist 403..

The roll stiffness that is to be added by the ARB will not be just a simple subtraction. Look at it this way.

When there is only spring, the spring and tyre make it a system of 2 springs in series. Keff = Ks*Kt/(Ks+Kt)
When the ARB come in it is 2 springs in parallel followed by a spring in series. Keff = (Ks+Karb)*Kt/(Ks+Kt+Karb)

Hope this helps clear the picture. Your Karb will be larger than the Subtraction.

[ben]

Originally posted by The_Man:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Garre88e:

The front seems acceptable (i still don't understand why it isnt just the desired rate minus the roll due to springs 400-293=107).

The rear seems very large considering the desired is 400 and the springs resist 403..


The roll stiffness that is to be added by the ARB will not be just a simple subtraction. Look at it this way.

When there is only spring, the spring and tyre make it a system of 2 springs in series. Keff = Ks*Kt/(Ks+Kt)
When the ARB come in it is 2 springs in parallel followed by a spring in series. Keff = (Ks+Karb)*Kt/(Ks+Kt+Karb)

Hope this helps clear the picture. Your Karb will be larger than the Subtraction. </div></BLOCKQUOTE>

No.

roll rate required - roll rate from springs = ARB required roll rate.

RCVD p604

Ben

[Garre88e]

Originally posted by ben:

roll rate required - roll rate from springs = ARB required roll rate.

RCVD p604

Ben

That is how I thought it should work. So then what is the equation on pg. 592 of RCVD referring to? They seem to contradict each other..

Thanks,
Brian

[The_Man]

On page 604 the required roll rate used is the required roll rate from the spring only with out the tyre springs.

The Ksr is calculated from the wheel rate and the tyre rate. The Kdes here is the torsional spring rate desired from the springs(and arb) alone. Therefore you can use the subtraction. The Roll rate you will end up with overall will be the Kdes||Kt

Even I am confused now(Time to go over RCVD again), but this seems logical.

[Ax]

i had a doubt regarding the total ride travel thats mentioned in pg589, from which the ride rates are found. is this the total wheel travel(bump and rebound both?) or just the chassis to ground travel(sounds like just the bump travel).
and for finding ARB rates, how do i determine the Ib term(installation ratio of ARB) in pg601 considering im designing a u-bar with blades and connected to the bell cranks of a push rod system.
also in his Engineer to Win book, Carroll Smith prefers the use of a linked-blade adjustable ARB with blades ending in 'guided rollers' over a simple blade ARB.. can anyone elaborate this design and its working to me?