Hello,

Does anybody know the numbers for "effective wheel rate" or spring rate of the front and rear swaybars on a G3 Integra? It depends on things like sway bar thickness, LCA mounting point, endlink angle, etc etc. Instead of me calculating it, I hope somebody else has already figured it out.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by beanbag &raquo;</TD></TR><TR><TD CLASS="quote">Hello,

Does anybody know the numbers for "effective wheel rate" or spring rate of the front and rear swaybars on a G3 Integra? It depends on things like sway bar thickness, LCA mounting point, endlink angle, etc etc. Instead of me calculating it, I hope somebody else has already figured it out.</TD></TR></TABLE>

Yes, I have calculated this for a G3 integra for an upcoming Sport Compact Car article on anti-roll bars. BTW, the effective wheel rate number will also depend on bushing material (harder material = higher eff. rate). Also, if you are using an aftermarket LCA like Omni, Sk2, ect. the eff. spring rate will depend on which hole in the LCA end link mount you decide to use.

 

that's great. so what are the numbers?

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by beanbag &raquo;</TD></TR><TR><TD CLASS="quote">that's great. so what are the numbers?</TD></TR></TABLE>

For which suspension spring rate and for which anti-roll bar? If you do not know for which anti-roll bar, you at least need to give me the dimensions of the bar for which you'd like the wheel rate numbers.

 

GSR front rollbar and itr rear rollbar. Once I know that, then I can pretty much figure out the total roll stiffness. Springs are 300/200. Did you add bushing compliance into your calculations?

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by beanbag &raquo;</TD></TR><TR><TD CLASS="quote">GSR front rollbar and itr rear rollbar. Once I know that, then I can pretty much figure out the total roll stiffness. Springs are 300/200. Did you add bushing compliance into your calculations?</TD></TR></TABLE>

Actually, I wrote two Matlab programs to compute time-step anti-roll bar stiffness for a visco-elastic material such as rubber or urethane and one for stiff bushings. The problem with visco-elastic materials is you need to determine the load verses time curve for the actual bushing being used. I modeled the visco-elastic material as a Kelvin-Voigt type material (dashpot and spring in parallel) which is a good model for polymeric compounds. I am currently updating the program to handle both stiff and compliant endlinks as well. I will find the ITR bar profile to compute this stiffness soon. Stay tuned.

 

I'm still tuned...

 

Wouldn't it be easier to experimentally determine the effective spring rate of the stock ITR swaybar? All those bends would make it extremely difficult to produce an accurate number analytically. I imagine you would simplify it by making it a straight bar with straight levers attached to the ends, but obviously this would not be as accurate.

IMO, it would be pretty high, like around 500-750 lbs/in. I'm pretty sure it's solid, it definitely does not feel hollow.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by ginsu2k &raquo;</TD></TR><TR><TD CLASS="quote">IMO, it would be pretty high, like around 500-750 lbs/in. I'm pretty sure it's solid, it definitely does not feel hollow. </TD></TR></TABLE>

And you'd be miles off. The front bar, for all its thickness, bends and weight, ends up comparitively quite soft.

Johnny Mac, I assume the numbers you'll publish are rate per degree of roll? Or will you have a chart so one can determine the bars effective rate for the spring rate they are using?

 

I was thinking about this problem, and it's a lot more complex than just adding a sway bar rate to find an 'effective' spring rate. First, we must consider one side to be a side that encounters a bump (i will call it the bump side), and then the other side.

For one, the sway bar will only twist when the force to move the suspension on the other side of the car is more than the force to twist the sway bar. If you do a static moment balance on the LCA you will find that:

Fr = Anti-roll bar force
Fs = Spring force
mg = force due to unsuspended mass
x1 = distance from center of rotation to ARB
x2 = distance from center of rotation to Spring Mount
x3 = distance from center of rotation to mass center of unsprung weight

Sum of the Moments: Fr*x1 - Fs*x2 - mg*x3 = 0

Fr = (Fs*x2 + mg*x3)/x1

But, Fs is equal to k*y

where
k = spring rate
y = deflection of suspension.

So, Fr = (k*y*x2 + mg*x3)/x1

everything is a constant except 'y', the suspension deflection

So the force that the ARB feels from the opposite side comes only from the mass at first and then ramps up quickly due to deflecting the spring. This means that the bump force is at first transmitted through the ARB until the suspension deflects enough to reach the point where the ARB then begins to twist, only then would the ARB actually be contributing to the 'effective' spring rate (on the bump side) of the suspension.

Also, my analysis does not include any of the effects of the angle between the endlinks and the sway bar lever, which is most likely adds a falling rate to the sway bar. It also does not include any compliance in the endlinks.

I think these effects are interesting because it highlights the fact that the sway bar really has to work against the springs, so if you have a really, really high spring rate, then the ARB is almost always twisting, thus adding to the effective spring rate. But if you have really soft springs, then it's transmitting a lot of bump forces back and forth (i think this would be bad).

 

Bump again for an answer. While I had the shock off one of the wheels, I pushed down on it. It was pretty soft, so I think a GSR front swaybar only gives an effective wheel rate of about 50-100 lbs per inch.

 

It is easy to actually measure the wheel rate while the bar on the car.

Put the car in the air, weight the trunk down (lots of weight as the rear of fwd car is really light). Take one of the shock off, and on the other side replace the spring with a pipe.

Put a scale on a floor jack and slide it under the wheel without the shock. Start jacking it up and measure the distance and the weight. Make sure the chassis doesn't move (you're not jacking the chassis)