ARB contribution to wheel rate?

[highboost]

Hi guys,

Long time reader, first time poster.

I understand you need the spring rate and motion ratio of the roll bar, but mathematically, how do you include this value into your overall wheel rate?

This has been bugging me for a while now.

Thanks

 

[NormPeterson]

1. If you already have the bar, actual measurements would be possible and you could ignore theory unless you were going to attempt to make it adjustable over some range.

2. If you don't have the bar, and the mfr is unwilling or unable to provide you with stiffness data, theory is all you've got.

With respect to item (2), I wish I could remember where I saw a picture of a front sta-bar that was experiencing a visible amount of bending as seen in front view along the central (mostly) torsional section. You know that one is going to be somewhat softer than Puhn's simplified formula suggests. Maybe a lot softer.

If chassis structure does not permit the D-blocks to be located close to the arms, the simplified formulas will again over-predict the stiffness.

It's not that more theory will get the bar designed or properly chosen in one shot. More like if it isn't as stiff as the quickie formulas suggest, you'll know why it's not (and have a better idea how to proceed).


Goran - when I ran problem A through my spreadsheet I was getting very large deflections for the 22.5° model as well, and I doubt that the small deflection thinking associated with standard beam formulas would still fully apply. Those results aren't on this computer (and I think there's a factor of 2 involved as well), but I'll post them later.


Norm

 

[NormPeterson]

One way I get 120, 125, 154, and 433 mm

The other way, 120, 129, 180, and 585 mm


Norm

 

[GregLocock]

On a production car the installation details (stiffness of D blocks, links and other compliances) have a pretty severe effect on calculated sta bar stiffnesses.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[NormPeterson]

Understood, Greg. Springs in series. The above numbers are based on negligible amounts of such flexibility (1E6 lb/in stiffness inputs used for both D-blocks and endlinks, 1.8E4 kg/mm).

The reason that my two sets of numbers above diverge as the angle tends from 90° toward zero is because it's a model of an entire bar rather than fixed at the midpoint of the torsional section as per Goran's sketches. Unlike torsion during pure roll, the bending in the central portion is antisymmetrical rather than symmetrical and the slope at midpoint will not be zero. The first set includes only center section torsion and arm bending (this essentially being Mr. Puhn's approach, except that I've held Young's modulus E out as a variable).


Norm

 

[hemipanter]

As we see here there is no easy way out on this one. There are numbers of inputs for calculations, and just as many different designed aftermarket bars. I think the point is to be aware of these things to make better use of our swaybars.
A straight torsion bar with a good mountings, and 90 dgr separate solid arms hooked to a good A-arm connection link, is a good idea from many aspects.
Easier to count on and a more precise function.
Goran

 

[evelrod]

A straight torsion bar with a good mountings, and 90 dgr separate solid arms hooked to a good A-arm connection link, is a good idea from many aspects.
Easier to count on and a more precise function.

Goran, I would love that to be the way that my race cars could be setup. Wow, would that simplify everything, especially on the front of the Lotus Cortina my son races.

Rod

 

[GregLocock]

What's that got - a true MacPherson using the arb as the leading link into a rubber bung on the lower arm? And doubtless the D blocks are a long way inboard.



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[evelrod]

Yep!

This configuration is the same as the OEM except with several changes for racing. I've kept it as it was in the 60's for the vintage racing clubs requirements.

Std. dia springs, but at 300 lb/in and fully adjustable Koni inserts. Suitable reinforcing at the spindles to help with the bending loads. A one inch bar/leading link with proper Al D blocks with suitable lubrication fittings and NAS bolts. The fully adjustable lower control arm mounting is via a Rose joint as is the ARB mounting. Upper strut mount is with the top mount from a BMW (Ford stuff no longer exists), again to maintain the originality per the time period when "camber plates" were not legal.

It actually "looks" pretty original, but performance is much better. The one inch Holman Moody bar is shorter allowing for more positive caster and a bit wider to allow for the three degree negative camber we use.

Even the rear axle housing is tweeked a bit to allow for a bit of negative camber. The full floating Quaife axles seem to tolerate it okay. The 7/8" rear bar is mounted in the boot on the roll cage extensions and project through the wheel wells to the outermost axle housing.

It seems to work. No more roll steer, no more funny wheels in the air...4 corners better than 3...;o)
Class legal weight is 1730 lbs.(we are a bit over that) and the engine output from the recent dyno run, is 197 hp at 8100 RPM...It's quick enough for vintage.

Rod