Front and rear roll angles

[Komodo86]

I have done a fair bit of mathematical and 3D modelling of independent automotive suspension systems, and have come to the conclusion that the in pure steady state cornering no matter what the combination of springs, ARB/sway bars, motion ratios or linkage geometry, the roll angle (ignoring the tyre flex component) is always determined by the deflection of the spring and that the load they see on each axle will directly correspond to the rate of the spring, meaning that they will deflect such that the wheel movement and thus roll angle, is the same at each end of the vehicle.

It has been suggested by an experienced engineer, that this is not the case and that the front and rear axles DO roll different amounts, however the mechanics of this were not explained. Surely if such a case was true, this means the chassis must be twisting to accommodate the difference in roll angle? In a typical modern chassis with say 15,000Nm/° torsional rigidity, surely the amount of differential roll, if any, must be negligible to the point of not being worth considering?

 

[Komodo86]

Greg, with regards to the calculated wheel displacements, I think I know what is causing them to come out much lower than expected. Just yesterday a thread popped up on the owners club about sway bar stiffness and someone had measured the stiffness of the bar and found the results to be some 60% of the values given by the calculations floating about for the bar rates. This is owing to the bending of the arm and the flexibility between the bushing and the arm, both of which are not accounted for in the calculations.

Not only this, but it seems I had the length of the bars a fair bit shorter than actual.

Adjusting the bar rates for this would suggest that the car has a roll rate of around 5°/g which is right on the money according to RCVD.

 

[TC3000]

this may help you along
the two angles φ_f and φ_r are the two individual angles as discussed and calculated yesterday.
c_tor is the torsional stiffness of your chassis.
I made a mistake in the formula yesterday, should have read (half track)^2, but I think you get the idea anyway

 

[TC3000]

if you calculate your spring rates as @ the wheel and lump your ARB rates into these wheel rates as well s_sp,f&r becomes your track width.

 

[GregLocock]

five deg/g is about right. There are many causes of sta bar inefficiency, the D block details being the big one, and people often make geometrical compromises that are not helpful.

Cheers

Greg Locock


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