A roll center based question...

[Pcar928]

To give an example of the knowledge I am after Ill make a theoretical example and ask what the effects might be. You have a 4 wheeled vehicle that has a monoshock arrangement front and rear. The front axle controls the roll rates and roll damping and obviously also has a spring and damper for pitch. The rear axle has no roll springs or damper only a spring and damper for pitch. Theoretically this should have similar effects to a three wheeled vehicle, with two front tires controling roll and a single rear which cannot control roll or in this case a 4 wheeled vehicle with evenly loaded rear wheels no matter the lateral load, correct? Then you throw in the roll centers. Assume the front is where ever, but the rear is above ground. Will the rear tires now have uneven loading under lateral acceleration?

 

[GregLocock]

Yes, if the cg is above ground and not at the front axle longitudinally. Draw a free body diagram of the rear axle.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[BrianPetersen]

Additionally, if the rear suspension is independent, it will have a jacking-up component during cornering if the roll centre is above ground (think swing-axles i.e. original VW Beetle and original Corvair and many other old cars), but if a regular axle is used, the jacking-up force will generally be much reduced or eliminated depending on the geometry.

 

[Pcar928]

Ah, yes jacking. So if the roll center at the rear is at ground level always, the front takes all the roll forces, it is a solid axle, and there is zero roll resistance at the rear then the rear tires will be evenly loaded?

 

[BrianPetersen]

Yup, and it's going to understeer like nothing you've ever seen.

 

[Pcar928]

Oh yeah! The 911 aftermarket guys go a similar direction although not nearly as extreme. Stiff front, soft rear, relatively, sometimes lifts the inner front tire powering out of corners. I would imagine if the weight bias in the above example were rearward enough and the rear tires were narrow enough, or the fronts wide enough, it might not understeer so badly, but would probably have problems three wheeling out of corners.