front/rear axle roll axis - relation

[uninformed]

how does the front and rear axle roll axis relate to each other?

is there a ideal set up on a road driven 4x4 that is PERMANENT 4WD?

like understeer front, oversteer rear, or oversteer front but more oversteer rear?

i know that roll center heights front and rear determine the vehicle roll axis: ie higher in the rear will be "loose" and the rear will break traction first....

just wondering what changing the axle roll axis will effect driving the rig onroad...

example:

wheelbase = 2794mm

front roll center = 410mm
rear roll center = 540mm

front axle roll axis = 7 degrees oversteer
rear axle roll axis = 12 degrees oversteer

what will happen if i change nothing but the rear axle roll axis to 9 degrees... i think the antisquat will be less but that wont effect onroad driving...

Serg

[junior80]

yes.

[sambo]

Can someone explain how you can have or measure oversteer in the rear axle.

[zookimal]

My impression is that the front and rear roll centres allow you to determine the roll axis of the vehicle as a whole by plotting a line between them.

The vehicle will rotate around the roll axis and weight will shift forward or back around that axis through a turn.

I think the ideal would depend on a myriad of things, as the roll axis will effect how it utilises that weight transfer in cornering and braking. Useful on a racetrack as it can be tuned to suit fuel load, track type/conditions and tyre wear, and get more or less weigh over the front or rear as a result.

I'm sure there's plenty out there who know way more than I do.

[uninformed]

there are different ways of measureing the axle (front or rear) roll axis, depending on the type of suspension used.. be it leaves, 3 link + panhard, 4 link, A frame + trailing arms, radius arms + panhard etc, etc...

on range rovers, disco 1, defenders...the front axle roll axis is parallel with the radius arms(center of axle case to center of chassis bush) and the height of axle roll center is centerline of vehicle(left to right) where it intersects the panhard...

for the rear, because it has parrallel (when viewed from above) trailing arms, the axle roll axis is parrallel with these, the height of the axle roll center is the center of the rear ball joint where the A frame meets the axle case...

there are 3 roll axis in a rover type vehicle(and other brands too) front axle roll axis, rear axle roll axis and vehicle roll axis, which is the line that intersects the front and rear roll centers...

my question is not about the vehicle roll axis, but the front and rear axle roll axis and how they effect each other on a road driven 4x4 that is permanent 4wd...

is there a better balance to have more of oversteer or understeer in the rear? that sort of thing...

Serg

[sambo]

Now I understand. I have had experience with suspension geometry with independant front and rear, but not with suspension on a 4wd. Thanks for explaining it.

[uninformed]

there is way way more to it than what i have added... im just learning this myself....

iwth independent suspensions there is a roll center but not a axle roll axis.... or i could be way off...

im only really concerned with rover type suspenison ie radius arm + panhard and parrallel trailing arms with A frame....

i do believe that constant 4wd vs 2wd plays a difference, but not sure how...

Serg

[KiwiBacon]

I presume you're using the individual roll axis to determine whether the axle steers into or out of a corner due to body roll?
From what I've read that has a big impact on how a vehicle feels, but there are many more variables in whether it's going to over or understeer. Roll stiffness (springs and bushings) seems to be the biggest.

You want a rangie to oversteer, take out the rear self-leveller and put in some big hard springs. The self leveller was an integral part of keeping rangies on the road before they introduced sway bars to them. When they did introduce sway bars the rear was only a token effort.

I'd expect the rear roll axis to pierce the instant centre of the linkage in side view. The rear axle setup on these steers outwards markedly with body-roll. This is due to the upwards angle of the rear links, as the outside compresses it gets pushed back, as the inside lifts it gets pulled forwards.

[1MadEngineer]

I started typing, then thought it would be better if you read this!!
http://www.pirate4x4.com/forum/showthread.php?t=701536