[ODE] Leaf springs, CFM and ERP with ball joints and hinges

[Amund Børsand]

I didn't get the patch for the ERP/CFM with ball joints to work. I ran
the patch command successfully with some directory level parameter
tweaking. However, I ran the command on Redhat Linux 8.0 on a FAT32
drive I usually use with Windows XP. When I came back to XP, it
complained about some currupt files (the ones I just patched) and
allocation units, and when I ran chkdsk it just deleted the newly
patched files... so I guess I'll have to do it manually, just haven't
gotten there yet.

So far I've tried using ball joints with global CFM. It works fine,
except from the unwanted effects the global CFM causes on the rest of
the system (that the wheels now can move around a little in all
directions on its bearings and such). 

What I was trying to simulate was leaf springs. I used three ball
joints: One in the middle of the car, attaching the axle to the car
there to keep the axle from spinning around when applying torque to the
wheels, and two ball joints right above/in the middle of the axle
besides eachother, to simulate the springs. How far these two are placed
from eachother controls how stable the axle will be sideways. For
offroading you'd want them fairly close to allow the axle to articulate;
but not too close or your chassis will fall over while the axles are
flat on the ground. This also depends on spring rates, of course.

This works fine for a live axle suspension, except of course for the
obvious side effects caused by the ball joints ability to move in every
direction, allowing the axle to move sideways as well. But this is
present in real cars too, to some degree.

But it's not a good approximation for leaf springs. Because the axles
will pivot around the point in the middle of the car, both when
accelerating/decelerating, and when the axle is pushed sideways. So I
came up with a better idea.

Leaf springs are just long flat steel "beams" (is that a correct word?),
which are made of a spring-like material allowing it to flex and bend a
lot without breaking. They are arched downwards, and attached to the
axle on the middle and bottom of the arch, while both (upper) ends are
attached to the car. In one end there is a "shackle", a short arm with
hinges in both ends allowing the spring to become longer and shorter as
the axle moves up and down, and straightens or sharpens the curve.

This spring can be thought of as a series of hinges (like the hinge
joint in ODE) placed very close to eachother along the whole length of
the leaf spring, each one bending by the same angle when the spring is
compressed. If you think of a very rough approximation of this, that
would be just one hinge in the middle, right? With two long straight
beams on each side. Connect the axle in the middle, and the car to each
end (with the shackle to allow for length changes) and voila, a leaf
spring! 

To be more precise, both leaf halves have to be attached to the axle,
maybe at the same point, maybe not. They should have their joint stops
set at the same (but opposite) angle, and both the high and low stop set
at the same angle. Then there should be used a quite stiff cfm/erp value.

Now, the problem I ran into when thinking about this, was that real leaf
springs move sideways as well. Not much, but some, enough to make
leaf-sprung vehicles handle oddly and not keeping a straight line. I
therefore thought that the hinges used should be able to twist around
the springs lenght axis, that is, an axis normal to the hinge axis. Is
there a CFM possibility in that direction, or can't hinges twist at all
that way? Is there a way to implement this, or could one use ball joints
with stops and CFM or something?

I haven't tested this yet, but it will be implemented very soon.


Also, is it easy to make one's own joints? It seems like all the joints
in ODE are just different combinations of locked position and rotational
axis. Is it easy to build one's own combinations of these?


Phew. Sorry for a long posting, I just get so carried away.. =)

-- 
Amund Børsand,
hooked on joints