Doug,
You're all wet!
If you read Kas Kastner's competition preparation manual, you will find that he
recommends dearching the rear spring to both bring down the center of
gravity and to induce negative camber so that the rear tires will have a better
patch in a turn and won't have such a tendency to tuck under.
THink about it this way:
Remove the spring attachment point and what happens? The wheel is only
attached by the axle and the radius rod. Since the radius rod hinges up and
down, the wheel will simply flop around (no upper control element).
With that in mind, visualize that as each wheel moves up and down, two things
happen, the spring flexes and the axle pivots on the U-joint. If it
goes up, the spring's arch gets less and since the axle is fixed in length, the
camber becomes more negative. Even easier to document is what happens
when you jack up the rear of a Spit. The spring develops more arch, the wheel
falls and the camber becomes greatly positive.
Adding a lower a-arm as on the GT6 rotoflex keeps this from happening, but the
rotoflex joint had to be added to absorb the changing relative length
of the axle through the limits of its travel. But dearching the spring on a
rotoflex rear also increases negative camber. Ask me how I know this?
(THat is what I have on Tiny Tim the race Spit)!
Let me address your items individually!
Douglas Braun & Nadia Papakonstantinou wrote:
>
> I stand by my assertion. Let me explain:
>
> 1: The Spit's swing angles are always perpendicular to the wheels
> (or you've got some REAL problems).
THe axles are perpendicular to the wheels but not to the diff. So collapsing
the spring will make the axle point upwards in relation to the diff and
that will produce negative camber (as does the longer axles on later spits).
>
> 2: Since the wheels are (hopefully) the same diameter, the outer ends of the
> swing axles must always be at the same height above ground.
Wrong. The diameter of the wheels does not come into play here. You could put
40 inch wheels on a Spit (if you could fit them into the arches) and
the camber would not change. The total amount of difference between the top
and bottom would but not the degree of camber.
>
> 3: Therefore, if the rear wheels have different cambers, the swing
> angles must be at different angles. Since their outer ends are the
> same height above ground, their inner ends must be at different heights.
True, but the angle would be different if one side had a different arch than
the other. The side with the lesser arch would also sag!
>
> 4: If the inner ends of the swing axles are at different heights,
> the left and right sides of the diff must also be at different heights,
> which means that the diff must be tilted at an angle.
>
THis has nothing to do with the argument. It may indeed cause some
differences, but does not need to be tilted in order to cause the anomaly. If
it
were straight, the tip of the spring would also be straight. And if one end of
the spring were lower than the other, that would cause the axle to be
higher or lower on the outside than the other and still not affect the overall
attitude of the diff. (in reality, it probably would tend to tilt
slightly due to the sag effect caused by all the parts interacting as they are
supposed to).
>
> 5: Since the diff is rigidly bolted to the car body, if the diff is tilted,
> the body must be tilted.
> See 4 above.
>
> 6: Since the length of a swing axle is something like 2X the half-width
> of the diff, a 10-degree change in differential rear wheel camber
> would cause something like a 20-degree change in body lean.
The length of the swing axle is far greater than 2 times the the half width of
the diff. In fact, it is something like 4 times.
The diff measures 5.25 inches from the centerline to the outside of the flange
that mates to the shaft flange. The shaft is something on the order of
21 inches from the outside end to the U-joint. The longer the shaft, the less
will be the resulting ratio of body sag to negative camber.
>
> Note that the spring and its condition did not enter into the above arguments.
> The spring does not directly determine the position of the wheels; it
> holds the body up, and if the body is held in a tilted position, the
> rear wheels' camber will be (slightly) different.
Your statements are in conflict. The spring does in fact determine the camber
and yes it does hold the body up. But if it sags and the body is
allowed to fall, the axle must move in relationship to the differential and
when it does, it changes the angle of camber. This is because on a swing
axle suspension the rear wheels pivot on the U-Joint around the differential.
As the pivot occurs, the angle increases or decreases. The most
negative point is when the axle is at its highest point relative to the diff.
>
> The rear spring is NOT really like a upper control arm. Changing
> its length or position would change the angle of the vertical link,
> but the the angle of the vertical link does NOT control the angle of the
> rear wheel.
Since the vertical link is fixed in relation to the wheel, and change in the
angle of the link would directly affect the angle of the wheel. There is
no way around it!
>
> BTW, this sounds like something out of a Monty Python sketch.
"And now for something completely different"!! :)
Joe
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