So I've had leaf springs on my mind.

"CalzoneGolem" (calzonegolem) 08/21/2013 at 14:05 • Filed to: None

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It seems to me that the axle would want to rotate in a circle described by a radius of the drive shaft, and the spring would just want to move lineally. But, then you bolt them both together.

Does this make any sense?

Formula4speed > CalzoneGolem 08/21/2013 at 14:08

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yup. The resultant effect is referred to as axle wrap.

matthewklein220 > Formula4speed 08/21/2013 at 14:11

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Nasty stuff...

CalzoneGolem > Formula4speed 08/21/2013 at 14:12

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Ah, that makes sense.

desertdog5051 > CalzoneGolem 08/21/2013 at 14:12

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Traction bars is the answer.

RamblinRover Luxury-Yacht > CalzoneGolem 08/21/2013 at 14:12

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Only with fixed driveshaft length would you get an arc per se. Otherwise, the U-joints prefer things in parallel, and otherwise wouldn't care much as long as precession was minimal.

A leaf spring isn't a clean straight line because it effectively elongates, and that elongation isn't necessarily linear.

CalzoneGolem > RamblinRover Luxury-Yacht 08/21/2013 at 14:15

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Are you saying my driveshaft isn't a fixed length?

wheelhop > CalzoneGolem 08/21/2013 at 14:15

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That is why traction bars are a hot rodders best friend

doodon2whls > CalzoneGolem 08/21/2013 at 14:17

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Axle Wrap is something different and is a result of the torque reaction of the drive torque through the springs mounted to the solid axle.

Kinematically speaking, however....

In most cases, the forward mounting position for the leaf spring is fixed and the rear has a shackle. This arrangement allows the leaf spring to behave both as a 'trailing arm' and spring simultaneously (the distance between the two points is allowed to vary via the shackle and bushings). It also allows for some recession of the suspension during jounce input as it follows a semi circular path about the forward mounting point. The result is that the spring and axle travel together on something similar to your red arc.

Again, in most cases, the driveshaft is _not_ responsible for suspension geometry/kinematics (there are a few exeptions), but is allowed to plunge on the trans end and high articulation vehicles (i.e. jeeps) might have both driveshaft ends fixed with plunging driveshaft tubes.

J. Walter Weatherman > RamblinRover Luxury-Yacht 08/21/2013 at 14:18

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I am far from an expert on this, but doesn't the slip yoke allow the axle room to move as the suspension compresses (i.e., not along a radius)?

Also, one end of the leaf spring is fixed, and the other is attached to the shackle, so as the spring compresses (and elongates) the shackle deflects, and I would presume creates a non-linear path for the axle.

???? I could be completely wrong on this, but that is how I understand things...

RamblinRover Luxury-Yacht > CalzoneGolem 08/21/2013 at 14:20

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Very often not. There's usually slip designed either into a slip-yoke on the axle, or a slip-yoke in the end of the transmission. GM TH400 and others, transmission:

Here's one from a Land-Rover Series II: mid-shaft slip:

LandSpeed-DSM > Formula4speed 08/21/2013 at 14:22

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What I came here to share.

CalzoneGolem > RamblinRover Luxury-Yacht 08/21/2013 at 14:22

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Ah, hah! That makes sense.

CalzoneGolem > wheelhop 08/21/2013 at 14:23

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Appropriate name for this discussion

J. Walter Weatherman > doodon2whls 08/21/2013 at 14:24

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Yes. This.

CalzoneGolem > desertdog5051 08/21/2013 at 14:24

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Looks lie a reasonable mod for me to make while I've got the rear end torn apart.

RamblinRover Luxury-Yacht > J. Walter Weatherman 08/21/2013 at 14:42

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All this is correct.

Demon-Xanth knows how to operate a street. > CalzoneGolem 08/21/2013 at 14:43

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There are two sources that allow for the play caused by the spring's arching. One is that the shackle isn't a fixed point, but a pivoting arm, and the other is the slip yoke at the back of the transmission. The axle also does wrap up and down.

If only EssExTee could be so grossly incandescent > CalzoneGolem 08/21/2013 at 22:18

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Perhaps a two-piece driveshaft could eliminate that. It would have splines like with a cv shaft, and one end would slide over the other allowing the driveshaft to change length while still transferring power. Rather than making the suspension travel in a circular path, the driveshaft would be able to follow the straight path of the spring.

I know that the shackles already solve your issue, I'm just proposing an alternative.

The WB > CalzoneGolem 08/22/2013 at 10:08

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Don't forget the rear of the leaf spring isn't fixed, its on a shackle that pivots at both ends, allowing the change in spring length eye-to-eye to be accomodated. They make sliders that do the same thing with less resulting rate change.