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Old 05-18-2017, 11:28 PM   #1 (permalink)
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Axle Articulation Question

So I'm very new, and probably very dumb, but I haven't found an answer anywhere no matter how I've tried.
When a beam axle articulates such that one side is higher than the other, where does that twisting force go? Is it just absorbed by the drivetrain, or is there some coupling between the differential and driveshaft or between the driveshaft and transmission that absorbs the twisting motion so it doesn't travel through the drivetrain and break something?
Please help, I'm deeply confused.
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Old 05-19-2017, 05:41 AM   #2 (permalink)
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Originally Posted by Jbc003 View Post
So I'm very new, and probably very dumb, but I haven't found an answer anywhere no matter how I've tried.
When a beam axle articulates such that one side is higher than the other, where does that twisting force go? Is it just absorbed by the drivetrain, or is there some coupling between the differential and driveshaft or between the driveshaft and transmission that absorbs the twisting motion so it doesn't travel through the drivetrain and break something?
Please help, I'm deeply confused.
If I'm understanding your question correctly, you're asking where torque is absorbed in an articulating axle under load? Ideally there is no twisting force between the planes of the two wheels. One goes up, one goes down. The links should keep the axle traveling up and down, not forwards or backwards (save the arc of the links during axle articulation). That's why trusses are employed on linked suspensions and not so much on leaf-sprung trucks. Shackles and springs do the same thing but in the vertical plane and allow some forward and rearward motion, but not very much.
Twisting motion traveling up the drivetrain? The driveshaft is either moving or not. You're probably talking about a few degrees max of rotational energy in an articulating axle.
Having said that, I fully acknowledge my own idiocy. i'm sure someone smarter than me will get to your question and answer it correctly.
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Old 05-19-2017, 06:58 AM   #3 (permalink)
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Originally Posted by Jbc003 View Post
So I'm very new, and probably very dumb, but I haven't found an answer anywhere no matter how I've tried.
When a beam axle articulates such that one side is higher than the other, where does that twisting force go? Is it just absorbed by the drivetrain, or is there some coupling between the differential and driveshaft or between the driveshaft and transmission that absorbs the twisting motion so it doesn't travel through the drivetrain and break something?
Please help, I'm deeply confused.
The way you're asking the question is confusing. I'm not sure what you are saying but I read this in a couple of different ways…. I'll try to answer what I think you are asking….

The first way I'm reading this is, if the axle articulates, where does the twisting force of the axle beam go? If one side of the beam is higher than the other, it is in relation to the chassis. The suspension and springs are the only devices that absorb that form of twisting motion.

The second way I'm reading this is, what does the drivetrain do when the axle articulates? When it comes to the chassis, we often look at various drivetrain parts as either being sprung or unsprung weight. Everything above the springs and supported by the suspension is sprung mass, and everything below the springs is unsprung mass. Part of the drivetrain is sprung mass. Those are the transmission and transfer case. Unsprung mass are your axles. The axles are free to articulate and move with the suspension in relation to the chassis, while the sprung mass of transmission and t-case are bolted to the chassis. The components that can "float" between the axles and the trans/t-case are the driveshafts. At each end of the driveshaft are universal joints. (or sometimes CV joints) These U-joints allow rotational motion from the trans/t-case to reach the axles via the driveshafts. Their flexibility also allows for suspension travel or articulation without binding. The driveshaft might also contain a slip joint to allow for differences in driveshaft length as the suspension moves up or down.
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Old 05-19-2017, 01:03 PM   #4 (permalink)
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Sorry if my phrasing is confusing, I'm not even sure there's a term for what I'm asking about.
If you think of the rear axle joined to the driveshaft as a T-shape, the crossbar of the T rotates around an axis passing through the driveshaft, like one of those T-shaped screwdrivers when you twist it.
Since the driveshaft already rotates around that axis when transmitting power, in one direction of articulation the axle will be turning in the same direction as the driveshaft, and in the other direction it spins opposite to the driveshaft.
What effect, if any, does that rotation have on the driveshaft? Doesn't it place a torque on the shaft as it goes around? Does that torque get absorbed harmlessly by the drivetrain, or is there some coupling that allows it to happen without applying a torque to the driveshaft?
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Old 05-19-2017, 02:08 PM   #5 (permalink)
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no coupling to absorb the torque. it would be the same force that propells the vehicle forward. when an obstacle causes the articulation the driveshaft is already under torque assuming your driving over the obstacle. if you were to just lift the axle straight up the force would try to rotate the axle and the tire would slip, rotate, whatever. this would be negligible compared to any slip from one tire traveling further from going over an obstacle or when turning with a spool.

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Old 05-21-2017, 08:19 AM   #6 (permalink)
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Originally Posted by Jbc003 View Post
Sorry if my phrasing is confusing, I'm not even sure there's a term for what I'm asking about.
If you think of the rear axle joined to the driveshaft as a T-shape, the crossbar of the T rotates around an axis passing through the driveshaft, like one of those T-shaped screwdrivers when you twist it.
OK, I think I understand what you're saying. Lets start by correcting a point. The driveshaft and axle aren't joined. They might work in harmony but they are totally independent of each other. It's not like a T shaped screwdriver where the screwdriver shaft and handle are a single structure. Think of this more like the screwdriver shaft has a pivoted handle, so that you can hold the shaft and spin the handle (Of course this would make this the worse screwdriver possible)

Look at the driveshaft. The d-shaft is turned by the power of the engine. After it is transmitted thru the transmission and t-case, the turning driveshaft then transmits power to the gears inside the axle housing where the power is split, and turned 90 degrees from the driveshaft plane. All of the driven axle components inside the axle housing are located by bearings, so there are no solid connections to the housing. The axle housing is free to turn via suspension movement and the driveshaft is free to keep transmitting engine torque to the tires.

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Last edited by RXT; 05-21-2017 at 08:20 AM.
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