Slip yokes versus bump stops

[ScrapIron]

The marbles in my head started rolling around again earlier and they came up with this question randomly.

I have seen pictures of guys that have done 4x4 swaps and literally destroyed their transfer cases with no real analysis as to what exactly caused the destruction in the first place.

So the marbles in my head asked... Should the length of travel in a driveshaft slip yoke be at least equal to the distance your axle travels before making contact with the suspension bump stops?

[boywonder]

I'm not a 4x4 expert, but the marbles in my head say no. Slip yoke travel is less than axle travel. Simple Trig should demonstrate that the relationship between the axle travel and the slip yoke travel is likely the sine of the angle of the front driveshaft.

To solve the equation you would need the angle of the driveshaft at full droop and the angle at full compression, but slip yoke travel is significantly less than the axle travel. The driveshaft would have to be completely vertical for the yoke to be the same travel as the axle; that would be the maximum travel.

When the driveshaft is completely horizontal (it may never be horizontal..) the travel of the slip yoke would be minimum compared to the axle travel.

*Edit: When thinking about this a little more you need to solve both the cosine and sine equations keeping the "adjacent" side of the triangle fixed.....

The Cosine function gives you the slip yoke travel and the sine function gives you the axle travel.

[kbeefy]

Most 4x4 vehicles get by with a 4" slip yoke. There are 'long travel' ones available for extreme cases.


Different suspension geometries behave differently as well.
A leaf spring can lengthen or shorten throughout it's travel, so you can think about that along with the angles and legs the trig boywonder referenced.


A Trailing arm system with arms mounted close to the t-case yoke will experience very little change in driveshaft length.


There is also change when the axle is flexing side to side, ie one side compressed and the other extended.


It's usually best to cycle the suspension and measure the extremes.

[ScrapIron]

Ok, so since i was an underachiever at everything in school beyond chasing girls and drinking beer... If i use ratchet straps to compress the front suspension until the front shaft is level and horizontal and then lift the truck until the axle is off the ground i can take those measurements and calculate how long of a slip yoke i need as well as driveshaft length?

[kbeefy]

Lifting the truck will give you the extended length, I'd be surprised if you can compress the springs.


Coil or leaf?

[ScrapIron]

coil springs, i should have stated horizontal as in a horizontal plane with the pinion. That would be maximum slip yoke compression and the maximum extension when i lift the axle off the ground.

[kbeefy]

That makes it easier. Just remove the springs and then yes, ratchet straps or floor jacks will compress the suspension.

[boywonder]

Originally Posted by ScrapIron

coil springs, i should have stated horizontal as in a horizontal plane with the pinion. That would be maximum slip yoke compression and the maximum extension when i lift the axle off the ground.

Yes full droop to horizontal should get you there.

[ScrapIron]

I would ask these kind of questions over on pirate4x4 but anymore they dont answer questions. They just tend to give you a verbal beating and tell you how stupid you are.
ok, thanks for the help.

[1der]

Originally Posted by ScrapIron

I would ask these kind of questions over on pirate4x4 but anymore they dont answer questions. They just tend to give you a verbal beating and tell you how stupid you are.
ok, thanks for the help.

Yep, this forum is an oasis.