Unequal length driveshafts, 4x4

[love machine]

I was toying with upping the power of my Suzi Jeep, I am aware that the axles aren't unimog items but have central diffs and equal length driveshafts. This to me makes me think about equal loads, I can't remember which ones snap (long?) usually, but if you blow a shaft on a Landrover or Suzuki SJ jeep, it's 99% certain that one side goes.

Anyone got an insight into the physics of snapping 4x4 driveshafts (no stupid comments please). I'm trying to work out if the unequal length situation makes a weak point into a snapping point or whether I'm just going to smash them anyway. I'd like to keep my central diffs but I could always cut an unequal length axle down and use 2 short shafts (the LJ is seriously narrow).

[steve_D]

I suspect the short shaft would fail first. Any shaft will twist under load and absorb shock loads. A longer shaft will twist more before failing.

Steve

[WLAcopilote]

In my experience it is the long side that fails first. I would suggest because it is longer more stress can build up (i.e. more twist) as power is applied and removed from the wheel at the end of it thus increasing the shock loading on the splines / spline to shaft body interface. The 24-spline Ashcroft shafts on the old Bowler racer suffered more spline twisting on the "long" side front and rear as did the "stock" 10-spline ones on all the other Rover axles I have dismantled.

[speedy_thrills]

love machine said:

I can't remember which ones snap (long?) usually, but if you blow a shaft on a Landrover or Suzuki SJ jeep, it's 99% certain that one side goes. Anyone got an insight into the physics of snapping 4x4 driveshafts (no stupid comments please).

I have to admit that I know very little about drive shafts but I hope that structural analysis applies here. I think what you need to be most concerned about are:
1. Torsional shear stress which can be calculated using:
Where T is torque (presumably measured as part of your revised engine configuration, it will be your peak torque value I expect), r is the distance between the axis of twist and the outermost fiber (in your case I suppose this would be the radius from the center of the shaft to the very outside of the drive shaft) and J is your Polar moment of inertia.
2. If the drive shafts change in length is restricted you may need to do some calculations to make sure that you do not apply a large axial stress through the strain (change in length of shaft) that occurs when you plant your foot on the throttle. I have a feeling one end of drive shafts are usually left such that they can expand and contract however so I think you are safe.

Clear both long and short shafts have equal chance of shearing due to excess torque but what I described in part 2. above (actually more technically described as Torsional deformation by engineers) because of the extra twist enabled in a long shaft it may place a greater axial strain making it appear weaker.

[danielgray]

From my experiance of my brother and his mates driving old landrovers off road is that the short shaft breaks dut to too much throttle usage, they have broken lots of the transmission because they have ford V6 petrol engine in one, too much power for 60's landrover.

[correlejco]

The stress in long or short shafts is identical. The strain is much greater in the long one, but it's stress that breaks things, not strain. If you don't understand the difference, read a text book.

There's a possibility that a vibration may set up in the drive shafts which can increase the stress. This might happen if the resonant frequency of the shaft coincides with some frequncy from the gearbox or final drive (or even from a tyre). But this is unlikely and would need serious and difficult analysis.

Hope this helps