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Lowering can invert the angle that the axle is at. Instead of the transmission being higher then the hub, the transmission can be lower then the hub. Armond noticed this to be a problem with launching and having axles break. Seems to be that when the car is launched, the transmission rocks down. If the differential is above the hub, then all is well. If the transmission starts lower then the hub, and goes lower then that when launching, it might be possible to pull the tripod inboard joint apart and strip it. I am just taking guesses on possible causes. This doesn't seem to be the only reason, though, as Armond broke axles everywhere, not just on the inboard CV.
Here is his writeup from: http://www.armond30.com/adp/smartquestions.html
Why do axles break?
That is a good question. From what has been seen here, axles will break from being twisted at odd angles, from large varaiations in axle angle, and from torsional shock. Most people think that it takes a certain ammount of torque alone to break a cv axle. NOT TRUE. 001 broke an axle with only 114 lb ft of towque to the wheels. 002 broke an axle with only 215 hp to the wheels. There are SEVERAL other factors that contribute to axle failure. First condition that can cause axle failure is excessive drivetrain motion. when 002 broke the driver side axle, the front motor mount bolt had come loose and was no longer holding the front of the drivetrain. This caused the drivetrain to rock backwards suring load, and this moved the axle into a different angle and brought the angle further away from 180 as load increased. the farther away from 180 degrees that the angle is positioned, the more stress is introduced to the axle joints. This extra stress creates heat, which weakens the metal and leads to cracks in heated areas (which include the Constant velocity joint, tri-pod, and the axle shaft area that connects to the joints). The axle grease helps to relieve some of the heat from this, but the grease is only designed to obsorb normal heat produced from slight joint movement. Once the axles create enough heat to fail, they usu fracture the weakest part that has been worn (in some cases the CV ring, in other cases the shaft into the tri-pod). Driving with axles at lower than 180 degrees only introduces more stress on teh joints when load is applied because of the increased axle angle when under load (the front will torque down just as the rear will torque down on rwd vehicles to an extent). This constant wear will make the axles weak in these points and easy to break. Wheel hop (a condition where traction causes the wheel to skip instead of grab or spin) will also cause axle failure. Wheel hop introducs another direction of motion to the axle's normal operational rotation, and makes the rotation irregular. this act can cause the axle to bind, crack the cv, or even fracture the hub end of the axle due to stress. it is almost like hitting the bottom of your axle with a hammer or driving down the road raising and lowering yrou car repetitively. This is why jumping several hills and such can cause drivetrain damage as well. To a certain extent, axles take shock from road conditions, and some of this shock is transferred to the drivetrain via. the motor mounts. if the motor mounts are too stiff, you will eliminate the chances fo drivetrain angle causing breakage, but you could also increase the risk of fracture due to torsuional shock. it's like thumping a flexi-straw stuck in pudding compaired to thumping a flexi-straw stuck in cement. the straw in the pudding will flex LESS than the straw in cement because pudding takes some of the shock. Same principle with solid motor mounts. Also, heavy wheels can reduce the life of axles. turning wheels takes torque. The farther out the weight is distributed on the wheel, the more torque it takes to turn it. This is one of the reasons why cars with larger wheels usu. have axle problems. havign axles not designed ot supprot the weight of your car will lead to failure for must fo the same reason. CV axles are not designed to have a LOT of torsional play. In most race cars, the impact of sudden torque is absorbed in the sidewall of the tire. As the sidewall wrinckes, less stress is applied to other crucial components like the drivetrain. On cars with stiff sidewalls, this creates a problem for instant torque transfer. If the tire tread is made of a sticky compound, the wheels may not spin, but all of the shock will be taken on my the drivetrain, and most of the force will be taken on by the weakest link, which is in most cases cv axles. if the car does not have good traction, teh tires will spin, and little is damaged. If the vehicle creates enough power and torque, and sufficient shock is NOT removed from the drivetrain via suspension trave, sidewall flex, tire spin, or drivetrain motion, all of the shock torque is sent to the axle. Most cv axles cannot take a large amount of angular momentum at once. This usu. leads to fracture of the weakest part of the shaft. in most cases, this has been found to be the end going into the transmission, as most are not designed to encounter extreme shock, but to transfer it. 001 has broken 2 of these. the ammount of power nessacessary to break an axle in this manner will varry, but unless an aftermarket-high strength differential is used, in most cases, this event will lead to differentaial failure first, as it then becomes the weakest link. Using a higher stregth alloy usu. solves this problem, unles the axle is put into a high load bind. Unfortunately, there is no real answer as to "how much power does it take to snap and axle". the life of a driveshaft or an axle is dependant on all of these variables. If the vehicle is safe on all of these issues, most axles will last for a long time. if the vehicle meets all of these standards and axles do break in noticabel areas, it is mroe than likely due to weak material.
Hope this wasn't too long to read, and helped out...
Thanks Armond!
-=B-=
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