Pajero Owners Club of South Africa

4ePajero wrote: Any locked axle - solid, independantly sprung, portal, you name it!

Then how come the SFA brigade are always touting the fact that you cant break a sideshaft as one of the ADVANTAGES of a SFA (and they always show the video of that Hummer breaking itself as an example)

It's a mechanical fact. A wheel turning at say 1500 rpm constitutes a lot of kinetic energy.
The worst case is where the is some wheel spin, (worst case is wheel spin on the other wheel on the axle), which means that the vehicle is not travelling at the speed it would have if none of the wheels were spinning.
The moment the spinning wheel touches the ground, the traction increases to the extent where the both wheels have to be 'stopped' to be in sync with the others (because there is now no wheel spin).
All that kinetic energy has to go somewhere - either in momentary wheel spin, or to snap something.
Due to the tolerances in the drive train, the shock loads will snap it.

If there is no wheel spin, the chances are less of breaking anything.

SFA don't have angles and that makes them stronger.

IFS have angles and CV angles to deal with. When the force upon impact goes through the sideshaft or CVs the IFS will more likely twist and snap due to the angles than SFA.

Just thought about something....... Is there any vehicle out there with IFS and quick disconnects for an antiroll bar? That would be awesome!

4ePajero wrote:One of the major advantages of TC over locked axles, is that you are unlikely to break either a CV or a side shaft when using TC.
Said breakages are real concerns when using locked axles. The breakage happens when a (locked) wheel is spinning in the air (with no traction) and then suddenly lands on the ground with full traction - "snap" !

Gerhard, I am not sure if I understand you correctly, but I am afraid I don't see where the traction control equipped vehicle would have the advantage in terms of drivetrain durability. In my opinion,the real danger to break drivetrain bits is with open differentials, where a wheel can spin out completely as it leaves the ground and then experience a shock load as it comes back down. Locked differentials help to eliminate this and traction control helps to alleviate this effect to a great extent as well.

If the diff on an axle is locked, both wheels on that axle will rotate at the same speed. This is exactly what the traction control system attempts to achieve as well. Let's look at a scenario where the vehicle is cross-axled as it heads up an undulating climb, say. Let's just consider the rear axle for simplicity:

With locked axle diff on the rear axle
The two rear wheels are locked together and always rotate at the same speed. One of the wheels always has grip (although it might slip to some degree), so the other wheel can not spin out; it spins at the same speed as the one that has grip and when the situation changes so that it regains grip, it will hit the track surface spinning at a rotational speed quite appropriate for the rate of travel of the vehicle.

With traction control
The brakes will attempt to get the two rear wheels to rotate at the same speed, although this will be achieved in an "average" sense as the brakes are typically pulsed on the wheel without grip; the rotation speed of the wheel without grip will fluctuate in a pulsating fashion and so the torque applied to the wheel with grip will also fluctuate in a pulsating fashion. The result is still that the wheel without grip can not spin out and will spin at the same speed as the one with grip, on average. When the situation changes so that the wheel without grip regains grip, it will hit the track surface spinning at about the appropriate rotational speed for the rate of travel, but perhaps not as close to the correct speed as with the diff lock.

SimonB wrote:Then how come the SFA brigade are always touting the fact that you cant break a sideshaft as one of the ADVANTAGES of a SFA (and they always show the video of that Hummer breaking itself as an example)

The old Series Land Rovers were quite famous for breaking the side shafts (the mechanical fuse ) in their solid axles.

I suppose any design concept can be prone to failure if the component sizing is not adequate. There are very solid idependent suspension based drivetrains out there (Ratel, anyone ).

I'll be the last one to argue with Gerrit Loubser on drive train issues.

The points I tried to make were:

1. With locked axles, there is still the possibility of a wheel spinning (on the ground) whilst another wheel is spinning in the air. When the wheel which was in the air hits the ground, there are large shock loads on the drive train, which can break a diff, side shaft or CV. I therefore question this statement:

   ... it will hit the track surface spinning at a rotational speed quite appropriate for the rate of travel of the vehicle.

   I have seen vehicles with locked axles spinning both wheels wildly (one in the air and one on the ground).
2. TC tends to (virtually) stop the wheel in the air from turning. When this wheel hits the ground, there are no shock loads transferred to the drive train.

I could (again) be wrong, but I cannot recall reading about side shaft / CV breakage under trail conditions on TC vehicles ?


PS Note my choice of words:
"locked axle" vs "locked diff"

"Locked differential is an oxymoron.

4ePajero wrote:PS Note my choice of words:
"locked axle" vs "locked diff"

"Locked differential is an oxymoron.

I don't agree. The differential, a physical component, is still there, but it's function has been overridden by a mechanical lock (the diff lock) rendering the diff locked. Anyway, its a case of poh-tay-toh poh-tah-toh; I am sure we both know what is meant .

4ePajero wrote:With locked axles, there is still the possibility of a wheel spinning (on the ground) whilst another wheel is spinning in the air. When the wheel which was in the air hits the ground, there are large shock loads on the drive train, which can break a diff, side shaft or CV. I therefore question this statement:

... it will hit the track surface spinning at a rotational speed quite appropriate for the rate of travel of the vehicle.

I have seen vehicles with locked axles spinning both wheels wildly (one in the air and one on the ground).

It is possible to spin wheels on locked axles with respect to the ground, but unless the vehicle actually jumps so that the wheels are in the air, it will not be a mad spin-out situation such as would occur with an open diff and one wheel in the air. When the wheel touches down, it will gain grip and torque will be transferred to that wheel suddenly, but the wheel would not be rotating any faster than the other one on the axle.

I do understand what you were saying, though. The key is not the wheelspin, though, but rather the fact that the wheel that was in the air suddenly starts to transmit torque as it contacts terra firma. This constitutes a torque spike. In the case of the traction control equipped axle, the wheel in the air would be transmitting torque against the brake and as it touches down, the brake torque will vanish and will be replaced with grip so that drive torque can be generated, so there might well be less of a torque spike.

On the other hand, the traction control system generates torque spikes (albeit of a smaller magnitude) all the time when it operates. These add up to create fatigue damage in the driveline.

4ePajero wrote:TC tends to (virtually) stop the wheel in the air from turning. When this wheel hits the ground, there are no shock loads transferred to the drive train.

The traction control system will not necessarily stop the wheel in the air completely, because it is trying to match the rotation speeds of the two wheels. Modern traction control systems are quite good at doing this when the rotational speed of the wheels is relatively high, but can bring the wheels to a stop when the wheel speeds are not that high . The brakes will be applied in a pulsed fashion, but the average wheel speed will be the same as that of the slipping wheel on the ground.

4ePajero wrote:I could (again) be wrong, but I cannot recall reading about side shaft / CV breakage under trail conditions on TC vehicles ?

Go have a look at some of the American websites where they use the Land Cruiser 100 Series in anger on tricky trails. Many of the post 2002 models have traction control and they do break CVs and differentials, but then again, with enough force anything is possible.

Both traction control and a locked diff can improve matters for the drivetrain in terms of shock loading as wheels slip and grip. Both of these systems should fare better than a system with open differentials that is driven through an obstacle with enough violence to clear it despite wheels losing and gaining grip.

One of the niceties with traction control systems is that they work on all four wheels for no real extra cost (ABS hardware is utilised), while each axle that is equipped with a diff lock mechanism incurs the cost of an actuator, wiring/piping and locking mechanism.