Quote:
Originally Posted by ex670c What you have mentioned is absolutely correct, but transmission windup does not disqualify the SOTF feature, makes is more difficult to use. |
One last time, let me try to explain this more clearly:
First think about this question:
What makes shift-on-the-fly possible or impossible in a modern transfer case or gearbox? What makes any gear-shifting possible or impossible?
The answer is “Shaft Synchronization”
What impedes synchronizing your front propeller shaft with your transfer case?
In two-wheel-drive mode in a modern part-time 4x4 with freewheeling front wheels, the front propeller shaft is stationary, while the transfer case is turning.
The front wheels are spinning freely, leaving the rest of the system upto the transfer case, stationary – not consuming fuel, nor wear-and-tear.
The front wheels have manually or automatically lockable hubs, or automatically lockable half-shafts. They are in the freed state.
To shift into four wheel drive, the system must
- engage and spin up the half-shafts, front differential and propeller shaft
- wait for the system to match the speed of the transfer case
- engage the transfer case dog-clutch, so that power is transmitted to the front-differential.
Any and every manual transfer case becomes “shift on the fly” if, you fortuitously, have your front prop shaft turning, because the front hubs are
- either permanently bolted to a steel flange as in the Mahindra,
- or with locking hubs, manually locked by you.
For people with Jeeps with free-wheeling automatic hubs, pneumatic and electronic shift-on-the-fly mechanisms come with pros and cons.
Pros: You don’t have to get out in the mud and muck to lock your hubs. The integrated "shift-on-the-fly" system does that for you. While the hubs are free, you have better fuel economy and reduced front axle wear-n-tear.
Cons: They need special maintenance. After some age they are fraught with vacuum leaks or stubborn solenoids.
Quote:
Originally Posted by ex670c The synchro is not mandatory in a SOTF mechanism. |
This is a completely contrary opinion to what the world of Jeeping thinks!
Quote:
Originally Posted by ex670c The real cause for transmission windup is the lack of differential action in the G-Box-Tcase. |
The native geometry of front-wheel steering causes a difference in speeds between the rear differential crown gear and the front differential crown gear.
When driving on unyielding terrain such as a hard or rocky surface, the driveline cannot reconcile these speed differences.
If a single wheel could lose traction and spin momentarily the torque-buildup would be defused.
This, IMHO, defines the "transmission windup" phenomenon.
I will explain with a diagram if Team-BHPians would like that!
That being said, even an open differential will not solve the problem of optimum torque distribution.
The Range Rover solves the problem of distributing torque without transmission windup with a viscous limited-slip center differential.
Expensive Toyota/Lexus 4x4s use a torque-sensing "Torsen" limited-slip center differential. Some other high-end 4x4s use a driver-lockable center differentials.
Quote:
Originally Posted by ex670c The Dana/Spicer 18 T-case is recognised as a SOTF capable world over. |
We disagree! This is just as a play with words, for the reason mentioned above, but ignorances the value of a true SOTF mechanism.
The Spicer-18 is a prehistoric transfer case, that comes from before the times of locking hubs, leave alone automatically locking hubs.
Because your hubs are permanently synchronized -- permanently locked with six bolts per hub, your Spicer-18 transfer case can be shifted-on-the-fly.
But please understand, that it has no SOTF synchronizer so, it will not
- engage and spin up the half-shafts, front differential and propeller shaft
- wait for the system to match the speed of the transfer case
In most Mahindras, the half-shafts are permanently engaged.
In GTO's Mahindra Classic, he must stop the Jeep and manually lock the hubs.
The fuel-wasting two wheel drive of the 4x4 Mahindra with bolted hubs, turns the half shafts, the crown and its planetary cage and the pinion and the front propeller shaft all the time. Not to mention the avoidable wear-and-tear on the front drive shaft “U” joints and the front differential. You lose the fuel economy and reduced wear-and-tear in two-wheel-drive possible with a true SOTF with autolocking hubs. You are deprived of the benefit of true two-wheel-drive.
Calling a Spicer-18, an SOTF mechanism is like saying a cheap fixed focus camera performs like an autofocus, because the subject is luckily at infinity.
Or like saying a stopped clock shows the correct time once in every twelve hours.