My current favorite combo is an automatic FRONT locker and selectable REAR locker.
There are a lot of little intricacies that come into play with locking differentials when you start having to actually manuouver the vehicle in situations when open/open is not enough traction. This is what I call the '3-wheel drive' problem. ( from this point forward I will refer to lockers in this format 'front/rear' to save a little time ). On a 4 wheel vehicle with only a steering front axle, the front tires are going to be pointing in the direction you want to go, and the rear tires are always going to be pointed directly forward. No matter what you do to the rear axle for a differential or locker, it is going to want to push you forward. If you run the math on all 4 tires when they go through a corner, the REAR tires actually have the largest left to right speed differential on the vehicle. The front tires have some, but they are turning a LARGE radius than the rear tires. This issue can get much worse with varying degrees of steering angle and wheelbase. There is also a front to rear axle speed difference, and this is why full time 4wd vehicles have center differentials.
If all you need to do is go in a straight line, lock everything up and make everything turn the same speed. This is basically what happens on 4wd vehicles with front and rear lockers. When we start to have issues is when we need to balance the ability of the vehicle to turn AND have traction at the same time. Some modern vehicles can do this ok with traction control systems that brake the wheel, but there are still a lot of limitations.
I am going to try to break down the common issues I see with a LOT of current locker systems including factory systems like the Rubicon, old fzj80s, etc.
#1. Should be be locking the REAR differential first if the rear axle is always trying to push us straight? This tends to GREATLY increase the turning radius of the vehicle. It also typically causes the rear of the vehicle to slip downhill when you are having issues. We see this issue with all rear lockers typically, and especially in vehicles with only a rear locker. I am proposing if at all possible, that the FRONT locker be locked first.
#2. Most maneuverability issues, including that bound up can't turn feeling we all know too well, is coming mainly from the REAR locker. We need to be careful to separate steering force issues. Mainly being how hard it is to turn the wheel, vs how the vehicle is actually maneuvering. Most steering force issues I see are actually from scrub radius induced issues especially with a spool ( or locked selectable ) where there is no mechanism to help that issue. It isn't that the vehicle doesn't want to maneuver really, it is that you might not have enough force to turn the wheel. What I have found is that if you get the REAR locker off when you need to change directions, the steering force goes way down. This is true for a spool ( or locked selectable ) but is AMAZINGLY better if you happen to have a front automatic locker. If you unlock the rear axle, pretty much every single bad front autolocker trait goes away. With the front locked, the front tires are going to be pointing in the direction you want to go. Conversely, if you have the rear locker only, it will be pushing you wide and downhill typically.
#3. The main issue with every selectable locker we commonly see, is that they typically don't like to unlock until they are unbound. When you are trying to manuver, they are typically going to be bound up, espeically in a front axle application with a decent amount of scrub radius. An automatic front locker, especially when not getting pushed over by a rear locker, is happy to lock and unlock happily on it's own using it's ratchet function. The action of steering and the scrub radius can actually make the locker ratchet. This works especially well at slow speeds where you need it when trying to technically maneuver a vehicle.
What I have found, is that in a lot of cases, that an auto/selectable can work even better in a lot of situations than a selectable/selectable setup. You end up having to worry about pushing buttons a LOT less. You will notice a lot less overall bound up and being pushed through corners where you need 4wd vs the other options. I will say, if you drive a LOT of snowy/icy roads at freeway speeds in 4wd at speeds above 45mph that a selectable/selectable system may be a better fit. I have put a LOT of miles on my auto/selectable system and don't really find this to be a huge issue however. If you have full time 4wd I would also suggest that selectable/selectable setup will be better.
In the end, one of the major things that I have found is that we should look at using the front locker first if possible. You will feel a little more steering force feedback under power, but the vehicle will generally maneuver a lot better in 3wd when you need to change directions. Get that rear locker off as soon as you don't need it. I have found that I can do very difficult trails with very little rear locker use. I notice a big difference when I try the same things with rear locker only and the front open. I do own both these combos.......auto/selectable and selectable/selectable. When I build the next vehicle, it will be auto/selectable. I will likely add rear left/right/both cutting brakes to further decrease rear locker use and increase maneuverability. Beyond that, I will be using a transfer case that allows front wheel drive only also to provide even more ability to pull the front end around things.
