Because there's always going to be differences in number of axle/wheel rotations, even driving straight forward, due to slight differences in tire diameter. But the real killer is because there aren't many places you can drive without making turns.

 

The Disco's front and rear diffs are not lock-able (in stock form). They are always 'open'. So any variance in wheel speed right to left is handled by the front and rear diffs.

Logic then says that the 'problem' of turning while the center diff is locked is front to back. If that is true, it must the the difference in the rotational speed between the 'slowest' wheel in front compared to the 'slowest' wheel in the rear. I suppose in tight turns the front does roll much more than the rear and that must be the problem now that I'm thinking it through.

Does that sound correct?

 

Yes, left/right wheel speed variation is balanced by the front/rear open diffs. However, variation in front/rear total wheel speed (driveshaft RPM) can only be balanced in 4WD by an open center differential. A locked center diff will prevent the front/rear driveshaft from spinning at the individual speeds they need and cause bind up in the drivetrain. In dirt this isn't a problem because the wheels themselves can slip on the dirt to relieve bind.

 

Perhaps you will find a better understanding by reading the RAVE specs. The 205R16 tires are spec'd at 28psi front, 38 psi rear. The 235/70R16 are spec'd at 26psi front, 34 psi rear. The difference in tire pressure alone will cause a ratio difference between the front and rear axles, even in a straight line. The difference will be reduced by running the same pressures front and rear, but unless the axles carry the exact same weight the difference will be enough to require a means to keep the driveline from winding up. The good folks at Land Rover were smart enough to provide that with the center differential. One can only assume the specs were determined by testing what worked best in the majority of situations(traction/handling/etc).

You can go ahead and drive your truck with the center diff locked if you wish. This should provide plenty of metal shards in the oil for your witch hunt.

You could also stop trying to argue with something that works and resign yourself to the fact that it's there for a reason and works as intended even if you can't understand it.

I'm not yet sure if you are trying to convince yourself the truck is broken because you don't understand how it works, or if you are trying to convince yourself the truck isn't broken because it is and you are trying to blame the design because you are too lazy to fix it?

 

Higgs, no arguing here. Just trying to light the bulb in my head. I get it now. Differing front to rear rotational speeds was something I honestly (and stupidly) hadn't considered. I appreciate the folks around here helping over that mental gap. Mine works perfectly and I use it as it was intended.