Under construction This page is still being built and checked.
4x4 drivetrains · Part-time, full-time & multi-mode · Lockers, CDL, low range
Flip the switches on a live 3D drivetrain and watch where the drive goes, through the transfer case and both diffs. Pick the system and the ground on the left, and press the dash switches on the right while it runs.
Drag to turn · scroll to zoom · click the ground under a wheel to change it
An illustration, not a calculator. The grip and pull figures are round numbers chosen to show how each system behaves, not measurements of any vehicle. What this page can't tell you.
No centre diff. In 2H only the rear axle drives and the chain to the front stands still. 4H and 4L slide a collar that locks the chain's sprocket to the rear shaft, so front and rear turn together and drive goes to whichever axle grips. Because they can't turn at different speeds, it winds up in turns on tar: use 2H on hard grippy roads.
A centre diff in the transfer case (planet gears between the front and rear outputs) lets the shafts turn at different speeds, so it stays in 4WD on tar. A plain planetary diff is open; a Torsen one can send the axle with grip a few times what the other holds. Neither helps when one end has nothing to push against. The CDL locks it.
One lever does both jobs: 2H for economy, 4H as full-time 4WD through an open bevel centre diff (fine on tar), and 4HLc / 4LLc to lock that diff for loose ground, in high or low range.
No centre diff: a stack of wet clutch plates, clamped by a computer-controlled electromagnet, feeds the front. In 4A it drives the rear and clamps the clutch as the rear starts to slip, so it's usable on tar. It can only pass so much before it slips and heats up. 4H and 4L clamp it solid, like a part-time 4x4.
Lets the two wheels on an axle turn at different speeds through its spider gears, so the vehicle can turn a corner. The catch: it always gives both wheels the same torque. If one wheel is in mud or in the air, the other gets only what that one can hold, and the free wheel spins.
An open diff with clutch packs that resist one side spinning faster, so it can send the gripping wheel several times what the slipping wheel holds. With a wheel right off the ground there's nothing to multiply, so it acts much like an open diff.
A dog collar locks the side gear to the carrier, so both wheels turn together and whichever has grip can use all of it. The strongest fix for a wheel in the air. It fights the steering, so it's for slow going on loose ground: unlock it on hard surfaces and before tight turns.
Brakes a wheel that starts to spin. The open diff then has something to push against, so drive goes across to the wheel with grip. It gets close to what lockers do at the wheels, but the engine has to push against the brakes too, and the brakes heat up on long climbs.
A reduction gear set in the transfer case (the planetary set at the front of the transfer case in the 3D view). It multiplies the engine's pull at the wheels and slows everything down, so you can crawl up steep or rocky ground with control. It adds pull, not grip: in mud it only spins the wheels harder.
Each wheel can push the ground with at most its grip times the weight on it. The wheels on the ground share the vehicle's weight evenly, so a lifted wheel carries none and has no grip at all.
The drive passes through the centre (the transfer case), then each axle's diff. An open diff can pass on twice what its weaker side can hold. A locked one passes on the sum of both sides. An LSD sits in between. Traction control acts like a locker at the ground, by braking the side that spins.
To keep moving up the slope it needs the slope's pull plus the drag of the surface. If the drivetrain can't pass that much to the ground, wheels spin. If it can but the engine can't pull that hard, it bogs down: that's when low range helps.