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Frequently Asked Questions
Universal joint or CV joint - which goes where?
A universal joint transmits torque through an angle at a fluctuating speed; a constant velocity joint does it at a steady speed, which is what steered driven wheels require.
A single universal joint speeds up and slows down twice per revolution when working at an angle. The effect is small at a few degrees and severe at large ones.
Propshafts use pairs of universal joints phased so the second cancels the first, which works while the angles stay modest and equal.
A driven front wheel steers through large angles, so the fluctuation would be intolerable - hence the constant velocity joint, which maintains an even output speed at any angle within its range.
CV joints are also used at the inner end of driveshafts, where a plunging design allows the shaft to change length as the suspension moves.
The practical consequence: universal joints tolerate less angle and more abuse, CV joints tolerate more angle and less contamination.
Why do boots matter so much?
Because the joint inside is fine until the boot splits, and scrap a few thousand kilometres afterwards.
The boot retains the grease the joint needs and excludes water and grit. Both functions are lost the moment it tears.
Grit is the killer. A joint running on contaminated grease wears rapidly and irreversibly, and no amount of later cleaning recovers it.
A split boot found early can be replaced with the joint cleaned and repacked, which is a fraction of the cost of a new driveshaft.
Boots split from age, ozone and heat as much as from impact, so they need inspecting on vehicles with low mileage as well as high.
Include boot condition in every routine inspection and treat a split one as a job for this week rather than next service.
What does a clicking noise on full lock mean?
A worn outer constant velocity joint, and few automotive symptoms come closer to conclusive.
The click is the worn joint's balls skipping in their damaged tracks, and it appears under the combination of steering angle and drive torque.
It is usually loudest on full lock and under acceleration, and it comes from the side that is on the outside of the turn.
By the time it clicks, the joint is beyond repair - the question is only whether to replace the joint or the complete driveshaft.
A complete shaft is often barely more expensive than a joint once labour is counted, and it renews both joints and both boots.
A vibration under acceleration in a straight line, by contrast, points at the inner plunging joint rather than the outer one.
How is a driveline vibration diagnosed?
By whether it tracks road speed, engine speed or load - each points at a different part.
A vibration that changes with road speed and is unaffected by gear or throttle is rotational: propshaft balance, a bent shaft, a worn centre bearing or a wheel.
One that appears only under acceleration and disappears when coasting points at joint angles under load, worn universal joints or tired engine and gearbox mounts letting the driveline move.
A vibration that tracks engine speed regardless of road speed is not driveline at all.
Propshaft balance weights are small and are sometimes lost; a missing one produces a speed-related vibration with no visible wear.
Always check the simple things first - wheel balance, a build-up of mud on a wheel, and the centre bearing - before removing a propshaft.
When is a limited-slip or locking differential worth having?
Wherever the vehicle regularly has one wheel on poor surface - site work, agriculture, recovery and off-road use.
An open differential sends equal torque to both wheels, which means the wheel with the least grip determines how much torque either gets. One wheel on ice or mud stops progress entirely.
A limited-slip differential resists the speed difference, transferring some torque to the wheel with grip while still allowing normal cornering.
A locking differential forces both wheels to the same speed, which gives maximum traction and must be disengaged for road use because it fights every turn.
Electronic traction control achieves a similar result by braking the spinning wheel, and on many modern vehicles is fitted instead.
Limited-slip units often require a specific oil with a friction modifier; using standard gear oil causes chatter on turns.
Can differential work be done in a general workshop?
Removal and replacement yes; setting up a crown wheel and pinion is specialist work with measured preload, backlash and a contact pattern.
The mesh between crown wheel and pinion is set by shims to a specification measured in fractions of a millimetre.
Getting it wrong produces whine, rapid wear and eventual failure, and the error is not visible on assembly.
Bearing preload is set by crush sleeve or shim and has its own specification, measured as rolling torque.
The contact pattern is checked with marking compound and read against reference patterns - a skill rather than a procedure.
For most fleets the practical answer is an exchange unit or a specialist rebuild, with the workshop doing removal, refitting and the oil change.
What maintenance does a drivetrain need?
Oil changes at the specified interval, greasing where grease points exist, and boot and seal inspection - most of which is inspection rather than replacement.
Differential and axle oils degrade and pick up wear metal. The interval is usually long but it is not indefinite, and severe duty shortens it substantially.
Wading, frequent trailer work and high loads all warrant more frequent changes than the standard schedule.
Greasable universal joints exist on many commercial vehicles and need doing on schedule; sealed ones cannot be greased and are replaced instead.
Breather valves on axles are routinely overlooked - a blocked breather pressurises the housing and pushes oil past the seals.
Boots, seals and the condition of the oil when drained tell you more about drivetrain health than any other check available.