Rear Axle and Differential

Date: 26 Oct 2021

Issue

My knowledge of rear axles is limited.

The rear axle comprises of a differential unit that transfers  power from the prop shaft to each half shaft. The differential performs two functions:

• Changes the ratio of revolutions from the gearbox to those of the wheels (creating torque)
• Balancing rear wheel revolutions when going around corners to prevent rear end ‘crabbing’

A differential unit is required to balance wheel speeds when cornering i.e. enable the wheel on the outside of the corner to rotate further than the wheel on the inside of the corner. Without a differential cornering becomes a complex and potentially dangerous affair!!

Issues usually manifest themselves as oil leaks or noisy whining sounds when the car is in motion.

Diagnosis

Oil leak – oil found in rear brake drum rendering the brake useless. This indicates the half-shaft bearing and seal are faulty (on the side where the leak is)

Oil leak – from the differential unit could be a blown gasket but is more likely a very worn unit

Whining noise – constant sound irrespective of rev’s is likely half shaft bearing wear

Whining noise – changes with rev’s of engine is likely something worn in the differential.

None of the above are good things – plan a repair as soon as possible.

Note: do not do what I did and ignore this and set off from Berkshire to Scotland. We got just across the border before coming to a shuddering stop.

Repair

Differential (the ‘diff’)

Problems often first appear as a new sound when the car is in motion. Changing a diff requires removal of the half shafts – to get these out the rear wheels and brake assemblies need removing. Then a shaft puller is needed (if you don’t have one of these find a good mechanic). With the shafts out, next remove the prop shaft from the diff. Now remove the diff from the rear axle – there is oil in there so use a tray to catch it. Replacement is reversal of removal.

Note: the above short paragraph describes about a days work and some level of swearing.

Half shafts

These transfer power from the differential to each wheel.

The half shafts can break, especially in cars being exercised around corners, this is fairly rare in standard cars, but for the sportier or uprated models the half shaft tensile (sheer) strength may be exceeded. No half shaft = no drive.

Half shafts are more prone to oil seal failure or bearing wear. Fixing oil seals and bearings is a relatively simple job for any mechanic with a sliding hammer and bearing press tool.

Bearing wear is simple to detect, a constant whining or grinding sound will be heard in the cabin whilst moving along (it is possible to distinguish prop shaft bearing from half shaft bearing wear as prop shaft wear is often accompanied by a clonk or jolt when moving away form stationary).

In my experience don’t leave a grumbling bearing unattended to unless you are a member of a recovery organization, in my experience total failure is always in the middle of the night, on a motorway in the rain – you have been warned. When a half shaft bearing collapses the half shaft presses on the brake shoes and jams the rear brake on which get smelly and hot after a few miles. When a prop shaft bearing fails the car shakes to hell and back, eventually either you or the car just gives up.

Half shaft oil seals

It is important to fix half shaft oil seal leaks when they start. Left unattended oil will eventually leak into the brake drum and ruin your rear brake shoes (and probably stop the brakes working effectively).

To change an oil seal requires a sliding hammer puller to pull the half-shaft and seal out of the axle casing. I used a local garage to change my leaking seal as once the half shaft is removed the oil seal needs extracting and replacing on the half-shaft itself.

The workshop manual best explains how to sort this out.

Parts

Approximate prices from: 2 Jan 2023

Part	Description	Supplier	Cost
ST45	Steering & Suspension: ST45 – Rear wheel bearing & retainer kit (per side)	Club Spares	£18
ST47	Steering & Suspension: ST47 – Rear axle oil seal (per side)	Club Spares	£2.64