A Caterham 420R is fitted with a Titan made Limited Slip Differential (LSD) unit designed for a BMW differential. The Titan LSD is a plate differential, meaning it contains a series of clutches which allow some slip when desired and lock when needed. These clutches wear over time / mileage based on the use and the material the clutches are made from. If the clutches wear excessively, other internal components in the LSD can fail. Unfortunately, BMW doesn’t offer spare parts for its differential; these parts can become damaged if the internal components in the LSD fail.
The good news is there is a simple way to measure and monitor the wear rate for your LSD clutches, by monitoring the LSD preload. The LSD preload is the torque needed to turn one wheel while the other is stationary. This post show how to do this.
Before you start, you need to know if your torque wrench can read anti-clockwise torque. I know mine can, but that’s not always the case. If you are unsure, measure the preload on the RHS rear wheel where the preload is measured in a clockwise direction. My pictures are of the LHS rear wheel, because I have more space to take the photo on the LHS, and I also know my torque wrench reads anti-clockwise torque, which is the direction used for the LHS.
The process starts by chocking the front wheel, especially in my garage which is built on a bit of a slope. Next, while the car is still on the ground, loosen (but don’t remove) the one of the rear wheel.
With a trolly jack under the centre of the DeDion tube (a small piece of carpet is the perfect way to protect the paint of the DeDion tube) lift the rear wheels just off the ground.
Remove one of the rear wheels, in my case the LHS rear wheel (the side I am going to measure). Ensure the handbrake is off, and the car is in neutral. Now lower the jack slowly, so the other wheel touches the ground, as soon as it does lock the jack off again it it is will taking the weight if the car.
My torque wrench can read “peak” torque, I also set it to read lb foot, because the advised setting I have are in lb foot. The socket is the same 41mm (or 42mm) used to torque the rear hubs (see Tightening the rear hub nuts and De Dion Ears), but this time I am using my 3/8″ torque wrench, because it is more accurate at lower torque than my 1/2″ torque wrench, so I have a 1/2″ to 3/8″ adaptor.
Using the torque wrench, turn clockwise for the RHS (or anti-clockwise LHS) and measure the torque needed to turn the hub / turn the prop shaft. The reading on the torque wrench is your differential preload.
If your wrench cannot read peak torque, just monitor the display as you turn the wheel and note the highest number. Mechanical (non-digital) torque wrenches can also be used, gradually increase the setting until it no longer clicks – the last click is your “highest reading”.
My 1000 mile 420R Titan LSD recorded a 46 lb foot preload.
The general rule of thumb – based on comment from a specialist from Road & Race Transmissions – is to expect 40 lb foot when the differential is new (often set a little higher like mine). This will reduce as the clutches wear. At 20 lb foot it’s time to speak to a differential specialist about a rebuild.
I have aways used Road and Race Transmissions in Kent https://www.rrtransmissions.com for any gearbox or differential issues. They were also used by Caterham Cars when my previous 360R had a gearbox warranty issue.
Running a preload below this 20 lb foot reading risks internal damage to the LSD and the complete differential unit.
All that remains is to lift the car again with the jack, and refit the rear wheel, refitting the wheel nuts and tightening the before lowering the car off the jack. Finally, with the car on its wheels, torque the rear wheel to 80Nm. I will check this again after 100 miles. Reapply the handbrake and remove the chocks.
- Trolly Jack (with carpet to protect paint)
- Digital 3/8″ drive torque wrench
- 1/2″ drive torque wrench
- 19mm 1/2″ drive socket
- 41mm 1/2″ drive socket
- 1/2″ drive extension
- 1/2″ drive ratchet
- 1/2″ to 3/8″ drive adaptor