FAQ

Helical LSD's & Metal Plate LSD's
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YCW
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FAQ

Post by YCW » Sun Feb 26, 2017 1:58 am

RWD Limited Slip Differential Noise
Also commonly referred to as "Diff Thump"

The two main causes of "thump", and both have to do with free play:

1) Worn or misaligned (during reinstallation) centre bearing of the propshaft. The only solution for this is to check the centre bearing, and replace if necessary.

2) Backlash. All LSD internals have free play in them (i.e backlash). This free play is a necessity because steel expands when heated. If there was no free play in the gears, they would end up welding themselves together due to the friction, and because of this, there is a compound effect i.e the backlash between all of the internal gears of the LSD (at least 12 of them for most Helical LSD's) plus the backlash of the Ring & Pinion.

What this means is that there is a small "lag" between turning the propshaft, and the axles locking (i.e turning), which results in the "thump" noise you are hearing. It is not noise from the diff that you are hearing, but noise due to the vibration through the chassis and the propshaft/axles locking into place. This is also why it is normally louder during hard acceleration, especially so if you've upgraded any chassis components (mounts, diff bushings, subframe bushings, braces, suspension etc) or upgraded your clutch.

There is no full solution for this, however, warming the diff up (thus reducing the overall backlash) will quieten it down slightly, as will using thicker gear oil. Baby'ing the clutch will also reduce it a lot, however, this will also increase the wear on your clutch.

When stationary and in neutral, your transmission gears are still turning, so when you put it into gear, this locks the gears together causing the propshaft to turn very slightly which causes the "thump" due to the free play between the propshaft turning and the axles turning. As the axles are basically locked into position and can't move due to not enough power being applied (i.e clutch is still disengaged), this is why the thump (i.e vibration) is more noticeable when putting it into gear from stationary, compared to changing gear when moving.

Most quality LSD's will be machined precisely to specification, based on a new set of "unworn" axle stubs i.e if new axle stubs are used, there should be very limited play between the meshing of the splines (there will be a little due to the fact that they are not press fit and just freely slide in and out). If old axle stubs are used, then there will be slightly more play due to wear/compression, and this will contribute to the overall backlash, thus the "thump".

Remember, the thump is not "noise" per se, it is vibration through the chassis.

The above is applicable to ALL LSD's, regardless of type.

No-Load Issue (Torsen/Helical type LSD's only)
A common misconception is that a Torsen type LSD is useless under a no-load situation (e.g One wheel in the air). This misconception has been further perpetuated by the marketing strategies of some LSD Manufacturers (e.g Wavetrac).

Whilst there is truth to this theory, the theory only really applies to vehicles with no Traction Control (i.e This no-load situation does not apply to the majority of cars manufactured post 2000), as your vehicles Traction Control will activate when it senses "slip" or "no-load" from a wheel and will apply a small amount of load via the vehicles braking system. To put simply, in any modern car with traction control, if one wheel does lift off the ground, the traction control will kick in by providing load to the spinning wheel (via braking), thus the LSD will continue transferring torque to the wheel with traction.

For the racer who prefers to switch off their Traction Control, there is a simple solution to the problem; simply balance left-foot braking with right-foot throttle control.

Do your Limited Slip Differentials require any special fluids or additives?
We highly recommend the use of Torco Transmission Fluid/Gear Oils when our Limited Slip Differentials are installed.

While it is not a necessity, we observe a 500 mile fluid change when first installing our Limited Slip Differentials.

In addition, for our Metal Plate LSD's, it is a necessity to do a standard figure-of-8 break-in for 15mins.

If you require further assistance in achieving the best performance possible from your Limited Slip Differential, please don't hesitate to contact us.
MFactory / YCW Engineering (LSD's, Gears, Flywheels, Clutches, Driveshafts, Axles) - http://www.teammfactory.com
YCW Suspension (Mid-Range Coilovers) - http://www.ycwsuspension.com
SWIFT Springs Europe (Exclusive Distributor) - http://www.swiftspringseurope.com
SWIFT Springs Asia (Exclusive Distributor) - http://www.swiftspringsasia.com

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