Limited-slip differential

A limited slip differential is to prevent a particular differential gear in a multi-lane motor vehicle with the purpose that the wheel spins with poor grip and the other - and thus the vehicle as a whole - no longer receives drive torque.

In extreme cases, leading to the wheels shafts are rigidly coupled together, the differential effect of the transmission is canceled, it will be blocked. Curves are complicated, because the wheel can not rotate at different speeds, the outer wheel due to its longer path faster than the inner. A small slip of one or both wheels is enforced mentioned design of this differential lock. Coupling is carried out for example by means of a claw clutch, which is operated at a standstill.

Younger, locking differential called types act independently and are correspondingly more complex. Each wheel is given a minimal large driving torque. About the wheel with good traction and its minimum torque of the drive of the vehicle is maintained. The speed compensation is not locked, but by means of a slip clutch that heats up, if necessary, braked. The locking differential is therefore in Switzerland often called differential brake.

The differential lock is of the preamble based limited slip differential, such a 100% locking action and the normal (open) differential gear with such a 0% locking action.

• 2.1 types
• 2.2 locking value and characteristics

Purpose of a locking differential

There are a number of situations in which a limited slip differential is advantageous:

• Starting under difficult conditions, terrain ride: The locking differential manages the drive torque from a slipping wheel to the wheel with better traction, thus improving traction.
• Straight: A locked axis runs better straight as an unlocked. Therefore, the effect is reduced, for example by crosswinds or the Bernoulli effect during fast driving past lorries.
• Handling during fast cornering: At high transverse acceleration, the inside wheel is unloaded and loaded the outside wheel. This causes the inside wheel slips stronger until it turns just as fast or faster than the outer turning wheel, the locking differential now prevents that the inside wheel can spin and thus the flow of power interrupts on the road. Besides the higher curve speed limit, the vehicle understeers less, because the uneven force distribution to the wheels ( the outer wheel now transmits more torque) generates a yawing moment around the vehicle's vertical axis, favors a tightening ( oversteer). Is the blocking effect in push mode exists, then the power flows are reversed, the resulting yaw moment causes an increased understeer, thus stabilizing the vehicle. This characteristic is used specifically for racing.

( Differential locks so ), and to some viscous couplings % locks in question - as traction differentials are mainly 100. In order to improve the handling, to torque -sensing locks recommend with low locking value.

Demarcation for traction control

Limited slip differentials are not to be confused with traction control, but normal differential gear are part of such traction control, and by the large spread of electronic stability programs ( ESP) offer traction control especially with torque -sensing limited-slip differential advantages.

The traction control systems are due to the translation from English ( Traction Control) also called traction control ', vendor-specific designations are ASC or ASR. You are now part of the electronic Fahrstabilitätssprogrammes of the vehicle ( ESP). These solutions mimic partially the effect of the locking differential after, but also have disadvantages as compared to locking differentials, these systems slow down engine power to a considerable extent away, which is disadvantageous for weakly motorized vehicles in difficult terrain and sporting enthusiasts.

Traction control, acting on a brake and engine intervention partially ( power reduction ), can therefore be seen as a precursor to cost- slip differential. Their advantage over simple locking differentials is located next to the lower costs that they can react appropriately by the electronic control to ensure on the one hand traction and stability on the other hand.

Disadvantages of simple (mechanical ) locking differentials

Simple locking differentials respond to different rotation speeds and torques, regardless of the causes. In some situations, therefore resulting disadvantages.

• Many types (particularly types with friction ) are subject to heavy wear, so that the blocking effect decreases with increasing mileage and eventually practically no longer exists.
• For certain types a separating clutch is often necessary to make it with an anti -lock braking system ( ABS), ESP compatible, because otherwise the feedback between individual wheels makes it difficult to control or impossible.
• Block sensitive to tires with different rolling circumference ( for example, by varying air pressure, different tread depth ), because they lead to differential speeds and have increased wear. Thus, the driving speed and driving distance is limited with Noträdern.
• When cornering with minimal wheel slip can lead to increased understeer, because here the required speed differences is counteracted.

Electronically controlled locking differentials can react appropriately and avoid these disadvantages.

In open axle differentials transmit both wheels the same amount of torque, thus no yaw moment generated about the vertical axis of the vehicle. When locking differentials, it can also lead to an asymmetric distribution of the Achsantriebskraft. This results in a yaw moment, the driver has to compensate for possibly with counter-steering. This additional effects arise:

• In a Vorderachssperrdifferential the traction difference for the driver on the steering wheel can be felt. Therefore, only a small locking value is chosen for reasons of comfort at the front mostly.
• For vehicles with Hinterachssperrdifferential sudden, unilateral smoothness can surprise the driver. Without timely counter-steering, the yaw moment causes a tightening of the vehicle towards the smoother side of the road and thus favors a spin.

Fitting

There are - as in ordinary differential - two applications for a limited slip differential:

• Between the two wheels of an axle as the axle differential;
• Is installed in an additional transfer case between the axes as a central differential, center differential and center differential, which behind the normal exchange gearbox (manual, automatic, sequential gearbox, ...).

Mechanics of the lock

Types

From the mechanical side, locking differentials can be divided into several types:

• For torque -sensing limited-slip differential ( Torsen such as A / B / C, GKN Super LSD, Helical LSD, Sensi torque as well as some limited slip differentials and helical jaw couplings ) depends on the blocking effect on the transmitted torque. In principle, will always be in the x -fold moment the other side to the side with the lower slip. The ratio of x is then referred to as the TBR ( see below).
• For speed -sensing limited-slip differential (especially viscous couplings, centrifugally operated locks and manufacturer-specific design principles such as GKN ViscoLok or Fuji AYC ) depends on the blocking effect of the differential speed of the wheels. The greater the difference in rotational speed is, the greater the blocking effect. ABS and ESP need to be adapted frequently an additional separating clutch or freewheel is necessary. Variants such as Powertrax Lockright or the No-Spin differential allow independent 100-percent locking action for one or both wheels.
• For a fixed value lock, the lock is biased so that is already a barrier effect even without speed difference and without drive torque. This simple form of lock is almost exclusively used in combination with other types of locks and there often realized by biasing with a feather.
• Wherein the electronically controlled locking differentials, the effect and the use of the boom of a vehicle dynamics control in the vehicle is controlled. Currently, the spread is still low (for example, Porsche Cayenne / VW Touareg, Toyota Land Cruiser, Land Rover Discovery, Ferrari F430 ), but in the future the greatest market potential is here assumed.

Mechanical locks are always a combination of these basic types.

These combinations also the ASD Mercedes belongs. It is a torque-sensing limited slip differential - forming lamellae, which can also be locked completely non-positively via an electronic control. Thus, it provides efficiency advantages over the brake intervention traction control, which means less drive power is lost. Registered via the ABS wheel speed sensors ASD slip (speed difference) to the driven rear axle and applies the integrated multi-disc locking differential ( basic locking value about 30%) in Hinterachsmittelstück hydraulic pressure. ASD locks at this moment 100% and makes the locking differential, a power differential lock. A yellow warning light in the form of a warning triangle in the speedometer shows the activity of the system. During braking, the full blocking effect is immediately canceled. The system operates at speeds up to about 30 km / h and was used in the 4 -, 5 -, and 6-cylinder models of the series W 126, W 140, W 129, W 124, W 201, as well as all-wheel drive 4MATIC models of the W 124

A special form are differentials with non-constant translation, for example, are used in construction machinery. By periodically fluctuating ratio of the differential gear, different torques are generated on the wheels.

Locking value and characteristics

A normal differential without locking effect is also referred to as an open differential. Ideally, free to rotate both wheels, the friction losses in the open differential would then be 0 ( in practical terms also behave like open differentials torque -sensing locks with low locking value ). At a limited-slip differential trying to decrease the efficiency of the open differential.

The characteristics of a barrier is determined by the locking value ( in %) and torque-sensitive limited slip differentials via the torque ratio ( ' Torque Bias Ratio ', TBR ). Constant value of the barrier or the TBR is only pure torque sensing differentials. Considering this, the torque transmitted to the left and the right wheel. With an open differential, the torque right ( MR) can not be different than the torque (ML ) at the left wheel, in the ideal case of frictionless open differential of locking value = 0% and the TBR = 1 For torque -sensing differential gears with symmetric (50: 50) torque split is the TBR equal to the reciprocal of the stationary transmission efficiency. For example, a differential on with an efficiency of 50 %, a TBR of 2.0.

The general formulas for symmetric differentials are:

Formulas for converting between the two values ​​:

Locking value and TBR are constant only in torque-sensitive locks and read- locks.