Hydratrak differential
Discussion
A diff is just a device that allows the two driven wheels to rotate at different speeds. Most diffs are 'open' meaning there's nothing but friction preventing the wheels from moving relatlve to each other, and both wheels nominally see equal torque.
The problem with this is that the torque applied to both wheels is limited by the wheel with least grip. For example if one wheel is on ice there will be no traction from either wheel even if the other wheel is on something grippy. You get a similar effect accelerating out of corners where the inside wheel has a lot less weight on it.
The point of a limited slip diff is to restrict this relative movement between the wheels so that torque is transfered from the faster wheel to the slower one. If you imagine pulling away with one wheel on ice, an LSD would transfer torque from the spinning wheel to the gripping one so that you still get some traction.
There are a couple of common types of LSD. The most common one uses a multi plate friction clutch between the two sides. The amount of torque transmitted between the two sides depends on the clamping force on the clutch. The clutch is pre-loaded by a spring giving you a certain amount of transfer, and the diff is designed so that torque applied to the prop shaft increases this clamping load so the diff gets stiffer the more power you apply to it.
The second type uses a viscous coupling between the two sides. Viscous diffs transmit torque according to the speed difference between the two sides. They act like an open diff when the two wheels are moving at similar speeds, but if either wheel starts to spin up the viscous coupling will gradually transfer torque to the other wheel. The key point is that no torque transfer takes place unless there is a significant speed difference between the two sides.
From the driver's point of view these two types behave very differently. Any time you drive round a corner, the inside wheel tried to go slower than the outer wheel. With a friction diff, the friction clutch fights against this and puts extra torque onto the inside wheel. This means that the inside wheel starts to slide relatively early. But because it is connected to the outside wheel, it can't slide by much. This results in a car which breaks away early but very progressively as you approach the limit of grip, and it always produces similar levels of wheel spin on both wheels which means you get similar slip angles and similar levels of grip.
Taking a viscous diff round a tight corner, the diff lets the inside wheel slow down without putting extra torque on it, so that wheel doesn't start to slide as early. When it does slide, the diff will not transfer any torque away from it until there is a lot of slip so you get a big imbalance between the wheels: the outer wheel is mostly pushing sideways and the inner wheel is mostly pushing forwards. Because the inside wheel has a lot of slip it also has less total grip as well less lateral grip. This results in a car which breaks away relatively late, but when it breaks away you suddenly let a lot less lateral grip and a lot less acceleration, together with lots of smoke from the inside wheel. From a driver's point of view this is great if you only drive it gently, but absolute pants if you ever approach the limit of grip. It's like a cross between a 'proper' friction LSD and a plain open diff.
Perhaps in the right circumstances viscous diffs have their advantages. But my experience is that they are a lot worse to drive than a friction LSD, and I would take £1000 off the value of a car with one fitted. That's roughly what it would cost to chuck that pos in the bin and fit a proper diff.
The problem with this is that the torque applied to both wheels is limited by the wheel with least grip. For example if one wheel is on ice there will be no traction from either wheel even if the other wheel is on something grippy. You get a similar effect accelerating out of corners where the inside wheel has a lot less weight on it.
The point of a limited slip diff is to restrict this relative movement between the wheels so that torque is transfered from the faster wheel to the slower one. If you imagine pulling away with one wheel on ice, an LSD would transfer torque from the spinning wheel to the gripping one so that you still get some traction.
There are a couple of common types of LSD. The most common one uses a multi plate friction clutch between the two sides. The amount of torque transmitted between the two sides depends on the clamping force on the clutch. The clutch is pre-loaded by a spring giving you a certain amount of transfer, and the diff is designed so that torque applied to the prop shaft increases this clamping load so the diff gets stiffer the more power you apply to it.
The second type uses a viscous coupling between the two sides. Viscous diffs transmit torque according to the speed difference between the two sides. They act like an open diff when the two wheels are moving at similar speeds, but if either wheel starts to spin up the viscous coupling will gradually transfer torque to the other wheel. The key point is that no torque transfer takes place unless there is a significant speed difference between the two sides.
From the driver's point of view these two types behave very differently. Any time you drive round a corner, the inside wheel tried to go slower than the outer wheel. With a friction diff, the friction clutch fights against this and puts extra torque onto the inside wheel. This means that the inside wheel starts to slide relatively early. But because it is connected to the outside wheel, it can't slide by much. This results in a car which breaks away early but very progressively as you approach the limit of grip, and it always produces similar levels of wheel spin on both wheels which means you get similar slip angles and similar levels of grip.
Taking a viscous diff round a tight corner, the diff lets the inside wheel slow down without putting extra torque on it, so that wheel doesn't start to slide as early. When it does slide, the diff will not transfer any torque away from it until there is a lot of slip so you get a big imbalance between the wheels: the outer wheel is mostly pushing sideways and the inner wheel is mostly pushing forwards. Because the inside wheel has a lot of slip it also has less total grip as well less lateral grip. This results in a car which breaks away relatively late, but when it breaks away you suddenly let a lot less lateral grip and a lot less acceleration, together with lots of smoke from the inside wheel. From a driver's point of view this is great if you only drive it gently, but absolute pants if you ever approach the limit of grip. It's like a cross between a 'proper' friction LSD and a plain open diff.
Perhaps in the right circumstances viscous diffs have their advantages. But my experience is that they are a lot worse to drive than a friction LSD, and I would take £1000 off the value of a car with one fitted. That's roughly what it would cost to chuck that pos in the bin and fit a proper diff.
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