Many cars in LFS are faster with locked diff. I have asked many friends about this issue but got no detailed answer. Some people say that the grip is too high now so the inner wheel usually stay in the air, minimizing the disadvantage of locked diff. But how about those wide body and low CoG cars ?

There are probably a number of factors - these two I suspect are main ones:

1. IIRC, Scawen mentioned that with the current tyre physics, tyres that don't have much load on them - the inside ones when going round corners, regardless of how stiff the suspension is - don't have enough grip. This means that the scrubbing effect of the inside wheel going at a different speed to the surface won't affect it as much as it should, and so the disadvantages of the locked diff are reduced.

2. LFS hasn't got helical diffs. The best (mechanical) diff for racing is helical type (Quaife etc) as it supplies the most torque to the wheel with the most grip. All other non-locking LSDs in LFS reduce the spinning of the unloaded wheel, but don't stop it. This means power is wasted (and tyre heat+wear produced) spinning up the wheel with least grip. This is made worse by 1. above.


I also suspect that there is a problem with the implementation of the clutch-pack (unless I misunderstand how clutch-packs work, which is a reasonable possibility) which may or may not affect its behaviour when cornering. If the car gets stuck in a gravel trap and one of the drive wheels is hardly touching the ground, this wheel will spin away merrily while the loaded wheel doesn't move at all. The only way to move the loaded wheel is to keep modulating throttle whilst pumping the clutch. I don't get how that should be possible when the diff is set to have reasonably high locking under power and a high preload torque.

First lets consider what's happening:

A locked diff does not ever permit tyres to rotate at different angular velocities. When driving round a corner in a car, the inside and outside wheels will travel different distances, so if they were allowed to free wheel, the inside wheel would rotate slower. Also weight transfer would put more load on the outside wheel, squishing it further, making the rolling radius smaller, further increasing the difference in free rolling angular velocities between inside and outside tyres.

As the wheels are not free rolling, this creates a slip ratio in the tyres when cornering. If no torque is being sent from the engine to the wheels, then this will be a negative slip ratio from the outside tyre (as it will be turning too slow), and a positive slip ratio from the inside tyre (it will be turning too fast).

These slip ratios create forces; how much depends on the braking stiffnesses of the two tyres (assuming the friction limit of the tyres is not breached). This force is negative for the outside wheel and positive for the inside wheel, these sum to create a torque in the opposite direction to that created by steering the car into the turn. So the car will not change direction as quickly, as the yaw torque will be reduced.

This reduction in yaw torque is the main reason cars are slower in tight bends with a locked diff (the smaller the radius of turn, the greater the proportion of the track width to turn radius, and thus the greatest slip ratios are produced when cornering).

So friction is not usually the limiting factor when it comes to tyre scrub caused by a locked diff, and even if it were wrong in LFS, it shouldn't be wrong by more than a few percent; not enough to explain the issues.

If the braking stiffness of the tyres was too low, however, the same slip ratios would give less longitudinal force, thus less yaw torque to slow the cars rotation, and thus less penalty for using one.

Braking stiffness is a very dynamic property, varing with load, tyre pressure, and slip angle, so my first point of suspicion would be that something is causing it to be too low.

So, generally, in today's LFS, the tyres slip curves are too progressive and traction peaks are too high ( or too "late" ) ?

Forgeting all the tyre physics the main reason for the locked diff to be faster than the others is becuase it has the most stable and predictable behaviour in transients thus giving the driver greater confidence and thus a more likely quicker lap time...

To get good performance out of the LSD in say the XXR requires much more setup skills (to tune to individual driver and track conditions) to get the same driver confidence levels and therefore likely good lap times. My experience with the XRR is that given the abiltiy to tune the LSD diff well for the driver and track in question it will be just as quick or quicker than a locked diff. Reality is though that it is far easier just to slap in a locked diff

Quote from Keling :

So, generally, in today's LFS, the tyres slip curves are too progressive and traction peaks are too high ( or too "late" ) ?

Maybe. Although looking at the peaks doesn't tell you enough, as you could change the peaks without changing the cornering or braking stiffnesses (especially in an empirical model).

Quote from Bob Smith :

So friction is not usually the limiting factor when it comes to tyre scrub caused by a locked diff, and even if it were wrong in LFS, it shouldn't be wrong by more than a few percent; not enough to explain the issues.

So it seems you did not answer the question

What I noticed (maybe wrong, it could be aerodrag :shy is that locked diff setups are usually faster on high speed sweepers (like AsNat twisted straights) where maximum power is needed with no loss due to weight balance. Prolly due to unproductive little slip ratio on inner tyre getting all the power left on engine.

i wonder if the disadvantages of a locked diff are really that big. I read a lot in tuner and drifter forums, and a lot of them weld the diff on their daily drifter and the only disadvantage seems to be that those cars are harder to park and manoeuvre at low speeds (and more tire wear, but its drift cars so :shrug

Quote from Bob Smith :

First lets consider what's happening:

A locked diff does not ever permit tyres to rotate at different angular velocities. When driving round a corner in a car, the inside and outside wheels will travel different distances, so if they were allowed to free wheel, the inside wheel would rotate slower. Also weight transfer would put more load on the outside wheel, squishing it further, making the rolling radius smaller, further increasing the difference in free rolling angular velocities between inside and outside tyres.

As the wheels are not free rolling, this creates a slip ratio in the tyres when cornering. If no torque is being sent from the engine to the wheels, then this will be a negative slip ratio from the outside tyre (as it will be turning too slow), and a positive slip ratio from the inside tyre (it will be turning too fast).

These slip ratios create forces; how much depends on the braking stiffnesses of the two tyres (assuming the friction limit of the tyres is not breached). This force is negative for the outside wheel and positive for the inside wheel, these sum to create a torque in the opposite direction to that created by steering the car into the turn. So the car will not change direction as quickly, as the yaw torque will be reduced.

This reduction in yaw torque is the main reason cars are slower in tight bends with a locked diff (the smaller the radius of turn, the greater the proportion of the track width to turn radius, and thus the greatest slip ratios are produced when cornering).

So friction is not usually the limiting factor when it comes to tyre scrub caused by a locked diff, and even if it were wrong in LFS, it shouldn't be wrong by more than a few percent; not enough to explain the issues.

If the braking stiffness of the tyres was too low, however, the same slip ratios would give less longitudinal force, thus less yaw torque to slow the cars rotation, and thus less penalty for using one.

Braking stiffness is a very dynamic property, varing with load, tyre pressure, and slip angle, so my first point of suspicion would be that something is causing it to be too low.

that just fried my brain, but has to be the best answer i've ever read

John - try reading some of the detailed physics explanation threads around the place, for a truly mind melting experience! The knowledge level of some of the people who come here is amazing, i'm afraid most of the time i need it "dumbed down" to a level i can understand :P