LSD

[Wiltshire Tony]

Could some kind soul please oblige this   with a concise and easy to follow explanation of what a limited slip differential is, what it does, and why is it preferred (or not) to standard differentials.

[Paul968]

I'll have a go:

A standard diff is there to allow the driven wheels to turn at different speeds when going round a corner, because if they were locked to the same speed then the inside one would scrub because it has a shorter distance to travel.

This is fine for a normal road car, but in a racing car there is a problem. The design of a standard diff will mean that if one wheel loses traction and spins up then the power is sent to that wheel.  The other wheel can spin at a different speed but it has reduced power until the inside wheel gains traction. This waste of power is obviously a bad thing in a racing car. The fact that  one wheel can lose traction independently is also bad because this means traction is limited by the wheel with least grip - not great, as the wheels rarely have the same grip mid corner.

One alternative is to get rid of the diff altogether and lock the wheels together but this is pretty radical and makes the car very reluctant to turn. A more effective solution is to use a limited slip diff. This will lock up to a certain extent and under certain circumstances, but these can be tuned to make sure that the locking does not compromise the car too much. There are normally different settings for when the car is on the power and when it is not, and also different rates at which the locking comes in and out.

In iRacing and GPL the ramp angles determine the amount of locking (more ramp = less locking) and the clutches and preload determine the rate of change (I think). In GTR2 and GTL the values are simplified to just locking amounts, but these are the other way round - higher = more locking. There is no way to change the rate of locking.

I hope that helps 

[Wiltshire Tony]

That's great Paul, thank's for taking the time to explain.

[spanner]

Lol, he's had some practise!

[quicksilver384]

I found this on Wikipedia if you want to go into more detail, but I think you will need a doctorate in mechanical engineering to understand it lol. HERE

[Wiltshire Tony]

I found this on Wikipedia if you want to go into more detail, but I think you will need a doctorate in mechanical engineering to understand it lol. HERE

I did try Wiki before posting here. Now you know why I posted 

[Dave 'Gizmo' Gymer]

A standard diff is there to allow the driven wheels to turn at different speeds when going round a corner, because if they were locked to the same speed then the inside one would scrub because it has a shorter distance to travel.

This is fine for a normal road car

Two small corrections...

1. A "standard" diff in this context is an "open" diff (anyone who's played with technical Lego will recognise the little square frame with three cogs inside it - that's an open diff).

2. It is not fine for a normal road car unless it's hideously underpowered. You just don't notice most of the time when you're just tootling along.

[Paul968]

2. It is not fine for a normal road car unless it's hideously underpowered.  You just don't notice most of the time when you're just tootling along.

I beg to differ on this. Only a tiny handful of cars have an LSD as standard. If by 'hideously underpowered' you mean 'less than 300HP' then perhaps, but these cars have the diff more for track work so are not just 'road cars'. Anyway, most road car diffs are not the same as the racing diffs being discussed as they are often simpler viscous types without the sophistication or performance of racing diffs.

Just as an example, I'm pretty sure that a standard 997 S (911) doesn't have an Locking diff, and that has 400 HP - hideously underpowered?

[Clive Loynes]

Perhaps Paul could explain why the PnG/GTR2 diff appears to me to operate in the opposite way to the GPL diff.

In GPL I was aligned with the fact that if I increased the amount of diff lock (lower angle) then I would increase the amount that the car would turn inwards at mid-corner when the power was applied.  Due to the outside rear wheel driving it around.

When I moved to PnG I went all the way up to 100% power side trying to achieve the same result and failed to notice any such thing.

Then Manteos provided me with a Cobra setup that had 20% power side diff and there it was.  I now don’t have a clue how the PnG diff is supposed to be working but if I want it to turn under power then the setting on the diff needs to be low.  It’s as if PnG never models the inside rear losing traction. 

[Dave 'Gizmo' Gymer]

2. It is not fine for a normal road car unless it's hideously underpowered.  You just don't notice most of the time when you're just tootling along.

I beg to differ on this. Only a tiny handful of cars have an LSD as standard. If by 'hideously underpowered' you mean 'less than 300HP' then perhaps,

300 is definitely hideously underpowered for RWD (100 for FWD).

[Paul968]

How did I know that you were going to say that 

[Paul968]

Just realised I never replied to this:

Perhaps Paul could explain why the PnG/GTR2 diff appears to me to operate in the opposite way to the GPL diff.

In GPL I was aligned with the fact that if I increased the amount of diff lock (lower angle) then I would increase the amount that the car would turn inwards at mid-corner when the power was applied.  Due to the outside rear wheel driving it around.

When I moved to PnG I went all the way up to 100% power side trying to achieve the same result and failed to notice any such thing.

Then Manteos provided me with a Cobra setup that had 20% power side diff and there it was.  I now don’t have a clue how the PnG diff is supposed to be working but if I want it to turn under power then the setting on the diff needs to be low.  It’s as if PnG never models the inside rear losing traction. 

I have no idea really, other than to say that the physics in PnG is rather simpler, plus the fact that the cars are rather different.  A 60s grand prix car is a lot lighter and has the weight over the back. I would also say that your GPL rule (more locking = more turn on power) is perhaps too simplistic as it only really applies once both wheels lose traction.

[spanner]

To me it sounds like in GPL your inside wheel is loosing traction as its tied to the outside wheel speed too closely so its relying on the outside wheel keeping traction and allows you to rotate the car easier.

But in PnG it sounds like your running it quite open so it allows a greater difference in wheel speed which sounds like your relying on the ouside wheel again to do most of the work?

I'd imagine the heavier, front biased cars might mean they dont behave like a light, rear engined 60's GP car, so might mean a slightly different style!

[Paul968]

Clive is saying that in PnG he can't get the car to steer on the power when the diff is fully locked, not open.

[spanner]

Yes but hes saying he can steer it on the power when the diff is open.