Diff modelling currently doesnt seem to include preload

[colcob]

Just a little something for my fellow physics geeks who are into this stuff.

We've suspected for a while now that something was a bit hinky with the clutchpack diffs, and also that the locked diffs can work out faster.
I always thought it was funny that there was no pre-load adjustment on the diffs, and my suspicion was that preload wasnt modelled, as generally if there is an adjustable parameter modelled, we can adjust it.

I did the following test and it seems like there is indeed no preload on the clutchpack diffs at all, which means that as you pass through the neutral torque zone between power and coast, there is no locking and the diff is effectively open. Its possible that this contributes to some of the difficulties people have with car behaviour on corner entry because as you come of the brakes and start to turn in, you have an open diff for a moment.

Anyway, the experiment went like this for those that are interested.
Run up to full speed at the oval, then put the car into neutral and drive a turn just coasting. This should meant that the diff is under a no-torque condition and so the only torque transfer should be as a result of the preload in a clutchpack diff.
With an open diff, this experiment saw a very small difference in longitundinal force between the rear wheels, probably just as a result of the outer loaded wheel having more rolling resistance (60N difference fact fans).
With a locked diff, as you would expect there was a big torque transfer.
But with a clutchpack diff with full coast and power locking, only the exact same tiny difference of 60N was observed, which effectively means that when not subjected to torque the clutchpack diffs act as open diff.
Ergo, preload is not modelled.

I think.

[mrbogeyman]

makes sense to me.

but what about the viscous differential? same results?

i have found it quite good to drive with in the LX4 recently. set to 19Nms/rad.

[Bob Smith]

Well, this was highly suspected, trust yourself to actually go test it. ;)

But at least we know for sure now (99%).

[Forbin]

makes sense to me.

but what about the viscous differential? same results?

i have found it quite good to drive with in the LX4 recently. set to 19Nms/rad.
I think the viscous diff may be a little different. It tries to equalize the speed of the driven wheels under all conditions, regardless of the torque applied. So, when you're going straight, it's effectively open. But as soon as you start turning, it begins to lock.

Try this: take an AWD car in LFS (RB4 or FXR). Set the torque split to full on the front wheels and the center differential to a viscous diff with the greatest locking factor. Get the car on it's roof somehow, put it in gear, hit the handbrake, and hit the gas for a while until the revs stop increasing. Hit the clutch and hold it in and then let off the handbrake while holding the clutch. Watch the rear wheels. :)

[Shotglass]

finally we have something we can take as a proof that lfs indeed doesnt simulate preload on diffs (something you and bob have speculated for quite a while now iirc)

and ill also take this as a prrof that my speculations that some of the problems that currenty are mostly attibuted to the tyre model are caused by the incomplete diff model ... namely the weird behaviour when you try to catch a slide by lifting off the throttle
my guess is that what happens is you momentarily get some serious open diff lift off oversteer making matters worse

[jtw62074]

I'd chalk that up more to the tire model than the diff. I do preload in my model and it doesn't make that much difference really to this sort of behavior. Not nearly so much as getting the tires running correctly.

[Forbin]

VRC != LFS

'Nuff said.

[Ball Bearing Turbo]

He isn't talking about VRC :P

[jtw62074]

Geeze, Forbin, I thought you'd been around longer than that ;)

http://www.performancesimulations.com/files/ToddSim11a.wmv <--750 HP at the flywheel, 2500 lb car. Watch the slip ratios. That's wheelspin, wheelspin, wheelspin on those short straights.
http://www.performancesimulations.com/files/ToddSim13a.wmv <--3500 lb car, 350HP, 0.95g skidpad, no downforce, and no problem either with or without diff preload ;)
http://www.performancesimulations.com/files/ToddSim7a.wmv

Best really to go in here and right click/save as:

http://www.performancesimulations.com/files

[Ball Bearing Turbo]

Still waiting for your thoughts on that other thread :D :hide:

[jtw62074]

You seemed to have it covered. I had nothing to add :scratchch

[Ball Bearing Turbo]

You seemed to have it covered. I had nothing to add :scratchch

LOL

Ok I'll quit hassling you now :shy:

[colcob]

Yeah, I'd agree with Todd that this isnt likely to be the biggest factor in the handling problems. Firstly because preload is a secondary factor in the behavour of clutchpack diffs*, secondly because its mainly the road RWD road cars that have the oversteer problems which points to the road tyre modelling, and thirdly because Scawen himself said he was investigating the tyres and something about how they lose and regain grip.

That said, it cant do any harm to include the preload, its there for a reason. Certainly in GPL setups, the 'more clutches' approach is a bit easier for beginners.
With minimal clutches in the differential, the back of the car now rotates much more freely. If you just suddenly lift off the throttle and try to turn into a corner, the car will probably spin through 180°. Instead, you have to ease off the throttle, ease onto the brake and steer the car into the corner by modulating pressure on the brake. So in other words, a lack of preload can contribute to making the car rotate more freely into the corners. Which is useful if you are a good driver, but can be difficult to handle if you're learning.


*Carroll Smith: “The most common misconception with these units is that the amount of preload on the clutch plates is a primary determining factor of the amount of lock up. It can be, but things are a lot better if it is not. The spreading force of the bevel gears is an order of magnitude greater than any conceivable preload"

[B2B@300]

The strangest behaviour for me with the clutch pack is under trail braking (towards the end of trail braking when in the final gear for the exit of the corner) a lot of us get this inexplicable sudden drop in RPM, which doesn't happen with viscous or locked diffs :shrug: Is that normal behaviour for a clutch diff? or is that some problem in its modelling or the tyres modelling?

[colcob]

Its probably the inside driven wheel dragging or locking but not actually screeching, which will pull the RPM down through the diff.
If you imagine that the engine RPM is the average of the speeds of the two driven wheels (factored my the gear ratio obviously), if one of them locks or near locks, the engine speed will drop.
It probably only seems wierd because of the apparent audio bug that means driven wheels sometimes dont screech when locked up.

It wouldn't happen with a locked diff because you cant just lock one wheel and you'd have to have a really bad setup to lock both rears under trail braking.

[B2B@300]

Thanks Colcob that makes sense :)

I guess then with preloading in the diff's that effect will be lessened and if we increase coast locking also it would help.

[axus]

I was recently in a race with the UFR on Aston Cadet and had to develop my own setup for both qualifying and the race. What I found with the qualy set was that when trail braking into a tight corner with a clutch pack LSD (even on a high locking factor), the rear would slide out unexpectedly and uncontrolably and it felt very weird. I then switched to a locked diff for the qualyfying and a clutch pack for the race (R3's on the front, used the clutch pack to dial out understeer on turn-in and it worked wonders). I'm sure that if there was preload in the diff it wouldn't slide out like that into tight corners... I'm no diff expert though.

[B2B@300]

What I found with the qualy set was that when trail braking into a tight corner with a clutch pack LSD (even on a high locking factor), the rear would slide out unexpectedly and uncontrolably and it felt very weird.

Did you notice a sudden drop in RPM just before you lost the back end? Because that would be exactly what I'm meaning. It will be interesting to see if it is the diff not having preload or something to do with the tyre model :scratchch I can't wait for the next update to see how all these things develop :D

But what Colcob describes sounds very plausible as the inside front wheel on a FWD car locking would cause the back end to step out, and until recently I didn't know that if one wheel locked you wouldn't hear any skid sound :x

[colcob]

The UFR is a bit of a non-standard case. Because it has huge forward weight distribution, and an almost square wheel layout, it has very little directional stability. And under braking, you end up with hardly any load at all on the rear wheels, which makes it even worse.
So in the first season of ESL UK, I think most people ran with locked diffs, because this basically prevents the front end from rotating under braking. I really couldnt drive the clutchpack setups hard because it was just such work to keep them straight under braking.

I guess some preload might help during trail braking as you come off the brakes, but the the UFR is even a bitch to brake in a straight line with 80% coast locking, and preload wouldnt affect that much.

[B2B@300]

I guess some preload might help during trail braking as you come off the brakes, but the the UFR is even a bitch to brake in a straight line with 80% coast locking, and preload wouldnt affect that much.

So the tyre model, specifically the issue of grip at or just beyond the limit, might be at play here as well :scratchch You tease me Colcob now I await with even more anticipation of the next update :x J/K I'm really fairly patient :D

[colcob]

Well maybe, but I dont see that the UFR behaviour is an issue really. If you take a FWD car with loads of power, huge forward weight distribution and short wheelbase, set it up for oversteer to counteract the natural understeer, then trail brake it into a corner, its going to want to swap ends. Just the nature of the beast really.
There may be some tyre or diff issues that contribute to the handling difficulty, but the characteristics of the car will remain.

[B2B@300]

I get this issue with the clutch pack even in the GTi but it doesn't happen with a locked diff :shrug: By tyre issue affecting it in this situation I mean the one that is locking is beyond the limit of grip and the other is at or near the limit of grip so it is possible this is contributing to the effect also :scratchch as Scawen has already identified this as an issue in similar situations.

[axus]

Yes, that sounds about right and the lack of sound could explain why it felt so unnatural.