Spool Diff

[gti_sdn]

Is the closest we can get to a spool diff to set the settings on the custom diff to Initial:60/Accel:60/Brake:60
Does this offer a full lock-up, or is there still the possibility of wheel speed differences across the axle?

 

[gti_sdn]

So i can confirm that setting all the values (Initial:60/Accel:60/Brake:60) on a Fully Customizable differential will make it act like a spool diff.

This first shot shows a car with its open differential, we can see the wheel speed difference across the rear axle during corner entry/mid/exit. Especially the inside wheel on exit.

The second shot shows a car with a customizable diff with the default settings, this shows the wheel speed across the rear axle a lot closer during the corner phases.

This last shot shows a car with the Customizable diff with the settings set to their maximum (Initial:60/Accel:60/Brake:60).
It clearly shows there is zero variation across the rear axle, acting exactly like a spool diff where there is a complete lock between left and right at all times.

 

[Greycap]

Excellent info! When I first saw the thread yesterday my idea was to dig into the wheel speed data but you got there yourself first.

Now, another question, since you have good software for it... how to make a Detroit locker for example. It would require 60 for acceleration and 5 for deceleration of course, but the initial value is a bit of a mystery. Logic says that 60 is always locked and 5 is open (well, as open as it can be) until either acceleration or deceleration occurs but you have the tools to see if that's the case.

 

[gti_sdn]

Excellent info! When I first saw the thread yesterday my idea was to dig into the wheel speed data but you got there yourself first.

Now, another question, since you have good software for it... how to make a Detroit locker for example. It would require 60 for acceleration and 5 for deceleration of course, but the initial value is a bit of a mystery. Logic says that 60 is always locked and 5 is open (well, as open as it can be) until either acceleration or deceleration occurs but you have the tools to see if that's the case.

If i have my theory right then the Detroit locker will only lock on throttle (acceleration), while all other states are open (deceleration/coast)
If this is correct then i would assume that the diff settings would have to be Initial:60/Accel:60/Brake:5
https://www.gran-turismo.com/au/gt7/manual/carsettings/02
Reading the manual the initial Torque is how responsive you want the locking up of the diff and with the Detroit it is instant from what i have read.
Now this instant lock up we want to happen only on acceleration so that has to be set to 60 as we know this is locked.
As you say with the customizable differential the lowest we can set the Brace sensitivity to is 5 (As open as possible).

Interested in what your application is as i assume this would make a car very nervous on the brakes and any acceleration mid corner would create a lot of understeer?

Older 60's and 70's Muscle car seems to be the only fit.

 

[Greycap]

Interested in what your application is as i assume this would make a car very nervous on the brakes and any acceleration mid corner would create a lot of understeer?

Older 60's and 70's Muscle car seems to be the only fit.

Those indeed, and it very probably sucks but it'll be interesting to try it. Then again, come to think of it, even top end sports prototypes such as the Porsche 917K we have in the game were often driven with a spool. No idea why someone would want a car like that with no differential at all but it happened.

The logic of high initial to make the application instantaneous sounds good... but GT being GT, I wouldn't be surprised if it plays a part in the deceleration behaviour too as it may just be a different name for preload with the manual saying "it may also hinder the car's ability to turn". That needs some looking at and you have just the right tools for that.

 

[gti_sdn]

Seems like having a Detroit locker or what Eaton call the no-spin differential needs to allow the outside wheel become decoupled but only at speed not based on load or lack thereof such as a wheel free wheeling.

Not sure how close to this we can get 🤔

It also seems that the Detroit locker is/was? Used in NASCAR.

It certainly seems a Detroit would be a slightly better option than the spool diff but surprisingly there are still racing series today using a spool diff 🤯

I will have a go at a few different settings when I get some free time.

 

[bread82]

I suspect the closest match for a Detroit locket in GT7 is max Accel, minimum Decel and minimum Initial Torque.

As far as I know, Initial Torque is the same as breakaway torque or preload in diff parlance. It's a bit confusing as in GT it's in the same units as the Accel and Decel, whereas in real life the preload is in Nm or ftlb, vs the Accel/Decel locking parameters are in locking percent, or torque bias ratio.

A 60 percent Accel lock means a torque split of 80:20 (60 difference), meaning one wheel can up to receive 4 times the amount of torque of the other. A real Detroit locker would be 100pc lock and therefore 100:0 split. But 80:20 should be aggressive enough in GT for most situations - you would have to have a greater than 4:1 grip difference between drive wheels for the diff to start to slip, which is quite a lot.

Leaving Initial Torque at minimum means the diff will be as open as possible until you stomp on the gas. It's my gut feeling that Initial Torque in GT is modelled as a percentage of something to do with engine output torque... My best guess is it's a percentage of max engine torque multiplied by final drive ratio, but I can't prove this. e.g. a 150 Nm engine through a 4:1 final drive gives 600Nm. Initial Torque of 5 is 5pc of this, 30Nm preload.

 

[gti_sdn]

I suspect the closest match for a Detroit locket in GT7 is max Accel, minimum Decel and minimum Initial Torque.

As far as I know, Initial Torque is the same as breakaway torque or preload in diff parlance. It's a bit confusing as in GT it's in the same units as the Accel and Decel, whereas in real life the preload is in Nm or ftlb, vs the Accel/Decel locking parameters are in locking percent, or torque bias ratio.

A 60 percent Accel lock means a torque split of 80:20 (60 difference), meaning one wheel can up to receive 4 times the amount of torque of the other. A real Detroit locker would be 100pc lock and therefore 100:0 split. But 80:20 should be aggressive enough in GT for most situations - you would have to have a greater than 4:1 grip difference between drive wheels for the diff to start to slip, which is quite a lot.

Leaving Initial Torque at minimum means the diff will be as open as possible until you stomp on the gas. It's my gut feeling that Initial Torque in GT is modelled as a percentage of something to do with engine output torque... My best guess is it's a percentage of max engine torque multiplied by final drive ratio, but I can't prove this. e.g. a 150 Nm engine through a 4:1 final drive gives 600Nm. Initial Torque of 5 is 5pc of this, 30Nm preload.

This is some great theory and I have been doing some more research and testing and what I have found out is.
The Detroit locker is actually an unlocker, what I mean by this is it is locked until there is a need for the speed difference between the wheel's

This may be hard to replicate in game.

The initial setting in game, as soon as you set this to 60 it seems the other 2 settings are redundant! If you set it to 60/5/5 it still acts like a fully locked diff and there is no speed difference between the two wheels.

 

[Greycap]

The initial setting in game, as soon as you set this to 60 it seems the other 2 settings are redundant! If you set it to 60/5/5 it still acts like a fully locked diff and there is no speed difference between the two wheels.

That's pretty much what I suspected, it's just a different name for a preload - the value of 60 means so much of it that it won't slip no matter what.

Mark Donohue was one of those known to always drive with a spool diff regardless of the car and when the rules said he couldn't have one he effectively made one, in a very similar way to what the 60 initial does in the game.

"For some reason locked differentials weren't approved by the SCCA, thought, so we left the positraction in and merely stacked up the clutch springs so that there was about a million pounds of pressure on the clutch packs."

https://forums.autosport.com/topic/90682-mark-donohue-and-the-locked-differential/

 

[bread82]

Ah, my bad. I thought a Detroit locker worked the other way around!

In that case, minimum Accel and Decel, and judicious use of Initial (I'm guessing sub 20) should allow wheels being locked together until you really start cornering at which point the grip bias of the wheels will overcome the diff's biasing capabilities, and you'll get the diff opening up.

One thing I never got round to experimenting with is, if my theory about engine Tq and final drive is correct (for Initial Torque setting), then running a low final drive should allow effectively lower IT behaviour; and you can still have the same overall gear ratio by using higher individual gear ratios with the lower FD.

 

[gti_sdn]

I have done a fair bit of testing but not got the correct workbook file to show it.
Another thing I have started to think about though is tyre compound! So far I have been using the E36 M3 on its stock comfort soft tyres. Will tyres with more grip need higher values?

 

[bread82]

Yes, in theory. Stickier tyres means more lateral G and therefore more load transfer. This means a bigger grip difference between the two rear wheels. The diff's locking strength is always fighting the tyres' grip capability difference. More grip difference can handle a stronger diff.

If the diff is set too strong it may never unlock on weak tyres, you effectively have a spool even though the diff numbers may not be maxed.

Even on sports softs I'd rarely need more than 25 Accel for my driving style. Comforts more like 15 Accel. IT values; I'd usually be on around half to one third of the Accel values, give or take.

Stickier tyres are less forgiving at the limit, so there's something to be said for using a slightly more open diff than you'd think, even on sticky tyres. Because if it's fully locked and you wheelspin on corner exit, you lose both rear tyres at once and therefore lose the back end.

 

[Greycap]

Stickier tyres are less forgiving at the limit, so there's something to be said for using a slightly more open diff than you'd think, even on sticky tyres. Because if it's fully locked and you wheelspin on corner exit, you lose both rear tyres at once and therefore lose the back end.

You're more right there than you may even realize. Most people use insanely high values in the belief that it gives more traction but in reality it does just what you've described - if there's enough power to break both tyres loose, you're gone.

I very seldom use more than 5/10/5 on anything RWD unless the car has some specific behavioural issue that needs to be addressed, no matter if a 550PP sports car on comfort softs or a GT3 on racing hards, because it just works. Maximize the manoeuvrability and give just enough locking to prevent spinning the inner wheel, and if it happens slightly every now and then, so be it. It's still ultimately faster as the car takes off pointed to where you want it to with an occasional faint squeal from the inner rear instead of either going straight or getting sideways, both of which need backing off a lot more to recover from.