We discovered that a "Double differential steering system" exists in the game and can be applied to user models

Community Technical Support CDK Problems & Advice
1

The fact that a “Double differential steering system” or “energy regenerative steering system” existed in real life, yet is still not implemented in the game, has been a topic of discussion in the community for many years.

This time, I discovered that Gaijin has already implemented a regenerative steering system in the game, and how to enable it on a user model.
This feature appears to have been first used in games in “Eldenhet 98”

To do this, edit the configuration BLK file for the user model vehicle.
Rewrite the block in the BLK that defines vehicle performance as follows: (We will use the m60a1 as an example.)

Before

mechanics {
steerType:t = “clutch_braking”
maxBrakeForce:r = 195000
driveGearRadius:r = 0.33
mainGearRatio:r = 5.42
sideGearRatio:r = 1.07
neutralGearRatio:r = 9
gearRatios {
ratio:r = -6.2
ratio:r = -8.4
ratio:r = 0
ratio:r = 6
ratio:r = 4.5
ratio:r = 3
ratio:r = 2.06
ratio:r = 1.52
ratio:r = 1.08
}
}

After

mechanics {
steerType:t = “double_differential”
maxBrakeForce:r = 195000
driveGearRadius:r = 0.33
mainGearRatio:r = 5.42
sideGearRatio:r = 1.07
neutralGearRatio:r = 9
doubleDiffGearRatio:r = 3
gearRatios {
ratio:r = -6.2
ratio:r = -8.4
ratio:r = 0
ratio:r = 6
ratio:r = 4.5
ratio:r = 3
ratio:r = 2.06
ratio:r = 1.52
ratio:r = 1.08
}
}

The meaning of the numbers in “doubleDiffGearRatio” will be explained later.

From what I’ve seen, the features of Double differential steering systems currently in-game are as follows:

  • The speed difference between the left and right tracks required for turning is achieved by energy distribution, not braking. This eliminates the need for extra torque when turning, allowing for more efficient turning.

  • What was discovered this time is a Double differential steering system. This transfers the energy no longer needed by the slower inner track to the outer track, increasing the outer track’s rotation speed. This theoretically allows a tank to turn at the same speed as when moving forward.

  • Vehicles equipped with tracks, such as tanks, experience extreme rolling resistance when turning. This is because the tracks drag sideways against the ground when turning. This is unavoidable as long as the vehicle is equipped with tracks. Therefore, even with a Double differential steering system, whether or not a vehicle can achieve the exact same speed as when traveling straight depends on the available torque output from the engine. If the engine’s total torque is used up when traveling straight, it’s obvious that speed will decrease when turning.

  • The track rotation speed distribution ratio changes depending on the number of gears connected to the main transmission. Specifically, the higher the gear, the larger the turning radius. This is a method used in real-life Tiger tanks and MBTs, and prevents unnecessary energy loss and tank breakdowns and accidents caused by sharp turns. This function is adjusted using “doubleDiffGearRatio.” Increasing this value increases the minimum safe turning radius.

  • Currently, it is not possible to perform a pivot turn by locking one track using just the normal turning key. To do this, you need to set a dedicated key for the track brake.

4 Likes
2

I’ve installed double-differential systems on several vehicles and done some research, and it’s true that this system allows for faster travel than current clutch and brake systems.

However, the current double-differential system in the game is a little too efficient.

From reading various sources, I’ve found that double-differential systems have more shafts and gears than clutch and brake systems, which tends to consume a certain amount of efficiency. Also, in the case of double-differential systems with a predetermined turning radius, turning outside of that radius requires the built-in brakes and clutches to slip, which reduces efficiency. (The Tiger tank is an example of this.)

Also, there are vehicles out there that are equipped with energy regenerative steering systems that can freely change the turning radius. This system has a continuously variable transmission built into the steering system, but depending on the structure of this transmission, some vehicles can have poor efficiency.

The current WT does not reproduce these “inefficiencies,” so the Double-differential system achieves high mobility that is almost like “magic.”

3

This is interesting to see, i hope they are actively working on something to implement to make the steering of tanks more true to how they work in real life.

Something that might be a way to do it is make it function like the thrust of planes where the amount of break applied increases the longer you hold the button or something like that perhaps? and then also add a key bind to toggle the differential on/off to still be able to do the “normal” hard break turns we have in-game now.

This would also explain why they used it on the EldE 98 as even WITH that “magic” unrealistic steering it’s still far from what it should be able to do compared to IRL.

Either way this was a fun read and interesting information :)

1 Like
4

Another thing that struck me as odd was the efficiency during pivot turns.

Whether it’s a clutch-and-brake system or a double differential steering system, if the pivot turn, performed with one track completely stopped, is performed at the same speed, the efficiency of both systems is said to be roughly the same. (This is what Soviet tank textbooks say.)

image image

However, in War Thunder, when a vehicle was tested for pivot turns with both a double differential steering system and a clutch-and-brake system installed alternately, the double differential steering system generated significantly more torque than the clutch-and-brake system. Even at the same engine speed, the double differential steering system generated more torque.

This is strange. As mentioned earlier, the efficiency of both systems should be roughly the same. Despite this, the double differential steering system generated higher torque during pivot turns than the clutch-and-brake system.

In other words, either the double differential steering system in the game has abnormally high pivot turn capabilities, or the clutch-and-brake system’s capabilities are unfairly limited.

5

This might also be a temporary thing they did to the system due to it being only present on the EldE 98 on live server so that will be the only vehicle effected by any changes to that subsystem.

The vehicle really needed something to at least make it somewhat playable at all and even then it’s still having issues (like for example if you’re standing still and hold left or right before starting to accelerate you hardly move AT ALL, you have to get up to speed before turning to have any chance of being able to move in a semi reasonable way)

If you can then i would suggest testing both the systems on the EldE 98 and seeing how they effect it and how you feel it handles with the different systems :)

3 Likes
6

I’ve been tinkering with the EldE 98, so I’ll share my thoughts on it.

  • To begin with, the original steering system of the EldE 98 is neither a clutch and brake nor a double differential. The EldE 98 is a vehicle that operates using a steering system called a tracked articulated vehicle. This works by forcibly adjusting the hinge angle between two vehicles with a hydraulic cylinder to create an angle between the vehicles, which allows the vehicle to turn.

  • When this happens, there is no difference in speed between the left and right tracks. This is because the left and right drive wheels of the EldE 98 are fixed with solid axles. Therefore, there is no difference in rotation speed between the right and left, as happens with clutch and brake or double differential systems.

  • Because the left and right drive wheels of a tracked articulated vehicle are connected, it is impossible to apply the brakes separately on the left and right.

  • So why does War Thunder’s EldE 98 have a double differential? Gaijin appears to have given up on accurately recreating a tracked articulated vehicle for now. Instead of implementing a forced hinge system, they appear to have attempted to recreate the EldE 98 by using a conventional method of turning based on track speed differences. The difference in track speed indirectly changes the hinge angle, thereby allowing the vehicle to change direction.

  • I believe Gaijin likely ran into a problem here. Trying to control the EldE 98 with a clutch and brake would make it very difficult to control turning at high speeds. The clutch and brake provide a constant amount of turning regardless of the vehicle’s speed. This creates a sudden turning force at high speeds, causing a sudden jackknife effect and potentially causing the vehicle to roll over. (At least, this is what happened in my tests.)

  • This is just my speculation, but I think Gaijin wanted a mechanism to control the amount of turning based on speed to solve this problem. This is likely why they decided to adopt the “double differential steering system” they were developing for future implementation on MBTs and other vehicles.

  • Also, the inconvenience of operating the controls while stationary is likely an inherent characteristic of tracked articulated vehicles. Due to the steering mechanism, the vehicle’s direction only changes when moving forward. Therefore, it would be easier to drive if the hinge angle could be forcibly changed, but that’s not currently possible…

  • The only thing Gaijin faithfully recreates is the inability to operate the left and right brakes independently. A new setting parameter was added just for this purpose.

7

A controlled diff should be about 70% efficient, whereas a double differential should be closer to 85% efficient, and a triple differential even more so at closer to 90%.

A double diff should have discrete steering ratio’s which the driver can switch between, allowing for multiple turn radii for a given speed, but not continuous radii.

Whereas the hydrostatic systems used on modern MBT’s are actually less efficient, back to the realms of 60-70%. But they are fully continuous and much lighter and simpler.

Or you have something in between, a cross drive as used on the Abrams, which has continuous steering by using pump’s but uses a double differential to bear most of the load to improve efficiency.

These efficiencies, ontop of the lack of transmission efficiency in game means these vehicles are substantially quicker in game.

8

Don’t know much about the suspension systems and tansmissions/gearbox beside the basic, but this would be a huge W for certain tanks, first one i thought of was challengers seeing as they crap out speed the minute you tap the A or D keys. Really hope they work more on implementing this feature!

1 Like
9

Not quite. What WarThunder has is indeed double differential.

A controlled differential steering system, or Cletrac like what is used on Shermans, Chaffees, Pershings, and a lot of other US WW2 tanks, provides a single efficient turning circle that does not change with speed. The Shermans have a fixed turning radius of 30 feet or roughly 10 meters independantly of whatever speed the tank is traveling at.

We can see from all the clips that this is not the case in WarThunder. As the selected gear increases, so does the turning radius, while with Cletrac or controlled differential steering you would achieve the same turning radius no matter the gear.

No, that’s just the Tiger’s double differential which specifically provides two distinct clutches for two different turning circles.

Tiger double differential schematic

Tiger double diff

Note: “Hauptantrieb” is the main drive input, which comes from the transmission and therefore the RPM depends on the engine RPM and whatever gear has been selected in the transmission, and “Lenkantrieb” is the steering input, which comes directly from the engine and is never disconnected.

When the driver provides a steering input, the right or left steering clutch is activated, together with either the short or wide turning clutches, all depending on the direction of and how much the steering wheel is turned. The wide and short turn clutches work by connecting the input from the steering drive to the “clutch shaft” (Kupplungswelle), which makes the right and left clutches rotate. These two clutches themselves provide a connection to the “support shaft” (Stutz-oder Nullwelle) whenever one is activated, which goes into the sun gears of the planetary differentials (there’s two of them, hence why it is called a double differential). Note that the right side has “intermediate gears” (Zwischenrad) which make it so left and right side rotate in opposite directions, which causes the different track speeds.

So, for example, this is the powerflow of the steering input whenever the left turning clutch and small radius clutch are activated.

Short left turn powerflow

Tiger steering powerflow
Tiger steering powerflow968×1034 786 KB

I’ve used red and blue to denote opposite direction rotation, red means same rotation direction as the main drive, and blue means the opposite. From the diagram we can see that the left side planetary sun gear receives an input that matches the incoming rotation from the main drive, which causes the left side track to speed up. The right side planetary sun gear receives an input that opposes the rotation of the main drive, which causes the right side track to slow down.

HOWEVER

As I said already, that’s just for the Tiger’s double differential, which is more complex in order to provide two turning radii per gear. More basic double differential systems (like on the Panther) provide only a single turn radius per gear.

Triple differential steering actually starts by just slightly modifying the double differential system.
Instead of having clutches for right and left side steering that can be activated or deactivated, you simply have a differential where each side of the differential has brakes to stop that specific side from rotating.

Basic triple differential steering

Triple differential

During normal, straight line drive, the differential receives direct input from the engine, neither side of the differential has brakes applied so both outputs rotate equally and into the sun gears of the planetary differentials.

Whenever the driver applies the brakes on one side of the differential, that side stops* and the other side rotates therefore twice as fast due to how differentials works. This goes into the sun gears of the respective planetary differentials, one sun gear stops and the other sun gear goes twice as quickly, which causes the speed difference in the tracks.

The reason I wrote stops* is because, theoretically, the driver can slip the brakes. This would mean the side doesn’t fully stop, and therefore a wider turn radius can be achieved since there would be a lower different between track speeds. However I doubt this is recommended, as slipping brakes causes more heat and wear on the brakes themselves. On the controlled differential, the brakes can also be slipped to allow wider turns if necessary, however as indicated in the technical manuals of the Shermans and other tanks with this steering, the steering levers should always either be fully applied or fully retracted in order to not slip the brakes.

NOW, onto how this steering is apparently implemented in game.

Both double and triple differential steering systems, at their most basic implementation, provide a single efficient turning radius per gear.
Most importantly, with the engine at a specific RPM, the difference in track speed is always the same. The different radii are achieved specifically because of the fact that there are two inputs into the planetary differentials of the tracks, one from the main drive and one from the engine. At a higher gear, the main drive is going to rotating much faster, which causes the difference in tracks to be comparatively smaller in relation to the movement of the tank. The opposite is true when at low speed.

As a very simple example, let’s say there are two selectable gears, one has a speed of 40 km/h and the other 80 km/h at a specific engine RPM, let’s say 3000 (doesn’t matter).
At this same 3000 RPM, when turning left the right track speeds up by 5 km/h and the left track slow down by 5 km/h.

When we are on the first gear, at 3000 RPM, and traveling at 40 km/h, let’s say we turn left. Now the right track does 45 km/h, and the left track does 35 km/h. The right track is, therefore, rotating 28% faster than the left track.
But now with the second gear, also at 3000 RPM and traveling 80 km/h, in the same left turn the right and left tracks would do 85 and 75 km/h, respectively. The right track spins only 13% faster.
This causes the different turning radii.

My understanding of the “doubleDiffGearRatio” is that it determines the speed increase or loss when turning. In the Elde, this would mean that when turning at max RPM on the engine, one track would gain 8.46 km/h and the other loses the same amount.

Math

image
image1320×569 44.9 KB

10

This doesn’t make sense? Radius is based on speed, the radius definitely isn’t constant for all speeds. It would also be undriveable if that were the case?

11

Given the ratio between the tracks is fixed, I don’t really see how a double diff is modelled here. I guess it could be considered an older double diff from what you suggest?

12

No, radius is based on the distance between the center of the turning circle and the line where the tank moves.
When turning, regenerative steering systems have a “steering ratio”. This is the ratio between the speeds of the two tracks, the lower this ratio, the less of a difference there is, and vice versa.

With double (and triple) differential steering, the steering ratio is different for every gear, with it increasing with higher gears. What this means is simply on low gears, the percentage difference in track speeds is high, and on high gears it is very low. I showed this with my example of 40 and 80 km/h gears. This causes ever increasing turning radii with higher gears.

Cletrac/Controlled differential/Geared differential (all the exact same thing) provides a single, fixed steering ratio for all gears. No matter what gear you select, the percentage difference in the track speeds will be the same. This means the same turning circle is theoretically achieved.

This is a diagram I’ve colored of this kind of system, showing the different gears and outputs.

image

One thing to note here, the orange and red gears on the inside of the differential are connected/meshed. It isn’t apparent from the drawing, but that is how this system allows for power transfer.

I explained how it could be a fixed ratio and still be a double differential at the end of my wall of text.

13

Nvm I’m the one who’s confused aha.
But iirc practically the radius did increase slightly at higher speeds which is what got me confused, tho I may be wrong?

14

No that’s the thing, on the Sherman (and other tanks) the turn rate doesn’t stay the same.

The turn rate actively gets faster because the tank is doing the same radius but with ever increasing speed.

That’s why these tanks have such huge turning radii (again, Sherman is something like 10 meters). If they gave the tanks small turning radii, they would be very hard to control and high speeds and potentially could flip over.

Practically, sure. The theory is that the radius would stay the same, but if you’ve ever played a racing game then you know that a car turns much tighter at low speed than at high speed simply because its not slipping and centrifugal force.

15

I see, thanks for the clarification.

16

Given it’s a ratio, the difference in track speed should be the same proportionally right? Which should give the same radius independent of speed then?

Or does the ratio effect the literal differences in track speed and thus have an increasing radius with speed?

17

If this ratio is applied on top of the selectable gears, then that would the case.

However, as is the case with how a double and triple differential steering system works, the speed difference between tracks stays the same no matter the selected gears, it’s just that as you select high gears the speed of the tracks themselves is faster (again, a 10 km/h difference matters more when you’re at 40 km/h then at 80 km/h).

So, from my understanding, this ratio would be applied entirely separate from the selectable gears. If you take the Elde on a test drive and put on manual transmission, the difference between tracks should always be ~17 km/h when you’re at max speed on a specific gear, one track gaining 8.46 and the other losing 8.46.

That said, what I’ve pointed out here is an assumption. I have not tested this to see if it is true/exactly how it works. Likely will.

18

This is the case if the steering shaft speed is constant right? That’s what causes the consistent addition of speed.

Thus do some tanks have the ability to vary the steering shaft and thus the addition of speed and radius?

19

The steering input is literally the engine’s output.
On the Tiger, you can have different steering radii because the steering system can selected effectively between two gear ratios to pass the steering input through, before it goes into the sun gears.

However if it were just a single turning radius per gear, the input into the sun gears would always be the same when turning. It’s only the input of the ring gears, which is the main drive, that changes.

20

I see, so the steering radius is constant with engine RPM.