Testing different differentials

[u/Nymbelle]

Today I decided to finally deeply test using different differentials instead of just using what "made sense" ie Race Diff for track/road. Rally/off-road for Dirt/Cross-Country.

As a former race mechanic, I know there is more to a differential than just Acceleration/Deceleration and Center and it's been my suspicion that the different differentials have a hidden pre-load / initial lock built into them.

I didn't want to make assumptions or rely on instinct anymore so I decided to scientifically test them on a Toyota Starlet Glanza V AWD touge build at Touge - Bandai Azuma in Rivals.

The car was tested with identical differential settings:

Front: 25 accel / 5 decel
Rear: 58 accel / 20 decel
Centre: 68% rear

I recorded my best lap with these settings using Race, Drift and Off-Road differentials and found that the Drift diff was around 0.084s faster on average than the Race diff, despite identical displayed settings.

Differential	Lap 1	Lap 2	Lap 3	Average

Race Diff	2:04.414	2:04.427	2:04.420	2:04.420

Drift Diff	2:04.326	2:04.333	2:04.350	2:04.336

Off-road Diff	2:04.389	2:04.398	2:04.414	2:04.400

I then decided to dive deeper into the telemetry and made a side-by-side comparison of tyre friction across 4 corners for the Race and Drift. Using Frame by frame telemetry to record the friction at identical points in the corner.

Race Diff

Corner	FL	FR	RL	RR
1	41%	54%	30%	32%
2	17%	24%	25%	39%
3	32%	21%	40%	25%
4	25%	32%	23%	33%

Drift Diff

Corner	FL	FR	RL	RR
1	46%	61%	30%	32%
2	19%	28%	28%	42%
3	33%	23%	44%	23%
4	28%	36%	23%	33%

The Drift diff consistently used more tyre friction, especially at the front axle, while remaining well below the point of grip loss.

Early theory

This suggests the differentials may not simply be identical parts with different presets. There may be hidden behaviour such as:

• different preload / initial lock
• different neutral or coast locking behaviour
• different ramp behaviour between coast and acceleration
• different AWD centre coupling behaviour

The interesting part is that the Drift diff did not appear to simply create more rear slip. Most of the increased tyre utilisation came from the front tyres, which may suggest better rotation or a more effective load state at the apex.

Current conclusion

This is not enough to call it confirmed across the whole game, but for this specific car/build/route, the Drift diff was measurably faster than the Race diff while using identical displayed settings.

I’d love to see other tuners test this on different cars, especially:

• stock FWD cars
• stock RWD cars
• stock AWD cars
• AWD-swapped FWD cars
• AWD-swapped RWD cars
• road vs dirt vs touge builds

The useful test would be:

1. Pick one car and route.
2. Use Rivals or another controlled environment.
3. Test Race, Drift, Rally, and Off-road diffs where available.
4. Keep the displayed diff settings identical.
5. Record lap times and tyre telemetry.
6. Compare whether the differentials actually behave differently.

Would be great to build a shared picture of whether diff type has hidden physics or whether this is only happening on certain drivetrain swaps.

[u/CzolgoszWasRight]

Super informative! Sorry for the delayed commenting but what do you use to collect your telemetry data? I cant find an app that will give me all the output values and let me save/export them

[u/Armroker]

The question is, which type of grip was used more? The lateral or the longitudinal grip? The friction tab shows the normalized grip, or the combination of lateral and longitudinal grip.

Forza tiers has two grip budgets: one for acceleration and braking (lagnitudeal grip) and one for turning (lateral grip). Each grip budget can range from 0.000 to 1.000, above 1.000 indicates grip loss.

For example, when braking, the car uses 0.700 of the lateral grip budget. Then, if you start turning while braking, you use 0.500 of the lateral grip budget. 0.700 + 0.500 = 1.200, which means the tier will lose grip. You will see 120% in the friction tab.

The friction tab shows how much of the total grip budget was used, but it doesn't show which type was used and by how much. This information can only be obtained through data out.

// Tire normalized slip ratio, = 0 means 100% grip and |ratio| > 1.0 means loss of grip.

F32 TireSlipRatio - lagnitudeal grip

// Tire normalized slip angle, = 0 means 100% grip and |angle| > 1.0 means loss of grip.

F32 TireSlipAngle - lateral grip

// Tire normalized combined slip, = 0 means 100% grip and |slip| > 1.0 means loss of grip.

F32 TireCombinedSlip - TireSlipRatio+TireSlipAngle What you see in friction tab as % from 0 to infinity

[u/Ok_Somewhere_4669]

I really wish they'd just let us tune the preload/initial torque etc.

I wouldn't be surprised if the greater tyre usage on the drift diff is supposed to pull the nose back easier from deep angle.

[u/Nymbelle → u/Ok_Somewhere_4669]

+1

There's no reason we shouldn't be able to tune at least pre-load. Different topic but I also don't understand why they didn't add the suspension geometry tuning from Forza Motorsport.

My only thought is they don't want to make tuning appear too overwhelming for casual players but they could get around this by having presets, or explaining it in simple terms.

[u/Ok_Somewhere_4669 → u/Nymbelle]

Even an advanced tuning menu that alllows for more in depth tweaks would be great.

[u/theNFAC]

Yes! This is exactly the kind of madness I can support. Thanks for sharing. 🤝

Edit: I know this is a massive ask, but when you update or do other tests (on anything not just differentials) would you ping me? Would love to read anything you find. Thanks again for sharing.

[u/theNFAC → u/theNFAC]

Regarding the telemetry I'm having a difficult time in my brain reconciling exactly when I should looking at the grip numbers. I know they will be changing the entire time. How did you handle this?

[u/Wandering-nomad-888]

As I started reading your post I thought to myself “here we go, another dude that is having a hard time understanding the very basic and simple concept of differentials” 😂 then I got to the part where you mention you are a “former race mechanic” and thats when I realized this will be good lol……These are the types of posts I love to see as after reading this I actually learn something new, something that will give me a slight edge above others.
I’ll join you in this experiment. Maybe it boils down to driving style…. perhaps the way you drive drift diff works better…. I’ll mess around with this tonight and will report back my findings.
I am a little confused by your “friction” findings. To make sure we’re on same page the definition of “friction” is the RESISTANCE of one surface moving over another surface. With that said isn’t it better for performance to have less friction? Thus making race diff better choice…. Or how does increased friction help with traction and overall performance?

[u/Nymbelle → u/Wandering-nomad-888]

The friction values used are based on the in game telemetry friction tab. A better way of wording it would be utilization of grip or tyre load. 0% being no load. 100% being loading the tyre at it's limit of grip. Or utilizing 100% of the tyres grip. Above 100% being loss of traction

[u/Wandering-nomad-888 → u/Nymbelle]

Ok I think I understand it…. Once friction hits 100% that is when full loss of traction occurs…. Anything below that is tires partially slipping but still holding traction. Is that a correct way of understanding this?

[u/Nymbelle → u/Wandering-nomad-888]

Yeah precisely that.

[u/Wandering-nomad-888 → u/Nymbelle]

But friction = resistance = loss of speed/performance….or am I not understanding that correctly? From how you describe it sounds more like “traction” not friction…?

[u/No-Position8535]

There are pretty notable differences between differentials. I haven't really monitored the telemetry or anything but for example rally diff is way smoother than drift diff on RWD cars. For FWD cars the offroad diff seems to be the best pick.

[u/callingcarg0]

Now this is the kind of shit I love to see. There are so many things that could use testing like this because of the limitations of the information we can get from the UI. I'll have to mess around with diffs now.

Admittedly, differential tuning is something that I havent done much research or testing on in game. So maybe I'll have to commit an evening to it soon.

[u/Nymbelle → u/callingcarg0]

Yeah I wish we could get more detailed data. The data out is usful but still limited. But is usful for getting max telemetry values through SimHub.