[vasillalov]

Hey folks,

Coming from the E9x platform, I have some questions about the XDrive vs RWD cars, particularly when it comes to track and AutoX.

1. In the E9x platform, it is well known and understood that the xDrive cars understeer more, given the same suspension geometry and tires. Is this rhe same with the G8x?

2. In the E9x, the steering rack ratio was 18:1, where the RWD steering rack had 16:1 ratio. Small difference on paper, but it is quite noticeable in AutoX. Is the steering rack ratio rhe same between RWD and Xdrive in the G8x?

Thanks

[Devils41]

I've AutoX my G80 Xdrive twice now and once in the wet. I cant speak to the steering rack but the biggest difference in the G80 Xdrive is the M Xdrive system - https://www.bmw-m.com/en/topics/maga...-m-xdrive.html
I found even with DSC on and in 4wd it would still rotate just fine.

[Limegrntaln]

Originally Posted by vasillalov

Is the steering rack ratio the same between RWD and Xdrive in the G8x?

Thanks

The xDrive steering rack is quicker.

[vasillalov]

Thank you for posting this!

The differences in steering ratio between RWD and Xdrive is negligible. In the E9x platform the difference was huge.

Would you mind sharing the link of where you got that screenshot? I am wanting to dive into other tech data if available.

Thank you!

[forcefed]

Here are some technical files i found on the platform and engine.

[vasillalov]

Thank you!

[jmg]

My G82 X-Drive does not understeer any more than my RWD F80 M3 CS. It is rear biased and you can oversteer to your heart's content!

[BMWRacer523]

X-drive behaves just like RWD, there is no tendency to understeer. You do notice the awd pulling you out of corners on the exit. It's actually pretty amazing. I've tracked/raced RWD M cars for the last 15 years and the G80 x-drive is awesome in my experience.

[M4ord]

As a novice to racing this seems like a good thread to ask this question. But let me first quantify the term novice.

I am in my early 50s and grew up "ragging" a lot of front wheel drive cars at the time and e-braking around corners to slide the back end out. Nothing really fast, but fun none the less. As I grew older my cars got slightly better. My first car that was a "sports car" was a 79 280ZX that I got in 1994, best cornering car for me at that point and it was rear wheel manual. I tried to break that car and in the process learned more about cornering. As I got older the cars got better, but they were designed to haul families around. Finally while living in Germany for the military I picked up a 96 Corvette LT4. The first true car I owned where you had to be midful of how much power you put into exiting corners. As you would imagine a fun car and fun place to drive it.

Recently I picked up a G83 and have been thrilled by what it provides. The car even inspired me to book a few hours of private track time in Spartenburg to find its limits. So long intro to ask probably a silly question. Here we go:

When I did the few hours in Spartenburg it was RWD competition models. I learned a lot about the platform especially feeling the amount of grip this car has. There is something I have noticed in what I assume is the xDrive platfrom but wanted to confirm with you all in this sub-group with the expertise.

There are times when entering a corner where the car will feel close to its grip limits (can be low speed corner or high speed corner), and the nose of the car starts to turn in deeper to the corner. I dont experience any understeer at this moment, it just suddenly feels like the car has found more grip and as a result the nose feels like it wants to dig deeper into the line I have established to the apex. Now my assumption is that I am feeling the moment where the front axle is begining to pull from the xDrive system. If this is the case then is this something that I can always expect or in other words, have those with xDrive systems had to change thier driving techniques when exiting corners compared to RWD only? Thanks for considering to respond.

[Ximian]

Originally Posted by M4ord

There are times when entering a corner where the car will feel close to its grip limits (can be low speed corner or high speed corner), and the nose of the car starts to turn in deeper to the corner. I dont experience any understeer at this moment, it just suddenly feels like the car has found more grip and as a result the nose feels like it wants to dig deeper into the line I have established to the apex. Now my assumption is that I am feeling the moment where the front axle is begining to pull from the xDrive system. If this is the case then is this something that I can always expect or in other words, have those with xDrive systems had to change thier driving techniques when exiting corners compared to RWD only? Thanks for considering to respond.

Without telemetry, this is just a a lot of assumptions, but it sounds like you're describing oversteer which is expected. You're entering a corner at a good speed, likely off the throttle or with a maintenance throttle. While cornering either off the throttle or maintenance throttle, the car is scrubbing off speed, load transferring to the front of the car, giving the front more grip. The result is that the car turns more with the same steering wheel input, increasing with each MPH lost. This is exactly the same for RWD, AWD/4WD, and FWD.

The differences are prominent when you're leaving a corner. With four wheel drive and to a much bigger extent with front wheel drive, you're giving the front wheels the extra thing to do of accelerating in addition to steering. You will have less grip overall for steering out of a corner. But with high HP cars, the benefit of all wheel drive outweighs that drawback because you can use more throttle out of a corner than the RWD car. If you're experiencing oversteer with xDrive while accelerating out of a corner, that's expected as well. A certain amount of throttle is used to "rotate" the car going into an corner but isn't really used coming out of one. You might be causing the car to rotate a bit with how you're applying throttle so more details are needed there.

You've probably heard the phrase "steering with the throttle" and it essentially means you can only give the car so much steering input so you purposefully induce oversteer and understeer at select moments by taking away or adding more throttle. More throttle means you would understeer and less throttle means you would oversteer. The typical example is when you feel like you're about to oversteer leaving a corner, just feed in a bit more power which will transfer the load (AKA weight transfer) to the rear, causing the front to lose a bit of grip and understeer. It won't help with oversteer skidding because that easily becomes unrecoverable caused by too much throttle out of the corner.

[M4ord]

Originally Posted by Ximian

Without telemetry, this is just a a lot of assumptions, but it sounds like you're describing oversteer which is expected. You're entering a corner at a good speed, likely off the throttle or with a maintenance throttle. While cornering either off the throttle or maintenance throttle, the car is scrubbing off speed, load transferring to the front of the car, giving the front more grip. The result is that the car turns more with the same steering wheel input, increasing with each MPH lost. This is exactly the same for RWD, AWD/4WD, and FWD.

The differences are prominent when you're leaving a corner. With four wheel drive and to a much bigger extent with front wheel drive, you're giving the front wheels the extra thing to do of accelerating in addition to steering. You will have less grip overall for steering out of a corner. But with high HP cars, the benefit of all wheel drive outweighs that drawback because you can use more throttle out of a corner than the RWD car. If you're experiencing oversteer with xDrive while accelerating out of a corner, that's expected as well. A certain amount of throttle is used to "rotate" the car going into an corner but isn't really used coming out of one. You might be causing the car to rotate a bit with how you're applying throttle so more details are needed there.

You've probably heard the phrase "steering with the throttle" and it essentially means you can only give the car so much steering input so you purposefully induce oversteer and understeer at select moments by taking away or adding more throttle. More throttle means you would understeer and less throttle means you would oversteer. The typical example is when you feel like you're about to oversteer leaving a corner, just feed in a bit more power which will transfer the load (AKA weight transfer) to the rear, causing the front to lose a bit of grip and understeer. It won't help with oversteer skidding because that easily becomes unrecoverable caused by too much throttle out of the corner.

Thank you for taking the time to respond and with detail. All your assumptions are correct on this and it makes perfect sense. What I am learning is that my observations are a byproduct of having a car with such performance and handling that I have not experienced these weight shift dynamics before. For example in the 280ZX there was never enough torgue to shift the weight to an extent that the front axle would experience oversteer and in the Corvette I was always worried about loosing the backend that I did not push the car around corners very hard and in hindsight the suspension on the corvette was not that good.

So is it safe for me to conclude that in the situation descibed above, that I have not kept my speed up high enough to have a smoother weight shift?

Thanks again Ximian! Responses like this are why I love this forum and the M community in general, peoples desire to help each other. Safe travels!

[Ximian]

Originally Posted by M4ord

Thank you for taking the time to respond and with detail. All your assumptions are correct on this and it makes perfect sense. What I am learning is that my observations are a byproduct of having a car with such performance and handling that I have not experienced these weight shift dynamics before. For example in the 280ZX there was never enough torgue to shift the weight to an extent that the front axle would experience oversteer and in the Corvette I was always worried about loosing the backend that I did not push the car around corners very hard and in hindsight the suspension on the corvette was not that good.

So is it safe for me to conclude that in the situation descibed above, that I have not kept my speed up high enough to have a smoother weight shift?

Thanks again Ximian! Responses like this are why I love this forum and the M community in general, peoples desire to help each other. Safe travels!

A smoother weight shift is just a slower one. If you didn't feel the car leaning in any direction, it's likely you were going too slow to be a drastic lean. The difference between smooth and jerky could be as little as half a second with an earlier but shallower turn of the steering wheel before continuing to the necessary steering angle input. But that's for track driving.

You should be able to feel load shift/weight transfer during regular driving. If you're going the speed limit on a 25-30 MPH curvy road, you'll feel the car lean a little bit to the opposite side of the turn for those turns that take a couple of seconds. Since you're holding a steering wheel, it might be harder to notice, but you can feel is as a passenger. It's also more noticeable with taller vehicles that have a higher center of gravity. In those types of vehicles, you can more easily practice smoother load transfer by slowing down your inputs.

At higher speeds, you lose speed quicker due to higher friction/drag. So going into a corner at 80 MPH and coasting, you'll lose more speed per second than at 30 MPH and coasting. With that higher rate of losing speed, the car will lean more toward the front (as the front of the car is slowing down quicker than the rear), giving the front wheels more grip. However, there is a speed you can go where it's too fast for the available grip from the tires, so you understeer. If the car felt balanced meaning more steering input causes the car to turn more, there was plenty more grip in the tires that could be exchanged for higher cornering speed.

If you want to get more experience with these types of car control dynamics, look into some auto-x events as it's a cheap way to experience it safely.

[M4ord]

Thanks Ximian!!

[BMWRacer523]

Ximian explained it well. With xdrive you can feed in more throttle earlier exiting the corner. Key word being "feed" of course

[hooked]

Originally Posted by forcefed

Here are some technical files i found on the platform and engine.

Just seeing this for the first time. Thanks for sharing!

Some interesting tidbits in them. Pulled a few out of the complete vehicle guide for those of you who don't want to slog through it.

Page 21
The use of a CFRP roof produces a weight saving of 3.3 lbs compared with the standard steel roof on the G20/G22 . The weight saving in the roof area increases the vehicle's agility and dynamics by lowering the center of gravity towards the road.

If the customer decides in favor of an electrical glass moonroof SA 403 in the G80/G82, the CFRP roof is deleted and a steel roof as in the G20/G22 is installed.

The electrical glass moonroof SA 403 is not offered for the G80/G82 with M Drivers Package 7ME. A pressure resistance of the glass moonroof cannot be guaranteed from 155 mph, therefore it is not used for the G80/G82 M Drivers Package 7ME

[I never knew the 155 MPH limit was based on the glass moonroof if equipped!]

Page 47
* During the first 3100 mile run-in distance, the Launch Control must not be used.
* The Launch Control is active from the factory. The activation of Launch Control is not restricted to the 1200 mile running-in check.
* Premature wear occurs as a result of the high load on the vehicle components when using Launch Control.

[I never knew the run-in distance is 3100 miles!]

Page 48
* The optimal slip-controlled Launch Control is based on Michelin Pilot Sport 4S and Michelin Pilot Sport Cup-2 tires at operating temperature.

[Interesting for 2 reasons; Pirellis not mentioned at all and PS4S and Cup-2s lumped together. Possible confirmation that PS4S BMW Star tires are more like Cup-2s than regular PS4S.]

Page 49
With the M8HP76 M Sport Steptronic transmission, which is based on the BMW AG 8HPTU2 gearbox, customers benefit from significantly improved gear shift responsiveness and even further optimized control of the converter lock-up clutch.

This has been made possible by the further development of converter technology to effectively dampen rotational irregularities in the drivetrain with a turbine torsional vibration damper. As a result, the operating ranges in which the converter lock-up clutch has to be controlled are reduced even further because the converter lock-up clutch is fully engaged in the vast majority of driving situations. This provides for an even more direct connection of the M8HP76 transmission to the complete drivetrain, resulting in an even sportier driving experience and reduced fuel consumption.

[I read this as significantly less power loss in the transmission!]

Page 56
The torque generated by the engine is stepped up in the automatic transmission and is supplied via the transmission output shaft to the transfer box.

[So the ZF Auto transmission adds torque?]

Pages 69-70
The M Servotronic (EPS) is an independent development for the BMW M3 and the BMW M4 Coupe. All components of the M Servotronic (EPS) have been developed specifically for the BMW M3 and the BMW M4 Coupe. With this measure the development of the steering was able to be coordinated to the typical M properties. Special attention was paid here to the typical M features:
* Direct steering sensation.
* Driving condition feedback.
* Dynamic driving in the limit range.

[Explains why the full M cars have better steering feel than other BMWs. Why is this not the standard module for all BMWs with user selectable controls to dial in the desired steering feel?]

Page 74
The design of the rear axle was created in close cooperation with the tires which were also specially developed for the G80/G82.

[Another reason the PS4S BMW Star tires are the best ones for this car!]

Page 76
A large-sized, perforated M compound brake disc combined with a six-piston fixed caliper from Brembo is used at the front axle.

[Assumed they were Brembo, but nice to see it confirmed.]

Page 77
A large-sized, perforated M Compound brake disc combined with a 1-piston floating caliper with electromechanical parking brake from TRW is used at the rear axle, which corresponds in design and function to the AG vehicles.

[Seems like a significant step down in braking out back though...]

Page 80
The brake dust tells the customer that his/her BMW M vehicle has been driven in an appropriate manner. What is important is to ensure that it is regularly removed by washing the vehicle, as otherwise it will eat into the surface of the wheel.

[Common sense, but interesting to see OEM note that the brake dust could damage the wheel finish.]

Page 83
* If the tires are not used for an extended period, it is recommended to remove the wheels from the vehicle and to reduce the tire pressure to half the usual value.

[Confirmation that tire pressure should be reduced significantly while in storage.]

Page 86
The shock absorbers have been developed with the supplier ZF Sachs and the system adapted to the G80/G82.

In addition, the ride height between the wheel suspension and body is used as a control, reference and load variable and is read off of the ride height sensors of the headlights. There is 1 sensor each at the front and rear.

[So what happens when people lower their car?]

Page 96
Actuator wheel slip limitation ARBx in the M all-wheel drive 
The key new feature of the actuator wheel slip limitation in the G80/G82 is the further shift of the slip controller for the longitudinal dynamics of the M xDrive from the virtual integration platform control unit (VIP) to the M transfer box (M VTG). This reduces the control times again significantly and therefore increases the performance of the traction control system.

The M transfer box can now adjust speed differences between rear and front axles within certain limits, without having to wait on the "slow" control loop via the virtual integration platform control unit.

This means that high differential speed peaks between front axle and rear axle in the multidisc clutch of the M transfer box can be avoided. The result is a significant improvement of the all-wheel drive performance of the M xDrive, both with regard to the driving performance such as dynamics and agility, as well as a thermal relief of the multidisc clutch and thus the transfer box oil of the M transfer box.

[Why is this not the standard control box for all BMW xDrive cars?]

Page 119
The ASD [Active Sound Design] characteristic and status are coupled to the engine dynamics control/Sound Control settings:
* Sound Control COMFORT = ASD deactivated

[Nice to know the fake engine sound is completely off in COMFORT mode!]

[forcefed]

Originally Posted by hooked

Page 80
The brake dust tells the customer that his/her BMW M vehicle has been driven in an appropriate manner. What is important is to ensure that it is regularly removed by washing the vehicle, as otherwise it will eat into the surface of the wheel.

OMG

[Forzanerazzurri]

This is so cool. Nerding out reading that PDF.