Daily driver thread re AWD poll

[ezmaass]

Originally Posted by stealth.pilot

ezmaas I agree its best to compare with the same car.

So, this is comparing winter tires to summer tires - completely irrelevant, as I don't believe anyone here (including myself) is suggesting summer tires are appropriate for snow. Of course the summer tires won't grip snow.

Your second video, although comparing winters on 2WD to A/S on AWD, is flawed in a few other ways:

- The 2WD car is a FWD car, not RWD. That's going to make a significant difference in handling. FWD cars have the advantage of applying force in the direction of the wheels... not the case with RWD, or hence any non-xDrive BMW.

- The two cars are not identical (Subaru and Mini), so there are more variables at play here than simply their tire type and drivetrain.

- The video does note, however, that the acceleration advantage (read: finding traction to get the car moving) is still given to the AWD car with A/S... essentially better grip from a stop.

Really what's necessary here is a comparison between identical cars - one with A/S + AWD, the other with Winters + RWD. Like I've explained above, the math just won't add up to allow the RWD car with Winters to out-perform (out-grip) the AWD + A/S unless you can find a way to at least double the coefficient of friction on the RWD car's tires... which would mean, finding a winter tire that offers u = ~0.8, which is HIGHER than the average u of 0.7 on an A/S tire in DRY conditions. That's one impressive tire!

I'll give you this - going and stopping are two different things. AWD will help get the car going on A/S tires, but it won't ever stop as well as the RWD car on winters, which is probably its major deficiency.

Anyway, this is all one big tangent from the OP's root question, though, which is does the M5 become a better car for winter weather with xDrive, and the answer is undoubtedly yes. I believe it would become a better car for dry weather, just the same.

[JS919]

If I were you I'd wait for the 2029 K50 M5 LCI with M tractor beam mag ride. It will have way better traction than AWD with studded ice tires.

[JOHNBMWM5]

Originally Posted by ezmaass

So, this is comparing winter tires to summer tires - completely irrelevant, as I don't believe anyone here (including myself) is suggesting summer tires are appropriate for snow. Of course the summer tires won't grip snow.

Your second video, although comparing winters on 2WD to A/S on AWD, is flawed in a few other ways:

- The 2WD car is a FWD car, not RWD. That's going to make a significant difference in handling. FWD cars have the advantage of applying force in the direction of the wheels... not the case with RWD, or hence any non-xDrive BMW.

- The two cars are not identical (Subaru and Mini), so there are more variables at play here than simply their tire type and drivetrain.

- The video does note, however, that the acceleration advantage (read: finding traction to get the car moving) is still given to the AWD car with A/S... essentially better grip from a stop.

Really what's necessary here is a comparison between identical cars - one with A/S + AWD, the other with Winters + RWD. Like I've explained above, the math just won't add up to allow the RWD car with Winters to out-perform (out-grip) the AWD + A/S unless you can find a way to at least double the coefficient of friction on the RWD car's tires... which would mean, finding a winter tire that offers u = ~0.8, which is HIGHER than the average u of 0.7 on an A/S tire in DRY conditions. That's one impressive tire!

I'll give you this - going and stopping are two different things. AWD will help get the car going on A/S tires, but it won't ever stop as well as the RWD car on winters, which is probably its major deficiency.

Anyway, this is all one big tangent from the OP's root question, though, which is does the M5 become a better car for winter weather with xDrive, and the answer is undoubtedly yes. I believe it would become a better car for dry weather, just the same.

Watch this and then!!!!!!!!!!

Convinced?.
These are on my LCI M5.

[MJones3]

Originally Posted by ezmaass

Yes, I should have mentioned that I have indeed owned snow tires - much earlier-on with my RWD BMWs, until I decided they were unnecessary that is.

Traction is the result of friction between the tire and the ground. The resulting coefficient of friction between the two directly determines available traction. That coefficient of friction is also impacted by the downward force on it (weight of the car over each tire) - the more weight, the more friction. And yes, the compound of the tire itself, along with the surface its driving on - dry, wet, snowy, icy, etc.

The car's locomotive force is determined by torque force on the driving wheels; however, if the torque forces overcome friction, the wheels spin. In a RWD car, all available torque is spread across two driving wheels. Those two driving wheels also only have half of the car's total weight applied to them, which as noted above is a key part of the wheel's coefficient of friction. Assuming you're spreading the torque of the engine evenly across all four wheels in an AWD car, each with a proportional distribution of the car's weight sitting on top, your longitudinal (locomotive) force is going to benefit from 4x, versus 2x, available "traction" where your rubber meets the road. In a RWD scenario where the car is attempting to start from a stop, the two front wheels, along with their potential traction, just goes to waste - there are no longitudinal forces applied to these wheels. The rear wheels will still have the same coefficient of friction as they do in an AWD car, and they'll still be overcome (slip) at the same threshold of torque.

So does a RWD car with snow tires get more traction than an AWD car with A/S tires? No. You're not likely to DOUBLE the coefficient of friction ("u") simply by using a winter tire over an A/S tire - results I've seen are roughly 20 - 30% increases. If the average A/S tire received a coefficient of 0.7 on dry surfaces and a low-end of 0.4 on wet/snowy surfaces (taken from research of Jones and Childer "Contemporary College Physics"), your winter tires would need to DOUBLE the available "u" value on a RWD car to "make up for" the lack of available locomotive forces on the 2 front wheels. Good luck with that as a racing slick provides a relative u value of 0.9 on dry surfaces to put things in perspective. And that's just talking about longitudinal forces. Once you add in lateral forces (car needs to steer/turn), the picture grows even more grim for a RWD car compared to AWD.

Fact is, even without getting into the physics of it, common sense should tell you that AWD cars wouldn't even exist on the market if their performance could simply be matched by putting "better tires" on two driving wheels. It just isn't possible. Winter tires will increase the coefficient of friction in snowy/icy conditions over A/S tires, but they will NOT provide for 2x the performance of an A/S tire, as that would require 2x the u value - said another way, locomotive forces of 4 driving A/S tires is greater than locomotive forces of 2 driving winter tires if all else is equal (car, weight distribution, road surface, etc).

So, just to be clear - I would never recommend using a summer tire on ANY car to drive in the snow. They're just not suited and provide little traction once the temperature drops below freezing. But, as I've said above, winter tires are certainly not necessary... and if given the choice between the same car with AWD and A/S tires versus a RWD version with snow tires, I'd take the AWD car. The maximum performance will obviously be obtained by putting winter tires on an AWD car... making the choices, from worst to best:

1. RWD car w/ A/S tires
2. RWD car w/ winter tires
3. AWD car w/ A/S tires
4. AWD car w/ winter tires

All of the above is based upon simple physics - where forces are applied evenly. A system like xDrive, where forces can be dynamically shifted up to 100%, will even further emphasize its advantage over RWD, even with a limited slip differential. Now you have a system that can pick and chose among 4 wheels, monitoring their specific thresholds of traction, and shift torque to exactly where it can be used. In a RWD car with the back wheels sitting on a patch of ice, no amount of spectacular winter tire performance is going to save it.

That's all well and good but totally irrelevant when you consider the most important factor, which is stopping..
It must be very interesting to live in the US in the winter time when no one seems to ever use their brakes. A bit impractical if you want to get out of the car though..

[3rd party]

It appears there is substantial interest in an M5 AWD, hopefully BMW's research indicates the same and they bring it to market as expected.

I will wait for the M5 AWD just like I waited for the 530ix in 2005. I doubt I buy more than one M5 in my lifetime so getting one w the biggest possible improvement, imho, they could make is well worth doing w/out it for a bit.

[ezmaass]

Originally Posted by Zlaatan

That's all well and good but totally irrelevant when you consider the most important factor, which is stopping..
It must be very interesting to live in the US in the winter time when no one seems to ever use their brakes. A bit impractical if you want to get out of the car though..

I agree stopping is very relevant, and snow tires will perform better than A/S there - xDrive will have no bearing. However, stopping is hardly relevant unless you have enough traction to get going in the first place, as well!

As I said above, the best and ultimate combination is obviously winters plus xDrive.

[JOHNBMWM5]

BMW UK informed me yesterday that here in the UK the current F10 in right hand drive is not capable of having the XDrive.

So left hand drive cars are OK but not ours>

We will have to wait till the next generation of this M5 IF BMW decide to go XDrive.

Not a real deal breaker............. but for some it will be.

[Stiggly]

Originally Posted by JOHNBMWM5;*****269

BMW UK informed me yesterday that here in the UK the current F10 in right hand drive is not capable of having the XDrive.

So left hand drive cars are OK but not ours>

We will have to wait till the next generation of this M5 IF BMW decide to go XDrive.

Not a real deal breaker............. but for some it will be.

We both might be in M4/3's then mate

[JOHNBMWM5]

Originally Posted by Stiggly;*****461

We both might be in M4/3's then mate

Maybe, I am enjoying my M5 though, not letting it go any time soon.

[Stash21]

I think I will be reluctant to move to the next gen m5 if AWD is not an option.

I don't see myself moving to a m3/4. Great cars undoubtedly they will be.

[3rd party]

Can't say I've ever chained up a BMW, but I have countless other times on 4x4's.