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      01-18-2013, 08:42 PM   #23
KoenG
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Strange that you seem to believe there really is a substantial difference in composure. Well, there isn't. The AWD just has better traction in all conditions and for the 600 extra it really is a no brainer, really.
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      01-19-2013, 02:41 AM   #24
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Originally Posted by KoenG View Post
Strange that you seem to believe there really is a substantial difference in composure. Well, there isn't. The AWD just has better traction in all conditions and for the 600€ extra it really is a no brainer, really.
I haven't driven it, but it would make sense if the steering is different. At least it used to be in older BMWs in AWD version. Drive shafts going to the front wheels will usually make a difference to the steering feel and alignment. The extra weight up front will also affect how the car drives.

For me it is not a no brainer. At equal cost, I would take the RWD version.

Last edited by ovekvam; 01-19-2013 at 04:46 AM.
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      01-19-2013, 04:31 AM   #25
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Strange that you seem to believe there really is a substantial difference in composure. Well, there isn't. The AWD just has better traction in all conditions and for the 600 extra it really is a no brainer, really.
Every dealer told me there is a significant difference in steering and handling.

I'm testing the RWD at the end of the week, and I've also asked to test two identical cars where one is a RWD and one is an xDrive, just to feel how they compare. So I'm going to be driving a 320D RWD, 320D xDrive and then the M135i.
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      01-19-2013, 03:25 PM   #26
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Extra weight is about 70kgs, distributed predominantly to the front which is a bit less interesting for the stability of the yaw but it remains far from a comparable Audi. The centre of gravity is a bit lower which is better for the stability of the roll and the pitch.

It prefers sending power to the back so it's willingness to turn in is only mildly impacted. Also consider that the extra weight is probably not worsening the rotational inertion since it's rather close to the center of gravity. All this makes me think that the vehicle dynamics are not at all compromised.

I'am eagerly awaiting your judgement then, but as for what me concerns, I would be surprised when I would notice it, except for the better traction in 1st and 2nd gear during full acceleration and sharply turned in of course.

Last edited by KoenG; 01-19-2013 at 07:27 PM.
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      01-19-2013, 04:43 PM   #27
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I would go for the xDrive with the 8 speed auto. Better traction, better grip, better acceleration, and better qualities cart and then you get hydraulic servo like the M cars. I do not like electric steering, losing much fead back on the steering wheel. And BMW X drive is 40% front and 60% rear so it behaves like a rear wheel drive car. M 135i xDrive is plenty fast as soon as it's a bit wet on the road than M 135i and the small seconds it loses from 100km / h to 200km / h is nothing compared to the benefits it has. Also keep secend hands value.
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      01-19-2013, 04:49 PM   #28
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I would go for the xDrive with the 8 speed auto. Better traction, better grip, better acceleration, and better qualities cart and then you get hydraulic servo like the M cars.
Better traction, yes.
Better grip, no.

But I think you are right about the second hand value being better than for the RWD version.
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      01-19-2013, 04:57 PM   #29
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Remember that if you go too early on the gas midcorner the rears break traction, and as the power is sent to the fronts, it will understeer just like an Audi, only this isn't as predictable. Why spoil a perfectly good and capable sports car with AWD?
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      01-19-2013, 07:29 PM   #30
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RWD. Just accelerate sensibly in rainy conditions and you will be fine.
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      01-19-2013, 07:45 PM   #31
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I think some of you deny reality: lots of ooooomph will convert automatically into 4WD transmission lines. All M cars will become AWD eventually. I agree there is no more grip from the tires... but what's the point, you can't use the reserves in a RWD at all. When you loose grip, you still have grip reserves left in the front tires that you can't access. You need AWD to exploit the limits that RWD don't even offer to you. For example look at what the facelifted E63 amg gains... and compare it to the poor M5 with only his RWD.

4WD is the future for all high output cars. The M135i will be a better all day performance car with it. I think it is make believe purist mumbo jumbo to think that an M135i pure experience is compromised with xDrive. That's like stating that a Ferrari can't be real with a power assisted steering in it. All this is bygone romance. New school requires that you use all technical trics you have access to, in order to exploit the potential under all conditions.

For all these reasons: when you go AT, best combine with xDrive. When you like purist RWD and rear end loosing grip, while steering wheel is total freed from torque effects, go for manual transmission RWD while avoiding spending additonal 3000€ then. But don't believe the latter is the better all rounded car that is faster under most conditions since it won't.

Last edited by KoenG; 01-19-2013 at 07:53 PM.
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      01-20-2013, 04:47 AM   #32
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No, it wont be faster but it will be better, and most importantly, it won't be dull.
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      01-20-2013, 05:03 AM   #33
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I've tested an S3 a few years back. It was just glued to the road. You could take on virtually any turn at any speed without even making the tires squeal. I could tell it was very fast in comparison to other cars on the road, but I didn't really enjoy driving it. It was very clinical and dull.

I've tested an E90 320D xDrive too, about two years back. It wasn't dull like the S3, even being underpowered, but I didn't really like the steering of that car either. I loved the grip though.
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      01-20-2013, 06:29 AM   #34
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My experience is that you can brake later and enter corners faster with RWD than AWD cars. The AWD cars are however more stable (easy to drive) for rookies), and they have a traction advantage on corner exits and standing starts.

My problem with AWD cars is that it is more difficult to get a feeling for how much grip you have to spare. It feels like it is on rails until it is too late, and the rails leave the track/road, no matter what you do. With RWD you get more signals as you are approaching the limit, and I find it easier to save the situation.

Traditionally, most sports cars and race cars have RWD because it is more rewarding to the driver, and faster on dry tarmac. For rally cars and VERY powerful sports/race-cars, it does however make sense to have AWD to get the power down.

For cars with tuned turbo engines with lots of turbo lag, AWD can make it less difficult to drive. RWD is more suited to engines with quick throttle response.
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      01-20-2013, 09:12 AM   #35
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Felt like explaining a little bit of the physics involved when driving in a corner.

When cornering on the limit, weight is distributed close to 50/50 front/rear, and all the grip is used for transverse acceleration. As you exit the corner, the driver starts feeding in power. The added forward acceleration will shift weight from the front to the rear wheels. That mens less grip up front, and more grip in the rear. This is the same for FWD, RWD and AWD cars.

In a rear wheel drive car, the aim is to use the extra grip in the back for forward traction, while still cornering at the limit with only transverse forces up front. The power you add, the more weight shift will happen, and then you can add more power again. Cars with short wheel base and high center of gravity has more weight shift than long and low cars.

With AWD (and FWD) cars, it is impossible to add power without using less front wheel grip for transverse forces. That means you get understeer and a wider line once you start using the throttle in a corner. Some grip is lost in the front due to weight transfer, and even more because you are using it for forward traction.

In low grip (slippery) conditions, there is a lot less weight shift to the rear wheels. If you still want to use a lot of power out of the corners, there will not be much left for transverse forces in a RWD car. You need to straighten the line a lot. With AWD you can get the same power down with only half the traction forces in the rear, so you can corner tighter.

Basically this means that a car should have pretty much 100 percent of the driven forces on the rear wheels in high grip conditions, and then gradually less the more slippery it gets. Having more than 50 percent of the traction on the front wheels does not make much sense, unless the car is designed to have a very front biased weight distribution.
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      01-20-2013, 09:51 AM   #36
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Unfortunately I'm without (BMW) car at this moment, but I wish I had my future RWD M135i right now.
Next winter, I will put on winter wheels, and have a Quaife LSD installed...
And then when it snows, turn off the ESP and boh oh boy, let the fun begin. OK not while commuting to work, but on some low-traffic roads or parking lots in the weekend, that's when you can really enjoy the car.

If you never had high perf RWD car before, then take a 1 or 2 day course first, or else keep your ESP/DTC turned on...
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      01-20-2013, 06:28 PM   #37
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Quote:
Originally Posted by ovekvam View Post
With AWD (and FWD) cars, it is impossible to add power without using less front wheel grip for transverse forces. That means you get understeer and a wider line once you start using the throttle in a corner. Some grip is lost in the front due to weight transfer, and even more because you are using it for forward traction.
Ovekwam, You did a really nice write up until this part... There is a transversal vector, which is orthogonal to the length ax of the car, and a longitudinal vector in the length of the car. the front tires on a RWD car leave the longitudinal vector rather unexploited. The AWD version will exploit it by pushing torque on the front tires.

When you check what the tire is going to do, you have to vectorial combine the longitudinal and transversal components together. When the combined forces exceed the grip of the tire, it slides.

When you check the grip "oval" of a front tire, you'll see that max g's are exploited when you combine transversal and longitudinal components. Assume theoretical that limits transversal and longitudinal are the same (they aren't since longitudinal are better, but anyway), you could converse 1.4142 times the transversal force to the FWD.

I agree that the transversal component will be a bit smaller on AWD, but the longitudinal one is clearly bigger. The overall G force on the AWD will be bigger during all circumstances. This will convert the AWD to a winner during circuit driving conditions while providing a comfort of mind to the one's not trying to set lap records.

Oh yeah, I brought it up... an xDrive AT will certainly lap faster on the Nordschleiffe than the RWD AT. Certainly with less swet involved, agree on that. But when you state, the max attainable lateral g's are best on the RWD, I agree but the difference is really small (about 1 percent) and not compensating enough for the lack of traction.
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      01-21-2013, 02:04 AM   #38
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Quote:
Originally Posted by KoenG View Post
Ovekwam, You did a really nice write up until this part... There is a transversal vector, which is orthogonal to the length ax of the car, and a longitudinal vector in the length of the car. the front tires on a RWD car leave the longitudinal vector rather unexploited. The AWD version will exploit it by pushing torque on the front tires.

When you check what the tire is going to do, you have to vectorial combine the longitudinal and transversal components together. When the combined forces exceed the grip of the tire, it slides.

When you check the grip "oval" of a front tire, you'll see that max g's are exploited when you combine transversal and longitudinal components. Assume theoretical that limits transversal and longitudinal are the same (they aren't since longitudinal are better, but anyway), you could converse 1.4142 times the transversal force to the FWD.

I agree that the transversal component will be a bit smaller on AWD, but the longitudinal one is clearly bigger. The overall G force on the AWD will be bigger during all circumstances. This will convert the AWD to a winner during circuit driving conditions while providing a comfort of mind to the one's not trying to set lap records.

Oh yeah, I brought it up... an xDrive AT will certainly lap faster on the Nordschleiffe than the RWD AT. Certainly with less swet involved, agree on that. But when you state, the max attainable lateral g's are best on the RWD, I agree but the difference is really small (about 1 percent) and not compensating enough for the lack of traction.
+1 well spoken
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      01-21-2013, 02:35 AM   #39
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If AWD was better than RWD then all Formula 1 cars would be AWD..
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      01-21-2013, 03:43 AM   #40
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Quote:
Originally Posted by KoenG View Post
Ovekwam, You did a really nice write up until this part... There is a transversal vector, which is orthogonal to the length ax of the car, and a longitudinal vector in the length of the car. the front tires on a RWD car leave the longitudinal vector rather unexploited. The AWD version will exploit it by pushing torque on the front tires.

When you check what the tire is going to do, you have to vectorial combine the longitudinal and transversal components together. When the combined forces exceed the grip of the tire, it slides.

When you check the grip "oval" of a front tire, you'll see that max g's are exploited when you combine transversal and longitudinal components. Assume theoretical that limits transversal and longitudinal are the same (they aren't since longitudinal are better, but anyway), you could converse 1.4142 times the transversal force to the FWD.

I agree that the transversal component will be a bit smaller on AWD, but the longitudinal one is clearly bigger. The overall G force on the AWD will be bigger during all circumstances. This will convert the AWD to a winner during circuit driving conditions while providing a comfort of mind to the one's not trying to set lap records.
You seem to assume that there is always a longitudinal component of the force on the contact patch. That is not the case. If you maximize the transverse component, there is no grip left for forward traction. If you choose to sacrifice the transverse force by adding forward traction on an AWD car, the overall force will not be any higher. The more forward force you add, the more transverse force you have to take away. If both components are the same, they will both be around 70 percent of the maximum transverse force. The vector sum is still not higher.

At this point, the cornering ability is down to 70 percent, and the car is accelerating. This combination indicates that the race line straighens a lot, so you can not do this before the very end of the corner. A RWD car can start feeding in power earlier and still keep a tighter line, since it is still using the maximum cornering ability by by not reducing the bottleneck transversal force up front.

In very tight corners, the AWD works well, since you need to use a lot of power and straighten the line quickly. In a long sweeper corner, RWD is more suitable, since it can corner better while accelerating moderately.

Quote:
Originally Posted by KoenG View Post
Oh yeah, I brought it up... an xDrive AT will certainly lap faster on the Nordschleiffe than the RWD AT. Certainly with less swet involved, agree on that. But when you state, the max attainable lateral g's are best on the RWD, I agree but the difference is really small (about 1 percent) and not compensating enough for the lack of traction.
Lap records are set with good tires on a dry day. In those conditions, I am pretty sure the RWD version would set the best time. The average speed around the track is quite high, so the cars will not be traction limited except for the slow corners. The traction advantage will not make up for the 70 kg extra weight on the AWD car and less optimal weight distribution.

On a wet lap, the AWD car would probably win.
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      01-21-2013, 03:44 AM   #41
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If AWD was better than RWD then all Formula 1 cars would be AWD..
Except for the fact that it's not allowed. A four wheel drive formula one car would be competetive on wet tracks.
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      01-21-2013, 06:45 AM   #42
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There must be a reason why rally cars are AWD, and (most) GT/circuit cars are RWD.
Different needs have different solutions...
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      01-21-2013, 05:17 PM   #43
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Originally Posted by ovekvam View Post
You seem to assume that there is always a longitudinal component of the force on the contact patch. That is not the case. If you maximize the transverse component, there is no grip left for forward traction. If you choose to sacrifice the transverse force by adding forward traction on an AWD car, the overall force will not be any higher. The more forward force you add, the more transverse force you have to take away. If both components are the same, they will both be around 70 percent of the maximum transverse force. The vector sum is still not higher.
OK, you're an intelligent guy (not that I didn't know)... assume the max attainable grip line is a perfect circle, like you take in your reasoning. Then I follow you: vectorial sum is the same size. So the AWD has the liberty to let you shape the force in the front-tire: or 100% transversal or 70/70% split or even further biased towards longitudinal traction or any combination in between.

Take one step back for now, you enter the corner and you're before apex. During these conditions you're either braking or approaching at constant speed. It's during this phase that understeer is felt most and a real joy killer. AWD and RWD are identical in this phase, except the extra weight and inertion and lower placed center of gravity. But that difference is not noticable (by an amateur like me at least). During this phase you want the max. attainable transversal grip.

The second phase after APEX, you want to pick up speed while "opening" up the turn and gradually reduce steering angle. Here the AWD will send torque to the front tires, and by consequence have a higher tendency to understeer. Because of the mechanism you describe, the max attainable transversal grip is smaller, but in this phase the max is rather never exploited, or you have done something wrong and need to recover. On top, a higher tendency to understeer is not felt as a joy killer in the second half of the turn since the car is already anticipating on reducing steering angle via the accelerator even before you actually steer out.

I believe that this is what intelligent AWD systems tend to do: before apex, don't engage Front wheels to avoid understeer in this critical phase and don't compromise max attainable transversal grip, after apex, engage the Front wheels to help straightening the car and max acceleration.
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      01-21-2013, 07:13 PM   #44
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This is my favourite thread since a few days. Pretty good discussion here. For me very simple, slippery road conditions and cold weather to handle most of the time then get the xdrive, dry and clear weather dominating where you live get a rwd. If it is your only car and you have a family with kids, be responsible and get the xdrive in 5 door regardless of road/weather conditions.

I would love to have a M135i xdrive five door, only to company my 1M. rwd-6 speed-manual-2 door coupe in VO vs. xdrive-8 speed auto-five door hatch in Estoril Blue. Different beasts for different moments. Perfect 2 car garage.
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