At the International Media Launch of the new 2011 BMW 5 Series, our very own Benny at BimmerToday, had the opportunity to speak with Jos van As, the man responsible for the chassis tuning and the suspension in the new sport luxury sedan. Some of the topics that we wanted to touch in our discussions also revolved around the active rear axle kinematics and the issue of run-flat tires that has been going through an intensive debate on our website.

BT: The new BMW 5 Series features again an active rear axle kinematics. It has been 10 years since a similar system was offered in the BMW 850CSi. Why the long wait?

JvS: In the early 90s, this subject was mainly interesting for added stability, but in the same time, electronic stability systems, like DSC, appeared and made mechanical systems, like the one we used in the 850CSi, unnecessary. But then the cars got bigger and heavier and we had to find a way to make them handle like before, despite the increased weight. In the 90s, we used an eletrohydraulic system for the rear axle,  and now we can use an electromechanical system that can be combined with the variable steering on the front axle.

BT: How does exactly the current system work?

JvS: The system brings additional customer benefits in two speed ranges. At low speed, such as in parking, turning or switchbacks in the mountains, you have a car that feels a lot smaller than it actually is. In these situations, the rear wheels are steered in the opposite direction than the front wheels and we get additional agility from this. At higher speeds, the customer benefits from increased stability because now the rear wheels are steered in the same direction as the front wheels .  Therefore, we can make the car drive as if it had a larger wheelbase. We launched the active steering in the E60 model, but back in those days, we only had it for the front wheels. Now we can use it for the rear wheels as well. This is the reason why you can not buy the system for the front wheels only anymore because we see this as one big package.

BT:  Are there any differences from the  system found in the new 7 Series?

JvS: Yes, in the 7 Series case, we are using an hydraulic steering system, but in the new 5 Series we are using an electromechanical system. There are no differences at the rear axle though.

BT: Why are you using different systems at the front axle?

JvS: We decided to do so because we, as the drivetrain team, wanted to contribute to the CO2-emission-reduction as well. The electric system we are using in the new 5 Series brings a benefit of about 4 gram CO2-emissions less compared to an hydraulic system.

BT:  Adaptive suspensions seem to be a lot better than in the past and in the new 5 Series you can almost talk about two different cars depending on the setting of the suspension.

JvS: Yes, this is mainly caused by the further development of the dampers in the last few years. The difference between the two opposite settings has been enlarged very much and therefore, we can give you a bigger difference between the settings as well. In the past, we also had to chose between specific settings that were indicated by the customer, but we could not do anything in-between. Now we can offer our customers the perfect setting for the current road conditions and the driving scenarios, because the systems adapts itself to the situation it is confronted with.

BT: Are there any disadvantages regarding security when driving in the soft settings, e.g. due to more body-roll and less ability to corner? For example, we could imagine a situation when the car comes to a corner and it cannot drive through with the comfort setting, and the DSC-sensors notice this, does this affect the settings of the dampers?

JvS: It actually works the other way around. When you are entering a corner, the dampers are immediately adjusted and become stiffer so that you have perfect handling, no matter which setting you are using.

BT: So when we’re in comfort mode, we can benefit from more comfort on straight roads, but if I have to corner…

JvS: …or if you have to evade some obstacles, the system automatically switches to a stiffer setting and you can corner with the maximum capability of the car. When the situation has calmed down, the previous setting is activated again. The focus is on optimal customer benefit: as stiff as necessary, but as comfortable as possible.

BT: We’re sure that lots of surveys were made among the E60 owners. Where they asking for more comfort or more sportiness?

JvS: It was rather more comfort. In the previous model, we already offered a sport suspension in combination with the active roll stabilization which is a combination that proved to be very popular in the U.S. and other markets. So we had some very sporty offerings and now we are combining this sportiness with more comfort.

BT: Many people see run-flat tires as one of the biggest faults in riding comfort. What do you think about this?

JvS: There has been a lot of development in the past and without a doubt the first run-flat tires had some issues with comfort levels. The Run-Flat tires offered today are way better and we even made some blind tests with some of our developers and with normal customers to see if they can tell the difference between run-flat and regular tires. And, they usually can’t.

So the normal customer will hardly notice any difference with today’s tires.

BT:  So will run-flat tires present an interest to BMW M for their future cars?

JvS: Thats a bit of a philosophical question and it also depends on how you are positioning a car. There are some things that are very important for the M GmbH, but not for every BMW. For example, they are using other tire sizes and they want as few unsprung weight as possible, so run-flats are not very attractive for their purposes.

BT: So, for the normal 5 Series customer it makes no difference.

JvS: No, except from the safety benefits of course. We also know from our 1 and 3 Series cars, where the run-flat tires are optional on some models, that our customers really want them, so the market acceptance is not a question at all. There are even markets like the U.S. and China where you simply have to offer run-flat tires because these customers want a high level of safety and a very high level of comfort. The chinese customer doesn’t leave his car when he has a flat tire, so in such markets run-flat tires are essential.

BT:  We assume xDrive 5 Series models will be offered again. How many of your customers decided to buy xDrive cars in the previous generation of the 5 Series?

JvS: It depends on the market. There are some markets like the U.S., Switzerland, Austria or Canada, where all-wheel-drive is very important and about 50% of our cars are equipped with xDrive. In other markets, it is less important.

BT: Jos, thank you for the informative interview!