THE ART OF DEVELOPING THE PERFECT TYRE.

BMW M Magazine: For the last few decades, BMW M GmbH has relied only on selected premium tyre makers. What advantages does working with a narrow selection of manufacturers have over a wider spectrum?

Frank Weishar: There are only a few manufacturers who can really do complete justice to the demands of high-performance vehicles. This means not only maximum grip. It’s more like a combined event in athletics: it’s a question of grip in the dry and the wet, rolling properties and acoustics, steering behaviour, wear – to name but a few aspects. Specific, focused collaboration with a small, selected circle enables us to get the most out of the tyre component in the interaction with a high-performance vehicle.

Why is continuous collaboration with just a few manufacturers fundamentally better?

With continuous collaboration, both BMW M and its partners know exactly what the requirements are and can draw on a broad spectrum of experience in order to fulfil them. It’s like in a successful human partnership – not everything has to be discussed in the tiniest detail on every project any more. This applies both to project work and the engineers who test the tyres both for BMW M and the manufacturers. You have to achieve a result and for that all the engineers have to speak a common language and make this comprehensible for the tyre designers. The better that works, the better the result.

How many development loops/compounds were created? How many are there as a rule?

At BMW M2 CS we were able to achieve a very good performance level fairly quickly and could concentrate on the fine tuning, because we had the experience of developing tyres for the M2 and M2 Competition. As a rule, because we’re dealing with a mixed set of tyres with different dimensions on front and rear axles, five so-called specifications are made per axle. These can differ in the substructure of the running surface and in the different rubber compounds that come into play in the construction of a tyre. These combinations are tested with the tyre designer on the target vehicle and combined according to handling in order to achieve the best result. This procedure is called a development loop. If the result isn’t right, the lessons learned are used for the next development loop. It’s an iterative process with the aim of approaching the optimum together with the chassis set-up. The exact number of loops is a company secret, sorry!

What is the Michelin Pilot Sport Cup 2, which was specifically created for the M2 CS, designed to tease out?

On the one hand, increased grip potential. But that’s not all that can be achieved by specific tyre development. Because of the mixed tyres on the front and rear axles, we were able to develop separate tyres for each, which means that a combination of improved/faster steering response and much greater rear axle stability could be achieved under lateral and longitudinal acceleration. Put simply – even more sheer driving pleasure.

Can one say that vehicles with a lot of torque – like the M2 CS – benefit in particular from a tyre with strong grip?

Yes. And that is measurable in lap times on the racetrack.

What can you say about the characteristics of this special Michelin Pilot Sport Cup 2 in detail?

This track tyre combines excellent performance on the racetrack with good everyday suitability. This is achieved via a bi-compound technology. For ideal dry grip, different compounds are used on the outer tyre surface than on the inside. On the inside, harder elastomers guarantee the highest level of steering precision and good grip on wet surfaces. On the outside, compounds are used that have been optimized for grip and use on racetracks and can also deal with the correspondingly higher temperatures.

What advantages arise when the vehicle is driven on the racetrack?

On the racetrack, you achieve maximum grip under lateral acceleration with further improved durability combined with good grip in the wet, which for a track tyre is not a given. Thanks to the explicit matching of tyres to vehicle, precision, feedback and driving pleasure can once again be raised to another level.

What is the difference to a normal sports tyre?

The difference lies, as you mentioned, both in the compound and in the tread compared to a normal sports tyre. A track tyre has fewer, bigger tread blocks, so it has a lower void ratio and a higher level of stiffness. Thus, the field of use can and is slewed more towards racetrack use and track days than everyday use on the road.

Does this translate into lap times? On the Nordschleife for instance?

It’s certainly in the area of several seconds. The absolute gain in time as always depends on the ambient and asphalt temperatures, and of course on the driver and their style of driving.
 

On the front 245/35 ZR 19 and at the back 265/35 ZR 19 tyres, why are the tyres different sizes?

With the M2 CS, as with other rear-wheel drive BMW M vehicles, we’re talking about high-performance automobiles with an extremely high performance level. To do justice to the performance, the rear tyres are wider than the front, as they have to transmit both the driving and lateral forces.

Do wider tyres automatically mean better traction?

Put simply, yes – with the same tread and the same compound compared to a narrower tyre, as they increase the contact of the rubber surface on the ground. But only going on the size, without taking the tread and the rubber into account, would be wrong. Rim width and tyre contour also influence the traction and are important parameters.

With such a dynamic high-performance vehicle with a short wheelbase like the M2 CS, how does one get comfort in everyday driving and perfect directional stability?

Thanks to the fast response from the tyres and the corresponding feedback from the axles, the driver has a constant response from the driving surface. Thus, drivers can react to the smallest disturbance. With unmatched tyres, even combined with a perfect suspension, there will be a delay in reaction, including the car’s response after a steering command driving error or a correction by the driver. This leads to a subjective feeling of bad directional stability, because it is not enough to keep hold of the steering wheel – one will also be forced to keep making corrections. Here too, the tyre plays an essential part, but this should always be seen in the context of the interplay between tyre and suspension.

What should drivers with semi-slick tyres look out for on wet roads?

In particular, standing water – puddles, rivulets in damaged road surfaces are a danger, as the emphasis on better dry performance means that the depth of the tread is lower. Thus, less water can be displaced compared to a sports tyre and the tyre aquaplanes sooner. The absolute level of grip, too, especially at low temperatures – when you drive in damp conditions at temperatures of 10°C and below – is lower than with a sports tyre. Here however, we take care during development that the threshold is not too acute and that transparent feedback is available at all times. That means that the transition between static and dynamic friction does not occur too abruptly and both driver and control systems have enough time to react.

The performance of high-performance vehicles is rising, at the same time suspensions are being tuned in more and more detail. Does this mean that in future there will have to be more specialized tyres developed for individual automobiles? Or to put it another way, to transfer the increasing performance to the asphalt, will uniform sports tyre types no longer work on more than one model?

It is already the case today that BMW develops and fits as standard tyres marked with a star for all vehicles in the BMW Group in order to guarantee the driving dynamics BMW is known for and the driving pleasure. At BMW M GmbH, we can match the tyres even more precisely to the relevant model and develop them with our partners, because our basic conditions are not so broad.
That means that for example per vehicle family in the high-performance-segment (M2, M3, M5, X3M and X5M) there is one engine as a rule and thus not the whole range of an engine portfolio and associated variations have to be taken into account in terms of weight distribution. With models like the BMW M2 CS, this is even more acute and we specifically develop as described just one tyre explicitly for this vehicle.

Thank you for talking to us, Frank!