BMW Run-Flat Problems

The run-flat tyre has been the desire of many tyre manufacturers over the last 30yrs. Its obvious safety features encouraged exhausting development advances but despite this the original embryo the Run-Flat tyre looked like it was deemed to failure.

In the late 90's Bridgestone launched their version of the RFT (Run-Flat-Tyre) which in essence is not too distant from an ordinary tyre in appearance. Nevertheless this tyre, undressed is far from similar to a conventional tyre.

Bridgestone's design required the tyres sidewall to be reinforced, "armoured" is a closer description so that the tyre is able to support the car when fully deflated. I, for one salute Bridgestone for finally bringing this necessary safety feature for the first time in a useable context, but unfortunately there are problems.

Most people would know that the tyre full of air has a reasonable level of compliance acting as our first line of defence deflecting the poor road condition however things appear to be quite different with the Run Flat. By compromise, all BMW's fitted with RFT's have downgraded (softer) suspension to absorb the migration of these road shocks. Unfortunately this doesn't appear to be perfect as we are starting to notice that this shock migration appears to transmit some of these forces to the wheel rim, and actually cracking it in some cases without amazingly, any apparent harm to the tyre itself. We are currently monitoring this but it is something that owners of Run Flat Tyres must keep a keen eye on and in the avoidance of doubt... stay away from pot holes.

As with a lot of things today we have to make compromises and the safety benefits of Run Flat Tyres are not in question, though we will continue our studies into this shock migration very thoroughly. However, "What about the actual inconsistent and premature tyre wear?" Wheels In-Motion, surprise surprise, has found the solution!

The BMW suspension allows superb handling characteristics providing that the tyres contact patch can be maintained. In order to do this the wheels vertical position (camber) is tilted and along with careful and sensitive suspension and geometry adjustments to compliment this, allows both tyre and suspension to follow the same radii thus maintaining that all important tyre contact patch.

The consequence of camber alone conically deforms a normal tyre sidewall, however if the correct maths are used in suspension adjustment, geometry and alignment thus allowing the cars weight to be evenly distributed across the tyre, all can be encouraged to work together in harmony.

The problem some BMW's have is that the RFT's do not deform and do not allow a conical profile because of the rigidity of the tyre and sidewall, meaning they cannot distribute the cars weight evenly easily, but before you depart your beloved BMW in despair, once again we have solutions to address this.

We can establish the camber curve and adjust the chassis accordingly allowing the RFT to maintain a solid, predicable footprint during the suspensions transitions. Our chassis adjustments belays the fact the RFT cannot deform, maintains the BMW handling and prevents the adverse tyre wear. Once again, regular alignment checks and calibration by Wheels In-Motion will maximise tyre life and handling without compromise. It just needs a little more thought, care and attention.

Tony Bones (Blackboots / Wheels In-Motion)

Further reading - RFT: Tramlining