Good structural stiffness makes a huge difference to the handling of any car. If a platform is highly resistant to tensional bending, the suspension geometry will remain accurate under load and work as its designers meant it to.
Scoring 14,500Nm/degree in BMW’s torsional stiffness test, the M Roadster is pretty stiff for an open car, and despite its wide 225/45 and 255/40 rubber on 8.0J and 9.0J x 18-inch alloys rubber, scuttle shake on poor roads in minimal. Even so, it does not come close to the Coupé’s impressive 32,000 Nm/degree, which is more than double the Roadster’s rigidity.
This extra structural strength means that the engineers have been able to dial in much more aggressive suspension settings knowing they will be maintained better even under high lateral loadings on a racetrack.
The springs and dampers are a completely new set-up. The front spring rates are increased by about five per cent, but because the rears are progressive rate it is hard to put a comparative number to them. It is easier to say that that the first half of their travel is similar in rating to the Roadsters, but once that point is reached, their rate rises faster. The characteristics of the Sachs/Boge dampers are calibrated to match.
What makes the Coupé more pointy however, is the combination of a quicker 12.8:1 steering ratio (Roadster 13.7:1) and a 1mm thicker rear anti-roll bar, which combine to mitigate some of the understeer you feel in the Roadster. “Because the Coupé shell is much stiffer, we can load the chassis more on initial turn-in and so improve overall handling response,” explained BMW M’s chassis guru, Gerhard Richter, who is also one red hot test driver
