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Kerbs a critical factor at Imola 20 Apr 2004

Jarno Trulli (ITA) Renault R23.
Formula One World Championship, Rd4, San Marino Grand Prix, Imola, Italy, 18 April 2003

Why San Marino punishes damping and transmission

The San Marino Grand Prix circuit has assumed very particular characteristics since the changes introduced at Imola nine years ago following Roland Ratzenberger’s and Ayrton Senna’s deaths.

The track essentially consists of a number of flat-out blasts, punctuated by chicanes which the drivers must attack in order to set a quick lap. The implications for the car are numerous, and Imola consistently sees one of the highest mechanical retirement rates of the year. Two areas that come in for particular punishment are the dampers, to cope with the severe impacts over the kerbs, and transmissions, as Renault’s Executive Director of Engineering Pat Symonds explains:

“An F1 car employs mono-tube dampers: these are high-pressure gas shock absorbers with a single pressure tube, inside which there are two pistons: a dividing piston and a working piston. The dividing piston separates the oil charge (located above the dividing piston) and the nitrogen gas charge, which fills the area below the dividing piston. The gas charge is pressurized to between 3.5 and 5.5 bar, helping to support some of the car's weight. During operation, the dividing piston moves up and down as the piston rod moves in and out of the shock absorber according to the movements of the wheel, thereby keeping the pressure tube full at all times.

“Imola registers some of the highest peak damper velocities of the season. Damper velocity measures the speed at which a damper is moving after striking a kerb, and thus gives an indication of the severity of each impact. To illustrate this, if we look at a graph comparing damper velocities at Imola, the most demanding circuit of the season, and Magny-Cours one of the easiest: the data from Imola reveals a greater number of severe spikes, which correspond to the car's frequent passage over the kerbs.

“However, when the team measures the work the dampers do, we do so in terms of the power that the damper has to absorb at a particular frequency: as a general rule, it is fair to say that high frequencies equate to high damper velocities. Here once again, a comparison between Magny-Cours and Imola illustrates how far beyond a 'normal' operating range the dampers are forced to work at the Italian circuit: even at a low frequency of 10 hertz (Hz), we register 78% more power input than at Magny-Cours; at 100hz, this figure rises to 263%, and at 200 Hz, we see 229% more work. However, this is actually far below the maximum the dampers can take: when Fernando Alonso's car flew into the air at the start of the 2003 Italian Grand Prix, the initial impact registered velocities over 23 times higher than those seen over kerbs!

“The kerbs also make the circuit very hard on transmissions; indeed, 62% of all retirements in the past three years have been engine or transmission related. In effect, as the drivers complete a lap, the car bounces over the kerbs: one of the best analogies of the sensation is to a jump jockey going over the fences. For transmissions, this cause damage not in the initial impact but through the second part of the chicane, when the cars are often at full throttle, and land in this state. When this happens, the transmission effectively works like a spring, loading up when the wheels land and grip, before needing to release this energy. This kind of impact produces extremely high shock loads on the transmission and while they are hard to measure because they are very quick events, it is fair to say that the values represent three times the torque produced by the engine in normal conditions - it would not be unreasonable (although not totally correct) to suggest that the forces when the cars land are equivalent to having an engine of 2500 bhp. Predictably, this can lead to failures of both the gear ratios themselves, and the driveshafts.”