As tractor manufacturers have more effectively isolated engine noise and vibration from the cabin, so other sources of disturbance have become more intrusive. Peter Hill reports on a tyre manufacturer’s efforts to redress the balance

Traction tyre development traditionally focuses on a new design’s ability to grip the field surface, shed itself of soil and carry loads at modest inflation pressures.

But, as tractors have become used on the road at faster speeds and over longer distances, factors such as vibration, ride comfort and handling characteristics have also come more important.

Czech tyre group CGS, which makes Continental agricultural tyres, paid particular attention to the tyre as a source of noise when developing its latest design, a large tyre with high-speed capability.

The aim was to reduce in-cab noise generated by the lugs hitting tarmac and the engineers involved appear to have succeeded, in no small measure.

CGS claims the noise level at the driver’s ear is halved in comparison with the same tractor running on other tyres and a test conducted by the German farmers’ organisation DLG largely supports that claim.

“The Continental SST is not only an advanced and effective design in terms of its speed, load and tractive capabilities, it also makes a significant contribution to driver comfort,” says Andrew Mabin of CGS Tyres.

The SST – SilentSpeedTyre – is a rear fitment traction design for tractors of typically 150-200hp. It is available initially in two popular low-profile sizes – 600/65R38 and 650/65R38. Both are “D” rated to run at up to 70kph.

Outwardly, the newcomer looks similar to the established ContiContract AC 65 and AC 75 but internally things are a little different, in terms of the materials used and the way the carcass components are arranged and formed into a single structure.

The design is claimed to deliver improved penetration, low rolling resistance and better damping to minimise bounce. But particular attention was paid to the vibration frequencies generated at fast road speeds.

“The tyre lugs transmit impulses to the driver’s cab, which forms a sound box,” explains Hans-Ulrich Klose, CGS automotive engineering and product manager. “Between 40 and 60kph, a critical frequency range of 110 to 120 hertz develops, resulting in a sharp increase in the noise level in the cab and which is perceived by the driver as a loud droning sound.”

Disrupting the transmission of lug-generated vibrations reduces the “booming” generated in the cab by the tyres to the extent that CGS claims a 3-6 dB(A) cut in noise level – which thanks to the linear scale of noise level is registered by the human ear as a halving of the noise.

When the DLG organisation put this claim to the test it compared the SST and AC 65 rear fitments by measuring noise just behind the driver’s head as the operator drove a 165hp six-cylinder tractor along a flat and relatively smooth tarmac surface. Measurements were taken in 1kph increments between 40kph and just over 50kph.

“At all the driving speeds, the noise level with SST tyres fitted was substantially quieter than with the conventional tyres,” says DLG test engineer Friedrich Uhlig. “With the rear window closed, the average difference was 2.4 dB(A) and with a maximum of 3.6 dB(A) occurring at 46kph.”

The difference was slightly less with the rear window open – 2.0 and 2.8 dB(A), respectively, the biggest margin occurring at 48/49kph.

Mitas traction tyre

Additional sizes are becoming available of the Mitas RD-03 traction tyre, a 65-series low profile radial for tractors up to 200hp.

Manufacturer CGS Tyres claims up to 40% traction advantage when a 75% aspect ratio tyre is exchanged for a 65% RD-03 of equivalent rolling radius but greater width, together with high load capacities.

Rated for use at speeds up to 70kph, the six new sizes comprise a 480/65R24 front fitment; three mid-range sizes – 600/65R28, 540/65R30 and 600/65R34 – and two bigger sizes – 710/70R38 and 650/65R42.

[Mit RD03 600/65R38 RZ.jpg]

Mitas RD-03 for 150-200hp tractors.