The Gran Turismo represents an entirely new way of producing bus seats. Based on widely differing requirements, the Italian company Grammer S.r.l. has developed a seat that surpasses the EU safety demands, while also improving comfort. The weight of the steel seat frame has been cut by 30 percent, which corresponds to 15 percent of the entire seat weight.

Although at first sight the Gran Turismo looks just like an ordinary bus seat, it actually represents a new generation compared to the heavy welded steel seat structures fitted to almost all touring coaches.

Before the tubular frame of the new seat is covered with polyurethane foam and fabric, the difference is easy to see. There are a few welded parts, and all important components are bolted together. This is part of the new concept that is inherent to the Gran Turismo.

The prototype of the Gran Turismo was shown at the bus exhibition in Kortwyk, Belgium at the end of 2003. This basically looked just like the seats that are now ready for going into production.

“But we then realized that the industry wanted much lighter seats,” explains Sandro Guidoni who, together with Agostino Mengoni and Eleonora Mancinelli, bore main responsibility for the development project at Grammer S.r.l. in the Italian town of Monsano.

Grammer then got in touch with SSAB Swedish Steel for discussions on whether high strength steel would be preferable to conventional steel grades. By that time, aluminium and magnesium had already been evaluated and ruled out as conceivable alternatives.

“The costs of using both materials would have been far too high,” says Sandro Guidoni. “Compared to steel, magnesium could lower the weight but would be much more expensive. Aluminium would not be all that much lighter, since heavy aluminium profiles would be needed to meet the EU requirements.”

Grammer uses subcontractors for basically all steel parts included in the seats produced by the company. For the development work on the new seat, close cooperation was therefore established with several companies, ranging from a steel supplier to tube manufacturers and presswork companies.

“But this could be regarded as a natural extension of the team organization we employ at Grammer,” explains Innocenzo S. Carbone, Managing Director of the company. “We have given the project new resources by taking on new players.”

As optimum compromise between cost, strength and formability, Grammer specified two different advanced high strength steels for the new seat. The frame structure of the seat consists of two “profiled tubes” made of cold-rolled, ultra-high strength, dual phase steel with a minimum tensile strength of 800 N/mm2. The same steel is used for the brackets that secure the seat to the chassis and enable the backrest rake to be adjusted. These pressed parts are simple, but they are subjected to high stresses during the expected useful life of the seat. They must be strong enough to withstand the massive forces occurring in an accident, but must also be able to withstand day-to-day fatigueloading. The bottom fixing points of the seatbelt are secured directly to these brackets.

Several other components of the seats produced by a forging process are also made of the same grade of steel. In its design work, the technical department assumed only 2 mm thick material for all of these metal parts in order to simplify the logistics and material purchasing. The remainder is mainly made of round tubes of cold-rolled, extra-high strength, dual phase steel with the minimum tensile strength of 600 N/mm2 and a thickness of 1.2 mm. Such tubes are used for forming the backrest frame. The backrest is designed to enable the seat to be equipped with integrated threepoin seatbelts.

“The Gran Turismo undoubtedly represents a new generation of bus seats,” considers Innocenzo S. Carbone. “A new generation of seats for the next generation of buses.”

Text: Håkan Johansson News Magazine No 1 2005