Economic and environmental aspects of using HSS in the trailer industry

Using high-strength steel (HSS) to develop lighter and stronger trailers can have a big impact on the economical performance of the vehicle. It is well known that both trailer manufacturers and transport operators can benefit financially, but the advantages in terms of lower CO₂ emissions should also be recognized. 

When looking into the financial benefits for the trailer manufacturer, it is important to consider all aspects that affect the overall production economy. Simply comparing price level per ton for different steel grades fails to provide an accurate picture of the manufacturing cost level. In most cases, reducing the sheet thickness will provide a significant cost reduction in both the processing of the material and material cost. Even if the price per ton is higher for HSS, less steel is consumed due to the weight reduction. Using thinner gauges in the workshop allows the cost of cutting, bending and welding to be reduced. Laser cutting in high-strength steel is no different from cutting in mild steel, and the producer will decrease the cutting time due to the thinner gauge. In most cases, welding thinner material provides the largest cost reduction due to the reduction of consumables and the opportunity to increase the welding speed. Introducing HSS with good bendability can also reduce the number of welds needed. Profiles in HSS generally do not require greater force to bend than a profile in a thicker gauge made from conventional steel.  However, the spring-back of HSS is greater compared to conventional steel and needs to be compensated for in the process. 

To give a better insight into these issues, a comparison between the production costs of a traditional flatbed trailer chassis and a lightweight solution manufactured from HSS was performed. The traditional trailer chassis studied here is manufactured from hot-rolled standardized I-beams, which are cut and welded back together in the goose-neck region to create the height transition of the main beams. The cross-members consist of bent profiles welded to the longitudinal beams and there are also some side-wing profiles to support the floor. The side rail profiles are manufactured from standardized U-beams. All parts are produced from a S355-steel grade. In the lightweight solution the hot-rolled I-beam has been replaced by a laser-cut and welded longitudinal I-beam manufactured from HSS. Upgrading the traditional chassis by introducing HSS allows for a reduction in the thickness of all major structural parts and in the weight of the chassis by up to 1,500 kg. 

In addition to the weight reduction, a production cost reduction of up to 30% can be observed (see graph). Cost have decreased for both cutting and welding operations. In this case, a slight increase in the bending cost was observed. Additional bending is required due to design issues, necessitating the introduction of new profile cross-sections. A 30% cost reduction offers obvious benefits for the producer, and when combined with a more attractive lightweight trailer, great market advantages can be expected. It is also noteworthy that this study was conducted on an existing chassis whose structural parts were primarily composed of hot-rolled profiles. If the existing traditional chassis had been produced from welded beams, even greater cost savings could have been achieved.

A lighter and stronger trailer also has a direct and obvious benefit for logistics operators. The maximum weight of the vehicle is limited by law, so a lighter configuration enables an increase in payload on every trip. In many cases, less fuel consumption and resulting fuel savings can also be observed, which directly affects the operational profit of any logistics company. Depending on the type of vehicle and upgrading approach, the introduction of Strenx^® performance steel can lower the total weight by 285 to 1,326 kg (628 to 2,923 lbs.). 

By selecting the appropriate HSS grades for the application, maintenance costs can also be lowered. Combining high-strength with abrasion or weather resistance can also help the vehicles withstand the tough demands on their performance.

In addition to the financial benefits, a lighter vehicle will reduce environmental impact by saving primary energy resources and reducing greenhouse gas emissions. 

In a life cycle assessment of a vehicle, different phases are often analyzed. When comparing an upgraded design to an original design in conventional steel, the influence of steel production and the service life is dominant. The latter often accounts for 90% of the total environmental savings for vehicles. 

When analyzing the service life of a vehicle with volume-limited cargo, the energy balance of the vehicle is considered. The basic energy consumption of road vehicles depends on several resistance factors that the vehicle has to overcome during its operation (see illustration). 

F_wi = Total resistance force
F_R = Rolling resistance 
F_L = Aerodynamic resistance
F_St = Gradient resistance of the road
F_B = Acceleration resistance

r = Density of atmosphere
c_w = Aerodynamic resistance coefficient
A = Front area
v = Speed
k_r = Rolling resistance coefficient
m = Mass
a = Gradient angle
k_m = Acceleration resistance coefficient
a = Acceleration

F_wi = F_R + F_L + F_St +  F_B
F_R = k_R • m • g • cos a
F_L = r • c_w • A • v_x² / 2
F_St = m • g • sin a
G = m • g
F_B = k_m • m • a_x

All resistance factors, except for aerodynamic resistance, are linearly dependent on mass. The aerodynamic resistance however, depends on the dimensions of the vehicle and the speed. As a result, energy consumption is also affected by mass, speed, acceleration, and gradient (hilly or flat). These factors are highly dependent on the driving situation and driving behavior. Assuming the same driving situation, the correlation between energy consumption and vehicle weight is linear. The energy savings corresponding to a specific weight savings is independent of the absolute weight of the vehicle. 

Vehicles with a fast, steady speed will therefore have a high aerodynamic resistance and low acceleration resistance, and thus will have moderate specific energy savings by weight reduction. In contrast, slow vehicles with frequent stops and accelerations will have high energy savings by weight reduction. 

Heavy trucks and trailers are the dominant modes of road freight transport in both Europe and the U.S., and account for a significant proportion of the fuel used in the transport sector. Either direct or indirect savings can be achieved by weight reduction. If the cargo is limited by volume, a lighter vehicle uses less energy for hauling, and if the cargo is limited by weight, additional cargo can be transported.

Let’s now consider the impact of weight reductions on lowering fuel consumption, emissions and raw material input for a tipper (dump) trailer with a gross vehicle weight of 44 tonnes (see tables below). We assume that the cargo is limited by volume and the vehicles mainly drive on highways and rural main roads. By using Strenx^® 700MC instead of S355 steel, you can achieve a tare weight savings of 285 kg, a 19% lighter chassis, and annual fuel savings of € 589 (around US$ 640). This positively impacts emissions, with a lifetime savings of 9.41 tonnes CO₂ gained from less steel produced, longer service life, and greater capacity with fewer trips. With high-strength steel in thinner dimensions, you can choose to increase legal payload while maintaining the same total weight.

When hauling weight-limited cargo, reducing the vehicle’s weight allows for a higher legal payload, so fewer vehicle-km are needed to transport the same amount of goods. This results in even greater energy savings than for volume-limited cargo. Let’s take the example of a heavy flatbed trailer in the tables below. By using Strenx^® 700MC instead of hot-rolled S235, you can achieve a tare weight savings of 929 kg, a 23% lighter chas-sis, and annual fuel savings of € 2,686 (about US$ 2,917). The reduced lifetime CO₂ emissions are substantial, at 62.66 tonnes, considering the reduction in raw material input, longer service life and greater payload. In our example, the Strenx^® chassis brings a total payload revenue increase of € 9,461 (about US$ 10,278) annually.