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Euclid began building off highway rigid frame trucks in the 1930's. Unlike the highway trucks that have a flexible frame designed to twist with the road, these trucks had massive rigid frames which were not intended to flex. They were normally made with a short wheel base and tended to be relatively wide providing excellent stability and a very short turning radius. This design was carried forward by a number of manufacturers and the trucks steadily grew in size steadily. By the 1960's sizes in the 25 ton capacity were as small as you found these trucks. Typical of these 25 ton rigid frame trucks is the IH Payhauler featured on this site.
A larger version of the Payhauler is shown here, and the Payhauler page itself has a larger truck featured on the bottom of it. What has happened though is that now we have the Articulated truck displacing the rigid frame truck on the small end. In the 1990's someone finally designed a better mousetrap. These artic trucks as they are called have displaced the rigid frame trucks on the small end of the market and are growing steadily. First you saw only 25 ton models, then 35 tonner and now 50 tonners. No one has yet built the largest rigid frame truck possible and I have no idea if ultimately all will be 'artic', but there is good reason in the smaller sizes.
The Artic truck is typically 'all wheel drive' and is much lower to the ground than the rigid framed truck. This provides both stability and the ability to get around on soft ground which is vastly superior to the rigid frame truck, which is often only rear wheel drive and which will spin out given only a little mud. Given that the smaller trucks are commonly used on construction sites where the haul roads are not the best, the extra traction and stability is a big plus. Additionally their fairly low box makes is possible to load them with smaller loaders which find the high rigid frame trucks 'out of reach'. Given that the large trucks are usually found in open pit mining operations where there are well developed haul roads, it is less clear whether these artic trucks will displace the rigid frame in the larger models, but the artics are getting larger.