North American Class 8 orders totaled 38,100 units in April, according to ACT Research. This was up more than 31 percent from the level set in March and over 157 percent above previous year.

This solid order level, the strongest since March 2006, resulted in the Class 8 order backlog surging past the 126,000 unit level. The Class 8 order backlog now sits at a 55-month high.

"Positively, Class 8 orders continue to be placed close-in. Of April's orders, 70 percent were scheduled for build by the end of Q3," says Kenny Vieth, ACT president and senior analyst. "Needless to say, open build slots remaining in 2011 are filling rapidly. At current fill rates, the 2011 build schedule could be completely filled by sometime in July. As has been the case throughout 2011, the industry's challenge is ramping up production. The industry's ability to build trucks, rather than demand for trucks will be the constraining factor on Class 8 production this year."

Truck OEs have been sounding the warning for the past couple of months. Last week, Jim Hebe, Navistar's senior vice president of North American sales operations, told reporters there's going to be a big gap between order intake and build rates, simply because the supply base isn't there.

Speaking strictly for Navistar, Hebe said his company can't raise production beyond where it is now, and he noted that he has heard similar concerns raised by his competitors.

In late March, Daimler Trucks North America President and CEO Martin Daum had much the same message. He told reporters at the Mid-America Trucking Show, "I would like to increase production further and faster, but I can't, due to alack of commodities in the market, starting with tires. We run from one shortage into another shortage, so you just don't have the production to sustain [higher numbers]."