Trailer makers experienced robust ordering activity to end 2011 and early this year, according to ACT Research Co. and interviews with manufacturers.

Private equity firm American Industrial Partners said it has purchased trailer manufacturer Heil Trailer International from Dover Corp. Terms were not disclosed.

Customers ordered 28,393 new trailers in November, a 29.5% increase from the same month a year earlier and the largest monthly total since March 2006, ACT Research Co. said.

Fleets today are running their trailers longer than they ever have, and they expect trailer suppliers to make products that can meet higher standards.

New orders for commercial trailers dipped 0.8% in October from the year-ago level after a rise in cancellations, but manufacturers and retail sellers reported strong late October and early November activity.

Frozen Food Express Industries said it will sell more than 400 dry vans and almost 300 tractors to truckload carrier Celadon Trucking Services.

A surge in tank truck business, spurred by oil and shale gas drilling, is fueling an “incredibly hot” market for tank trailers to haul crude oil, sand and related products, industry officials said.

Trailer manufacturers said they received 12,524 net orders in August, a 10% increase from a year ago, but below July’s level, ACT Research Co. reported last week.

Truck fleets ordered 13,200 new commercial trailers during July, a 20% increase over July 2010, but the fourth straight month of declining figures, ACT Research Co. reported.

The more than year-long surge in orders for new trailers showed signs of leveling off in June, increasing just 6% from 2010 levels, compared with an overall 65% gain over the first half of 2011, ACT Research Co. said.