The upgrades are being done by individual soldiers and units as the Pentagon decides how Humvees should be changed, and follow public criticism of the Bush administration for not armoring all Humvees ahead of the war.
Nearly three years after rolling into Iraq in trucks covered in many instances only by canvas roofs, the 101st Airborne Division's 3rd Brigade is adding extra layers of armor to its Humvees.
Col. Michael Steele, the brigade's commander, said he ordered the improvements because the insurgents' roadside bombs, known to the military as "improvised explosive devices," have become bigger and harder to detect.
"The responsibility of the commander is to figure out what we need to respond to this evolving threat. The easiest, the fastest and most appropriate answer is add additional armor," Steele said.
Iraqi insurgents are also using more anti-tank mines and making bombs that can penetrate the Humvee's current armor. Among the more deadly devices are explosives shaped to funnel a blast through Humvee plating sophisticated bombs that officials suspect are being imported from neighboring countries like Iran.
Because additional armor won't always stop such explosives, one bomb destroyed an Abrams battle tank last month, for instance, a National Guard unit in Baghdad has added detection devices and other measures to protect its Humvees.
Drawing on the part-time soldiers' backgrounds as mechanics, electricians and carpenters, the 126th Armor Battalion based in suburban Grand Rapids, Michigan, added thermal imaging cameras and a 6-foot boom that can be lowered in front of the Humvee. Dangling chains and an infrared countermeasure on the boom can help trigger explosives before the Humvee is directly over them, said Lt. John Caras.
Caras, a former Marine, was the driving force behind the improvements, which have been made to six of the unit's Humvees.
"Right from the beginning, I was looking for ways to go on the offensive," he said of the upgrades, which also include extra bulletproof glass around the Humvee gunner and lights and sirens to help with traffic control.
Many Humvees around Iraq also jam signals like cell phones, garage door openers and other remote-control devices used by insurgents to detonate explosives.
U.S. troops in the past have hardened soft-skin Humvees by using upgrade kits or by whacking spare steel onto their vehicles, and the Army's chief of staff now requires that all combat vehicles in Iraq be armored. The military now has more than 25,000 armored Humvees in the country.
Commanders in Iraq and at the Pentagon have debated how to further improve the Humvee. The Army also has tested several vehicles to replace it, but a successor has not been developed.
There have been 43 bomb and mine attacks on Humvees operated by the 101st Airborne Division's 3rd Brigade since it came to Iraq in September, killing nine soldiers and injuring dozens.
Army officials would not comment on where Humvees have failed or detail how the armor improvements differ from current designs.
Caras said the additions like the infrared camera, which might detect the thermal footprint of a bomb hidden among roadside debris help turn the Humvee from an armor-wrapped defensive shell into an offensive vehicle.
"It's about moving to where the problem is and counteracting it," he said. "Your purpose is to move against any enemy that's out there."
Commanders in both units say insurgents are adept at hiding their work and improving their bombs. And they are quick to learn.
"All the stupid ones are dead," said Capt. Jamey Turner of Baton Rouge, Louisiana, a brigade commander in Beiji.