DAYTONA BEACH, Fla. – Dale Earnhardt Jr. started a 12-car accident at Daytona International Speedway on Friday that essentially shut down a three-day test session designed to hone NASCAR's redesigned cars.

Stock car racing's most popular driver was trying to bump draft with Marcos Ambrose on the back straightaway when he lifted Ambrose "like a forklift" and turned him into the wall. Ambrose's Ford bounced back across the track and triggered a pileup that collected a host of others. There were no injuries.

"It was a big mess and tore up a lot of cars down here trying to work on their stuff," Earnhardt said. "Definitely the drafting is not like it used to be. You can't really tandem certain cars; certain cars don't match up well."

Two of Earnhardt's Hendrick Motorsports teammates, Jeff Gordon and Kasey Kahne, also were involved. So were defending Sprint Cup champion Brad Keselowski, new teammate Joey Logano, Carl Edwards, Kyle Busch, Jamie McMurray, Martin Truex Jr., Aric Almirola and Regan Smith.

The wreck caused several teams to leave Daytona. At least 10 teams, including Michael Waltrip Racing, Penske Racing and Richard Petty Motorsports headed back to North Carolina.

"It is unfortunate, but sometimes you have to wreck them to learn," Keselowski said.

Chevrolet, Ford and Toyota are using new cars in 2013 that have unique front ends. The cars have considerably less downforce than their predecessors and perform differently on the track.

The previous models had identical designs that made it easier for bump drafting because the front and rear bumpers lined up squarely.

But with redesigned parts in low supply as vendors try to keep up with demands, many teams came to Daytona without backup cars. Drivers took a cautious approach to the test, trying to gain information about speed and handling while hoping to avoid the kind of huge wrecks typically associated with racing in tight-knit packs at nearly 200 mph.

NASCAR asked teams to simulate race conditions Friday by forming the large drafting packs, and there were 18 cars on the track when Earnhardt rear-ended Ambrose.

"You can't push, which I think is a good thing," Gordon said. "The bad thing is you can still get to the guy's bumper, but the cars just don't line up very well. … It's something that is going to have to be dealt with very carefully."

Five-time series champion Jimmie Johnson and crew chief Chad Knaus decided before the session not to race.

"It doesn't make any sense to go out there and draft," Johnson said before the crash. "You don't learn anything. You're just taking a chance of ruining your best race car."

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