Dale Earnhardt Jr. started a 12-car accident at Daytona 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 Marcus 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.

“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.

There were no injuries, but the wreck caused several teams to leave Daytona. At least 10 teams, including Michael Waltrip Racing, Penske Racing and Richard Petty Motorsports, packed up their haulers and headed back to North Carolina.

“It is unfortunate, but sometimes you have to wreck them to learn,” Keselowski said. “The sport is rewinding. That is the important thing to say. The sport advanced to the two-car tandem three or four years ago, and there were certain things you could do then that you couldn’t do in the past without wrecking.

“Now the rules package is back to where we were in the early 2000′s when the fans enjoyed the racing better. We as drivers have to rewind to how we used to drive these cars. This is how you do it. You make mistakes and learn and that is part of it. I might be the guy who makes the mistake next time, so I can’t be mad about it.”

Manufacturers Chevrolet, Ford and Toyota are using new cars in 2013, ones that have unique front ends that make race cars more closely resemble those on the streets and in the showroom.

The new 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 triggered the big one.

” 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. You are going to have to be cautious when you do it and do it with the right guys, but most of the time you’re going to need to stay away from it. That is certainly something we learned.”

Backing up Gordon’s sentiments, Earnhardt said the nose of his Chevrolet slid under the rear bumper of Ambrose’s Ford.

“It’s going to take a lot more care and concentration and just knowing kind of what is at stake,” Earnhardt said. “Certain cars you line up OK with and can push fine and for whatever reason mine and Marcus’ car didn’t line up good. We got our bumpers together and it hooked him. For whatever reason, you’ve got to be careful who you are working with.”

One driver who avoided the melee was five-time series champion Jimmie Johnson. He and crew chief Chad Knaus decided before the session to stay.

“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.”